Acta Metall Sin-Forthcoming Articles

Interaction Between Coarse and Fine Grains in the Inhomogeneous Microstructure of Stainless Steel Weld and Its Influence Mechanism on Residual Stress Distribution

Stainless steel components are widely used in aerospace, marine engineering, and railway transportation owing to their outstanding corrosion resistance and high mechanical strength. Welding is a key joining technique for stainless steel structural components. However, substantial residual stresses are inevitably introduced into components owing to the complex coupling effects of force and heat during welding. Because of the critical role of residual stress in the performance and service life of components, considerable attention has been given to its experimental measurement and theoretical modeling, as well as to the effects of microstructure (e.g., phase transformation and grain size) on residual stress. However, the microscopic mechanism through which grain size, especially the interaction between coarse and fine grains, influences residual stress distribution remains an open question. To clarify this mechanism, this study investigates grain-size distribution and interactions between coarse and fine grains in the weld zone of stainless steel samples prepared via electron beam welding using a combination of X-ray and neutron diffraction. A clear correlation is observed between grain-size variation in the weld zone and residual stress distribution. Samples with a narrow grain-size distribution (~32 μm) exhibit minimal transverse residual stress fluctuations, with a stress amplitude of ~160 MPa. However, samples with a wide grain-size distribution (~53 μm) show substantial transverse residual stress fluctuations, with a stress amplitude of ~316 MPa. Owing to the high dislocation density at grain boundaries and within grains, fine grains possess greater deformation resistance than coarse grains. The interplanar spacing of fine grains is larger than that of coarse grains, indicating that fine grains undergo less deformation and exert an extrusion effect on coarse grains. The size difference between coarse and fine grains is a key microscopic parameter that determines the distribution of welding residual stresses. By regulating the uniformity of grain-size distribution, local stress concentration can be reduced, enabling effective control of residual stress and enhancing the structural stability of welded components.

Influence of Equiaxed Dendrite Movement and Columnar-to-Equiaxed Transition on Macrosegregation Formation in Fe-0.45wt.%C Steel Ingots

Large steel ingots are widely used in heavy-duty equipment for the nuclear power, hydropower, and metallurgical industries. However, severe macrosegregation induced by thermosolutal convection, grain sedimentation, and solidification shrinkage during solidification deteriorates the chemical homogeneity and service reliability of ingots. To understand the macrosegregation formation mechanism and develop effective control strategies, a mixed columnar dendritic-equiaxed three-phase solidification model was developed within the framework of the Eulerian-Eulerian approach. By tracking the position of the columnar dendrite tip and capturing the equiaxed dendrites at the columnar front, the effects of the equiaxed-dendritic grain movement, interphase momentum exchange, and columnar-to-equiaxed transition (CET) on macrosegregation in a 2.45 t Fe-0.45%C (mass fraction) steel ingot were investigated. The predicted maximum negative segregation at the ingot bottom reaches approximately -0.27, and the length of the conical negative segregation zone is approximately 2/3 of the ingot’s height. These results are consistent with the experimental measurements reported in the literature. Increasing in interphase momentum exchange intensity between the liquid and equiaxed phases enhances the liquid-solid coupling and weakens the relative slip of equiaxed grains, thereby shifting the CET position and further reshaping the morphology and intensity of the V-segregation bands. Reducing the heat transfer intensity at the hot top decreases the maximum positive segregation there from 0.76 to 0.26. However, the positive segregation bands in the transition zone beneath the hot top worsen. The combined scheme of sidewall gradient cooling and hot-top insulation reduced the maximum positive segregation along the ingot centerline from 0.76 to 0.10 and the maximum negative segregation from -0.27 to -0.12, effectively suppressing the positive segregation at the ingot top, A-segregation beneath the hot top, and the negative segregation at the ingot bottom.

Fast Prediction of Flow Fields in a Slab Continuous Casting Mold by an Attention-Gated Dual-Head U-Net

The flow field of molten steel in the mold play an important role in the slab quality, and rapid and accurate prediction of flow-field is an important foundation of process optimization and on-line control for continuous casting. Because high computational cost of computational fluid dynamics (CFD) simulations for flow field in a slab continuous-casting mold cannot satisfy the demand of the rapid evaluation, an attention-gated dual-head U-Net model is proposed on the base of small-sample on a two-dimensional flow-field. First, three-dimensional steady-state flow field is obtained by OpenFOAM, and the related two-dimensional flow-field is obtained by interpolation. Next, a data extension method based on the statistical characteristics of turbulent kinetic energy is developed to expand the flow-field dataset according to the relationship between turbulent kinetic energy and fluctuating velocity. Furthermore, an attention-gated dual-head U-Net model with a multi-channel input integrating the signed distance function (SDF) and inlet velocity masks is proposed to adaptively focus on key flow regions, which include the jet from the nozzle and the upper and lower recirculation zones. The results show that, after training, the inference time for a flow-field is less than 1 s. Under the conditions of a fixed nozzle angle and various casting rates, the velocity mean absolute error (MAE) is on the order of 10^-3 m/s. Under the unseen nozzle-angle and casting-rate conditions, the velocity MAE remains on the order of 10^-2 m/s. These results demonstrate that the data extension method based on turbulent-kinetic-energy statistics combined with the attention-gated dual-head U-Net model can enable rapid reconstruction of molten-steel flow fields in the mold, and provide a feasible approach for subsequent multi-physics digital-twin modeling of continuous casting.

Machine Learning-Driven Compositional Design of Refractory High Entropy Alloys with Synergistically Optimized Strength and Ductility

Refractory high-entropy alloys (RHEAs) exhibit outstanding high temperature stability and mechanical strength, making them promising candidates for aerospace and other high temperature applications. However, their widespread application is constrained by the inherent trade-off between high strength and limited ductility, while conventional trial-and-error compositional design strategies remain inefficient and costly. To address this challenge, a Machine Learning (ML)-driven compositional design framework is proposed for the synergistic optimization of compressive Yield Strength (YS) and Fracture Strain (FS) in RHEAs. The data comprising 298 reported literature on compressive property of Al-Hf-Mo-Nb-Ta-Ti-V-Zr-W alloys was constructed. By systematically developing feature pools and model pools, optimal feature subsets and ML models were identified for accurate prediction of YS and FS. Feature importance analysis reveals that local atomic size mismatch and modulus mismatch dominate the yield strength, whereas electronegativity difference and the structural stability parameter γ play critical roles in governing the fracture strain. An expected improvement (EI)-based utility function was further integrated with the NSGA-II multi-objective optimization algorithm to efficiently explore the nine-element compositional space and identify the Pareto-optimal strength-ductility trade-off. The developed models achieve high predictive accuracy with R2 values of 0.94 for YS and 0.87 for FS. Guided by the optimization results, a promising alloy composition, Mo18Nb26Ti25V11Zr20, was prepared. The alloy exhibits a single-phase BCC structure, delivering a high compressive yield strength of 1713 MPa, while maintaining a fracture strain of 36.5%, in excellent agreement with model predictions. These results demonstrate that the proposed machine learning framework enables efficient and synergistic optimization of strength and ductility in RHEAs.

Effect of He⁺ Irradiation on the Microstructural Evolution and Properties of the Diffusion-Strengthened Copper Alloy Cu–Y₂O₃

The development of fusion energy is an important direction for meeting future energy demands, and the divertor, as a key component of fusion reactors, operates in harsh service environments characterized by high heat flux and intense radiation. Yttrium oxide powder-reinforced (Cu–Y₂O₃) copper alloy can effectively capture irradiation defects, suppress their aggregation and growth, and enhance the radiation resistance of copper-based materials by introducing highly stable nano-Y₂O₃ particles into the matrix. Therefore, it is urgent to investigate the service performance of the candidate heat sink material Cu–Y₂O₃ under similar working conditions. To investigate the effects of service-like irradiation on Cu–Y₂O₃ copper alloy, this study employed mechanical alloying and spark plasma sintering to prepare the alloy, followed by high-energy He⁺ irradiation at 450°C to examine the effect of irradiation dose on the microstructural evolution and properties of the alloy. The results show that under high-energy He⁺ irradiation, defects such as He bubbles and dislocation loops gradually form within the Cu–Y₂O₃ copper alloy. As the irradiation dose increases from 1.0 × 10¹⁶ to 1.0 × 10¹⁷ ions/cm², the average size of He bubbles increases from approximately 7.3 to 19.0 nm, and the total bubble number density increases from 3.4 × 10²¹ to 9.2 × 10²¹ m⁻³ and finally decreases to 2.6 × 10²¹ m⁻³, showing a trend of initial increase followed by a decrease; that is, the coalescence of He bubbles dominates at high doses. The size and number density of irradiation-induced dislocation loops increase steadily from 4.65 nm and 6.6 × 10²¹ m⁻³ to 9.45 nm and 8.7 × 10²¹ m⁻³, respectively. Dislocation loops are a key contributor to irradiation hardening in Cu–Y₂O₃ copper alloy. As the irradiation dose increases, defects gradually reach saturation, and the material hardening also tends to saturate.

Microstructural Inheritance Behavior and Mechanical Property Control of TA18 Alloy From Ingot to Tube Blank#br#

LI, Shuai-Yu, Yang, Jieren

In metallic material processing, the microstructural and textural characteristics developed during each manufacturing stage establish the structural foundation for subsequent processing operations, creating substantial hereditary effects throughout the multistage manufacturing chain. This progressive, cross-process accumulation of microstructural and textural evolution makes precise tracing and effective control of microstructural inheritance pathways particularly complex during full-scale manufacturing from the initial ingot to the final TA18 tube blank. Microstructural inheritance forms the basis for structural design and property optimization in TA18 alloys. This investigation employed a 700-mm diameter TA18 alloy ingot as the starting material. The thermomechanical processing route comprised three stages: initial multipass forging in the β-phase field (1150℃–950℃) followed by forging in the α+β phase field at 900℃ to produce a 170-mm diameter rough-forged bar; subsequent processing of the rough-forged bar in the two-phase region at 850°C to obtain a 125-mm diameter finish-forged bar; finally, peeling, drilling, and canned hot extrusion of the finish-forged bar to fabricate tube blanks with inner and outer diameters of 28 and 42 mm, respectively. This study systematically investigates the hereditary evolution of microstructure and texture throughout the processing route from ingot to tube blank in the TA18 alloy and evaluates its impact on mechanical properties to guide the integrated control of multiscale microstructure and performance. The results demonstrate that microstructural hereditary characteristics can be categorized into three primary types. (1) Once established during rough forging, the micron-scale grain size and equiaxed α-phase morphology remain stably inherited in subsequent processing stages. Grain refinement serves as the fundamental basis for increasing axial tensile strength from 400 to 550 MPa and improving elongation. (2) The inheritance of low-angle grain boundary (LAGB) content exhibits dynamic evolution. The high content inherited during the rough- and finish-forging stages contributes to work hardening but negatively affects plasticity; in contrast, the low content inherited after extrusion, achieved via complete recrystallization, results in plasticity recovery. (3) The inheritance of crystallographic texture is governed by multiple competing mechanisms. The {0001}∥axial direction (AD) basal texture and the1010∥AD texture originating from the central region of the ingot are strongly inherited during subsequent processing, with the intensity of the1010∥AD texture continuously increasing. The c-axis orientation of the α-phase undergoes controlled evolution during processing: it is randomly distributed after rough forging, transforms into a radial texture after finish forging, and finally develops into a circumferential texture after extrusion. The TA18 alloy controls the inheritance and evolution of crystallographic orientation through the competition and synergy of various mechanisms, including initial crystallographic orientation, deformation, and dynamic recrystallization. The integrated design of TA18 alloy tube blanks, possessing high strength and good plasticity can be achieved by reinforcing beneficial hereditary features (such as fine grains and favorable texture) and interrupting harmful features (such as high LAGB content and unfavorable texture).

Effects of Electrically Assisted Forming on the Microstructure and Mechanical Properties of 7050 Aluminum Alloy Thin-Walled High-Ribbed Forgings

XING, Zi-Han, HU, Jian-Liang

7050 aluminum alloy thin-walled high-ribbed die forgings are essential structural components in aerospace engineering, necessitating a lightweight design coupled with high mechanical performance. However, conventional hot die forging often results in a substantial microstructural gradient distribution, characterized by coarse surface grains and severe inhomogeneity between the surface and core of the forgings due to rapid surface cooling when the billet contacts the die. These defects significantly degrade the mechanical properties and reliability of the components. In addition, conventional process optimization methods cannot fundamentally resolve these issues, often necessitating the removal of unacceptable surface layers, which leads to considerable material waste. Pulsed current-based electrically assisted forming (EAF) has demonstrated unique advantages in regulating metal plastic deformation and microstructure via the skin, electrothermal, and electroplastic effects. However, its systematic application in complex thin-walled high-ribbed die forgings has not been documented. To address these challenges, this study incorporates pulsed current into the conventional hot die forging process, taking 7050 aluminum alloy H-shaped thin-walled high-ribbed die forgings as the research object. Key parameters include a pulse voltage of 400 V, pulse width of 50 μs, and energizing time of 60 s. Two current frequencies, 600 and 1000 Hz, are selected as experimental variables to assess the regulatory effects of EAF on the microstructure and mechanical properties. The study employs finite element simulations to analyze current density and temperature field distributions, alongside multiscale experimental characterizations such as OM, EBSD, SEM, TEM, and tensile testing. This comprehensive approach reveals the microstructural evolution and strengthening mechanisms under electro-thermal-mechanical multifield coupling. Results demonstrate that the 600-Hz medium-frequency EAF process yields the optimal comprehensive regulation effect. The pulsed current mitigates surface heat loss during die forging, facilitates sufficient dynamic recrystallization, and generates uniform equiaxed grains throughout the cross-section, thus eliminating microstructural inhomogeneity. Moreover, the electro-thermal-mechanical coupling effect enhances solute solid solubility and boosts the contribution of precipitation strengthening to yield strength, leading to simultaneous improvements in strength and ductility. The mechanical properties of the surface and core of the forgings are highly balanced, with yield strength, tensile strength, and elongation measuring 495 MPa, 511 MPa, and 13.6% for the surface, and 492 MPa, 507 MPa, and 13.5% for the core, respectively. By contrast, the 1000-Hz high-frequency current exacerbates the skin effect, causing excessive current concentration on the surface and insufficient energy penetration into the core, resulting in incomplete recrystallization and increased structural inhomogeneity. Consequently, the mechanical properties of the 1000-Hz-processed forgings are inferior and less uniform. This study elucidates the regulatory mechanism of current frequency on the microstructural homogeneity and mechanical properties of complex aluminum alloy die forgings under electro-thermal-mechanical multifield coupling. The findings provide a novel and efficient technical approach for integrated microstructure-performance control in high-strength aluminum alloy thin-walled high-ribbed components, offering considerable potential for reducing material waste and enhancing component reliability in aerospace manufacturing.

Deformation Behavior of Mg–Zn–Y–Sn Alloy Containing Long-Period Stacking Ordered Phase Under High Strain Rates

Lightweight Mg alloys are desirable for transportation and defense applications owing to their high specific strength and energy‑absorption capacity under impact loading. Among these, Mg–Zn–Y alloys containing long-period stacking-ordered (LPSO) phases exhibit exceptional comprehensive mechanical properties; however, their deformation behavior and underlying micromechanisms under high-speed shear remain poorly understood. Therefore, this study performs a comparative analysis of the microstructural evolution and dynamic mechanical behavior of as-cast and extruded Mg–Zn–Y–0.3Sn (mass fraction, %) alloys containing LPSO phases using a split Hopkinson pressure bar at strain rates ranging from 800 s⁻¹ to 1800 s⁻¹. The aim is to elucidate the strength response, identify the dominant deformation mechanisms, and clarify how microstructural evolution, including features characteristic of LPSO-containing Mg–Zn–Y alloys, couples with the high-strain-rate shear response. The results show that as the strain rate increases from 800 s⁻¹ to 1800 s⁻¹, the ultimate compressive strength of the extruded Mg–4Zn–12Y alloy increases from 279.16 MPa to 354.78 MPa, showcasing an enhancement of approximately 27.1%. Similarly, increasing the strain rate from 800 s⁻¹ to 1600 s⁻¹ raises the ultimate compressive strength of the extruded Mg–4Zn–12Y–0.3Sn alloy from 342 MPa to 433 MPa, corresponding to an improvement of approximately 26.6%. The Sn addition enhances the alloy’s strength and work-hardening capacity. The {1012} tensile twinning primarily serves to coordinate plastic strain, whereas kinking the LPSO phase plays a critical role in coordinating the deformation. High densities of geometrically necessary dislocations accumulate at grain-boundary triple junctions, producing considerable local lattice distortion and providing favorable conditions for twin nucleation. Furthermore, dynamic recrystallization occurs around the LPSO phase, driven primarily by the particle-stimulated nucleation mechanism and the complex morphology of the LPSO phase/matrix interface. These findings provide crucial insights into the deformation micromechanisms of LPSO-containing Mg alloys under high-strain-rate conditions.

High-temperature Microstructural Stability of High-Si Austenitic Steels for Lead-Cooled Fast Reactor Fasteners

Currently, there is rapid development of lead-cooled fast reactors (LFRs) as a main type of Generation IV nuclear reactor in China. High-Si austenitic stainless steel is a candidate structural material for important LFR components, such as fasteners, due to its excellent high-temperature mechanical properties and corrosion resistance to liquid Pb–Bi eutectic. However, high-Si steel accelerates the precipitation of secondary phases, such as M₂₃C₆, M₆C, χ phase, G phase, and ferrite, during long-term high-temperature aging, thereby reducing microstructural stability and degrading mechanical properties. Therefore, while high-Si steel enables better corrosion resistance, the problem of high-temperature microstructural stability remains a critical issue to resolve. This review focuses on the use of high-Si austenitic stainless steel for LFR fasteners, with the introduction of the Si alloying design principle and a summary of the evolution characteristics of secondary phases induced by Si addition in austenitic steels. Furthermore, an alloying strategy to improve microstructural stability based on experimental findings is proposed along with an outlook toward future developments.

Research Progress and Future Prospect on New High-Speed Extruded Magnesium Alloys

High-speed extruded magnesium (Mg) alloys demonstrate tremendous potential for lightweight applications in aerospace and rail transportation owing to their low cost and excellent extrudability. With the increasing demand for low-cost and reliable service in high-end equipment manufacturing, large-sized Mg alloy profiles are facing significant challenges in achieving both high-efficiency processing and excellent mechanical properties. Overcoming the trade-off between extrusion processability and service performance has thus become a critical issue that urgently needs to be addressed in Mg alloy research. Therefore, it is crucial to develop low-cost Mg alloys with both excellent extrusion processability and reliable service performance. In this paper, the compositional design strategy of high-speed extruded Mg alloys is systematically elucidated based on three core perspectives: low alloying, solidus temperature, and dynamic recrystallization. Furthermore, the research progress and underlying mechanisms of high-performance Mg alloys fabricated via high-speed extrusion are reviewed, with a focus on thermal stability, corrosion resistance, strength, and ductility. This review aims to provide a reference for future material design and engineering applications of high-speed extruded Mg alloys, offering significant engineering value and scientific relevance for the development of cost-effective, high-performance lightweight materials.

Microstructure and Mechanical Properties of Laser Additively Fabricated TiBw/Ti65 Composite After Shot Peening

Titanium matrix composites hold immense application potential in the field of aerospace high-temperature load-bearing components. Laser additive manufacturing technology, known for its short production cycles and high efficiency, enables rapid fabrication of these composites and their components. However, the introduction of ceramic reinforcement phases leads to issues such as high surface roughness and the concentration of residual tensile stress in Ti matrix composites, which hinders their engineering applications. This study investigated the effect of shot peening on the microstructure and mechanical properties of a laser deposited 1.0%TiBw (volume fraction)/Ti65 composite. The results reveal that after shot peening treatment, the surface roughness of the 1.0%TiBw/Ti65 composite increases from 2.02 μm (in the as-laser-deposited state) to 5.21 μm at an air pressure of 0.25 MPa. Meanwhile, the surface microhardness increases from 381.1 HV (in the as-laser-deposited state) to 683.9 HV under the same air pressure. The maximum residual compressive stress is located at a depth of 30–90 μm from the surface, reaching −571 MPa at a shot peening air pressure of 0.2 MPa. Shot peening treatment causes the α-colonies of the 1.0%TiBw/Ti65 composite to be fragmented into lamellar α or equiaxed α grains. When the air pressure is 0.2 MPa, the room-temperature tensile strength of the 1.0%TiBw/Ti65 composite reaches achieves 1238 MPa. According to the calculation based on the strengthening theory, the increase in the room-temperature yield strength of the 1.0%TiBw/Ti65 composite is mainly attributed to the dislocations introduced during the shot peening process.

Formation Mechanism and Processing of Solute Clusters in Mg Alloys

As the lightest metallic structural material, magnesium (Mg) alloys hold promising application prospects in the automotive, aerospace, and defense industries. However, their widespread use is hindered by severe intrinsic limitations, including limited precipitation hardening capability, a tendency for precipitate coarsening, and a pronounced strength-ductility trade-off. Through strategic alloy design and processing optimization, fully coherent solute clusters can be introduced into the matrix. Further control over their evolution and stability offers a viable pathway toward achieving an improved balance between strength and ductility. This review systematically outlines the preparation methods, microstructural evolution, and thermal stability of solute clusters in Mg alloys. It also provides an in-depth analysis of the mechanisms through which these clusters influence alloy strength, plastic deformation behavior, and corrosion resistance. Finally, the future research directions for solute clusters in Mg alloys are discussed, focusing on compositional design and property optimization.

Achieving Enhanced Electrical Contact Performance of Ag/Ti₃AlC₂ with Novel Spherical Micro/Nano Composite Reinforcing Phase

Ding, Jianxiang, Yang, Yang, ZHANG, Shi-Hong

Ag-based electric contact materials (ECMs) critically determine the performance of low-voltage switchgear for low-frequency and high-current control. However, these materials suffer from poor environmental friendliness, low tensile strength (< 200 MPa), high material loss (> 5%), and short service life (< 1 × 10⁴ cyc). To improve the comprehensive performance of Ag/Ti₃AlC₂ ECMs, this study designs a reinforcing phase with a novel composition and structure. For electrical contacting processes, the layered structure of MAX phases and their excellent wettability with metals ensure strong anti-material transfer performance of Ag/MAX composites. However, the interdiffusion between the A-layer atoms of MAX phases and the Ag matrix leads to interfacial solid-solution behavior that deteriorates the electrical conductivity and electrical contact reliability. To create an innovative design of the morphology and structure of MAX phases, this study combines mechanical ball milling, spray granulation, and vacuum heat treatment to prepare large-sized spherical Ti₃AlC₂-reinforcing phase powders with surface-loaded nano-Al₂O₃ particles that are subsequently combined with the Ag matrix. The spherical micro- and nano-composite structured reinforcing phase is found to improve the microstructure uniformity of the Ag/Ti₃AlC₂ material, promote unidirectional diffusion of Ag along the grain boundaries of the spherical reinforcing phase into its interior, and restrict the Ag-Al interdiffusion behavior within the reinforcing phase. Consequently, the resistivity reduced from 5.462 to 4.431 μΩ·cm. The nano-Al₂O₃ particles on the surface of the spherical reinforcing phase are mainly distributed at the Ag/Ti₃AlC₂ interface; however, some are detached and dispersed through the matrix. The unidirectional Ag diffusion and distribution of nano-Al₂O₃ particles collectively enhance the bonding quality of the composite interface, thereby increasing the tensile strength (228.8 MPa), interface surface hardness (140.2 HV), and interfacial nanohardness (3.261 GPa). During 2 × 10⁴ cyc of arc discharging, the microsized spherical Ti₃AlC₂ reinforcing phase and nanosized Al₂O₃ particles synergistically increase the viscosity of the molten pool at high temperatures caused by electric arc discharging, effectively restricting the movement of liquid Ag, diminishing the material loss to 1.7%, and thus improving the antiarc erosion performance of Ag/Ti₃AlC₂ composites. This work provides research ideas and technical references for the performance improvement of Ag-based ECMs.

Effect of Tempering Time on the Hydrogen-Induced Delayed Fracture Behavior of Medium-Carbon Cr–Ni–Mo–V Type High-Strength Bolt Steel

With the advancement of modern industry, further improvement of the strength of high-strength steels is highly required without compromising their resistance to hydrogen-induced delayed fracture (HIDF). Quenching and tempering is the most convenient and effective method for regulating the HIDF performance of high-strength steels along with their mechanical properties, when their chemical composition is fixed. In addition to tempering temperature, tempering time can influence the HIDF performance of high-strength steels by controlling their microstructural characteristics. Herein, the effect of high-temperature tempering time (t_temp) at 600 °C on the HIDF behavior of a medium-carbon Cr–Ni–Mo–V-type high-strength bolt steel was investigated through slow strain rate tensile tests using pre-hydrogen-charged notched round bar specimens. The results reveal that when t_temp is extended from 2 to 6 h, the size of the nanoscale plate-like V-rich MC precipitates remains almost unchanged, and when t_temp is further extended to 24 h, a considerable increase in size is observed. The hardness and strength of the experimental steel first increase and then decrease with increasing t_temp, with a peak at t_temp = 2 h. Further theoretical calculations reveal that precipitation strengthening and dislocation strengthening are the dominant factors causing the differences in the strength of the experimental steel tempered at different t_temp. The HIDF resistance, characterized by the notch tensile strength (NTS_H) of hydrogen-charged samples, decreases exponentially as the diffusible hydrogen content (H_D) increases. At the same H_D level, NTS_H increases gradually in the order of 2, 24, and 6 h tempered samples, whereas the hydrogen embrittlement susceptibility index, characterized by the relative notch tensile strength loss ratio, decreases gradually in the same order. Furthermore, the critical hydrogen content H_C, defined as the H_D corresponding to 90% of the notch tensile strength of the un-hydrogen-charged sample, gradually increases in the order of 2, 24, and 6 h tempered samples. This indicates that the 6 h tempered sample exhibits the greatest resistance to HIDF, the highest hydrogen tolerance, and the lowest susceptibility to hydrogen embrittlement. Further hydrogen thermal analysis and hydrogen permeation test results reveal that the experimental steel exhibits the strongest hydrogen trapping capability and the lowest hydrogen permeation coefficient at t_temp = 6 h. In conclusion, changes in the hydrogen trapping characteristics of the V-rich MC precipitates with t_temp are the main reason for the changes in the HIDF performance of the experimental steel. Therefore, selecting an appropriate t_temp allows the attainment of the desired strength level and helps achieve excellent resistance to HIDF.

Research Status and Development Trend of Precipitation Phase Regulation and Strengthening Mechanism in Ultra-High Strength Steel

As a key structural material for aviation, aerospace, and major equipment, the lightweight and high-performance development of ultra-high strength (UHS) steel is essential for the advancement of related fields. However, traditional UHS steels have long faced critical challenges: the difficulty in achieving a synergistic combination of high strength, high toughness, good weldability, and corrosion resistance, along with the constraint of high alloy costs that limit their widespread application. To address these issues, we systematically review the development history, composition, microstructure, and performance characteristics of traditional UHS steels, including low-alloy UHS, secondary hardening, maraging, and precipitation-hardening stainless steels. We also review the research progress in developing novel UHS steels through two main pathways: the regulation of composite nanoprecipitates and the design of multiphase composite microstructures. Special attention is paid to exploring the potential and preliminary practices of the “hybrid” design concept, which breaks the boundaries of traditional steel classifications and integrates multiple strengthening mechanisms to develop cost-effective, high-performance, and easily weldable “all-round” UHS steels. The current research status regarding the evolution laws of precipitates and strengthening mechanisms in UHS steels is detailed. Finally, the future development directions of UHS steels are proposed, particularly regarding the synergistic control of complex precipitates, high-temperature stability, and industrial application.

Molecular Dynamics Study on the Nb Regulation Mechanism in the Primary Irradiation Damage of Zr-Nb Alloys

Zr alloys represent the key material employed in the fabrication of fuel cladding for pressurized water reactors. The primary irradiation damage behavior displayed by the alloys exerts a direct influence on the operational safety of the reactors. However, the atomic-scale regulatory mechanism of Nb in this behavior remains to be elucidated. Therefore, this study employs molecular dynamics method to examine the primary irradiation damage characteristics of Zr-xNb (x = 0.0, 0.5, 1.0, 2.5; atomic fraction, %) single-crystal alloys with various primary knock-on atom (PKA) incident crystallographic directions and system temperatures. The findings suggest that the impact of the PKA incident direction on the total number of Frenkel pairs and displaced atoms is less than 1%. However, the PKA incident direction has been shown to considerably alter the proportional distribution of Nb vacancies. As the system temperature increases, the size of vacancy and interstitial clusters displays a non-monotonic variation, initially increasing and then decreasing. At 573 K, which is the typical service temperature of nuclear reactor cladding materials, there is a heightened propensity for coalescence, as evidenced by a high tendency for such processes to occur. The proportion of interstitial Nb atoms is notably higher than the nominal Nb content of the alloy. Additionally, increasing the Nb content markedly weakens the anisotropy of interstitial atom diffusion. With respect to Nb concentration, the size of interstitial clusters is maximized at 0.5%Nb; however, at 1.0%Nb, the augmented pinning effect of Nb results in a slight reduction in cluster size. The three-dimensional pyramidal structures characteristic of these systems simultaneously undergoes a transition to a two-dimensional basal-plane structure, accompanied by a notable weakening of the anisotropy of interstitial diffusion. This effect suppresses the growth of interstitial <a>-type nanoclusters on the prismatic planes and ultimately enhances the irradiation growth resistance of Zr-Nb alloys. This study clarifies the underlying mechanisms of primary irradiation damage in Zr-Nb alloys, thereby providing a theoretical foundation at the atomic scale for enhancing the irradiation resistance of these materials.

In Situ Study on Tensile Deformation and Damage of Particle-Reinforced Aluminum Matrix Composites Based on High Spatiotemporal Resolution Synchrotron Radiation CT

Lin, Hao, Hu, Hongjie, Li, Ke, Wang, Jun, Zhang, Junfan, Deng, Biao, Xiao, Bolu

Particle-reinforced aluminum matrix composites are widely used in aerospace and rail transportation fields due to their excellent specific strength and specific stiffness. However, their further development is limited by brittle fracture and the strength–toughness trade-off caused by the introduction of reinforcements. Traditional quasi-static characterization methods are inadequate for capturing the dynamic damage evolution of materials under load. To address this challenge and reveal the dynamic fracture mechanisms of Particle-reinforced composites, this study developed an in situ dynamic continuous tensile X-ray computed tomography (CT) method with a temporal resolution of up to 5 Hz at the BL16U2 beamline of the Shanghai Synchrotron Radiation Facility. This approach enabled 4D (3D space and time) observation of the entire fracture process of a 12 vol.% Ti2AlC/Al composite. By combining deep learning and ambient occlusion (AO) algorithms, the challenges associated with accurately segmenting reinforcements and microcracks in low signal-to-noise ratio dynamic images were successfully addressed. The findings revealed that: (1) Damage initiation exhibited significant size dependence, with microcracks preferentially nucleating inside larger and agglomerated Ti2AlC particles; (2) Damage evolution followed a three-stage characteristic of “multiple-site crack initiation–matrix connection–localized coalescence”; (3) At the critical point of failure, isolated microcracks rapidly coalesced by the Al matrix cracks, leading to a sharp decrease in the number of cracks and a drastic increase in the volume proportion of the main crack and ultimately triggering unstable fracture. This study elucidated the dynamic fracture mechanism of 12 vol.% Ti2AlC/Al composite and demonstrated the unique advantages of sub-second high spatiotemporal resolution CT in capturing nonlinear failure behaviors in materials.

Progress in Exploring Plastic Deformation Mechanisms of Pure Tungsten

Tungsten ( W ) has become the preferred plasma facing materials ( PFMs ) for the divertor in nuclear fusion reactors due to its high melting point, low physical sputtering rate, low deuterium retention and excellent mechanical properties. At the same time, because of its high density, high hardness, high conductivity and thermal conductivity, it is regarded as an important strategic material in the fields of national defense, military industry, electronics and electrical engineering. However, pure W exhibits obvious brittle behavior at room temperature due to its poor toughness and high ductile-brittle transition temperature. This not only increases the difficulty of processing at low temperatures, but also as a fusion material, W is subjected to continuous thermal damage and irradiation damage during the operation of the fusion reactor. This harsh service environment will aggravate the brittle cracking of W, which greatly reduces the service life of related components. The plastic deformation behavior and toughening modification of W have become a hot issue in current research, but there are still defects in the essence of the plastic deformation mechanism of tungsten. At the same time, existing studies have shown that single crystal W exhibits good ductility at low temperatures, and its deformability varies significantly with different crystal curves. At the same time, it is also found that the plastic deformation of nanocrystalline W is dominated by twinning deformation, which means that there are still loopholes in the understanding of the nature of plasticity and toughness of pure W. Based on the existing research results at home and abroad, this paper expounds the improvement of people 's understanding of the plastic deformation mechanism of pure W, and summarizes the research progress of plastic deformation of pure W under different conditions, hoping to improve people 's understanding of the plastic deformation mechanism of W and provide new ideas for the subsequent modification of pure W.

Aluminum alloys are widely used in marine engineering applications. However, due to their inherent properties, Al alloys commonly face severe challenges of corrosion and wear failure. Current single-surface strengthening techniques for aluminum alloys, like plasma implantation, micro-arc oxidation and physical vapor deposition, still exhibit numerous limitations and considerable difficulties in process design. This thesis employs a combined magnetron sputtering and ion nitriding technique to fabricate a gradient-structured coating on 7075Al alloy, which consisted of Ti-N compounds/Ti-Al intermetallic compounds/aluminum substrate. By changing the nitriding temperature (460 °C to 520 °C), the comprehensive properties of the coating are enhanced. The experimental results show that the titanium nitride gradient coating exhibits well-defined interfacial boundaries and excellent adhesion. The primary phases are TiN0.3 and Al2Ti, and the relative content of TiN0.3 increased with the rising nitriding temperature. HRTEM analysis reveals the formation of TiN nanocrystalline particles on the surface of the N520 sample. The mechanical properties of the titanium nitrided coating also improved with increasing nitriding temperature. The N520 sample exhibited the highest hardness and lowest wear rate, with a wear rate of 9.24 × 10-16 m3/N·m at a load of 5 N. Under heavy loads, increased shear forces caused the surface coating of the N490 sample to fracture into discrete micro-zones due to the presence of net-like microcracks. Anisotropic slippage occurred during wear, leading to mutual collision and compression that resulted in complete coating spalling and failure. The N520 sample exhibiting higher H/E and H3/E2 values did not experience wear failure. However, due to the presence of net-like microcrack structure on the surface of the titanium nitrided coating, the electrolyte can penetrate through the nitriding layer into the intermetallic compound (IMC) layer, even reach the coating/substrate interface during the initial stages of corrosion, forming a corrosion cell that significantly accelerates the corrosion rate finally. The corrosion wear rate of the samples was higher than the dry friction wear rate.

Microstructure control and energy storage mechanism of porous Cu-Al-Mn alloy for ultra-high near-elastic energy storage

Elastic materials that store and release elastic energy play pivotal roles in both macro and micro mechanical systems. However, it remains a significant challenge as metals generally suffer from a limited linear elastic strain of < 2% or large hysteresis due to its microscopic defects. Here, we presented a new strategy for enhancing the linear-elastic deformation of shape memory alloys (SMAs) and this put forward strategy was demonstrated in a Cu-based SMAs through experiments. This strategy relies on the interaction between the second soft phase including Kirkendall and interstitial pores and the soft-oriented strong texture matrix phase caused by powder metallurgy preparation process as elemental powders used as raw materials. In this work, the second phase pore soft phase with porosity of 11.4~31.1% and pore size of 0.5~10 μm was introduced into Cu71Al18Mn11 SMAs matrix through the combining of low temperature reactive sintering and spark plasma sintering process. The abnormal grain growth process realized the preferred texture of <101>-oriented and small angle grain boundary of matrix phase, as well as the reduced γ2 phase at the grain boundary. As a result, the designed porous Cu71Al18Mn11 SMAs demonstrated excellent elastic energy storage performance (elastic energy density Ue > 21.1 MJ/m3, efficiency η > 0.86), large recoverable near linear strain (near-elastic strain,~8%), and low energy dissipation simultaneously as well as ultra-low temperature dependence rate. This strategy may be applicable to other SMAs such as Ni-Ti-based and Fe-based SMAs with high performance to achieve high elastic deformation of bulk metals or construction.

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Insufficient intergranular corrosion (IGC) resistance limits the use of 7xxx-series ultra-high-strength aluminum alloys in aerospace and other critical structural applications. {111}/{111} near-singular boundaries (NSB) are special boundaries featured in a inter-connection of two {111} closest planes. They show higher corrosion resistance than the random grain boundaries. A higher fraction of these boundaries can markedly improve IGC resistance in ultra-high-strength aluminum alloys. In order to understand how {111}/{111} NSB form in aluminum alloys, 99.99% (mass fraction) high-purity aluminum were used as the experimental material. This choice can avoid the interference from precipitates and compositional fluctuations. {111}/{111} NSB were examined by EBSD measurement and the grain boundary inter-connection characterization method. The samples were processed by accumulative roll bonding (ARB) either at room temperature or at 200 ℃ (true strain ε = 4.6) followed by recrystallization annealing at 370 ℃. The results indicated that the sample ARB-ed at 200 ℃ and recrystallized had a higher {111}/{111} NSB fraction compared to the sample ARB-ed at room-temperature and recrystallized. Specifically, the {111}/{111} NSB fraction in the former was 16.71%, about 68% higher than that in the latter. In situ EBSD annealing experiments, at the temperatures ranging from ambient to 370 ℃, show that the sample ARB-ed at 200 ℃ undergoes mainly continuous recrystallization. The texture of recrystallization keeps almost the same as that observed in the ARB-ed state. After ARB-ed at 200 ℃, well-recovered Taylor- and Goss-oriented sub-grains constitute the main part of the sample. During annealing, these sub-grains continuously absorb dislocations by the associated interfaces, driving sub-grains rotation and coalescence and promoting continuous recrystallization. Taylor and Goss orientations show a <111>/θ (or near-<111>/θ) misorientation. When grains of these orientations impinge, local interfacial adjustment and re-orientation give rise to the formation of {111}/{111} NSB. High-resolution transmission electron microscopy reveals periodically spaced arrays of edge dislocations at these {111}/{111} NSB. These dislocations accommodate the mismatch caused by the deviations from the exact {111}/{111} inter-connection. The grain boundary energy of such {111}/{111} NSB is about 251 mJ/m², substantially lower than that of random grain boundaries (about 500 mJ/m²).

Research Progress and Trends in Characterization Techniques for Ultra-Fine Stainless Steel Wires: A Perspective Review

Ultra-high-strength (≥3 GPa) and ultra-fine (≤16 μm) stainless steel wires are critical components in solar photovoltaic manufacturing, yet their domestic production poses distinct technical challenges. Countries such as Japan and Germany currently dominate this field, having developed the capability to manufacture wires with tensile strengths of ≥3.5 GPa and diameters of ≤11 μm. Localizing ultra-high-strength ultra-fine stainless steel wire technology has therefore become an urgent priority. The extreme true strain and fine diameter attained through cold drawing produce a complex three-dimensional lamellar and heterogeneous microstructure that conventional two-dimensional characterization techniques cannot fully capture. Moreover, the absence of systematic three-dimensional characterization methodologies and standards further impedes localization efforts. This study presents a comprehensive overview of the characterization techniques currently employed for ultra-fine metallic wires and critically evaluates existing approaches for analyzing microstructure, mechanical properties, and performance, along with their respective limitations and strengths. The study also identifies opportunities for developing multiscale, three-dimensional in situ characterization techniques that would provide essential technical support and a theoretical foundation for optimizing wire performance and advancing localization.

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Xu, Shu-Xian

The defect of coarse grains on the surface of cast-rolled aluminum slabs significantly impacts the yield of subsequent rolling processes. These coarse grains manifest not only in size variations but also in pronounced orientation differences. Therefore, investigating the orientation characteristics of surface grains in cast-rolled slabs is crucial for improving the microstructure and surface quality of cast-rolled products. Traditional inspection methods, which primarily rely on macroscopic observation after acid etching, struggle to quantitatively reflect grain orientation variations. To address the challenge of accurately extracting grain orientation information solely through optical characteristics—which are inherently unstable—due to the fine grain size and complex orientation of cast-rolled aluminum alloys after acid etching, this study proposes a method that integrates optical and height features to obtain grain orientation characteristics of cast-rolled aluminum slabs. This method first utilizes feature engineering to construct a multidimensional feature set—including grayscale co-occurrence matrices(GLCM) and geometric curvature—based on raw optical and height measurements of grain orientation and microstructure surfaces. This approach extracts additional etching micro-morphology information. Subsequently, post-processing trains and optimizes the prediction outputs of the constructed stacked ensemble model. The selected stacked model achieved an accuracy of 78.3% on the test set, with a macro-average F1-score of 0.784. In the validation experiments, the predicted results showed good consistency with the actual orientations in spatial distribution, achieving an accuracy rate of 80.1% , and successfully enabling the prediction of grain orientation features across larger regions. Finally, the SHAP (SHapley Additive exPlanation) interpretability model was employed to substantiate the scientific validity of the model. SHAP analysis results indicate that optical features and texture features within the microstructure of acid-etched cast-rolled aluminum effectively reflect grain orientation variations in the model. Height features primarily serve to regulate and stabilize the model. The combined action of these three elements collectively achieves the model's excellent predictive performance. This study provides a viable technical approach for obtaining grain orientation information from the surface microstructure of acid-etched cast-rolled aluminum alloys and similar alloys, and for efficiently achieving quantitative identification of crystallographic orientation across large areas.

Effects of Long-Term Thermal Aging at 700 °C up to 10000 h on Mechanical Properties and Microstructure of 14Cr-ODS Steel With the Addition of Al and Si

Structural materials for generation-IV nuclear reactors must withstand extremely harsh environments involving high temperatures, corrosive coolants, and severe radiation, which render conventional nuclear structural materials ineffective. Owing to its excellent high-temperature stability, remarkable mechanical properties, and outstanding resistance to irradiation damage, oxide dispersion-strengthened (ODS) steel is considered a promising candidate for advanced nuclear power systems. The addition of Al and Si to 14Cr-ODS steel can improve its corrosion resistance. However, research on the long-term high-temperature stability of 14Cr-ODS steel with the addition of Al and Si, which is critical for ensuring reactor operational reliability, remains inadequate. In this study, the microstructural evolution, hardness and tensile - properties of two 14Cr-ODS steels containing 4%Al + 0.6%Si and 4%Al + 1.5%Si (designated as 0.6Si-ODS and 1.5Si-ODS, respectively) were investigated after long-term aging at 700 °C for 10000 h. The results showed that the addition of 0.6%–1.5% Si exerted no notable influence on the microstructure and mechanical properties of the steels before and after aging. Nanoparticles uniformly dispersed were identified as YAlO₃, Y₄Zr₃O₁₂, Y₂(Zr_0.6Ti_0.4)₂O₇, and Y₂Si₂O₇ by TEM. In addition, a small amount of 200 nm Si-rich oxide particles were found in 1.5Si-ODS steel. The nanoparticles with an average size of (6.3 ± 0.8) nm were observed in 1.5Si-ODS, whose number density reached 2.3 × 10²³ m⁻³. After aging at 700 °C for 10000 h, the hardness and tensile properties of both the steels at room temperature and 700 °C remained essentially unchanged. The ultimate tensile strength of 1.5Si-ODS decreased only slightly from 214 MPa to 210 MPa, and the elongation decreased from 37.5% to 36.8% at 700 °C, which is markedly higher than that of most reported Si-containing ODS steels. Microstructural observations confirmed that the grain size, nanoparticle size, and number density of nanoparticles exhibited excellent stability. These stable nanoparticles continuously pinned grain boundaries, effectively hindering the movement of dislocations and recovery of grain or subgrain structures, ensuring the stability of mechanical properties.

Preparation of NiCrBSi–ZrB₂ Composite Powder and Coating and Their Hot Corrosion Behavior in NaCl–KCl–Na₂SO₄ Molten Salt

The critical heat exchange components in boilers (water wall tubes, superheater tubes, reheater tubes, and economizer tubes) are susceptible to premature failure under high temperature, high pressure, and complex corrosive media. This remains a significant challenge for the reliability and economic operation of thermal power plants, especially with the trend towards higher parameters and capacities. Statistics indicate that failures of these components account for nearly half of all unplanned shutdowns in power plants, approximately 60% of which are directly attributable to high-temperature corrosion, resulting in substantial economic losses. A primary cause of such failures is hot corrosion induced by the deposition of molten salts such as NaCl, KCl, and Na₂SO₄, which originate from impurities in low-grade coals or biomass fuels. These salts form low-melting-point eutectic mixtures that destroy protective oxide scales and initiate catastrophic corrosion. To mitigate this issue, thermal spray coatings, particularly those applied via high-velocity oxygen-fuel (HVOF) spraying technology, offer an effective solution due to their low flame temperature, high deposition efficiency, and excellent bond strength. While traditional NiCrBSi coatings provide good protective properties, their performance limits are being challenged in increasingly harsh environments. This study introduces ZrB₂ as a reinforcing phase to enhance performance. ZrB₂ possesses an ultra-high melting point and high hardness. Most importantly, it oxidizes to form protective ZrO₂ scales, effectively blocking sulfur and chlorine permeation. However, its standalone use is limited due to poor sintering properties and high brittleness. In this work, NiCrBSi–ZrB₂ composite powders with high sphericity and uniform composition were successfully prepared through planetary ball milling, spray granulation, and vacuum sintering. Subsequently, dense and well-bonded composite coatings were fabricated using the HVOF spraying process. The hot corrosion behavior of the coating was systematically investigated in a NaCl–KCl–Na₂SO₄ molten salt mixture at 600, 700, and 800 °C for 100 h. The results demonstrated that the composite coating exhibited a dense layered structure with a bond strength exceeding 70 MPa and uniform distribution of phases. The hot corrosion resistance was highly temperature-dependent. At 600 °C, the coating displayed optimal performance with a mere weight gain of (13.13 ± 0.08) mg/cm² and a kinetic constant of 1.19 mg²/(cm⁴·h). Analysis via XRD, SEM, EPMA, and TEM revealed that the corrosion products primarily consisted of protective oxides (such as ZrO₂, SiO₂, and Cr₂O₃) alongside spinels and chromates (including NiCr₂O₄, K₂CrO₄, and Na₂CrO₄). The preferentially formed ZrO₂ and SiO₂ created a mixed oxide layer that acted as a robust barrier, significantly hindering the diffusion of corrosive elements and resisting alkaline dissolution. Thermodynamic analysis indicated that the ZrCl₄ and SiCl₄ generated within the coating possessed low oxidation Gibbs free energy, leading to a sluggish oxidation process that effectively interrupts the destructive chlorine cycle and reduces the corrosion rate. However, at 800 °C, the chlorination-oxidation cycle accelerated dramatically. The thermal stress generated within the oxide scales exceeded their cohesive strength, leading to extensive cracking and spallation. Consequently, the corrosion rate increased significantly, and the protective capability deteriorated. This study identified a critical temperature threshold of approximately 700 °C for the safe long-term application of this coating.

Prediction Method for High- and Low-Cycle Fatigue Life of Welded Joints Considering Multiscale Residual Stresses Relaxation

Recently, in the manufacturing industry, advancements toward high-parameter and lightweight designs has resulted in an increasing prominence on the influence of multiscale residual stress on the fatigue failure of pressure equipment. However, there is no unified conclusion on the influence of multiscale welding residual stress relaxation behavior on the fatigue failure of pressure equipment. This could be due to the following factors: (i) the multiscale coupling characteristics of residual stress, (ii) complexity of material microstructure, and (iii) dynamic evolution of stress relaxation behavior under cyclic loading. These factors hinder traditional fatigue life prediction methods to accurately evaluate the service performance of welded structures with residual stress. Therefore, this study utilizes SAF 2205 duplex stainless steel welded joints and reveals the relaxation behavior of multiscale residual stresses with the number of fatigue cycles. Additionally, this study proposes a predictive model for multiscale residual stress relaxation and clarifies the influence mechanism of residual stress relaxation on fatigue damage behavior. The results show that under low-cycle fatigue conditions, residual stresses are completely relaxed and converted into plastic deformation, which markedly increases the strain amplitude of the weld and broadens the stress amplitude range leading to weld fracture. Under high-cycle fatigue conditions, high-level residual stress shifts the crack initiation site from the specimen side surface to the specimen central surface. The substantial mechanical property difference between grain boundary austenite and surrounding grains, together with the constraint effect induced by the mismatch between the tensile and compressive attributes of residual stresses reduces the crack initiation life. Based on the above research, this study establishes a fatigue life prediction model considering residual stress relaxation. The results agree well with the experimental data, with a prediction error of less than 20%.

First-Principles Study on the Effects of Rare-Earth Er Doping on the Structural, Mechanical, Electronic and Dislocation Properties of (Ti_0.25Ta_0.25Hf_0.25Nb_0.25)₂Zr₂O₇ High-Entropy Ceramics

High-entropy ceramics (HECs) are promising materials for advanced applications; however, the effects of rare-earth doping on their structural and mechanical properties remain underexplored. Using density functional theory, this study investigates the effects of Er doping on the structural, mechanical, electronic, and dislocation properties of (Ti_0.25Ta_0.25Hf_0.25Nb_0.25-xEr_x)₂Zr₂O₇ (x = 0%–25%) HECs. The disordered ceramic supercell is modeled as a 2 × 2 × 2 special quasirandom structure. The influencing mechanisms of Er doping are elucidated by calculating the critical parameters (formation enthalpy, lattice distortion, elastic constants, mechanical properties, and density of states). Er substitution at the Nb sites minimizes the formation enthalpy, i.e., optimizes the structural stability. Increasing the Er content intensifies the lattice distortion, as evidenced by increased atomic size differences and root-mean-square atomic displacements. The (Ti_0.25Ta_0.25Hf_0.25Nb_0.1875Er_0.0625)₂Zr₂O₇ (x = 6.25%) and (Ti_0.25Ta_0.25Hf_0.25Nb_0.0625Er_0.1875)₂Zr₂O₇ (x = 18.75%) HECs exhibit the highest bulk modulus, Young’s modulus, shear modulus, fracture toughness, and dislocation energy factors. The ductility decreases with an increase in Er content. Moreover, the isotropy increases and anisotropy decreases with an increase in Er content up to 18.75%. Er doping elevates the density of states at the Fermi level, enhancing the metallic and electrical conductivity properties of the HECs. The energy factors are lower at screw dislocations than at edge dislocations, aiding the nucleation of screw dislocations. The edge-dislocation width peaks at 18.75% Er, promoting twinning deformation and improving the plasticity.

Deformation Mechanism and Multiaxial Constitutive Modeling Method of High-Temperature Tension–Torsion Fatigue in FGH96 Superalloy

To elucidate the deformation mechanisms and mechanical behavior of FGH96 superalloy under high-temperature multiaxial fatigue conditions relevant to gas turbines, tension–torsion combined fatigue experiments were conducted at 650 °C. The cyclic stress evolution under uniaxial fatigue, 0° proportional loading fatigue, and 45° and 90° non-proportional loading fatigue was systematically investigated. EBSD and TEM were employed to examine the deformation mechanisms of FGH96 superalloy under both proportional and non-proportional loading conditions. Based on the Chaboche viscoplastic framework, a unified constitutive model incorporating non-proportional hardening was developed. The results indicate that FGH96 superalloy exhibits pronounced non-proportional hardening under non-proportional loading. The additional hardening mainly arises from intense dislocation multiplication and interaction, accompanied by severe dislocation entanglement. With increasing degree of non-proportional loading, the dislocation density gradually increases, and slip lines evolve from straight to curved configurations. Compared with the traditional Chaboche viscoplastic unified constitutive model, the proposed model incorporating non-proportional hardening significantly improves the prediction accuracy of the hysteresis loop stress range, achieving relative errors below 5%, which satisfies typical engineering requirements for deformation prediction.

Influence of Strain Rate on the Dynamic Failure Behavior of Q345 Steel Welded Joints in Ultra-High Voltage Transformer Tanks

Ultra-high-voltage transformer tanks are critical structural components of large-scale power and converter transformers and require exceptional explosion-proof performance to ensure grid stability. These tanks are typically fabricated from low-alloy high-strength steels such as Q345 and are characterized by complex geometries and a high density of welded joints. During internal arc discharge events, welded joints serve as primary sites for crack initiation and propagation under transient impact loading, ultimately leading to structural failure. Although welded joints are recognized as potential weak links, their strain rate-dependent deformation behavior and underlying failure mechanisms have not been systematically elucidated. In particular, it remains unclear how dynamic loading conditions affect fracture initiation sites and the evolution of microstructural damage within welded joints. In this study, the deformation behavior and failure mechanisms of Q345 steel welded joints were systematically investigated. Uniaxial tensile tests were conducted over a strain rate range of 1.0 × 10⁻⁴ s⁻¹–1.0 × 10⁻¹ s⁻¹, coupled with digital image correlation to monitor the evolution of localized strain fields. Impact toughness was evaluated using standard Charpy tests, and microstructural characterization was conducted using optical microscopy and SEM. Furthermore, X-ray computed tomography was employed to visualize and quantify internal defect evolution. The results indicate that Q345 steel welded joints exhibit more pronounced strain rate sensitivity than the base metal (BM). As the strain rate increased to 1.0 × 10⁻¹ s⁻¹, a substantial reduction in elongation was observed. Meanwhile, the fracture initiation site shifted from the BM to the weld metal (WM) with increasing strain rate. Digital image correlation analysis revealed a transition in fracture location. At lower strain rates, plastic deformation was primarily accommodated by the BM, leading to necking and final failure within the BM region. In contrast, at higher strain rates, strain localization rapidly developed within the heat-affected zone and WM, resulting in fracture initiation in the weld region. Quantitative impact testing further demonstrated a substantial disparity in toughness. The BM exhibited a high impact toughness of (326.1 ± 14.1) J/cm², whereas the welded joints showed a significantly lower value of (52.7 ± 2.7) J/cm², corresponding to an approximately sixfold reduction. This marked decrease identifies the weld region as the dominant failure site and underscores its critical role in improving explosion resistance. SEM fractography confirmed a transition from ductile dimple morphology to brittle cleavage fracture, characterized by distinct cleavage facets and river patterns. Adiabatic heating during rapid plastic deformation was found to promote the formation of oxide particles within the welded joint. These oxides induced localized stress concentrations and acted as critical crack initiation sites, ultimately triggering structural failure. In addition, X-ray computed tomography results verified that large internal defects within the WM served as preferential sites for strain concentration and crack propagation under dynamic loading. This study clarifies the intrinsic mechanisms governing dynamic failure in Q345 steel welded joints and provides essential theoretical insights for improving the explosion resistance and structural integrity of welded steel components under extreme dynamic conditions.

Influence of Al Doping on Thermoelectric Properties of CuInTe₂

CuInTe2, can be regarded as two cubic ZnTe stacked on top of each other, with Cu and In atoms alternately replaced Zn atoms to form a crystal structure with short-range disorder and long-rang order. This gives it a lower thermal conductivity than ZnTe, making it a promising P-type thermoelectric material. The weak electrical conductivity and relatively high inherent lattice thermal conductivity of CuInTe2 induce the low ZT and conversion efficiency of harvesting waste heat, and thus hinder commercial application in thermoelectricity field. A series of Al-doped CuInTe2 compounds were prepared successfully by solid reaction, annealing, and spark plasma sintering techniques and the influence of aluminum doping on the structure and thermoelectric performance was systematically explored through multiple advanced characterization methods. Al doping remarkably improves electrical transport performance by modulating carrier concentration and mobility, remarkably regulates electrical transport performance. Meanwhile, Al doping induces the substituted point-defects, dislocation, strain fluctuation and nano precipitations of CuInAl4Te8 as extra barriers, inhibiting the phonon transport and thus achieving the reduction of ?L so that minimum lattice thermal conductivity of 0.72 W?m-1?K-1 at 823 K are obtained for x=0.2 sample. Eventually, an optimized ZT value of 0.88 is achieved for CuIn0.8Al0.2Te2 at 823 K with an enhancement of 115%. And ZTavg (323-823 K) value of 0.34 and ZTavg (523-823K) value of 0.6 are obtained, respectively, with about double enhancement, compared to that of pristine CuInTe2. This work proves the effective utilization of doping at In sites of CuInTe2 and advance the development of the chalcopyrite thermoelectric materials with diamond-like structure.

Effect of Heat Treatment on Interfacial Microstructure of Inertial Friction Welded Joints for GH4065A/IN718 Dissimilar Superalloys

There has been a relentless pursuit for high thrust-to-weight ratio development in the aeroengine field. To tackle this, the study successfully achieved high-quality joining of GH4065A/IN718 dissimilar superalloys via inertia friction welding that demonstrated excellent interfacial bonding and no significant welding defects. A systematic investigation was performed on the influence of post-weld heat treatment on the microstructural evolution at the joint interface. The results indicated that post-weld heat treatment increased the width of the weld nugget and thermomechanically affected zones via dynamic recrystallization and grain boundary migration, which confirmed its regulatory effect on the microstructure. Heat treatment promoted uniform precipitation of γ′ and γ′′ phases on the GH4065A and IN718 sides, respectively, thereby increasing the volume fraction and average size of the γ′/γ′′ strengthening phases. After heat treatment, minor grain coarsening was observed in all the joint regions, along with a considerable reduction in local misorientation in the thermomechanically affected zone, indicating effective alleviation of stress concentration. Moreover, fine γ′/γ′′ phases precipitated in the heat-treated weld nugget zone, and dislocations that cut through these strengthening phases resulted in the formation of stacking faults, which further impeded dislocation motion and enhanced deformation resistance.

Optimization Design of Mechanical Properties for Bucket Teeth Cast Steel Based on Machine Learning

As a key and vulnerable component of excavator engineering machinery, the performance of bucket teeth directly determines the machine's working efficiency, economic cost, and operational safety. In traditional material development models, reliance has primarily been placed on trial-and-error methods and the experience of experimental personnel. This approach not only results in lengthy development cycles and high costs but also often fails to achieve a balance among the multiple performance indicators of cast steel, significantly limiting the efficiency of developing new high-performance materials. In recent years, with the rapid advancement of artificial intelligence technology, particularly machine learning methods, revolutionary tools have been provided to address the mapping relationship between material composition, processing, and performance. This study combines machine learning and optimization algorithms to construct a composition space for cast steel, thereby screening cast steel materials with high hardness and strength. A dataset of 800 cast steel samples, including composition, heat treatment processes, and mechanical properties (Rockwell hardness, tensile strength, and elongation), was compiled from the literature. Five machine learning models (SVM, XGBoost, RF, DT, MLP) were developed to predict the Rockwell hardness, tensile strength, and elongation of cast steel. The results show that the trained gradient boosting model (XGBoost) achieved the best overall prediction performance for Rockwell hardness and elongation, with mean absolute errors of 2.146 and 1.6442, and coefficients of determination (R2) of 0.9082 and 0.9166, respectively. The support vector machine model (SVM) performed best in predicting tensile strength, with a mean absolute error of 70.1750 and a coefficient of determination (R2) of 0.9234. Subsequently, the trained machine learning models were combined with an optimization algorithm (NSGA-Ⅱ) to search the composition space of cast steel. One cast steel composition was selected from the feasible solutions obtained through the search and experimentally validated. The experimental results showed that the cast steel had a Rockwell hardness of 50.54 HRC, a tensile strength of 1736.9 MPa, and an elongation of 14%, which aligned with the model's search objectives. This study provides a more efficient new strategy for the development of high-strength, high-hardness cast steel for bucket teeth.

Superplastic deformation behavior of an advanced Ti750S high-temperature titanium alloy sheet

The 750 ℃ high-temperature titanium alloy is a lightweight, high-strength material with potential for the development for advanced hypersonic vehicles, requiring urgent research. The primary prospective application of this alloy is in the fabrication of thin-walled components through superplastic forming. The superplastic deformation behavior of an advanced, high-temperature titanium alloy Ti750S sheet was investigated. The Ti750S sheet exhibited optimal superplastic properties at a strain rate of 5.00 × 10⁻³ s⁻¹ and temperature of 960 ℃, achieving a tensile elongation of 120%. The average strain-rate-sensitivity exponent (m) of the sheet was 0.31 and the calculated thermal activation energy (Q) was 358.338 kJ/mol. Based on these parameters, a constitutive equation for the superplastic deformation of the Ti750S alloy was established. Microstructural and textural analyses after superplastic tensile deformation revealed that the lamellar-structured sheet underwent dynamic spheroidization, dynamic recrystallization, and texture weakening during deformation. The superplastic deformation process involved the transformation of the initial lamellar structure into an equiaxed microstructure via dynamic spheroidization and dynamic recrystallization, followed by grain-boundary sliding, coordinated by dislocation motion, diffusion creep, and grain rotation.

Microstructural Evolution and Strengthening Mechanisms of Cu-Ni-Si/1010 Steel Bimetallic Composites via Direct Annealing and Cold Rolling + Annealing

王, 明飞, Jie, Jin-Chuan

High-performance materials are critically demanded in the electronics and automotive industries. Cu-Ni-Si alloys are widely used for the excellent combination of strength and electrical conductivity, while 1010 low-carbon steel offers superior formability, high modulus, and cost-effectiveness. Integrating these two materials into laminated bimetallic composite presents the promising strategy to achieve structural-functional integration, overcoming the trade-offs inherent in single component metals. However, the preparation of such composites faces significant challenges, particularly the poor wettability and limited mutual solubility between Cu and Fe, which often lead to weak interfacial bonding and coarse as-cast microstructures. Consequently, developing effective preparation method and appropriate post-processing to tailor the microstructure and optimize the mechanical properties is essential for the industrial application. In this study, Cu-Ni-Si/1010 steel laminated bimetallic composite was successfully fabricated using the solid-liquid bonding method. To investigate the effects of annealing and rolling + annealing on microstructural evolution and mechanical behavior, the Cu-Ni-Si/1010 steel laminated bimetallic composites were subjected to four distinct conditions: the as-cast state (S1), direct annealing at 450 °C (S2), 70% cold rolling followed by annealing at 450 °C (S3), and 70% cold rolling (S4). Comprehensive characterizations were conducted using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tensile testing to reveal the correlation between processing parameters, interfacial characteristics, and strengthening mechanisms. Microstructural analysis shows that the Cu-Ni-Si layer in sample S1 consisted of coarse columnar grains with minor β-Ni3Si phase at α-Cu grain boundaries. Direct annealing promotes the formation of nanoscale δ-Ni2Si precipitates in the α-Cu matrix, providing strengthening effect. However, the combined cold rolling and + annealing process induces larger microstructural changes. The severe plastic deformation introduces high dislocation density and refines the grain structure into the fibrous morphology due to incomplete recrystallization. No brittle intermetallic compounds are observed at the interface in any condition. The S3 and S4 samples exhibit the wavy and tightly bonded interface, indicating enhanced interfacial diffusion and metallurgical bonding. Tensile testing reveals that the sample S3 achieved the higher ultimate tensile strength of 613 MPa, significantly exceeding that of S1 (379 MPa) and S2 (415 MPa), attributed to the synergistic effect of work hardening and precipitation strengthening. Although ductility decreased slightly in S3, its elongation remains at 18.4%, while S4 shows the lowest elongation of 12.6% which is detrimental to subsequent forming. Importantly, all samples fracture through the matrix without interfacial delamination, confirming excellent metallurgical bonding. These results indicate that direct annealing can simultaneously enhance both strength and ductility of Cu-Ni-Si/1010 steel laminated bimetallic composites, whereas cold rolling combined with annealing represents an effective strategy for substantially improving strength while preserving interfacial integrity.

Formation Mechanism of Heterogeneous Microstructure and Strengthening–Toughening Mechanisms in High-Strength, High-Toughness AZ91 Magnesium Alloy

Magnesium alloys have attracted significant attention owing to their low density and high specific strength, offering broad application prospects in lightweight aerospace and automotive components. However, the limited number of activatable slip systems inherent to their hexagonal close-packed structure severely restricts ductility and work-hardening capacity, posing a persistent challenge in achieving a synergy between strength and ductility. Heterogeneous structure design is considered an effective strategy for attaining such a balance in metallic materials. In this work, AZ91 magnesium alloys with a lamellar grain structure (LGS) and a bimodal grain structure (BGS) were fabricated via thermomechanical processing combined with controlled annealing, achieving a synergistic enhancement of strength and ductility. The LGS was produced by rapid extrusion, resulting in a lamellar microstructure characterized by alternating dynamically recrystallized (DRX) fine grains (average ~3.6 µm) and deformed coarse grains (average ~24.0 µm). The BGS was obtained by annealing the extruded alloy, leading to fully statically recrystallized coarse grains (average ~13.0 µm) coexisting with DRX fine grains (average ~5.2 µm). Mechanical testing showed that both heterogeneous structures exhibited pronounced work-hardening capability and improved uniform elongation. The BGS alloy demonstrated a yield strength (YS) of ~234.5 MPa, an ultimate tensile strength (UTS) of ~337.5 MPa, and elongation (EL) of ~23.8%. In contrast, the LGS alloy exhibited significantly higher strength (YS ≈ 274.9 MPa; UTS ≈ 377.1 MPa) while maintaining excellent ductility (EL ≈ 21.3%). This improvement is attributed to the combined effects of higher dislocation density in the deformed coarse-grained regions and the finer grain size in the fine-grained domains. In this study, multi-scale characterization techniques, including SEM-based slip trace analysis and digital image correlation, were employed to elucidate the deformation mechanisms. The results indicate that, at the early stage of deformation (~8% strain), both basal and non-basal slip systems were activated in the fine-grained regions of the two heterostructured alloys. In addition, basal-to-basal and basal-to-non-basal slip transfer occurred between adjacent grains, effectively alleviating intergranular stress concentration. Meanwhile, the coarse-grained regions exhibited a high dislocation storage capacity, and deformation twinning was activated at later deformation stages to accommodate c-axis strain. As a result, both heterostructured alloys (LGS and BGS) demonstrated strong work-hardening behavior, with the LGS alloy achieving a superior combination of strength and ductility. The findings contribute to a deeper understanding of the plastic deformation mechanisms in heterostructured magnesium alloys and provide a feasible pathway for enhancing their mechanical performance.

Effect of Heat Treatment on Microstructural Evolution and Mechanical Properties of Inertial Friction Welded Joints for FGH96 Superalloy

Aero engines are a critical indicator of national comprehensive strength and technological advancement. However, their performance enhancement poses considerable challenges to the manufacturing technologies of hot-end components. Hot-end structures undergo inertial friction welding, which effectively avoids the defects associated with fusion welding and has been widely adopted for high-quality joining of these structures. This study addresses the inherent process limitations of inertial friction welded joints, including strengthening phase dissolution within the welding zone (WZ), along with axial and radial microstructural heterogeneity, and proposes a customized post-weld heat treatment (PWHT) strategy for inertial friction welded joints of FGH96 superalloys. A systematic investigation was conducted on the effects of solution aging and double aging on microstructural evolution and mechanical properties. The results indicated that increasing the solution temperature leads to grain coarsening and enhanced recrystallization, accompanied by pronounced precipitation and growth of the γ′ strengthening phase, a more uniform carbide distribution, and intensified grain boundary serration within the WZ compared to joints at low solution temperature. At a solution temperature of 1140 °C, abnormal grain growth occurred, along with coarsening and inhomogeneous distribution of the γ′ strengthening phase. After solution aging, the impact toughness of the joints improved with increasing solution temperature, whereas the tensile strength initially increased and then decreased. At the solution temperature of 1080 °C, the grain size and γ′ strengthening phase were moderate and uniformly distributed, the degree of recrystallization and grain boundary serration were relatively high, carbides were uniformly dispersed, and microstructural inhomogeneity was notably improved. Under this condition, the ultimate tensile strengths of the joints at room temperature and 750 °C were 1455 and 1042 MPa, respectively, and the impact toughness reached 41 J/cm², demonstrating an optimal strength–toughness synergy. Double aging promoted the uniform precipitation of tertiary γ′ strengthening phase in the WZ, resulting in tensile strengths of 1574 and 1279 MPa at room temperature and 750 °C, respectively, approaching base metal levels. Meanwhile, the hardness was considerably enhanced beyond the base metal, although the impact toughness was slightly reduced compared with the as-welded joints.

Study on high-temperature creep and phase evolution of the Fe-Ni-Cr-W-Al alloy ethylene pyrolysis furnace tubes

The relationship between the high-temperature creep behavior and microstructure of an Fe–Ni–Cr–W–Al alloy used as the centrifugal casting material for the radiant section tubes of ethylene pyrolysis furnaces was investigated, along with the precipitate phases. Analyses were performed using a high-temperature creep testing machine, OM, SEM, TEM, and other instruments. The creep behavior was evaluated at 900–1200 ℃ and 10–48 MPa. The data reveal that the as-cast microstructure of the 27Cr44Ni5W3Al+MA alloy consists of γ and eutectic precipitates, where the eutectic precipitates mainly comprise herringbone-shaped M₇C₃ and blocky or lamellar M₂₃C₆, as well as coherently precipitated, dispersed γ′ particles and a small number of blocky γ′ particles within the γ matrix. A plot of the Larson-Miller parameters indicates that this alloy has excellent creep performance, where the high-temperature creep data fit a master curve according to the relation lgs = 3.67436 –0.04401P–0.00121P². As the temperature in the creep test increases, the average widths of the M₇C₃ and M₂₃C₆ precipitates gradually increase from 0.9 and 4.6 μm for the original as-cast state to 1.8 and 8.1 μm at 1200 ℃, respectively. During the creep process at 900℃–1050℃ and 10–48 MPa, M₇C₃ is gradually transformed into blocky M₂₃C₆, which precipitates with blocky γ′. The precipitation of γ′ is most pronounced at 950 ℃ and the average particle width also increases from 1.4 μm for the as-cast state to 3.3 μm. The creep cavities mainly nucleate at the interfaces between the coarse, blocky M₂₃C₆, blocky γ′, and thin, lamellar M₇C₃ eutectic precipitates and the γ matrix. During the creep process at 1100℃–1200℃, M₇C₃ is further transformed into blocky M₂₃C₆, with no observed precipitation of blocky γ′. The creep cavities undergo further nucleation at the interfaces of the coarse, blocky M₂₃C₆ or thin, lamellar M₇C₃ eutectic precipitates with the austenite matrix, all of which are intergranular fractures.

Exploring the Influence of Experimental Conditions on Indentation Relaxation Behavior of a Heat-resistant Steel Based on Finite Element Method

The indentation technique imposes high requirements on the accuracy of testing equipment in practical applications and is susceptible to disturbances from the testing environment and factors such as sample preparation quality. To optimize the key parameters of indentation experiments and enhance the reliability and scientific validity of this method, this study investigates the influence of different experimental conditions on the indentation relaxation behavior of Sanicro25 austenitic heat-resistant steel using the finite element method. The simulation results indicate that the friction coefficient has an obvious impact on the test results. When the friction coefficient increases from 0.00 to 0.30, the corresponding maximum force increases by 18.41%. At the same time, the surface morphology of the indentation changes significantly. As the friction coefficient increases, the material stacking height decreases. However, when the friction coefficient exceeds 0.15, the change in stacking height becomes less pronounced, and the results under different friction coefficients tend to be consistent; The indentation response is also affected by the ratio of sample thickness to indentation depth (thickness-to-depth ratio). The results show that when the ratio is ≥ 20, the relaxation curves are essentially consistent. Further increasing the thickness-to-depth ratio does not lead to significant changes in the relaxation curve. Under the same indentation depth conditions, a quadrangular pyramid indenter produces a larger equivalent creep strain and a faster initial relaxation rate than a conical indenter.

Orientation Relationship Between Cold-Rolling Shear Band and Matrix Grain in Grain-Oriented Silicon Steel: Crystal Plasticity Calculation and Experiment

Superior magnetic properties of grain-oriented silicon steel arise from sharp Goss texture after secondary recrystallization. Shear band which is formed during cold rolling acts as preferential nucleation sites for recrystallization, and is a critical factor influencing accuracy of recrystallized Goss texture. The deviation angle of shear band (Δφ₁ʹ and Δφ₂ʹ) from ideal Goss orientation ({110}<001>) is closely related to that of deformed matrix (Δφ₁and Δφ₂) from ideal {111}<112> orientation. Accordingly, it is essential to elucidate the relationship of orientation deviation between deviated {111}<112> orientation matrix and Goss shear band for precise orientation control of grain-oriented silicon steel. This study incorporates two-dimensional (2D) shear band plane (shear band parallel to transverse direction) and three-dimensional (3D) shear band plane (shear band parallel to slip plane) into a visco-plastic self-consistent (VPSC) model to calculate the crystal orientation rotation of shear bands within deviated {111}<112> orientation matrix, and simulated results are compared with experimental data. The findings indicate that both magnitude and mode of matrix orientation deviation significantly affect the corresponding shear band orientation deviation. When Euler angle φ₁ of deviated {111}<112> orientation matrix is less than 90° and Euler angle φ₂ is less than 45°, |Δφ₁ʹ| decreases while |Δφ₂ʹ| either initially decreases and then increases or monotonically increases during shear band orientation rotation, and final shear band orientation exhibits a smaller |Δφ₁ʹ| than |Δφ₂ʹ| relative to ideal Goss. When φ₁ of deviated {111}<112> orientation matrix is less than 90° and φ₂ exceeds 45°, both |Δφ₁ʹ| and |Δφ₂ʹ| decrease during shear band orientation rotation, and final shear band orientation exhibits a greater |Δφ₁ʹ| than |Δφ₂ʹ| compared to ideal Goss. Shear band plane is also found to depend on direction of matrix deviation. For φ₂ < 45°, the slip plane where shear band locates deviates significantly from the maximum shear stress plane, and shear band plane is more accurately represented by 2D shear band plane. For φ₂ > 45°, the slip plane where shear band forms exhibits a smaller deviation from the maximum shear stress plane, and shear band plane is more accurately represented by 3D shear band plane. Identification of shear plane in deviated {111}<112> orientation matrix is thus fundamental to clarify the mechanism of shear band formation and to enable reliable crystal plasticity simulations of shear band orientation.

Research on Optimization of Laser Powder Bed Fusion Process for 304L Stainless Steel Based on Machine Learning and Multi-Objective Optimization

Laser powder bed fusion (LPBF) has emerged as a promising additive manufacturing technique for producing high-performance 304L austenitic stainless steel, which is widely used in the aerospace and automotive industries and in biomedical engineering because of its excellent mechanical properties, corrosion resistance, and high-temperature stability. However, the mechanical properties of LPBF-fabricated materials are influenced by the complex interplay of various process parameters, including laser power, scanning speed, layer thickness, and hatch spacing. Traditional trial-and-error methods for process optimization are costly and time-consuming, and they often fail to precisely control the material’s microstructure, resulting in suboptimal performance. Therefore, there is a pressing need to adopt advanced approaches to systematically optimize the LPBF process and ensure reliable, high-performance outcomes. This study proposes an innovative hybrid intelligent framework that integrates stacking-ensemble learning, interpretable machine learning techniques (SHAP), and multiobjective optimization (NSGA-II–TOPSIS). The primary objective is to develop an all-encompassing framework for the simultaneous prediction and optimization of ultimate tensile strength, yield strength, and elongation in LPBF-fabricated 304L stainless steel. The framework is designed not only to improve the accuracy of predicting the mechanical properties but also to provide a clear understanding of the influence of process parameters on material behavior. The stacking-ensemble model demonstrates superior performance in terms of accuracy, generalization, and stability compared to individual machine learning models, such as random forest (RF), gradient boosting (GBDT), and extreme gradient boosting (XGBoost). SHAP analysis has revealed that laser power plays a critical role in determining the mechanical properties of the material, making it the most important factor to consider in the optimization process. The multiobjective optimization approach facilitates the identification of the optimal process parameters, resulting in a balanced strength–ductility trade-off that is crucial for practical applications. Experimental validation was conducted to confirm the effectiveness of the proposed framework. The optimized LPBF samples exhibited refined, uniform cellular substructures, increased dislocation density, and the formation of twin boundaries, which significantly improved the material’s mechanical properties. These findings validate the accuracy of the model’s predictions and demonstrate the framework’s potential to achieve improved material performance. This work presents a novel, intelligent approach to process–performance co-design in LPBF-fabricated 304L stainless steel, contributing to the development of a transferable methodological framework that can be applied to other metallic materials.

Microstructural Evolution and Slip Mechanisms in TC4 Titanium Alloy During Cyclic Deformation

LUO, Zhong-Bing

The cyclic damage behavior of TC4 alloy, which is widely utilized in aerospace and other fields, is critical to the structural integrity of its components. The aim of this study is to elucidate the underlying microstructural damage mechanisms, from the aspect of microstructural evolution, slip activity, and dislocation configurations, during cyclic loading through advanced characterization techniques including EBSD and TEM. The results indicate an initial rapid hardening stage, during which strain is highly localized in microtextured regions due to deformation incompatibility with the surrounding grains. The material subsequently reaches a quasi-steady state, which is marked by accumulated plasticity. Influenced by crystallographic texture and loading direction, the pyramidal 1011 slip system exhibits the highest Schmid factor and is preferentially activated, dominating the deformation process and promoting a gradual grain reorientation toward the 1120 direction. TEM analysis indicates that dislocations multiply and align parallel to α/β phase interfaces during cyclic deformation. These interfaces function as both dislocation sources and barriers, thereby enhancing the material’s fatigue life. The synergistic coupling between dislocation activity at α/β interfaces and pronounced strain localization within microtextured regions is identified as the dominant mechanism governing cyclic deformation damage in TC4 alloy.

Tuning local chemical order and multi-properties in high-entropy alloys via interstitial filling by non-metallic small atoms: a percolation-theory perspective

This review summarizes the regulatory effects of non-metallic small atoms (SAs) on the crystal structures, local chemical ordering (LCO), and multi-properties of high-entropy alloys and films. SAs occupy interstitial sites, induce local lattice distortions, and drive structural transitions to ordered compounds or amorphous states, thereby reconstructing short- and medium-range order. Furthermore, the percolation theory effectively explains the relationship between the SAs-induced LCO connectivity and abrupt changes in macroscopic properties, revealing critical behavior in the resistivity, saturation magnetization, and mechanical performance near percolation thresholds. Specifically, SAs enhance strength, hardness, and wear resistance through the solid-solution strengthening, grain-boundary segregation, and secondary phase formation. Nitrogen provides the most pronounced strengthening effect, while carbon offers additional lubrication capabilities. Oxygen alters magnetic exchange interactions, enabling tunable magnetic ordering. Electrically, SAs achieve wide-range control of resistivity from metallic to insulating states through the electronic localization and reconstruction of transport pathways. In future studies, the percolation theory can be employed to guide the composition design and properties optimization of high-entropy alloys, thereby propelling their applications across multiple fields.

Multi-Scale Fatigue Crack Propagation Prediction of 304 Austenitic Stainless Steel Based on Physics-Informed Neural Network

Although machine learning methods have demonstrated promising potential in fatigue crack propagation prediction, they are currently limited by insufficient incorporation of physical constraints, low model interpretability, and suboptimal predictive accuracy. These limitations restrict the reliability of data-driven models on complex engineering structures, in which fatigue crack propagation is governed by nonlinear interactions across multiple length scales and loading conditions. When explicit physical information is lacking, models may output unstable predictions and cannot be generalized by extrapolating beyond the training data domain. To accurately predict multiscale fatigue crack propagation behavior while enhancing the physical consistency, interpretability, and generalization capability, the present authors investigated the multi-scale fatigue crack propagation behavior of 304 austenitic stainless steel samples using an artificial neural network (ANN). As purely data-driven models cannot properly represent complex crack propagation across different scales, the authors incorporated physics-informed features into the ANN framework, creating a feature-extended neural network (FENN) that incorporates prior physical knowledge related to fatigue crack propagation. Next, a multi-scale fatigue crack propagation mechanism was coupled into the PENN model, establishing a physics-informed neural network (PINN) model that can explicitly integrate physical constraints into the learning process. The ANNs more accurately predicted multi-scale fatigue crack propagation than the traditional light-gradient boosting machine, extreme gradient boosting, and ridge regression machine learning methods. The neural-network-based models well-fitted the nonlinear data of fatigue crack growth data under multi-scale conditions. Expanded with physics-informed features, the FENN model improved the prediction accuracy by 7.23% and 18.75% on the training and testing datasets, respectively, relative to the baseline ANN, confirming the effectiveness of physical feature embedding. Incorporating the physical information into the learning framework significantly enhanced both the training performance and generalization capability. The prediction accuracy was further improved in the PINN model, which integrates multi-scale physical mechanisms to capture the complex patterns of fatigue crack propagation across different scales. The PINN model is robust with good error controllability and high generalization capability. This performance improvement demonstrates the advantage of combining data-driven learning with explicit physical constraints into multiscale fatigue crack propagation prediction. To improve the transparency and physical interpretability of the model, the decision-making process of the PINN model was interpreted through the SHapley Additive exPlanations (SHAP) method, which quantitatively evaluates the contribution of each input feature to the model’s prediction. The SHAP-based analysis determines the relative importance of different input variables, further supporting the physical rationality and reliability of the proposed PINN model.

Microstructure and High Temperature Oxidation Behavior of Silicide-Boride Composite Coatings on the Surface of Mo

Wu, Fan

Molybdenum and its alloys exhibit considerable potential for aerospace high-temperature components, electronic thermal management systems, and high-temperature power-generation structures due to their high melting point, excellent elevated-temperature mechanical strength, and good creep resistance. However, their application is severely limited by rapid oxidation at temperatures above 700°C, where the formation and volatilization of MoO₃ lead to accelerated material loss and structural degradation. This oxidation susceptibility can ultimately result in disintegration and catastrophic failure under extreme service conditions. The application of silicide-based coatings is an effective strategy to mitigate high-temperature oxidation by forming a protective barrier that isolates the substrate from the environment. Nevertheless, monolithic silicide coatings often suffer from premature failure caused by thermal expansion mismatch with the substrate and inward silicon diffusion during prolonged high-temperature exposure. In this context, silicide–boride composite coatings have emerged as a promising alternative for further improving oxidation resistance. Despite their potential, the mechanisms governing gradient microstructure formation and the origins of performance variability in such composite coatings remain insufficiently understood. In this study, silicide and silicide–boride composite coatings were fabricated on pure molybdenum substrates using halide-activated pack cementation, and their microstructural evolution and high-temperature oxidation behavior were systematically investigated. The results demonstrate that boron incorporation promotes the formation of a silicide–boride composite coating with a five-layer graded structure: MoSi₂/(MoSi₂ + MoB)/Mo₅Si₃/MoB/Mo₂B. Notably, boron facilitates the preferential formation of an initial MoB interlayer at the coating–substrate interface. This interlayer not only inhibits the directional diffusion of Si but also induces a displacement reaction between Si and MoB to form MoSi₂, thereby suppressing the (001) preferred growth orientation of MoSi₂. In addition, volume contraction associated with MoB formation within the MoSi₂ + MoB mixed layer generates pores and a roughened interface, which act as high-density nucleation sites and significantly refine the surface MoSi₂ grain structure. The refined grain structure accelerated the formation of a dense and continuous SiO₂ protective film, thereby effectively inhibiting oxygen diffusion. After 30 h of oxidation at 1200°C, the silicide–boride composite coating exhibited an oxidation weight gain of 1.28 mg/cm² and an oxidation rate constant of 0.29 mg/(cm²·h), representing a 53% reduction relative to the silicide coating. Moreover, the MoB interlayer suppressed inward Si diffusion into the substrate, thereby enhancing long-term stability under high-temperature oxidative conditions.

Effect of Deep Cryogenic Treatment on Microstructure and Mechanical Properties of 18Ni(200) Maraging Steel

LI, Xiao-Lin

With advances in cryogenic engineering, increasingly stringent performance requirements are imposed on cryogenic structural alloys. Conventional heat treatments are often inadequate to satisfy the demanding performance requirements of 18Ni maraging steel used in aerospace and other high-end applications, creating a need for alternative heat-treatment strategies. Although deep cryogenic treatment has been reported to enhance the properties of various steels, its effects on maraging steels remain insufficiently understood. Accordingly, this study investigates the influence of cryogenic treatment temperature and duration on the microstructure and mechanical properties of 18Ni(200) maraging steel. The material was subjected to a combined process comprising solution treatment, cryogenic treatment, and aging. After solution treatment at 800 °C for 1 h, cryogenic treatments were performed at −78 °C for 4, 8, and 12 h, and at −196°C for 12 h, followed by aging at 550 °C for 4 h. Mechanical properties were evaluated using tensile testing and hardness measurements, while microstructural evolution was characterized by XRD, EBSD, and TEM. The results indicate that deep cryogenic treatment not only promotes the transformation of reversed austenite into martensite and refines martensitic laths, but also induces lattice distortion in martensite. This distortion is manifested by high stress concentrations and a high dislocation density, which increase the internal energy of the material. During subsequent aging, the release of this stored energy enhances the diffusion driving force of alloying elements, thereby promoting the formation of fine precipitates with a high number density. Among the investigated conditions, the specimen cryogenically treated at −78 °C for 12 h exhibited the best overall performance, achieving a yield strength of 1778.9 MPa, a hardness of 484.9 HV, and an elongation of 7.2%. In comparison, specimens treated at −196 °C showed a lower content of reversed austenite after aging. The lower cryogenic temperature induced greater lattice strain in the martensite, leading to the accumulation of a large amount of internal (potential) energy within the material. During subsequent aging, this stored energy was rapidly released, leading to strain recovery, a reduction in dislocation density, and a decrease in both the size and number density of precipitates. These effects ultimately resulted in lower strength and ductility compared with specimens treated at −78 °C. At −78 °C, increasing the cryogenic treatment duration progressively reduced the content of reversed austenite and refined its morphology. Concurrently, the fraction of low-angle grain boundaries and the precipitate size decreased. These microstructural evolutions enhanced grain-refinement and dispersion strengthening, resulting in significant increases in the strength and hardness of the steel. Overall, deep cryogenic treatment effectively optimizes the microstructure and mechanical properties of 18Ni(200) maraging steel by promoting martensite refinement and a favorable precipitate distribution. However, excessively low cryogenic temperatures may accelerate strain recovery during aging and diminish strengthening effects. This study provides practical guidance for designing cryogenic treatment protocols for high-performance maraging steels intended for cryogenic applications.

Study on ordering process of interfacial intermetallic compounds and strengthening mechanism of QAl10-5-5/TC6 bimetals

2026-01-15 00:00:00.0

Structural-Functional integrated copper alloy/titanium alloy bimetals can effectively combine the excellent properties of both copper and titanium alloys, exhibiting broad multifunctional application potential. In consideration of the difficulties in controlling the interfacial structure and the low interface strength of copper alloy/titanium alloy bimetals, the present work mainly focuses on investigating the evolution of interfacial microstructure, the ordering process of interfacial intermetallic compounds and the relationship between interfacial microstructure and mechanical property, and to clarify the formation process of interfacial intermetallic compounds and its effect on the interface strength of copper alloy/titanium alloy bimetals. Thus, QAl10-5-5/TC6 bimetals were fabricated by diffusion bonding using Ag interlayer (adding Ag foil as interlayer or electrodeposited Ag interlayer on QAl10-5-5) in the present work, the interfacial microstructure and shear strength of the QAl10-5-5/TC6 bimetals were characterized, and the strengthening mechanism was clarified. For the QAl10-5-5/TC6 bimetals adding Ag interlayer, a solid-solution transition layer formed in the interface with the bonding temperature increases from 800℃ to 850℃, The interfacial strength reached maximum of 200 MPa at a bonding temperature of 850°C, while the occurrence of micron-sized Cu-Ti intermetallics in the interfacial transition region at the bonding temperature of 875℃ substantially deteriorated the shear strength of the QAl10-5-5/TC6 interface. Meanwhile, for the Ag layer deposited QAl10-5-5/TC6 bimetals, the fine-grained structure of the electrodeposited Ag layer promoted metallurgical bonding, and the QAl10-5-5/TC6 bimetals achieved a higher interfacial strength of 217 MPa at a lower bonding temperature (835°C). Moreover, an "affected zone" formed on the TC6 alloy side, consisting of lamellar TiAlCu2 (thickness <1 μm), submicron/nanoscale Ti2Cu and Ti3Al phases, in which a semi-coherent interface occurred between Ti3Al and α-Ti. Theoretical strengthening calculations of Ag layer deposited QAl10-5-5/TC6 bimetal revealed that the synergistic effects of grain refinement, solid solution strengthening, and precipitation strengthening significantly enhanced the interfacial bonding strength, where precipitation strengthening playing the dominant role.

High-Temperature Oxidation Behavior of Novel Null-Matrix Alloy#br#

2026-01-12 00:00:00.0

Neutrons possess characteristics such as deep penetration, isotope sensitivity, and a magnetic moment, which make them pivotal in fields including engineering materials, polymers, energy materials, and condensed matter physics. Their strong penetrating ability enables applications in complex external-field sample environments, providing an ideal platform for investigating structure–property relationships of materials under extreme conditions. However, background signals originating from sample-environment equipment can interfere with the precise analysis of neutron diffraction data. Null-matrix alloys, characterized by the absence of neutron diffraction peaks, are ideal structural materials for neutron scattering experimental sample environments, as they do not interfere with diffraction signals from the target sample and thus enable accurate structural characterization during in situ experiments. A key challenge in current high-temperature in situ neutron diffraction experiments is the lack of null-matrix materials with sufficient high-temperature resistance. Conventional Ti–Zr alloys have melting points of only about 1550 °C, undergo softening and recrystallization above 750 °C that lead to interfering diffraction peaks, and present poor processing safety. Vanadium and its alloys, meanwhile, are prone to oxidation above 675 °C and suffer significant embrittlement beyond 1200 °C, which limits their reusability. These deficiencies severely constrain the development of high-temperature in situ neutron diffraction experiments. To address these limitations, a high-melting-point, oxidation-resistant Ti–Ta–Al null-matrix alloy (Ti–27.24Ta–9.08Al, atomic fraction, %) has been developed to meet the requirements of high-temperature in situ neutron diffraction experiments. To further evaluate its oxidation resistance, a systematic investigation of its high-temperature oxidation behavior in air was conducted. The results reveal a distinct two-stage oxidation kinetics behavior at 1000 °C. During the initial oxidation stage (< 10 h), the oxidation kinetics follow a parabolic rate law, and the oxide layer is mainly composed of TiO₂, Al₂O₃, and Ta₂O₅, forming a dense barrier that impedes inward oxygen diffusion. With prolonged exposure (> 10 h), the oxidation behavior transitions to linear kinetics, driven by abnormal coarsening of TiO₂ grains. The resulting structural discontinuities facilitate rapid oxygen penetration along grain boundaries, thereby accelerating the oxidation rate.

A Review on the Evolution Mechanism and Regulation Strategy of Strengthening Phases in Laser Powder Bed Fusion Nickel-Based Superalloys

2026-01-09 00:00:00.0

Laser Powder Bed Fusion (LPBF) technology offers significant advantages in the manufacturing of complex components made from nickel-based superalloys. However, due to its unique thermal history, the evolution of strengthening phases in LPBF differs significantly from traditional processes, becoming a key factor influencing material performance. This review summarizes the formation mechanisms, evolutionary characteristics, and their impacts on the microstructure stability and mechanical properties of multiple strengthening phases in LPBF nickel-based superalloys. It focuses on the multi-scale regulation mechanisms of strengthening phases through laser processing parameters and heat treatment, revealing the potential risks associated with phase instability in service performance. Furthermore, this review addresses the challenges faced in current research, including the synergistic control of strengthening phases, multi-scale modeling, and the integrated design of process-microstructure-performance. It envisions future advancements in high-performance nickel-based superalloys through the synergistic design of multiple strengthening phases, intelligent process optimization, and advanced post-processing techniques. This review aims to provide theoretical guidance and process references for the precise control and performance optimization of strengthening phases in LPBF nickel-based superalloys.

Effects of Cold-Rolled Pre-Deformation on Microstructures and Mechanical Properties of 2219

2026-01-08 00:00:00.0

This study investigates the effects of cold-rolling pre-deformation (0?20%) on the mechanical properties of peak-aged 2219 Al alloy sheets via hardness, tensile tests, and microstructural characterizations (EBSD, TEM). Using the improved Orowan model, it quantifies strengthening mechanisms to explore how pre-deformation enhances strength. Results show cold-rolling pre-deformation improves dislocation and precipitation strengthening, increasing peak-aged strength. Strength first increases, then decreases, and increases again with pre-deformation. Specifically, 5% and 20% pre-deformation promote precipitate nucleation and uniform distribution, boosting strength/hardness, while 10% pre-deformation causes precipitate coarsening and reduction, weakening strengthening. This study contributes to a deeper understanding of the mechanism by which cold-rolling pre-deformation improves the strength of 2219 aluminum alloy and provides a reference for selecting appropriate pre-deformation amounts.

Study on the Effect of Lattice Distortion Induced by SrZrO3 Doping on the Structure and Energy Storage Properties of Bi0.5Na0.5TiO3 Lead-free Ferroelectric Ceramics

2025-12-31 00:00:00.0

Lead-free relaxor ferroelectric ceramic capacitors, owing to ultrafast discharge rates and ultrahigh power density, find widespread application in pulsed power devices. However, conventional Bi0.5Na0.5TiO3-based ceramics are limited in their commercial applications due to the severe lack of energy storage density caused by high remnant polarization and low breakdown electric field strength. In this study, the solid solution properties of (1-x)Bi0.5Na0.5TiO3-xSrZrO3 (x=0, 0.025, 0.05, 0.075, abbreviated as BNT-xSZ) lead-free ferroelectric ceramics were systematically investigated, and the effect of lattice distortion on their structure and energy storage performance was explored. The study revealed that the lattice distortion introduced by SrZrO3 doping caused the c/a ratio of the ceramic to gradually approach unity. This enhanced local structural disorder suppressed long-range polar ordering, resulting in an increase in the ceramic's relaxation coefficient γ from 1.62 to 1.86. However, excessive doping can readily induce structural instability. Furthermore, SrZrO3 doping hindered elemental diffusion and inhibited grain boundary migration, which reduced the grain size from 5.79 μm (pure) to 1.83 μm (x = 0.025). Impedance analysis revealed that the resistance and activation energy Ea of BNT-0.025SZ were significantly higher than those of BNT. The excellent insulating properties and high Ea of BNT-0.025SZ contributed to increased oxygen vacancy migration resistance, thus improving the breakdown electric field. Therefore, BNT-0.025SZ achieved a high energy storage density of 2.08 J/cm3 and an outstanding efficiency of 83.14% under an electric field of 300 kV/cm. These findings clearly elucidate the effect of lattice distortion on the structural stability of BNT-xSZ binary solid solutions and provide theoretical guidance for designing lead-free ferroelectric ceramics with high energy storage performance.

Microstructure and Mechanical Properties of AC-CMT Arc Welded 7075Al Alloy Joint

2025-12-19 00:00:00.0

The 7075 Al alloy has garnered widespread attention in various industrial fields including aerospace, aviation, high-speed rail, and automobile manufacturing owing to its high strength, excellent ductility, and corrosion resistance. However, welding this alloy using conventional arc welding processes is challenging because the high heat input and the application of dissimilar filler materials result in high cracking susceptibility and remarkable reductions in the strength and corrosion resistance of the welded joint compared with the parent material. Recently, alternating current cold metal transfer (AC-CMT) arc welding has been extensively used as a promising solution. Herein, AC-CMT arc welding, which is an inherently low heat-input welding process, was used to weld 7075 Al alloy using a self-made ER7075 welding wire. The microstructure and mechanical properties of the welded joints before and after heat treatment were studied using optical microscopy, SEM, TEM, and a universal tensile testing machine. The results revealed that the AC-CMT arc-welded 7075Al alloy exhibited a smooth weld appearance; no cracks were observed, except for occasional arc-crater cracks in the extinguishing area. In the as-welded metal, a large number of strip-shaped and dot-like T phases (Mg(Zn, Cu, Al)₂ + Al eutectic phase) and h phase (Mg(Zn, Cu, Al)₂ phase) were present along the grain boundaries, whereas no dispersed precipitates were observed within the grains. The average tensile strength of the as-welded joint was 349 MPa, and the elongation was <4.48%. After postweld T6 heat treatment, the grain size of the weld metal remarkably increased, whereas the grain size of the heat-affected zone remained almost unchanged. The precipitates at the grain boundary almost disappeared, and a majority of dispersed GP regions and strengthening h¢(MgZn₂) phases precipitated inside the grains and along the grain boundaries.The dispersed GP regions and strengthening h¢ phases can effectively hinder grain boundary migration and significantly improve material strength. The continuous precipitated phases on the original grain boundaries disappear due to solid solution, reducing the fragile area of the grain boundaries and making the overall mechanical properties of the joint close to the level of the base material. The tensile strength of the T6-treated welded joint was increased to 538 MPa, which is almost equal to the tensile strength of the base material (540 MPa), whereas the elongation increased to 6.11%, reaching 55.50% of the base material. The postweld T6 heat treatment also shifted the fracture location from the center of the weld to near the fusion line.

Texture Controlling and Superelastic Tension-Compression Asymmetry of NiTi Alloy via Laser Powder Bed Fusion

2025-12-18 00:00:00.0

This study systematically investigates the effect of laser powder bed fusion (LPBF) process parameters (hatch spacing: 40-120μm; scanning velocity: 800-1400 mm/s) on the relative density, microstructure and crystallographic texture of Ni51.47Ti48.33 (at.%) shape memory alloys. Furthermore, the mechanisms underlying the superelastic tension-compression asymmetry of LPBF NiTi alloys with <001>{100} cubic texture and <110>{001} Gross texture are elucidated. At hatch spacing of 40 and 80 μm, sufficient overlap between adjacent melt pools ensures densities exceeding 99.5% across the scanning velocity range. Melt pool morphology analysis reveals that the inheritance of grain orientations among melt pools facilitates texture formation, while scanning velocity modulates texture types by altering the angular deviations between grain-growth directions and thermal gradients. Superelasticity tests demonstrates that {100}-textured specimen exhibits superior recovery rate (97.1%) and cyclic stability, with exceptional recoverable strain (5.77%), but suffer dramatic reduction in tensile recoverable strain (1.24%). In contrast, {110}-textured specimen displays higher recoverable strain (4.12-4.5%) under both tension and compression, yet with significant irrecoverable strain (0.89~4.00%) and inferior cyclic stability. The tension-compression asymmetry originates from the strong orientation dependence of martensitic transformation characterized as unidirectional shear-mode as well as the differential activation tendencies of slip systems under varied loading orientations. This work establishes intrinsic relationships among LPBF parameters, texture evolution, and superelasticity, providing theoretical foundations for the regulation of LPBF NiTi alloys' superelasticity and design of multimodal actuator devices.

Research Progress of the Niobium-stabilized Austenitic Stainless Steels for Generation IV Nuclear Reactors

2025-12-18 00:00:00.0

Austenitic stainless steels have been utilized in in-core and out-of-core structural components of various Generation IV nuclear reactors due to their excellent comprehensive properties, mature manufacturing processes, and decades of service experience in pressurized water reactors. However, after prolonged exposure to high temperature and high-intensity irradiation, these steels gradually exhibit some performance limitations, including lower creep strength, inadequate microstructural stabilities, and higher to radiation embrittlement sensitivities. Recently, alloying with niobium has proven to be effective in regulating the precipitation behavior of secondary phases, and the formation and evolution of irradiation defects in austenitic stainless steels, which holds significant promise for an simultaneous improvements in creep resistance and irradiation tolerance, thereby emerging as a key pathway for the development of next-generation austenitic stainless steels with enhanced high-temperature performance, irradiation resistance, and extended service life. This paper provides a systematic review of the research progress on niobium-stabilized austenitic stainless steels for Generation IV nuclear power applications. This paper reviews the compositional design and optimization to improve creep properties and irradiation resistance, the effects of niobium additions on austenite stability, the formation mechanisms of primary NbC, and the precipitation behavior of Nb-bearing secondary phases. Furthermore, the mechanisms about Nb alloying effects on strength-ductility balance, creep resistance, high-temperature microstructural stability, and irradiation-induced defect evolution are discussed. The insights summarized herein aim to provide a theoretical foundation and technical guidance for the compositional design, microstructural control, and engineering application of niobium-stabilized austenitic stainless steels in Generation IV nuclear reactors.

Phase Transformation Behavior During Multi-Step Heat Treatment Processes in High-Carbon Chromium Bearing Steel

2025-12-16 00:00:00.0

High-carbon chromium-bearing steel is widely used in rolling bearings in various mechanical equipment. The martensite in this steel provides high hardness, wear resistance, and contact fatigue resistance. However, its relatively low toughness compared to medium- and low-carbon steels increases the risk of failure during service, limiting its application under high-impact environments. To overcome this limitation, developing a martensite–bainite multiphase microstructure through multistep heat treatment has recently gained extensive attention. This approach considerably improves mechanical properties and extends service life. Compared with conventional martensite quenching and bainite isothermal quenching, the phase transformations in multistep heat treatment are more complex and interdependent making traditional characterization techniques inadequate for thorough explanation. These factors make in-depth investigation challenging. To optimize the multistep heat treatment process and clarify the relationship between processing parameters and the resulting martensite–bainite microstructure, this study designed a heat treatment sequence involving martensite–bainite–martensite transformations. Following this route, martensite–bainite multiphase structures with distinct microstructural features were constructed in high-carbon chromium-bearing steel. A comprehensive method integrating multiple testing techniques was also proposed to quantify phase fractions and transformation kinetics. This method enables the quantitative determination of phase fractions and the bainite transformation rate during heat treatment, reveals the influence of prior martensite on bainite transformation kinetics and elucidating the transformation behavior during heat treatment. In situ laser confocal microscopy was used to observe phase transformation characteristics during the multistep treatment. The results reveal that the initial martensite transformation considerably shortens the incubation period of subsequent bainite formation, demonstrating the catalytic effect of martensite on bainite transformation. Moreover, the martensite transformation provides nucleation sites for bainite, causing bainite to grow preferentially on the surfaces of martensite blocks rather than at austenite grain boundaries. This effect leads to a dispersed and uniform distribution of bainite sheaves and produces a uniformly refined martensite–bainite multiphase structure. Furthermore, when the volume fraction of martensite formed during the first quenching step reached 33.7%, the martensite and bainite generated during earlier steps exerted the strongest dividing and refining effects on the subsequent transformations. The accumulation of these refining effects resulted in a considerably refined multiphase microstructure.

Molten Pool Characteristics and Inclusions Behaviors in Ni-Based Superalloy During Vacuum Arc Remelting

Peng ZHAO Shu-feng YANG — 2025-12-16 00:00:00.0

As the final step in the production of nickel-based wrought superalloys, the characteristics of the molten pool in vacuum arc remelting (VAR) have a decisive impact on the cleanliness and metallurgical quality of the ingot. In this study, a multiphysics coupling model integrating electromagnetic effects, fluid flow, heat transfer, and inclusion motion was developed based on the process characteristics of a large-scale vacuum-arc remelted GH4738 superalloy ingot with a diameter of 690 mm. Using this model, the molten pool characteristics, along with the motion trajectories and distribution patterns of inclusions during VAR, were systematically investigated, with particular focus on evaluating the influences of arc current intensity, helium gas cooling pressure, and arc morphology evolution on molten flow and inclusion migration behavior. Results indicate that during remelting, the molten pool primarily exhibits a stable annular flow structure, with inclusion movement exhibiting distinct flow-driven behavior and pronounced size-dependent effects. As the melting current increases from 8 kA to 10 kA, the electromagnetic field strength and thermal input are enhanced, thereby promoting more active inclusion migration within the molten pool. Optimizing the helium cooling pressure to 400 Pa effectively improves molten pool morphology, suppresses turbulence development, and mitigates inclusion entrapment. The arc morphology transition from a diffuse to a constricted configuration fundamentally alters the electromagnetic field distribution, establishing dual circulatory flow patterns that drive inclusion accumulation toward the ingot's central region. Therefore, by rationally regulating the melting current and helium pressure, maintaining a shallow and flat molten pool, and stabilizing the diffuse arc mode, efficient inclusion removal can be achieved as a key processing strategy.

Correlation Between Large-Area Magnetic Domain Morphology and Saturation Magnetic Flux Density/Iron Loss in High-Permeability Grain-Oriented Silicon Steels

2025-12-12 00:00:00.0

Grain-oriented silicon steel (GOSS) is a key soft magnetic material used in electrical devices such as motors and transformers. Optimizing its magnetic properties remains a research focus in materials science and electromagnetics. Magnetic domain morphology, a critical factor influencing magnetic properties, directly determines the key characteristics of silicon steel, including saturation magnetic flux density (B₈₀₀) and core loss, thereby impacting its energy efficiency in practical applications. Therefore, elucidating the intrinsic relationships among domain morphology, crystallographic orientation, and magnetic properties is important for developing high-performance silicon steel. However, current research predominantly focuses on local domain observations, while the relationships among large-area domain morphology, crystallographic orientation, and macroscopic magnetic properties are rarely studied. To address this gap, the large-area domain morphology, fine microscale domain structures, and crystallographic orientations of GOSS were systematically investigated using large-scale domain observation method (Bitter method) combined with forescatter detector domain imaging and electron backscatter diffraction. The relationships between domain structures and magnetic properties, including B₈₀₀ and iron loss, were thoroughly examined. The results show that increasing both the area fraction of 180° primary domains with less than 2° deviation from the rolling direction and the average width of the primary domains enhances B₈₀₀ while reducing iron loss. Moreover, smaller average in-plane deviation angle (α) and out-of-plane deviation angle (β) of the ⟨100⟩ easy-magnetization axis from the rolling direction or rolling plane, as well as more concentrated α and β distributions, are associated with higher B₈₀₀ and lower iron loss. Furthermore, the presence of island grains and fine-grained microstructures possibly leads to a decrease in B₈₀₀ and an increase in iron loss. Microdomain observations and crystallographic orientation analyses reveal that low-angle grain boundaries do not impede domain transfer and that wedge-shaped domains form near boundaries with increasing misorientation. Additionally, unlike low-angle grain boundaries, high-angle grain boundaries completely block domain transfer. The orientations of both island grains and fine grains substantially deviate from the Goss texture, typically forming high-angle grain boundaries with surrounding coarse secondary recrystallized grains. The magnetic domains within these finer grains—primarily supplementary domains such as strip and lancet domains—obstruct the movement of 180° primary domains during magnetization, thereby degrading the overall magnetic properties of the material.

Dynamic mechanical response and spallation behavior of 60 steel under shock loading

2025-12-12 00:00:00.0

Plate impact experiments are conducted on 60 steel via a single-stage gas gun. Free surface velocity histories are measured to obtain the impact mechanical properties such as Hugoniot elastic limit and spall strength. Spall strength shows an initial increase followed by saturation with increasing peak stress. The microstructural damage of 60 steel under shock loading is characterized using scanning electron microscopy and electron backscatter diffraction. Damage morphology analysis indicate that the dominant damage mechanism is brittle fracture, manifested by the nucleation and propagation of cleavage cracks. As the peak stress increases, ductile damage mechanism becomes more pronounced, characterized by the nucleation, growth, and coalescence of microvoids. Further analysis reveal the influence of microstructure on damage behavior. Grain boundaries, cementite lamellae, and ferrite/cementite interfaces serve as preferential sites for damage nucleation. Within pearlite colonies, microcracks exhibit a tendency to propagate along directions that form larger angles with the cementite lamellae. Electron backscatter diffraction analysis confirms that grain orientation plays a key role in cleavage crack propagation, with microcracks preferentially propagating along the {001} crystallographic planes of the corresponding grains.

Effect of Rare Earth Treatment on the Friction and Wear Property of Bainite/Martensite Bearing Steel

2025-12-10 00:00:00.0

Bainite/martensite (B/M) multiphase bearing steel has become a research focus owing to its superior combination of strength and toughness compared with conventional martensite bearing steel. Our previous study confirmed the significantly improved toughness and fatigue properties of B/M bearing steel and demonstrated that rare-earth (RE) element incorporation can enhance its overall mechanical properties, highlighting the promising application potential of RE-incorporated B/M bearing steel. However, reports on the tribological properties of B/M bearing steel remain limited. Therefore, it is necessary to investigate the friction and wear behavior of B/M bearing steel and the effect of the RE element on the behavior. In this study, oil-lubrication sliding wear tests were conducted to evaluate the friction and wear behavior of B/M bearing steel and the effects of RE element incorporation. After the wear tests, the surface morphology and microstructural evolution beneath the wear tracks of the B/M bearing steels with and without RE elements were comprehensively characterized and analyzed using contact profilometry, a white-light interferometer, SEM, TEM, EBSD, transmission Kikuchi diffraction, and XPS. The results indicated that RE element incorporation significantly enhanced the wear resistance of B/M bearing steel, reducing material loss by >25% and achieving performance comparable to that of conventional martensite bearing steel. During the initial 20 min of the sliding wear test, B/M bearing steel with and without RE elements exhibited abrasive wear. The presence of smaller martensite/austenite blocks and a reduced fraction of retained-austenite transformation in the RE-containing steel inhibited microstructural evolution beneath the wear surface, reduced the generation of large wear debris, and decreased the debris-induced stress and plowing-induced wear volume, improving wear resistance. When the sliding duration was extended to 1 h, the primary wear mechanism for B/M bearing steel with and without RE elements transitioned to oxidative wear. RE element incorporation facilitated the formation of hard Fe₃O₄ oxide films with strong adhesion to the matrix, resulting in the excellent resistance of the RE-containing steel to oxidative wear.

Active modulation of eutectic growth and creep mechanism of Co₃₅Ti₃₅Nb₃₀ alloy under electrostatic levitation condition

2025-12-10 00:00:00.0

Cobalt alloys, a series of high-temperature alloys with high thermal corrosion resistance, excellent fatigue resistance and oxidation resistance, have been commonly used as structural materials in the recent decades, especially in aerospace engineering and biomedical fields. In experimental, a sample was levitated between two vertical electrodes coupled with four side electrodes in a chamber evacuated to 1.0 × 10-5 Pa. When the levitated sample was heated and melted for several times by SPI SP300 laser, it solidified rapidly at high undercooling owing to the high-vacuum environment and containerless state. The rapid solidification mechanism and micromechanical properties of ternary Co35Ti35Nb30 alloy were investigated by means of electrostatic levitation (ESL). The microstructure of the alloy consists of Co(Ti,Nb) + (Nb) eutectic. An orientation relationship of {110}Co(Ti,Nb) // {110}(Nb) and <-113>Co(Ti,Nb) // <-113>(Nb) formed between Co(Ti,Nb) and (Nb) phases. Under ESL conditions, the microstructure transformed from lamellar to anomalous eutectic with increasing undercooling. The microstructure of the alloy underwent a transformation into a completely anomalous eutectic and had no crystallographic orientation relationship because of the independent growth of the two phases, at the maximum undercooling (273 K). The nanoindentation creep behavior and mechanism were analyzed by estimating the strain rate sensitivity and activation volume. Owing to the refinement of the rapidly solidified microstructure and the Ti solute trapping in the (Nb) phase, the resistance to dislocation motion of the alloy increased, and the microhardness and creep resistance of the alloy improved. With the undercooling increased from 30 to 273 K, the creep strain rate during the steady stage creep stage decreased by about an order of magnitude, from 1.03 × 10-4 s-1 to 2.79 × 10-5 s-1. In addition, increasing the loading strain rate promoted the proliferation of dislocations during loading, leading to an increase in strain rate sensitivity from 7.7 × 10-5 to 8.7 × 10-5 and a decrease in activation volume from 26.7 b3 to 25.7 b3.

DECOUPING HYDROGEN DIFFUSION PARAMETERS OF SUB-LAYERS IN ALUMINOSILICONIZED STEEL: INTEGRATED EXPERIMENTAL AND MODELING

2025-12-09 00:00:00.0

Current hydrogen permeation testing techniques can only measure the average hydrogen diffusion parameters of materials, making it difficult to directly determine the sub-layer parameters of multi-layered hydrogen barrier coatings on steel surfaces. This study employed TP80SS steel as the substrate to fabricate aluminosiliconized steel via a hot-dip aluminizing process. This process formed a multi-layered hydrogen barrier structure comprising an outer Al-Si alloy layer and an inner Fe-Al-Si intermetallic compound layer. Utilizing high-pressure gaseous hydrogen permeation experiments, EDS layer thickness measurements, a multi-layer diffusion theoretical model, and Python-based numerical calculations, the hydrogen diffusion parameters of each sub-layer were resolved. The results demonstrate that the hydrogen diffusion coefficients of the TP80SS steel substrate, the Fe-Al-Si intermetallic compound layer, and the Al-Si alloy layer are 2.03×10-6 cm2·s-1, 4.83×10-9 cm2·s-1, and 7.48×10-10 cm2·s-1, respectively. Under a 10 MPa hydrogen environment, the aluminosiliconized coating reduced the hydrogen concentration on the hydrogen-charging side of the steel surface from 1.04×10-6 mol·cm-3 to 7.76×10-8 mol·cm-3. Significant hydrogen concentration discontinuities exist at material interfaces: the hydrogen concentration ratio (k1) across the interface between the outer Al-Si alloy layer and the inner Fe-Al-Si intermetallic compound layer is 26.11, while the ratio (k2) across the interface between the intermetallic compound layer and the steel substrate is 9.52. This indicates that the Al-Si alloy layer possesses superior hydrogen barrier efficiency compared to the Fe-Al-Si intermetallic compound layer. By integrating experimental methods with theoretical modeling, this study has, for the first time, achieved precise resolution of the hydrogen diffusion parameters for each sub-layer within a complex multi-layered hydrogen barrier structure. This approach provides a crucial theoretical foundation for the design and optimization of advanced hydrogen-resistant coatings.

Current-Carrying Friction Properties and Mechanism of Cu–Ti₂AlC Composite Coating

2025-12-08 00:00:00.0

Copper-based materials are the primary choice for current-carrying friction components and are widely used in applications such as electromagnetic railguns, electrical contacts, electrically conductive slip rings, and pantograph plates. However, their friction losses under current-carrying conditions require increased attention. Pure copper is prone to adhesive wear against counterface materials during dynamic current-carrying friction. Furthermore, traditional Cu alloys exhibit significant fluctuations in the friction coefficient under low contact pressure and high current density, and they are susceptible to arc ablation and other forms of interface instability. These problems lead to substantially increased wear rates, which severely compromise service reliability. In recent years, researchers have explored various coating systems to improve the current-carrying friction performance of copper-based materials across diverse service environments. As a representative MAX phase material (general formula M_n_{+ 1}AXₙ), Ti₂AlC exhibits a unique layered hexagonal structure combining metallic and ceramic attributes. This ternary carbide demonstrates exceptional performance under extreme conditions, including elevated temperatures, corrosive environments, and tribological stresses. During frictional contact, the surface-initiated oxidation of Ti₂AlC facilitates the formation of a protective oxide layer, effectively reducing friction coefficients through its intrinsic self-lubrication mechanism. With electrical conductivity comparable to that of metals, Ti₂AlC demonstrates particular promise for applications such as electromagnetic railguns and electrical contacts, exhibiting stable contact resistance and low wear rates under current-carrying conditions. Hence, Cu–Ti₂AlC composite coatings were fabricated on Cu alloy surfaces using spray granulation and supersonic flame spraying techniques. The phase composition, microstructure, mechanical properties, and electrical characteristics of the coatings were systematically characterized through XRD, SEM, microhardness testing, and scratch testing. The Cu–Ti₂AlC coating exhibits a conductivity of 28% IACS, an average hardness of 209.2 HV_0.1, a bonding strength of 48 MPa, and a fracture toughness of 9.6 MPa·m^1/2, demonstrating excellent comprehensive performance. Comparative investigations of the tribological performance and wear mechanisms of Cu alloy and Cu–Ti₂AlC coatings under current-carrying and non-current conditions were conducted using a current-carrying friction–wear equipment. The Cu–Ti₂AlC coating demonstrated superior current-carrying performance compared with the Cu alloy. The results demonstrate that the Cu–Ti₂AlC coating effectively mitigates adhesive wear, suppresses the formation of Al-rich deposition layers, the friction coefficient is lower compared to Cu/Al. , and enhances wear resistance. The effects of current (0–120 A) and friction load (20–40 N) on the current-carrying friction properties of the coatings were investigated. The optimal performance of the coatings occurred at 80 A and 30 N, under which the coating exhibited the minimum wear rate, and a synergistic wear mechanism was identified, involving electrical erosion, fatigue wear, abrasive wear, and oxidative wear.

Effect of Lanthanum on the Solidification behavior of GH4151 Superalloy

2025-12-03 00:00:00.0

The high alloying degree of GH4151 alloy gives rise to severe elemental segregation. Rare earth elements (REEs) have a great influence on the as-cast microstructure. In the present study, we delved into the evolution of the both the as-cast and homogenized microstructures of GH4151 alloy, modulating the content of the REE lanthanum (La) at varying concentrations (0, 0.01, 0.1, 1 wt.%, respectively). The results demonstrated that the inclusion of 0.01 wt.% La into GH4151 alloy led to a notable reduction in both primary and secondary dendrite spacings. Furthermore, in the 0.1 wt.% La alloy, a precipitate phase comprising La, O, and S was distinctly identified. Notably, the introduction of La significantly impacted the precipitation behavior of key phases such as (γ+γ′) eutectic, Laves phase, MC carbide, and η phase within the interdendritic regions, concomitantly mitigating the degree of segregation of Ti and Nb elements. Specifically, the (γ+γ′) eutectic content was conspicuously diminished in the 0.01 wt.% La alloy, whereas the proportion of η phase was increased. As the La content increased, the (γ+γ′) eutectic content gradually augmented, while the abundances of MC carbide and η phase diminished. Crucially, the modulation of trace La content facilitated a substantial reduction in the residual segregation indices of Ti, Nb, and Mo post homogenization, thereby enhancing the homogeneity of these major segregation elements. This improvement underscores the potential of REEs, particularly La, in refining the microstructure and mitigating segregation in GH4151 alloy.

Effect of tempering temperature on stress corrosion resistance of F22M Steel for Blowout Preventer in Ultra-deep Well

2025-12-03 00:00:00.0

Existing low-alloy Cr-Mo steels of 35CrMo, 20CrMoV, and 25CrNiMo, are inadequate to meet the requirements of heat resistance, corrosion resistance, strength and toughness necessary for the pressure components of blowout preventers (BOPs) with large and thick sections in ultra-deep wells. Recently, a novel F22M steel, synchronously micro-alloyed with vanadium, boron and rare earth elements, has been developed and applied in a test ultra-deep well. Under the extreme conditions about 200 °C and 140 MPa in ultra-deep wells, hydrogen atoms derived from H2S decomposition synergistically interact with tensile stress and corrosive environments, potentially inducing stress corrosion cracking (SCC). The resulting crack growth rate could reach up to 10-6 mm/s, severely compromising the service safety of critical pressure-bearing components such as blowout preventers. Tempering, as a critical post-quenching heat treatment for Cr-Mo steels, significantly modifies microstructure features including dislocation density, carbide morphology, and grain boundary characteristics, thereby directly governing the resistance to SCC. The present work systematically investigated the effects of tempering temperature on microstructure evolution and stress corrosion resistance of F22M steel, using the slow strain rate test, electrochemical analysis, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. As the tempering temperature elevated from 610°C to 670°C, the dislocation density in F22M steel decreased from 6.29 × 1015 m-2 to 3.51 × 1015 m-2, the morphology of carbides transitioned from needle shape to spherical particle, and the proportion of high-angle grain boundaries increased from 41.0% to 54.3%. The reduction in dislocation density and change of carbide morphologies could prominently diminish the potential for local hydrogen accumulation and stress concentration, while grain boundary characteristics improvement further influencing hydrogen distribution and crack propagation. As a result, the stress corrosion susceptibility index has been decreased by 84.21%, indicating that the stress corrosion cracking resistance was substantially enhanced.

TMechanism of Carbides in Promoting Stray Grain Formation at the Bottom of Laser Remelting Pools in DD32 Superalloy

2025-12-01 00:00:00.0

Nickel-based single-crystal superalloys are widely utilized for manufacturing turbine blades in aerospace engines owing to their excellent oxidation and hot-corrosion resistance. However, these components are susceptible to various types of damage under high-temperature conditions, and their high replacement cost makes repair essential. Laser additive manufacturing (LAM) is commonly used for repairing single-crystal blades; however, it often introduces stray grains, which form grain boundaries that degrade the alloy's properties. Therefore, suppressing stray grain formation is a critical objective in the laser repair process. During laser melting, nickel-based single-crystal superalloys are highly prone to stray grain formation at the bottom of the melt pool. According to the stray grain formation theory, the propensity for stray grain formation is inversely proportional to the ratio of the temperature gradient to the growth rate at the columnar dendrite front. As the melt-pool bottom has a high thermal gradient and low solidification rate, stray grain formation should theoretically be suppressed in this region; however, experimental observations yet contradict this prediction. Carbides may act as potent nucleation sites and play a critical role in facilitating stray grain formation at the melt-pool bottom; however, the detailed mechanism remains elusive. To clarify this mechanism, laser remelting was performed on DD32, a high-carbon nickel-based single-crystal superalloy produced in China. Particulate inclusions within the stray grain area at the melt-pool bottom were detected via optical microscopy and subsequently identified as MC-type carbides (TaNb)C via scanning electron microscopy and energy-dispersive X-ray spectroscopy. Their considerable size difference from that of carbides reprecipitated after remelting indicates that they originate from the alloy matrix, and they exhibit resistance to complete melting or dissolution during the remelting process. As these carbides have a considerably higher density than the alloy melt, they settle to the melt-pool bottom due to gravity, thereby increasing the nucleation site density and promoting the formation of stray grains. By integrating electron probe microanalysis (EPMA) with numerical simulation, this study quantitatively elucidated how carbide-induced nucleation sites increase nucleation density at the melt-pool bottom and promote stray grain formation. These findings explain the phenomenon of stray grain formation at the bottom of the laser-melted pool in nickel-based single-crystal superalloys and provide a theoretical basis for controlling stray grains in future single-crystal repair processes.

Effect of Process Parameters and Post-Treatment on Room-Temperature Tensile Properties of GH3536 Superalloy Fabricated by Laser Powder Bed Fusion#br#

2025-11-21 00:00:00.0

Laser powder bed fusion (LPBF) is a typical additive manufacturing technology that offers several technical advantages such as high geometric freedom, few forming processes, and the capability for large-scale customization. It provides a reliable and feasible method for manufacturing complex metal components used in fields such as aerospace, nuclear energy, and new energy. The development of GH3536 alloy fabricated by LPBF is beneficial for improving the design of complex, high-precision components. For alloys fabricated by LPBF, the process parameters and post-treatment conditions influence defect formation, microstructural characteristics, and mechanical properties. Improper selection of process parameters can introduce internal defects into the alloy and deteriorate its mechanical properties. By contrast, appropriate post-treatment can modify the defects and microstructure of the alloy. Therefore, the rational design of process parameters and post-treatment conditions is crucial for ensuring the service reliability of components fabricated by LPBF. However, most studies have investigated the influence of process parameters and post-treatment independently, or first optimized the process parameters of the alloy to minimize defects and then investigated the influence of post-treatment under these optimized process parameters. A comprehensive design approach that considers process parameters and post-treatment simultaneously is required to achieve optimal performance in GH3536 alloy components fabricated by LPBF. In this study, five groups of GH3536 alloys were fabricated under different laser energy densities by regulating the laser power (205–305 W) and scanning speed (700–1200 mm/s). Two post-treatment conditions were designed: heat treatment and heat treatment followed by hot isostatic pressing (HIP). The combined effects of process parameters and post-treatment on defect formation, microstructural characteristics, and room-temperature tensile properties of GH3536 alloys fabricated by LPBF were comprehensively investigated. The results show that the presence of internal defects leads to relatively low strength and plasticity, as well as a pronounced dispersion in the alloy under heat-treated conditions fabricated at 39 J/mm³. Increasing the laser energy density or performing HIP treatment can enhance its plasticity by ~50%. Meanwhile, HIP also increases its yield strength by ~15.3%, mainly because HIP eliminates internal defects, thereby improving the strength and plasticity of the alloy. When the defect content is reduced, the tensile properties of the heat-treated alloys fabricated at 60–99 J/mm³ transition from being jointly influenced by defects and microstructure to being primarily dominated by microstructural factors. Consequently, the effect of HIP treatment on tensile properties becomes less pronounced. After HIP treatment, the plasticity and yield strength of the alloy fabricated at 99 J/mm³ increase by only 3.5% and 8.4%, respectively.

Phase-Field–Lattice Boltzmann Simulation on the Effect of Shrinkage Flow on Solute Dendritic Growth

2025-11-21 00:00:00.0

Liquid flow significantly influences dendritic growth behavior. Previous studies have shown that under forced convection, flow directed toward the dendrite tip enhances thermal dendrite growth. In contrast, shrinkage flow directed toward the tip suppresses thermal dendrite growth, although the underlying mechanism remains unclear. It also remains uncertain whether this effect similarly influences solute dendrites. In this study, we incorporated a density field into a quantitative phase-field model and coupled it with the Lattice Boltzmann method to simulate liquid flow. We began by examining the density-coupling and flow effects to investigate how shrinkage flow influences solute dendrite growth. Our findings indicate that shrinkage flow increases the liquid composition ahead of the dendritic tip, thereby suppressing dendritic growth—a trend consistent with that observed in thermal dendrites. Our analysis further shows that although the density-coupling effect promotes dendritic growth, the inhibitory effect of shrinkage flow is stronger. Unlike forced convection, shrinkage flow not only facilitates advective transport of solute but also introduces a source term in the composition field at the interface due to density variations. This source term expels solute, increasing the liquid composition ahead of the tip. Its presence is the fundamental mechanism by which shrinkage flow suppresses dendritic growth and also explains the inhibitory effect of shrinkage flow on thermal dendrites.

Effect of alloying elements on phase interface behavior in Co-Ti based superalloys

2025-11-10 00:00:00.0

The mechanical properties and thermal stability of novel Co-based superalloys are critically dependent on the interface characteristics between precipitates and matrix. The influence of alloying elements on the evolutions of L12-γ′/γ and D019-χ/γ interfaces was systematically investigated by first-principles calculations. The preferred precipitation sites of alloying elements near the phase interface were identified, and their effects on nucleation and growth of precipitates were comprehensively evaluated. The results demonstrate that Mo and Ta significantly enhance the stability of interface between precipitates and matrix. In contrast, the addition of Al and Ni tends to weaken the stability of interface and inhibit D019-χ phase nucleation. The charge analysis reveals that the strengthening effect of alloying element doping on interfacial strength is primarily attributed to the enhancement of metal bond strength across the interface. Furthermore, the impact of various alloying elements on diffusion behavior was also examined and experimentally verified. These findings provide valuable insights for the development of novel Co-based superalloys with superior performance.

Hierarchical Lamellar Heterostructured Metallic Materials

2025-11-05 00:00:00.0

Microstructural Characteristics and Properties of Antibacterial Low-Alloyed Mg–Ag–In Alloys

2025-10-30 00:00:00.0

Magnesium alloys are regarded as highly promising biomaterials for medical applications due to their low density, high strength, excellent biocompatibility, and ability to degrade safely within the human body. China, which possesses the world’s largest magnesium reserves, has a significant resource advantage for the research and development of medical magnesium alloys. In recent years, China has faced an increasingly severe societal challenge—population aging—which has led to a growing demand for biomaterials, particularly bone implant materials. However, existing magnesium alloys suffer from several limitations, including excessively rapid degradation, an imbalance between strength and toughness, and insufficient antibacterial properties. These drawbacks substantially hinder their clinical translation and large-scale application. To address these challenges and following the low-alloying design principle, an extruded Mg–Ag–In alloy was developed. This study focuses on a low-alloyed Mg–1Ag–0.5In composition with intrinsic antibacterial properties. The investigation examines its microstructural characteristics, mechanical performance, corrosion behavior, osteoblast compatibility, and antibacterial activity. The results showed that the extruded Mg–1Ag–0.5In alloy exhibited a favorable balance between strength and ductility. This synergy was primarily attributed to the coordinated deformation of tensile twins and multiple slip systems. During corrosion, the alloy transitioned from an initial mixed pitting and filamentous corrosion mode to a uniform filamentous corrosion pattern, which reduced the overall corrosion rate and facilitated the formation of a denser corrosion product layer. The corrosion driving force was mainly influenced by the grain orientation and the distribution of regions with high kernel average misorientation values. Furthermore, the extruded Mg–1Ag–0.5In alloy demonstrated excellent osteoblast compatibility, effectively promoting the proliferation of mouse embryonic osteoblast (MC3T3-E1) cells, and exhibited remarkable antibacterial activity.

Microstructural Evolution and Mechanical Property Enhancement in SiC-Reinforced AA7075 Aluminum Matrix Composites Processed via Laser Powder Bed Fusion Additive Manufacturing

2025-10-29 00:00:00.0

This study centers on mitigating solidification cracking in laser powder bed fusion (LPBF) processed 7075 aluminum alloys. An in-situ reaction strategy employing SiC ceramic particles was introduced to modify molten pool solidification behavior, aimed at enhancing formability and mechanical properties. 7075 aluminum matrix composites with varying SiC contents (0-8 wt.%) were fabricated using LPBF combined with mechanical mixing. The influence of SiC particles on microstructural evolution, crack suppression mechanisms, and mechanical performance was systematically examined. Results revealed that in-situ reactions between SiC particles and the aluminum matrix generated Al4C3, Al4SiC4, Si, and Mg2Si phases. These phases formed intergranular network structures that fortified grain boundaries and diminished cracking susceptibility.The synergistic strengthening from micron-sized SiC particles and nano-scale precipitates (Al4C3, Al4SiC4, Si, Mg2Si) substantially improved mechanical properties. At 6 wt.% SiC content, solidification cracking was entirely eliminated, with tensile strength achieving 307.0 ± 37.0 MPa and elongation improving to 5.0 ± 0.3%. Further increasing SiC content to 8 wt.% yielded a microhardness of 161.4 ± 10.5 HV0.1, although ductility diminished to 3.5 ± 0.4% due to excessive particle content. This research demonstrates the feasibility of micro-scale ceramic particles in regulating the formability, microstructure, and properties of LPBF-processed Al-Zn-Mg-Cu alloys, offering significant implications for economical additive manufacturing of high-strength aluminum alloys.

Influences of High-Entropy Alloy Particles on the Microstructure and Mechanical Properties of Selective Laser Melted Al12Si Alloy during Solution Treatment

2025-10-27 00:00:00.0

为了探究固溶处理过程中耐高温第二相对Al12Si合金微观组织及力学性能的影响，本工作采用选区激光熔化(SLM)工艺制备了AlCrCuFeNi高熵合金(HEA)改性Al12Si合金，结合后续的固溶处理实现了对Al12Si合金微观组织和力学性能的协同调控。重点研究了固溶处理过程中耐高温第二相对Al12Si合金微观组织和力学性能的调控机制。结果表明，打印态Al12Si试样具有典型的初生α-Al和连续共晶Si组成的胞状组织；而HEA颗粒的加入使Al12Si-HEA试样的打印态组织发生显著转变，由Al12Si试样中的共晶Si相转变成共晶Si + α-Al(Fe, Cr)Si相。固溶处理后，Al12Si试样中连续的胞状共晶组织解体，转变成在基体中弥散分布的颗粒状Si相。此外，随着固溶时间的延长，Si相由于发生Ostwald熟化而逐渐粗化，平均直径变大，同时数量密度降低。值得注意的是，α-Al(Fe, Cr)Si相能够阻塞Si元素在基体中的扩散通道，从而显著降低颗粒状Si相的粗化速率。力学性能测试表明，在所有固溶时间下，Al12Si-HEA试样均表现出比Al12Si试样更优异的综合性能。这可归结于经固溶处理后，Al12Si-HEA试样形成了双粒径原位增强颗粒的微观组织，即更为细小的微米级Si相及纳米级α-Al(Fe, Cr)Si相。这种独特的微观组织使Al12Si-HEA试样在保持较高的塑性(约15%)的同时，也具有优异的极限抗拉强度(311 MPa)。

Micro- to Nanoscale Mechanical Behavior of Irradiation Hardening in Zr-Sn-Nb Alloys Under Ion Irradiation

2025-10-17 00:00:00.0

To address irradiation-induced hardening and growth in zirconium alloys for nuclear cladding, this study investigated Zr-Sn-Nb alloys irradiated by 1.8 MeV Ar+ ions at 300°C (corresponding to peak damages: 2, 5, 10, 24 dpa). By integrating microstructural characterization of irradiation-induced defects (dislocation loops, argon bubbles, and secondary phase particles) with micromechanical testing, the correlation between defect evolution and irradiation hardening mechanisms was systematically elucidated. The results indicated that irradiation defects are dominated by -type dislocation loops and high-density argon bubbles. The size of -type dislocation loops exhibited continuous growth with increasing dose, and their density saturated at 10 dpa. Argon bubble nucleation displayed significant dose dependence, with both density and size increasing markedly with irradiation dose. The average bubble size reaches the largest value of ~2.48 nm at 24 dpa. Secondary phase particles underwent amorphization under low-dose irradiation, while their geometric characteristics (size distribution and spatial density) remained unchanged. A comparative investigation based on nanoindentation measurements and in-situ TEM uniaxial compression tests, supported by theoretical frameworks including the Nix-Gao model and dispersed barrier hardening (DBH) theory, revealed that irradiation-induced hardening in Zr-Sn-Nb alloys predominantly arises from the synergistic interaction between dislocation loops and argon bubbles. Furthermore, the irradiation hardening trend derived from nanoindentation measurements was statistically consistent with that obtained from in-situ TEM compression tests. The proposed "microstructure-mechanics-modeling" methodology provides a systematic framework for evaluating irradiation damage in zirconium alloys.

Microstructure and Mechanical Properties of Mg-5Zn-0.5Ca Alloy Controlled by Ti Microslices

2025-10-15 00:00:00.0

To enhance the mechanical properties of magnesium alloys and elucidate the regulatory mechanisms of flake-like structures in composites, while also exploring the feasibility of using deformable metallic particles with varying morphologies to reinforce magnesium matrix composites, this study systematically investigated the influence of deformable Ti microslices (Ti_ms) with varying slicing degrees on the microstructure and properties of magnesium matrix composites. Ti particles were processed into sheets via ball milling process to control the slicing degree of the Ti_ms. These Ti_ms were introduced into an Mg–5Zn–0.5Ca (ZX50) alloy via semisolid stirring casting to successfully fabricate Ti_ms/ZX50 composites. The composites subsequently underwent hot extrusion to examine the effect of Ti_ms’ slicing degree on the microstructure and mechanical properties of the ZX50 alloy. The results indicate that the hardness of Ti_ms increased significantly with higher slicing degrees but decreased markedly during the hot-extrusion process. Ti_ms effectively regulated dynamic recrystallization and precipitation by altering the strain and load distribution within the matrix. As the slicing degree increased, the recrystallized grain size of the ZX50 matrix decreased, and the number of precipitated MgZn₂ phases increased. The influence of Ti_ms on recrystallization exhibited directionality, with finer grain sizes parallel to the extrusion direction than those perpendicular to it. Furthermore, the grain-size disparity grew with increasing Ti_ms’ slicing degree. Additionally, the strength, work-hardening rate, and softening rate of Ti_ms/ZX50 composites generally increased with increasing Ti_ms slicing degrees, although the softening ratio initially increased and subsequently decreased. When the average aspect ratio of Ti_ms reached 1.2, the softening rate progressively decreased as the quantity of MgZn₂ phases increased. Under the synergistic effects of load-transfer, dislocation, and grain-refinement strengthening, the strength of the Ti_ms/ZX50 composites gradually increased, with grain-refinement strengthening being the most significant contributor.

Microstructural Characteristics and Electrochemical Behavior of a Mg-0.1Sn Anode for Magnesium-ion Batteries

2025-10-14 00:00:00.0

With the global emphasis on renewable energy, the development of large-scale energy storage technologies has become essential for achieving carbon neutrality. Lithium-ion batteries (LIBs) are currently the most widely used and technologically mature energy storage systems; however, resources such as Li, Co, and Ni are scarce in China, resulting in high production costs. Moreover, the growth of lithium dendrites poses serious safety risks, as these structures can penetrate the separator, cause short circuits, and potentially lead to thermal runaway, fires, or explosions. In light of these challenges, while significant progress has been made in developing protection strategies for LIBs, researchers are increasingly exploring alternative metal-ion battery systems, including magnesium-ion, sodium-ion, and zinc-ion batteries. Among these, rechargeable magnesium-ion batteries (MIBs) are considered one of the most promising next-generation energy storage technologies. Compared with metallic Li, metallic Mg offers a higher volumetric capacity and a relatively low reduction potential. In addition, Mg is more abundant in the Earth's crust and theoretically resistant to dendrite formation under appropriate conditions due to its low self-diffusion barrier. Its chemical stability in air and lower flammability also contribute to improved operational safety. MIBs thus present a promising route toward safe, high-capacity energy storage. However, challenges such as uneven stripping and high overpotential at the Mg anode lead to pitting corrosion and premature failure. To address these issues, a Mg-0.1Sn (mass fraction, %) anode material with a homogeneous equiaxed structure was prepared via extrusion, yielding an average grain size of (15.6 ± 2.4) μm and a texture strength of 17.40 MRD. The electrochemical performance of this alloy was compared with that of a pure magnesium anode to achieve more uniform and rapid Mg deposition/stripping behavior. At a current density of 1.0 mA/cm² and a deposition capacity of 0.5 mAh/cm², the overpotential of the T0-E//T0-E cell was only 0.21 V, maintaining stable operation for up to 1000 cycles, whereas the P-Mg//P-Mg cell sustained only 505 cycles. The T0-E//Cu half-cell also exhibited excellent cycling stability for over 450 cycles with an average coulombic efficiency of 98.88%. In contrast, after 230 cycles, the coulombic efficiency of the P-Mg//Cu cell fluctuated significantly and remained comparatively low. Furthermore, when paired with a Mo₆S₈ cathode, the T0-E//Mo₆S₈ full cell maintained good cycling stability for over 1800 cycles at a rate of 1 C. The superior cycling performance of the Mg–0.1Sn anode is attributed primarily to the uniform solid solution of Sn within the Mg matrix. Furthermore, the Sn component participates in the charge–discharge processes, effectively mitigating volume expansion and suppressing microcrack propagation. Meanwhile, the formation of SnCl₂ on the anode surface reduces the local Cl⁻ concentration, thereby slowing electrolyte corrosion and enhancing overall electrode stability.

Microstructural Evolution and Dynamic Failure Mechanism of B10 Cu-Ni Alloy Under Multiple Stress Coupling in Flowing Seawater#br#

2025-09-19 00:00:00.0

To address the critical issue of microstructural degradation and dynamic failure of Cu–Ni alloys subjected to multiple stress couplings in flowing seawater, this study systematically investigates the microstructural evolution and failure characteristics of the alloy in a complex service environment. Focusing on the widely used B10 Cu–Ni alloy in pipeline systems, a combined experimental and computational simulation approach was employed. A molecular dynamics method was used to construct an alloy/seawater solvation model, with emphasis on the effects of seawater flow velocity, pressure, and their coupled interactions on microstructural evolution and corrosion kinetics. In flowing seawater, the strain level of the alloy increases markedly, accompanied by a rise in microstructural defects. The results reveal that corrosion of Cu–Ni alloys proceeds through a multi-step coupled mechanism governed by the migration–dissolution energy barrier of Cu atoms. Density functional theory calculations show that as seawater pressure increases from 0.1 to 12 MPa, the migration–dissolution energy barrier decreases from 1.76 to 1.54 eV, significantly accelerating the corrosion rate. Furthermore, increasing seawater flow velocity induces atomic-level axial elastic tensile strain on the (111) crystal plane of the alloy, further reducing the migration–dissolution energy barrier. A critical flow velocity of 4 m/s has been identified as exacerbating corrosion. Under the coupled influence of flow velocity and pressure, the migration–dissolution energy barrier is further reduced, the work function decreases, and corrosion kinetics are significantly accelerated. The corrosion rate constants obtained from simulations align well with experimentally measured corrosion rates, enabling reliable assessment and prediction of the corrosion tendency of the B10 Cu–Ni alloy during dynamic failure under real operational conditions.

Molecular Dynamics Simulations of Tensile Deformation of Cu/Ta Nano-Bilayer Films and the Effect of Al and W Atoms Doping on the Deformation#br#

2025-09-17 00:00:00.0

Nano-multilayers composed of immiscible metals have been widely investigated over the past decades due to their exceptional microstructural stability. The Cu/Ta system, which is also immiscible, is particularly notable because tantalum offers higher melting point, strength, and superior wear and corrosion resistance compared to metals like niobium. Cu/Ta nano-bilayer films are extensively used in the electronics industry for semiconductors, microelectronic devices, optical systems, and magnetic applications. However, the presence of interfaces and the distinct deformation responses of the Cu and Ta layers during processing, fabrication, and service conditions make mechanical deformation and subsequent failure inevitable, potentially compromising the performance of microscale devices. Therefore, understanding the deformation mechanisms and enhancing the mechanical strength of Cu/Ta nano-bilayers at the microscopic scale is essential. In this study, molecular dynamics simulations were employed to investigate the tensile behavior and deformation mechanisms of Cu/Ta nano-bilayers. In addition, the effects of Al and W doping in the Cu layer on the deformation behavior were analyzed. The results indicate that tensile loading direction significantly influences the plastic deformation mode. When the bilayers were stretched parallel to the interface, both Cu and Ta layers exhibited sequential plastic deformation. In contrast, when the loading was applied perpendicular to the interface, only the Cu layer deformed plastically, while the Ta layer remained elastically constrained throughout. Doping with Al or W atoms enhanced the overall hardness and yield strength of the nano-bilayers. Furthermore, W doping induced stacking faults in both loading directions when stretched parallel to the interface, and a martensitic transformation from fcc to bcc structure occurred in the Cu layer under perpendicular tension.

The Effect of Cumulative Deformation on the Recrystallization Mechanisms of F316 Austenitic Stainless Steel

2025-09-05 00:00:00.0

The forming of large and complex forgings typically relies on multistep cumulative deformation processes that optimize microstructures and improve mechanical properties. However, the mechanisms of the recrystallization that occurs during these processes are not fully understood. This study examines the effects of deformation temperature, strain rate, inter-pass holding time, and deformation distribution on the dynamic recrystallization (DRX), static recrystallization (SRX), and metadynamic recrystallization (MDRX) behaviors of F316 austenitic stainless steel. With a combination of thermal simulation experiments and EBSD techniques, the evolution of the geometrically necessary dislocation (GND) density, grain boundary characteristics, and grain orientation under various deformation conditions was comprehensively analyzed. Increasing the deformation temperature and decreasing the strain rate facilitate DRX occurrence by reducing the critical energy. Prolonged holding time results in excessive SRX and MDRX, which depletes the stored deformation energy and hinders subsequent DRX. When the initial deformation amount is large, sufficient MDRX tends to suppress DRX activity. When the initial deformation amount is small, SRX contributes additional nucleation sites. Increasing the final pass deformation amount tends to promote DRX occurrence by preventing MDRX. Further analysis indicates that although SRX consumes part of the deformation energy, the effect of its grain refinement provides a greater number of nucleation sites for subsequent DRX. MDRX is a process of grain growth that proceeds directly from DRX grains and that consumes deformation energy, thereby suppressing subsequent DRX and potentially resulting in a mixed grain structure that would adversely affect the material’s final microstructure and properties. An accumulated deformation energy model for F316 stainless steel is established to evaluate the effects of various recrystallization processes. This model calculates the energy consumption of SRX and MDRX during hot deformation, determines the critical energy required for DRX in subsequent passes, and quantifies the specific contributions to DRX from SRX and MDRX. The findings provide theoretical support for the microstructural control and performance optimization of large forgings, thereby offering practical guidance for optimizing forging processes, especially under high-temperature and high-strain-rate conditions.

Influence of Capsule Wall Thickness on Preparation and Mechanical Properties of Powder Metallurgy-Hot Isostatic Pressed Titanium Alloy Fan Impeller

2025-09-03 00:00:00.0

Powder metallurgy-hot isostatic pressing (PM-HIP) has emerged as an important technology for near-net-shape fabrication of complex, high-integrity components, particularly for demanding applications in aerospace and power generation. Herein, using simple cylindrical specimens and a complex fan impeller, the influence of capsule wall thickness on the microstructure and mechanical properties of PM-HIPed Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy was systematically investigated. Additionally, the service performance of the PM-HIPed fan impeller was assessed under extreme operational conditions via spin testing conducted at room temperature (25 °C) and elevated temperature (300 °C). Results show that, under HIP conditions of 940 ℃, 120 MPa, and 3 h, capsule wall thickness variations of 5–40 mm had a negligible effect on the microstructure or tensile properties of the PM-HIPed alloy formed within these cylindrical capsules. Similarly, the complex fan impeller, manufactured under comparable HIP conditions, exhibited excellent microstructural uniformity throughout its intricate form. During spin testing, the impeller demonstrated remarkable stability at 300 °C, successfully achieving a stable rotational speed of 35746 r/min. At 42494 r/min, however, the impeller began to experience significant vibration, culminating in a loss of structural stability. To elucidate the mechanism of structural stability loss, finite element simulations were performed, complemented by transmission electron microscopy (TEM) analysis of the stability loss region. These investigations highlighted significant stress concentration at the fan impeller–shaft connection region as the primary initiator of the stability loss. TEM observations provided micro-scale evidence that the concentrated stress likely promoted extensive dislocation activity, with dislocations shearing through α₂-phase particles. This process facilitated planar slip behavior, intensifying localized plastic deformation within the shaft connection region and ultimately causing instability at ultrahigh rotational speeds. Furthermore, based on simulation results and interrupted experiments, a method for designing capsule wall thickness using threshold control of stress and temperature was proposed and successfully validated using cylindrical capsules.

Regulation of Heat Treatment and Synergistic Mechanism of Strength–Ductility in Precipitation-Strengthened High-entropy Alloy Fabricated by Laser Powder Bed Fusion#br#

2025-08-29 00:00:00.0

High-entropy alloys (HEAs) offer exceptional mechanical properties due to their unique solid solution structures. However, conventional face-centered cubic single-phase HEAs exhibit limited strength. While introducing L1₂ precipitates can simultaneously improve strength and plasticity, conventional casting methods cannot effectively produce large, complex components. Laser powder bed fusion (L-PBF) technology has emerged as a solution for fabricating complex geometrically intricate HEA parts. However, the extremely high cooling rates associated with this technology present a dual challenge: suppressing the precipitation of strengthening phases and generating significant residual stresses, which negatively affect the material’s high-temperature performance. Current research in additive manufacturing of HEAs primarily focuses on optimizing performance at room temperature and intermediate temperatures (≤ 800 °C). Studies exploring non-equilibrium microstructural evolution and mechanical behavior under ultra-high temperatures (≥ 900 °C) are lacking, thus failing to address the urgent need for extreme heat-resistant materials in fields such as aerospace and nuclear energy. In this study, a precipitation-strengthened NiCoCr-based HEA was fabricated using L-PBF technology, and a systematic investigation was conducted into how different heat treatment processes (including aging treatment and solution treatment immediately followed by aging treatment) affect its microstructure and mechanical properties. The results show that optimized L-PBF parameters (laser power of 200 W and scanning speed of 1000 mm/s) enable the production of defect-free samples with a density of 99.64%, characterized by columnar crystals and dendritic segregation in the as-fabricated state. After undergoing solution and aging treatment, the sample exhibited a 21.4% increase in room temperature yield strength ((732 ± 7) MPa) and a 24.3% enhancement in tensile strength ((1075 ± 13) MPa) compared to the as-fabricated sample. This was attributed to recrystallization and modulation of the precipitation phases (L1₂ and σ phases). Notably, the elongation after fracture remained at (18 ± 4)%. By contrast, aging the samples alone resulted in significant loss of plasticity (only (3 ± 1)% elongation) due to extensive grain boundary precipitation of the σ phase and high residual stresses. During high-temperature (900 °C) tensile testing, the solution and aging-treated sample demonstrated the highest tensile strength of (273 ± 15) MPa and an elongation of (13 ± 2)%. Microstructural analysis indicated that the solution and aging treatments effectively balanced the strength–ductility synergy by promoting recrystallization, modulating the properties of the precipitates (such as distribution and quantity etc.), and introducing annealing twins.

Effect of Mo Content on the Microstructure and Mechanical Properties of 1000 MPa Grade High-Strength Steel Weld Metal

2025-08-25 00:00:00.0

High-strength steels are essential materials in various sectors, such as engineering machinery, marine engineering, and hydropower. Welding is a crucial thermal processing technique for fabricating structural components made of high-strength steels. The weld metal, as a vital component of the welded joint, plays a pivotal role in determining the applicability and service life of weldments through its microstructural characteristics and properties. At present, steel manufacturers globally have developed 1000 MPa grade high-strength steels. However, the welding consumables associated with these steels exhibit inadequate strength–toughness matching, which significantly hinders their widespread adoption. In this study, weld metals of 1000 MPa grade high-strength steels with three different Mo contents were produced via the gas metal arc welding process. A comprehensive investigation of the microstructure and mechanical properties of weld metals of 1000 MPa grade high-strength steels were conducted using SEM, EBSD, TEM, tensile testing, and Charpy impact testing. The influence mechanism of Mo content on the microstructural evolution was elucidated. The microstructural characterization revealed that the weld metals predominantly comprised lath bainite (LB) and coalesced bainite (CB). As the Mo content was increased, the proportion of high-angle grain boundaries initially decreased and then increased. The morphology of LB transitioned from an interwoven structure to a more parallel arrangement, which was accompanied by an increase in the CB content. Mechanical testing revealed that a higher Mo content enhanced the metal hardenability, resulting in increased yield strength, tensile strength, and hardness. In contrast, the impact toughness initially decreased and then slightly increased. Analysis of the crack propagation paths on the cross-sections beneath the impact fracture surfaces demonstrated that the cracks readily propagated through the CB regions. The presence of CB considerably impaired the impact toughness of the weld metals. The optimal balance between strength and toughness in the weld metals was achieved at 0.71% Mo, resulting in a yield strength of (939 ± 10) MPa, a tensile strength of (1181 ± 2) MPa, and a room-temperature impact energy of (60 ± 3) J.

Effect of High-Density Ultrafine Twins on the Mechanical Anisotropy of As-Rolled AZ80 Magnesium Alloy

2025-08-22 00:00:00.0

Magnesium alloys, as low-cost commercial metallic materials with light weight, low density, good castability, and dimensional stability, are widely used in lightweight industries such as transportation and aerospace. However, the traditional hot deformation process limits the performance enhancement of magnesium alloys, significantly restricting their industrial applications. Through a novel multidirectional forging (MDF) process, tensile twins can be introduced to the maximum extent, effectively refining the grain structure and markedly improving mechanical strength. However, the twin structure may alter the texture, further affecting mechanical anisotropy. In this work, microstructural characterization, tensile testing, and failure analysis were conducted to investigate the effects of high-density ultrafine twins, formed via the MDF technique, on the mechanical anisotropy of as-rolled AZ80 Mg alloy. Microstructural observation shows that the average grain sizes of the rolling direction–transverse direction (RD–TD), normal direction (ND)–TD, and ND–RD surfaces for the as-rolled sample are 15.7, 16.2, and 18.5 μm, respectively. After six passes of MDF under a single-pass strain of 6% at room temperature (25 ℃), high-density ultrafine twins were formed within the grains, and the average grain sizes of the RD–TD, ND–TD, and ND–RD surfaces of the MDF-processed sample were reduced to 3.1, 2.8, and 3.5 μm, respectively. The formation of high-density ultrafine twins not only weakens the basal texture intensity but also causes the c-axis of grains to deflect by a larger angle with respect to RD than to TD. Tensile testing demonstrates that the introduction of such twins significantly improves mechanical properties but induces pronounced mechanical anisotropy. For the as-rolled samples, the yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) along RD are 132, 303 MPa, and 15.4%, respectively, whereas along TD they are 138, 302 MPa, and 14.5%. After MDF, the YS, UTS, and EL along RD are 259, 357 MPa, and 8.8%, while along TD they are 361, 459 MPa, and 6.4%, respectively. Failure analysis indicates that, in the as-rolled samples tested along RD and TD, microcracks preferentially initiate at twin boundaries, whereas in MDF-processed samples, microcracks mainly occur at grain boundaries.

High-Strength and High-Conductivity Cu-10Fe In Situ Composites Fabricated via Cold Spraying and Subsequent Thermo-Mechanical Treatment

2025-08-19 00:00:00.0

Cu–Fe alloys are promising for electrical engineering applications due to their ability to combine high strength with excellent electrical conductivity. Conventional casting, however, suffers from severe limitations: metastable liquid-phase separation during solidification causes pronounced compositional segregation, while dissolved Fe atoms scatter conduction electrons, significantly reducing the Cu matrix conductivity. To address these issues, this study employs a novel fabrication route integrating cold spraying (CS) with post-deposition heat treatment (HT) and cold rolling (CR) to produce high-performance Cu–10Fe in situ composites. Spherical gas-atomized Cu and Fe powders were mechanically mixed and deposited onto an Al substrate using nitrogen propellant gas. The CS deposits underwent vacuum annealing at 600 °C for 24 h, followed by severe CR with 99% thickness reduction. Microstructural evolution, strengthening mechanisms, and electrical conductivity were systematically characterized using SEM, EBSD and TEM/EDS. The CS deposit exhibited a gradient nano-grained structure, comprising ultra-fine Cu nanograins (80–800 nm) surrounding deformed micron-sized Fe grains (3.5–10 μm) and coarser micron-sized Cu grains (1.5–10 μm), with low porosity (0.13%). Subsequent HT induced static recrystallization in the Cu matrix, producing equiaxed grains (~3.9 μm) and promoting recovery/recrystallization in the Fe particles. Interestingly, HT increased porosity to 0.86% and weakened some Cu/Fe interfaces, likely due to thermal expansion mismatch. Severe CR (HTCR state) markedly transformed the microstructure: equiaxed grains elongated along the rolling direction, forming Cu grains with strong <111> and <211> fiber textures and Fe fibers with a dominant <110> texture. This HTCR process enhanced interfacial bonding and reduced porosity to 0.09%, yielding a dense composite. Importantly, no mutual solid solubility between the Cu matrix and Fe fibers was observed, except for a narrow (~20 nm) atomic diffusion layer at the interfaces, as confirmed by TEM/EDS. The absence of Fe solute in Cu is critical for maintaining high electrical conductivity. The HTCR composite exhibited an exceptional combination of properties: an ultimate tensile strength of 550 MPa, far exceeding the CS (190 MPa) and HT (233 MPa) states, and a high electrical conductivity of 72%IACS. Its remarkable strength arises primarily from fine-grain strengthening, dislocation strengthening, and load transfer strengthening via Fe fibers. Notably, the experimentally measured strength (495 MPa) exceeds the calculated contribution from these mechanisms alone (367 MPa), highlighting the significant role of hetero-deformation-induced strengthening and potentially underestimated fine-grain strengthening from sub-EBSD-resolution grains. Together, these mechanisms account for the composite’s ultrahigh strength. The high electrical conductivity results from the absence of Fe solute scattering in the Cu matrix, improved interfacial bonding and pore elimination during HTCR, and the alignment of elongated Cu grains and Fe fibers along the current path, minimizing electron-scattering interfaces. Compared to conventionally fabricated Cu–Fe alloys (e.g., casting, powder metallurgy), the CS + HTCR Cu–10Fe composite exhibits a superior strength–conductivity balance. This study demonstrates that CS combined with tailored thermo-mechanical processing (HT + CR) provides an effective, industrially viable route to high-strength, high-conductivity Cu–Fe in situ composites with uniform composition and dense microstructure.

Site Occupation of Alloying Elements in α₂ Phase in High-Temperature Titanium Alloys: A First-Principles Study

2025-08-19 00:00:00.0

The site occupation of alloying elements on various sublattices of intermetallic compounds directly affects their properties. The precipitation of the α₂ phase (Ti₃Al intermetallic compound) serves as a key strengthening mechanism in high-temperature titanium alloys. However, due to the complex composition of these alloys and the nanoscale size of the α₂-Ti₃Al phase, the occupation behavior of alloying elements in α₂-Ti₃Al remains unclear. Theoretically, this behavior can be predicted by calculating the formation energies of alloying atoms on different sublattices, which requires knowledge of the chemical potentials of the matrix atoms (Ti and Al in Ti₃Al) as reference energies. Conventionally, these chemical potentials are approximated by the energies of the elements in their standard states. However, this approach is inadequate for determining site occupations in α₂-Ti₃Al precipitated in high-temperature alloys, as it neglects the phase equilibrium between the precipitate and matrix, which governs the chemical potentials. In addition, the influence of temperature on site occupation in α₂-Ti₃Al has not yet been theoretically addressed. In this study, the chemical potentials of Ti and Al are evaluated based on the phase equilibrium between the α-Ti matrix and the α₂-Ti₃Al precipitate. The formation energies of alloying elements on Ti and Al sublattices are then computed using first-principles methods, and their site preference is assessed by comparing these formation energies. Furthermore, the temperature-dependent partitioning of alloying elements between the two sublattices is determined based on site preference energy. The results show that, at service temperatures of high-temperature titanium alloys, elements such as Sc, V, Cr, Mn, Fe, Y, Zr, Nb, Mo, Tc, Ru, Hf, Ta, W, Re, and Os preferentially occupy the Ti sublattice; Si, Ni, Cu, Zn, Ga, Ge, Ag, Cd, In, Sn, Pt, Au, Hg, Tl, and Pb preferentially occupy the Al sublattice; while Co, Rh, Pd, and Ir distribute over both sublattices.

Effect of Aging Temperature on Microstructure and Tensile Properties of Cu-containing Maraging Stainless Steel

2025-08-13 00:00:00.0

Maraging stainless steel is a high-strength material with excellent corrosion resistance and weldability, making it widely used in the aviation, aerospace, marine, and energy sectors. This study systematically investigates the microstructural evolution and tensile behavior of a Cu-containing maraging stainless steel across different aging temperatures to overcome the strength–ductility trade-off and elucidate the role of Cu in microstructural tuning and tensile property optimization. The thermodynamic mechanism of reversed austenite formation was analyzed based on the double-spherical-cap model. In addition, the contributions of various strengthening mechanisms to yield strength evolution were quantitatively evaluated. The results demonstrate that as aging temperature increases from 480 °C to 560 °C, the number densities of fcc Cu-rich precipitates and Laves-phase Fe₂Mo type Mo-rich phases exhibit a nonmonotonic trend, first increasing and peaking at 500 °C, then decreasing, while their sizes increase continuously with temperature. Higher aging temperatures accelerate reversed austenite formation and slightly increases the effective grain size and reduce the density of geometrically necessary dislocations. Furthermore, tensile properties exhibit a nonmonotonic response, yield strength first increases and then decreases with increasing aging temperature, whereas elongation improves continuously across the full temperature range.

EFFECT OF MICRO-ALLOYING ELEMENT ON INTERFACIAL ENERGY BETWEEN TWO LIQUID PHASES IN A MONOTECTIC ALLOY

2025-07-29 00:00:00.0

Influence of Heat Treatment Cold Rate on the Organization and Properties of Powder Metallurgy GH4099 Alloy

2025-07-28 00:00:00.0

GH4099 alloy, a typical precipitation-strengthened nickel-based superalloy, exhibits exceptional structural stability at temperatures up to 900 oC without forming topologically close-packed (TCP) phases. This superior high-temperature performance positions it as an ideal material for critical hot-end components in aero-engines, where its service reliability relies on synergistic strengthening mechanisms. Hot isostatic pressing (HIP), a powder metallurgy (PM) technique, enables near-net-shape fabrication of components with tailored microstructures and cost efficiency. However, inhomogeneous cooling rates in large-scale HIP-processed parts may compromise microstructure uniformity, necessitating systematic exploration of cooling rate effects on PM GH4099 alloy. In this study, GH4099 pre-alloyed powder was prepared via plasma rotating electrode process (PREP), and fully dense billets were consolidated by HIP at 1230 oC /150 MPa/4 h. The dual-stage heat treatment (solution: 1175 oC /1 h; aging: 850 oC /5 h) with varied cooling rates (argon quenching >50 oC /min, furnace cooling≈8 oC/min, controlled cooling 5 oC /min) was applied to investigate their regulatory roles in multi-scale microstructure evolution and high-temperature mechanical properties. Orthogonal experiments demonstrated that controlled cooling at 5 oC /min during both solution and aging stages (S3-A3) yielded optimal tensile strength (404.3 MPa) and elongation (34.7%) at 900 oC, with a 79.3% enhancement in high-temperature plasticity compared to gas-quenched solution combined with slow-cooled aging (S1-A3). Microstructural analysis revealed that slow cooling modulated γ' precipitation kinetics, suppressed continuous brittle phase (e.g., TiC) distribution at prior particle boundaries (PPBs), and promoted annealing twin formation (59.3% area fraction). These microstructural advantages synergistically enhanced grain boundary compatibility and inhibited crack propagation. This work elucidates the coupling mechanisms among cooling rate, γ' phase behavior, grain boundary characteristics, and PPBs evolution. The proposed HIP-graded cooling strategy offers a theoretical and technical foundation for microstructure-property integrated design of aerospace components under extreme thermal-mechanical conditions.

Effect of miniature specimen thickness on fatigue life and determination of its critical dimension in additively manufactured GH3536 alloy

2025-07-25 00:00:00.0

As additive manufacturing (AM) technology advances toward industrial applications, constrained by the unique microstructure and defect characteristics, and structural/dimensional limitations, it has become particularly urgent to develop effective and scientific certification and evaluation methods based on miniature specimen for mechanical properties of AM component. In this study, the effect of specimen thickness to grain size ratio (t/d) on the mechanical properties of the GH3536 alloy fabricated by AM technology were investigated by using miniature specimen with varying thicknesses conducted on tensile and cantilever beam bending fatigue tests. The results show that as t/d increases, the tensile strength, uniform elongation, and fatigue life of the alloy first increase significantly and then stabilize. Combined experimental results and finite element analysis, the critical dimension of miniature specimen using for cantilever beam bending fatigue tests was established, ensuring that t/d ≥ 5.0. Reasonable selection of miniature specimen thickness based on this critical dimension contributes to improve stability and accuracy in mechanical property evaluation of AM components, thereby providing a more reliable and practical approach for fatigue performance assessment.

Effect of Mg content on morphology evolution and mechanical properties of Fe-rich phase in regenerated Al-7Si-0.4Fe alloy

2025-07-17 00:00:00.0

The influence of Mg content on the microstructure and properties of recycled Al-7Si alloys using scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermodynamic software Jmat Pro, and synchrotron radiation X-ray computed tomography (SRXCT). The results show that as Mg content increases, the secondary dendrite arm spacing of the primary α-Al phase decreases. The morphology of the Fe-rich phase evolves from fine needle-like to coarse needle-like, Chinese-script, and granular forms, accompanied by a transition from a single β-Fe phase to a mixed structure of β-Fe and π-Fe phases. The three-dimensional (3D) morphology of the Fe-rich phase transforms from complex coral-like to independent flake-like and multi-branched structures. Simultaneously, the number, volume fraction, and sphericity of the π-Fe phase significantly increase. In contrast, the number and volume fraction of the β-Fe phase decrease by 95.3% and 73.6%, respectively, despite a 73% increase in its equivalent diameter. The increase of Mg content lowers the eutectic silicon formation temperature, promoting the coarsening of the β-Fe phase. When the Mg content is 0.4% and above, the β-Fe phase undergoes a peritectic reaction with the melt, resulting in the formation of a large number of small-sized, highly spherical π-Fe phases, which significantly reduces the volume fraction of the β-Fe phase. The 0.7Mg alloy demonstrates substantial improvements in tensile and yield strengths, increasing by 36.8% and 137%, respectively, compared to the Mg-free alloy. However, its elongation decreases by 51.1%. The formation of large-sized β-Fe phases intensifies crack propagation, leading to a marked reduction in the plasticity of high-Mg Al-7Si alloys. This study revealed the influence mechanisms of Mg content on the type and morphology of Fe-rich phases in Al-Si alloys with high Fe content.

Characterization of Anisotropy in Single Crystal Superalloys Based on Ultrasonic Surface Longitudinal Wave#br#

2025-07-14 00:00:00.0

The performance characterization of single-crystal materials faces numerous technical limitations owing to their intrinsic anisotropy. Ultrasonic testing, a mature and cost-effective technique, provides rich information about both macroscopic and microscopic structural properties of materials. While it has been widely used to characterize the microstructures and mechanical properties of isotropic materials, its applications to anisotropic materials remain limited owing to the complexity of their acoustic behavior. Recent advancements driven by the competitive demands of the aerospace industry have made this field a research hotspot. To leverage ultrasonic technology for characterizing anisotropy in single-crystal superalloys, this study proposes a surface longitudinal wave omnidirectional scanning (SLW-ODS) method that enables the visual representation of ultrasonic wave propagation fields in anisotropic media. The core approach involves exciting SLWs in the workpiece, combined with surface acoustic field imaging and morphological image processing techniques, to map and analyze the macro- and micro-structural and mechanical anisotropy of single-crystal superalloys. First, the technical conditions for SLW excitation and reception in single crystal materials were clarified through theoretical analysis and simulation. Then, a water-immersion ultrasonic ring-scanning test platform was developed to examine nickel-based single crystal superalloy plates with <001> and <011> crystal growth orientations as well as isotropic stainless steel and electrolytic nickel plates. The SLW propagation sound fields in the four specimens were obtained by detecting waves leaking into water during propagation. By extracting annular sound field signals in polar coordinates, an omnidirectional amplitude of flight-time distribution map (OATM) was constructed. Image processing techniques were then employed to obtain accurate relative velocities in all directions, which closely matched theoretical velocity curves derived from the Christoffel equation. The results demonstrate that OATM, with distinct morphological patterns obtained via the SLW-ODS method, effectively reflects the anisotropic acoustic characteristics of bulk waves and multimode ultrasonic waves in single crystal superalloys. These acoustic features can be further utilized to characterize anisotropy in the macro- and micro-structures and mechanical properties of single crystal superalloys.

Geometric Dynamic Shear Refining Grain Behavior in Cryogenic Helical Channel Angular Pressing Process #br#

2025-07-11 00:00:00.0

To investigate the mechanism of grain refinement during severe plastic deformation at cryogenic temperatures, the helical channel angular pressing (HCAP) process was employed, and the grain refinement behaviour under twin- and dislocation-mediated plastic deformation modes was examined. Cryogenic HCAP (cryo-HCAP) experiments were carried out at −196 and −76 °C on C18150 copper alloy and 6005 aluminium alloy, respectively. Microstructural evolution in both alloys was characterised using EBSD and TEM, and the results were compared with those obtained under room temperature processing. A geometric dynamic shear grain refinement mechanism is proposed to elucidate the observed behaviour in the cryo-HCAP process, based on the formation of serrated subgrain boundaries. At cryogenic temperatures, when the equivalent strain reaches 3–5, twin bundles or deformed grains are thinned to approximately 1–3 times the subgrain diameter, and torsional deformation occurs. In the twinning-dominated plastic deformation mode, twin bundles oriented perpendicular to serrated twin boundaries are sheared off at twin gap intersections, leading to grain refinement and the formation of deflected, chain-like twin bundles within the matrix. In the dislocation-dominated deformation mode, equiaxed refined grains emerge when the deformed grains are pinched off perpendicular to the serrated subgrain boundaries. Under the combined effects of cryogenic temperature, severe plastic deformation and torsional strain, both deformation modes share the following characteristics: when serrated (sub)grain boundary structures and critical thinning dimensions are achieved, equiaxed refined grains are produced. Furthermore, the fraction of high-angle grain boundaries increases as temperature decreases.

The Study of the low-temperature deformation characteristics and fracture behavior in dual-phase medium Mn steel

2025-07-10 00:00:00.0

The lightweight and high-strength medium manganese steel (MMnS with 3%-12% Mn, mass fraction) is a typical representative of the third-generation steels for automotive applications, which can achieve excellent strength and plasticity by fully utilizing the coupling effect of metastable austenite, induces twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effect during plastic deformation. However, the low-temperature deformation characteristics and fracture behavior of dual-phase MMnS remain unclear at present. The mechanical properties, deformation mechanism and the ductile-to-brittle transition characteristics of dual-phase MMnS are investigated under low-temperature conditions The yield strength (TS) and ultimate tensile strength (UTS) of the MMnS increase, whereas the total elongation (TE) decrease as deformation temperature decrease form 20℃ to -60℃. The comprehensive mechanical properties of the samples reach peak value at 0°C, with an excellent combination of strength and ductility (72.97 GPa%). The TRIP effect of metastable austenite(γ) plays a crucial role in the low-temperature mechanical properties of MMnS. This paper established a quantitative kinetic model of strain-induced martensite transformation (SIMT) under low-temperature deformation progress, which indicated SIMT rate is faster at lower deformation temperatures. The α′-martensite nucleation mechanism is characterized by the combined action of γ→twins and γ→ε→α′ mechanisms at 20°C. This indicates that both the TWIP and TRIP effects are acting together at 20°C. As the deformation temperature decreases to 0℃, the TWIP effect was inhibited, and the γ→ε→α′ transformation dominated the SIMT. The sample demonstrates a direct nucleation mechanism from γ→α′ as the deformation temperature decreases to -40℃, leading to a rapid increase in the content of α′-martensite. In addition, the fracture mode of MMnS exhibits a ductile-to-brittle transition as the deformation temperature decreases. As characterized by SEM and EBSD analysis of the microstructure surrounding the crack, the crack initiation in MMnS was primarily due to damage at the ferrite/martensite (α/α′) interface at 20°C. Crack propagation along the α/α′ or α′/α′ interface. The α′-martensite impedes crack propagation, resulting in ductile fracture of the sample. The crack initiation location is the same at α/α′ interface in low-temperature environments. However, the crack rapidly propagates through the α′-martensite due to the increased brittleness of α′-martensite at low temperatures. This results in brittle fracture of the sample in low-temperature deformation.

Effect of austenite stability on the cyclic plasticity behavior of TRIP-assisted duplex stainless steels and its micromechanisms

2025-07-02 00:00:00.0

TRIP-assisted duplex stainless steels (DSSs) have gradually gained widespread application in the new energy vehicle industry due to its significantly reduced density and the excellent ductility and work hardening capability induced by the TRIP effect. However, the differences in the stability of austenite in the steel can affect the TRIP effect, thereby altering the material's cyclic plasticity behavior. This study investigates two DSSs with different austenite stabilities. By comparing and analyzing the mechanical responses and microstructural evolution of these materials under cyclic loading, the influence of austenite stability on the cyclic plasticity behavior of TRIP-assisted DSSs is explored, along with the underlying microstructural mechanisms. The results indicate that with a decrease in austenite stability, the degree of α' martensitic transformation increases. In addition to nucleation at the intersections of the ε-martensite bands, nucleation is also observed directly on the ε-martensite bands. The reduction in austenite stability weakens the cyclic softening effect of the material, enhances its secondary hardening capacity, and increases the transformation rate and volume fraction of α' martensite. This leads to a higher degree of performance mismatch and strain discontinuity between the phases, making it easier for microcracks to initiate at phase interfaces and at the α' martensite transformation sites, thus reducing the material's fatigue life.

Synergistically Enhancing the Mechanical Properties and Hydrogen Embrittlement Resistance of Low Alloy Steels via Vibration-Assisted Thermoforming Process

2025-06-30 00:00:00.0

Hydrogen embrittlement (HE) remains a critical issue in industrial applications and academic research, particularly for low-alloy steels. To address this issue, this study developed a novel mechanical vibration-assisted thermoforming process, accompanied by self-developed equipment, to improve the mechanical properties and HE resistance of forged steels. The physical behaviours induced by the vibration field were leveraged to effectively generate hydrogen traps, thereby considerably improving mechanical performance and reducing HE sensitivity. Results revealed that the mechanical vibration-assisted process synergistically enhanced the strength and ductility of the forged samples while mitigating their HE susceptibility. In particular, the strength of the vibration-treated samples increased by 53.9% to 1330 MPa, while their ductility improved by 48.9%. Notably, the strength–elongation product increased by 131.0%, indicating a balanced enhancement in both properties. After H-precharging, HE sensitivity—characterised by the loss in elongation—decreased from 72.68 for the non-vibration samples to 63.14 for the vibration-treated samples, indicating a 15.1% reduction. Similarly, HE sensitivity based on area reduction declined from 71.15 to 60.57, demonstrating a 14.87% decrease. These findings highlight the effectiveness of the mechanical vibration-assisted process in improving both mechanical properties and HE resistance. To elucidate the underlying mechanisms, advanced characterisation techniques, including SEM, EBSD, FIB, and TEM, were employed. Based on the characterisation results, the improved strength of the vibration-treated samples was attributed to refined grain structures, uniformly distributed dislocations, finely dispersed precipitates and numerous slip bands within the grains. The primary strengthening mechanisms included grain boundary, dislocation and precipitation strengthening, with grain boundary and dislocation effects contributing prominently. Moreover, the stable structures formed by slip bands and grain boundaries further reinforced the overall strength. Fractographic analysis revealed conspicuous differences between the vibration-treated and non-vibration samples. The non-vibration samples exhibited brittle fracture features, including cleavage planes and intergranular cracks, while the vibration-treated samples displayed a mixed morphology of dimples and quasi-cleavage, indicative of enhanced toughness. TEM observations of the fracture zones in the vibration-treated samples revealed high-density dislocations and severe plastic deformation near the crack tips, suggesting the concurrent operation of hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced local plasticity (HELP) mechanisms during HE. In contrast, the non-vibration samples primarily exhibited HEDE, with hydrogen accumulation at precipitates and grain boundaries initiating and propagating secondary cracks, ultimately leading to brittle fracture. The reduced HE sensitivity of the vibration-treated samples was primarily attributed to the mitigation of conditions conducive to the HEDE and HELP mechanisms. Uniformly distributed dislocations, grains and precipitates promoted the dispersion of hydrogen and limited its accumulation at grain boundaries. Furthermore, interactions between hydrogen and slip bands broadened dislocation migration zones, enhancing resistance to hydrogen-induced cracking. In conclusion, this study presents a new approach for developing HE-resistant materials through mechanical vibration-assisted thermoforming. This process not only enhances the mechanical properties of forged steels but also considerably reduces their HE susceptibility, providing valuable guidance for industrial applications and future research in materials science and engineering.

First-Principles Simulation of Hydrogen Capture Behavior at the bcc-Fe/fcc-Fe Phase Interface in Steel

2025-06-30 00:00:00.0

Hydrogen-induced delayed fracture is an unpredictable and extremely hazardous failure mode that often occurs suddenly under applied stress levels significantly below a material’s yield strength. This issue must be addressed to ensure the promotion and application of advanced high-strength steels (AHSSs). The development of third-generation AHSSs has transformed automotive lightweighting strategies by achieving an exceptional balance between strength and ductility through precise microstructural engineering. A key microstructural feature enabling this balance is the coexistence of bcc and fcc phases within the steel’s microstructure. The interface between bcc and fcc phases, an important microstructural defect in AHSSs, acts as a trapping site for hydrogen atoms. However, hydrogen-trapping behaviour at the bcc-Fe/fcc-Fe interface, along with the influence of alloying element segregation, remains poorly understood. In this study, the hydrogen-trapping behaviour at Kurdjumov–Sachs oriented bcc-Fe/fcc-Fe interfaces was systematically investigated using first-principles density functional theory calculations. Parameters such as the binding energy, interfacial separation work and hydrogen Bader volume were analysed to quantify the hydrogen-trapping behaviour. In addition, the effects of alloying elements on the bcc-Fe/fcc-Fe interfacial stability and hydrogen-trapping behaviours were evaluated. The results show that hydrogen atoms preferentially segregate to the fcc-Fe side of the bcc-Fe/fcc-Fe interface with a binding energy of 34.5 kJ/mol, indicating a strong tendency for interfacial trapping. Notably, the hydrogen Bader volume was found to be a reliable indicator of hydrogen-trapping behaviour at the bcc-Fe/fcc-Fe interface. Further analysis of alloying elements revealed varying effects on the bcc-Fe/fcc-Fe interface. Nb and Mn had a minimal effect on interfacial stability. V slightly increased the interfacial separation work, improving interfacial stability. Tc, Mo, and Cr significantly increased interfacial stability by increasing the interfacial separation work. Conversely, Ni decreased the interfacial separation work, thereby weakening the interfacial stability. This study provides an atomic-scale understanding of the hydrogen-trapping behaviour at the bcc/fcc interface and offers theoretical guidance for designing the chemical composition of AHSSs with enhanced resistance to hydrogen embrittlement.

Microstructure, Mechanical Property, and Crack Formation Mechanism in Electron Beam Welded Joints of Powder Metallurgy Ti2AlNb Alloy

2025-06-27 00:00:00.0

This paper primarily studies the microstructure, mechanical properties, and crack formation mechanism of electron beam welded joints of powder metallurgy Ti2AlNb alloy. In the electron beam welded joints of powder metallurgy Ti2AlNb alloy, the weld zone is mainly composed of β/B2 phase and a small amount of α2 phase. The heat-affected zone (HAZ) can be divided into two parts: the heat-affected zone near the weld is mainly composed of β/B2 phase and a small amount of α2 phase, while the heat-affected zone near the base material is mainly composed of β/B2 phase, α2 phase, and O phase. As the welding speed increases, the α2 phase at the grain boundaries in the heat-affected zone and base material gradually becomes coarser, and the needle-like O phase gradually transforms into coarse plate-like O phase. The α2 phase has poor resistance to plastic deformation during tensile processes, and the coarse O phase has poor pinning ability for cracks, resulting in a decrease in the tensile strength of the joint. Mechanical performance test results indicate that when the welding speed is 8 mm/min, the maximum tensile strength of the joint can reach 499 MPa. The fracture of the joint occurs in the heat-affected zone and exhibits cleavage fracture. The main reason for the formation of weld cracks in the joint is the presence of brittle phases and the high residual stresses generated during the welding process.

Interface Characteristics and Thermal Deformation Behavior of 4D Printed 316L/Invar36 Heterogeneous Metal Structures

2025-06-23 00:00:00.0

Heterogeneous metal 4D printing presents a promising technological route for fabricating structures with controllable thermally induced deformation. This approach addresses limitations inherent in shape memory alloy components, particularly their restricted number of stable deformation cycles, thereby offering broader application potential. Nonetheless, heterogeneous metal 4D printing remains at a conceptual stage, with limited research on process selection, interfacial bonding characteristics and thermally induced deformation behaviour. This study investigates the 4D printing of heterogeneous metals via laser solid forming of 316L/Invar36 alloys, focusing on the influence of process parameters and deposition configurations on interface flatness, compositional transition across the interface and thermally induced deformation. The results demonstrate that, with all other parameters held constant, decreasing the laser power within the 1000–1800 W range leads to a flatter interface and narrows the compositional transition zone between 316L and Invar36 alloys. At a laser power of 1000 W and scanning speed of 15 mm/s, strip-shaped specimens fabricated through vertical stacking of the two materials exhibit pronounced deformation under thermal actuation. In contrast, side-by-side deposition results in a jagged interface and a broader compositional transition zone, significantly diminishing the thermal actuation deformation of the resulting strip specimens. When 316L and Invar36 are alternately arranged in both vertical and horizontal orientations, the strips exhibit S-shaped deformation under thermal stimulation. Assembled from multiple such strips, a heterogeneous metal spring structure capable of substantial deformation is obtained, displaying a deformation magnitude 2.58 times greater at 270 °C than at 25 °C.

lEffect of Ga Microalloying on Structure and Soft Magnetic Properties of a Fe_81.5Si₄B₁₃Cu_1.5 Nanocrystalline Alloy

2025-06-20 00:00:00.0

The rapid advancement of modern industry and information technology has led to higher demands for efficiency, power density and reduced energy consumption in power electronic devices, with soft magnetic materials playing a critical role in their performance. Traditional Fe–Si–B–Nb–Cu nanocrystalline alloys (Finemet) have been widely employed due to their excellent overall soft magnetic properties, including low coercivity (H_c), high permeability and low core loss. However, their relatively low saturation magnetic flux density (B_s) remains a limitation. Developing new nanocrystalline alloys with enhanced B_s and industrial applicability is thus of great importance. In this study, with the aim of producing high-B_s nanocrystalline alloys possessing good manufacturability, minor Ga was added to a Fe_81.5Si₄B₁₃Cu_1.5 alloy. The effects of Ga addition on the as-quenched and annealed structures, as well as the soft magnetic properties, were investigated. The results show that the incorporation of Ga changes the as-spun structure of the Fe_81.5−xSi₄B₁₃Cu_1.5Ga_x(x = 0–2.0) alloys from a fully amorphous state to one containing a high number density (N_d) of pre-existing α-Fe grains, with average grain sizes (d̅_α_-Fe) below 7 nm dispersed within the amorphous matrix. Both N_d and d̅_α_-Fe increase progressively with increasing Ga content. Ab initio molecular dynamics simulations reveal that Ga addition lowers the atomic packing density and weakens interatomic chemical bonding within the alloy, thereby reducing long-range atomic diffusion resistance during rapid solidification and consequently decreasing the glass-forming ability (GFA). The reduced GFA, along with the formation of (Cu, Ga) clusters due to Ga-promoted Cu aggregation, facilitates the precipitation of pre-existing α-Fe grains. Upon annealing at a low heating rate, the Ga-containing alloys develop a fine and uniform dual-phase structure comprising amorphous matrix and α-Fe nanocrystals, resulting in excellent soft magnetic performance. Specifically, the alloy with x = 0.5 exhibits an average α-Fe grain size of 16.3 nm, B_s of 1.77 T, H_c of 9.0 A/m, and an effective permeability of 8,700 at 100 kHz, significantly outperforming the x = 0 alloy, which has corresponding values of 47.6 nm, 1.78 T, 213.6 A/m and 500, respectively. The competitive growth between pre-existing and newly formed α-Fe grains during annealing effectively suppresses grain coarsening, leading to a refined nanocrystalline structure in the Ga-containing alloys. This refined structure reduces the average magnetocrystalline anisotropy and promotes the formation of regular magnetic domain structures, thereby significantly improving magnetic softness.

High-temperature Oxidation Behavior and Surface Microstructural Evolution of Ti–Cu Alloy

2025-06-20 00:00:00.0

Understanding the evolution of oxidation in Ti alloys under high-temperature conditions is crucial for enhancing the long-term service stability of engineered components. In this study, the high-temperature oxidation behaviour and kinetics (1000‒1200 °C) of Ti‒14Cu alloys were investigated, and the evolution of microstructural stress on the alloy surface and the oxidation mechanism were clarified. The results demonstrated that the oxide layers on the alloy surface gradually evolved into a multi-layer structure with increasing temperature. After oxidation treatment for 5 hours, the oxidation rate increased from 3.50 × 10^-3 mg²/(cm⁴·h) at 1000 °C to 4.17 × 10^-2 mg²/(cm⁴·h) at 1200 °C. The significant residual stress generated by the difference in the thermal expansion coefficient of TiO₂ and that of the α-Ti matrix induced rapid crack initiation and propagation in the oxide layer. These cracks act as rapid diffusion pathways for oxygen, thereby accelerating the oxidation rate of the Ti–14Cu alloy. Consequently, the dominant factor governing oxidation shifted from elemental diffusion to interfacial reactions. Molecular dynamics simulations revealed that tensile stress was concentrated within the oxide layer, whereas a combination of tensile and compressive stresses developed at the matrix/oxide layer interface. A large number of semi-coherent interfaces were formed between TiO₂ and the α-Ti matrix during high-temperature oxidation, leading to enhanced structural disorder and elevated atomic potential at the matrix/oxide layer interface. Furthermore, a thin and dense Cu-rich layer formed on the inner side of the oxide layer after oxidative treatment at 1200 °C, effectively suppressing oxygen diffusion and inhibiting crack propagation.

FORMATION MECHANISM OF EQUIAXED β PHASE INTERMATELLIC COMPOUNDS IN Al/Mg SOLID-STATE WELD UNDER THERMO-MEHANICAL ACTION

2025-06-19 00:00:00.0

The experimental characterization and multiphase-field simulation methods were combined to explain the reasons why the different morphologies of two intermetallic compounds (IMCs) were generated in dissimilar Al/Mg alloys solid-state welding process. The Gleeble thermo-mechanical device was used to obtain solid-state welded joints of dissimilar Al/Mg alloys with varying degrees of deformation. The results shown that, regardless of the degree of deformation, the morphology of the two IMCs (β-Al3Mg2, γ-Al12Mg17) after welding is a single layer of columnar crystals. After annealing the joints at 400°C for 1 hour, it was found that, regardless of the degree of deformation, the morphology of β-Al3Mg2 was a multi-layer equiaxed crystal structure. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) analysis showed that the formation of multi-layer Al3Mg2 equiaxed crystals is not caused by precipitation phases or the Kirkendall effect. A multiphase-field model was established to analyze the growth process of IMCs during the annealing period, and the reasons for the formation of multi-layer Al3Mg2 equiaxed crystals were investigated. By analyzing the driving force for IMCs growth, it was found that a supersaturated Al-based solid solution exists at the Al/Al3Mg2 interface, providing a nucleation driving force for the formation of Al3Mg2 nuclei. In contrast, at the Mg/Al12Mg17 interface, the nucleation driving force for Al12Mg17 is very small. Comparing the diffusion coefficients of the phases, it was observed that the diffusion coefficient of the Al3Mg2 is much larger than that of Al12Mg17. When solute atoms diffuse to the Al3Mg2 layer, it cannot accommodate a large number of solute atoms, causing them to be expelled to the adjacent Al matrix, forming a supersaturated Al-based solid solution that provides a nucleation driving force for Al3Mg2. The finer grains of the deformed Al matrix provide more nucleation sites for Al3Mg2. Therefore, the growth of Al3Mg2 manifests as the nucleation and growth of multi-layer equiaxed crystals, while the growth mechanism of Al12Mg17 grains remains the growth of the original single-layer columnar crystals.

Effect of Homogenization Temperature on the Microstructure and Mechanical Properties of 3003 Aluminum alloy

2025-06-09 00:00:00.0

To eliminate the urgent problem of insufficient mechanical properties in 3003 aluminum alloys, this work systematically investigates their solidification behaviors, as well as the influence of homogenization temperature on both microstructure and mechanical properties by using thermodynamic simulation, SEM, EBSD, and TEM. During the initial solidification stage, the Mn content in the FCC phase is approximately 0.94%. When the solid phase fraction reaches about 50%, the Mn content in the FCC phase increases to ~1.20%, at which point the α-AlMnFeSi phase begins to form. On the contrary, by the end of solidification (with a solid phase fraction of about 95%), the Mn content in the FCC phase decreases to 0.33%. After homogenization heat treatment, multitudinous second-phase particles precipitate within the grains. For the sample homogenized at 555±5 ℃, the intragranular precipitates are smaller and have a higher volume fraction, compared to the sample homogenized at 600±5 ℃. In addition, the average width of PFZ at the grain margin in the former sample is narrower. Furthermore, the contribution of precipitates to the yield strength of the 3003 aluminum alloy homogenized at 555±5 ℃ is about 28.5 MPa; which is approximately 23 MPa higher than that of the sample homogenized at 600±5 ℃. This difference matches well with the actual measured value, indicating that the precipitated phase is the main factor responsible for the difference in yield strength between the 3003 aluminum alloys homogenized at 555±5 ℃ and 600±5 ℃. This study will provide a theoretical basis for the large-scale industrial production of high-strength 3xxx series aluminum alloys.

Effect of pre-aging treatment on the hot deformation behavior and microstructure evolution of Al-Zn-Mg-Cu alloy

2025-06-06 00:00:00.0

The effect of pre-ageing treatment on the hot deformation behavior of Al-Zn-Mg-Cu alloys is studied by thermal simulation experiments. Thermal simulation of the flow stress curves showed: at a low strain rate of 0.01s-1, the flow stress is higher than that of the aging state in the solid solution state during hot deformation at room temperature and 473 K, While the flow stress in the aging state exceeds that in the solid solution state at 573K-743K, and the difference of the flow stresses decreases with the increase of the temperature; At a high strain rate of 0.5s-1, the difference between the flow stresses of the solid solution state and the aged state increases abnormally at 743K. The microstructural evolution of the aged and solid solution Al-Zn-Mg-Cu alloys was studied by means of SEM, EBSD, and TEM for the above phenomena, and the results showed: at low strain rate, the high density of precipitates entangled dislocations at 473 K-573 K, the low deformation storage energy and the high thermal deformation activation energy of the aged Al-Zn-Mg-Cu alloys resulted in the un-recovery of the matrix, and the deformation resistance is improved; At 573 K-743 K, the discontinuous coarsening of the precipitates and the increase of deformation storage energy increase the DRV volume fraction of the aged Al-Zn-Mg-Cu alloy, and the small-sized spherical particles of which pinning dislocations increase the deformation storage energy and the volume fraction of DRX as the temperature increases, so that the Al-Zn-Mg-Cu alloy under aged state has a similar flow stress to the solid-solution flow stress. At high strain rates, the appearance of GDRX in the aged Al-Zn-Mg-Cu alloy at high strain rates further increased the volume fraction of DRX, which reduced the flow stress of Al-Zn-Mg-Cu alloys.

Preparation and Properties of Mo Coating on the Surface of Biodegradable Fe–Mn–C Alloy

2025-06-05 00:00:00.0

为了解决在可降解支架应用中铁合金降解速率过慢的关键性问题，本工作在Fe-Mn-C合金表面通过直流电沉积的方式制备出可降解Mo涂层，以实现铁合金的加速降解，详细研究了沉积参数对Mo涂层微观结构、降解性能和血液相容性的影响。结果表明，随沉积时间和电流密度增大，Mo涂层的厚度和析氢效率增加，导致涂层表面微裂纹、气孔等缺陷尺寸增大。电化学和离子溶出结果表明，Mo涂层显著加速了铁合金的降解，降解速率随沉积时间和电流密度的增大而增大。沉积Mo涂层的铁合金，其降解过程是主要受电位差驱动的电偶腐蚀机制主导，导致基体阳极溶解速率显著提高。涂层的表面缺陷为腐蚀介质提供扩散通道使得涂层缺陷处和基体/涂层界面处优先发生局部腐蚀；涂层的溶解减少了腐蚀产物的沉积，进一步加速了基体腐蚀。此外，Mo涂层独特的微观结构可有效抑制血小板黏附，表现出良好的血液相容性。

Low-Temperature Fatigue Crack Propagation Behavior of Bainitic Rail Steel

2025-05-30 00:00:00.0

Bainitic rail steels have been extensively studied as a solution to the considerable fatigue and wear issues experienced with conventional pearlitic rails. With railway constructions in high-cold regions, the effect of low temperatures on the fatigue performance of railways has garnered considerable attention. Despite several studies on the room-temperature fatigue performance of high-strength steels, the effect of the microstructure, particularly retained austenite (RA), on the low-temperature fatigue crack growth behavior in bainitic rail steels remains unclear. This study examined the low-temperature fatigue crack propagation behavior of U20Mn2SiCrNiMo (U20Mn) bainitic rail steel subjected to hot rolling and air cooling. The microstructure of the bainitic rail steel was characterized employing SEM, TEM, EBSD, and XRD. The conventional mechanical properties and fatigue crack propagation rate (da/dN; where a is the crack length and N is the number of stress cycles) were determined at room temperature (25 ℃) and −40 ℃. The results demonstrated that the U20Mn bainitic rail steel was primarily composed of a granular bainite/martensite multiphase structure, with approximately 10% (volume fraction) RA. A decrease in the experimental temperature increased the tensile and yield strength of the U20Mn rail steel; however, its impact toughness decreased. The da/dN vs. stress intensity factor (ΔK) curves for U20Mn rail steel at 20 ℃ and −40 ℃ indicated that the fatigue crack propagation rate reduced at low temperatures within the ΔK range of 8–20 MPa·m^1/2. However, an examination of the fatigue fracture surface revealed a transition from ductile to brittle fractures at −40 ℃. This indicates that the combined effects of the increased strength, decreased toughness, and changes in the stability of RA at low temperatures are the underlying factors responsible for the variation in the low-temperature fatigue crack propagation rate of bainitic rail steel. These findings are expected to guide the design of low-temperature fatigue-resistant bainitic rail steel by optimizing the volume fraction and RA stability.

Effect of the Initial Structure on the Microstructure and Mechanical Properties of a New Press-Hardening Steel

2025-05-28 00:00:00.0

New press-hardening steels (PHSs) have been developed recently to further improve strength and ductility, meeting the demand of the automobile industries. PHSs are usually formed by retaining austenite grains with sufficient fraction and stability for enhancing the transformation-induced-plasticity (TRIP) effect by deliberately introducing spherical carbide particles, where austenite grains can be nucleated and grow in company with their gradual dissolution. However, lamellar pearlite (LP) microstructure, rather than spherical pearlite (SP), is commonly formed in automotive steels during industrial hot rolling. This study investigated the influence of the initial microstructure (LP or SP) on the resultant microstructures and tensile properties of a new PHS (Fe-0.29C-1.75Si-1.2Mn-2.1Cr-0.027Nb) after press-hardening with a short solution period at 915°C and baking at 170°C. The results show that compared to an initial SP structure, an initial LP structure resulted in higher yield strength (YS), ultimate tensile strength (UTS), and total elongation (TE) after the press-hardening at 915°C for 20-30 s. On increasing the solution period to 25 s, the YS, UTS, and TE of the LP specimen were higher than those of the SP specimen (by approximately 35 MPa, 75 MPa, and 1%, respectively) and the 22MnB5 specimen (by 440 MPa, 490 MPa, and 3%, respectively). The austenitic reversion kinetics in the solution were simulated using DICTRA. The simulation results indicate that lamellar cementite dissolves more rapidly than the spherical one under identical solution treatments. Moreover, the austenite that nucleated and grew on the lamellar cementite had a higher C content than the austenite on the spherical cementite. Thus, compared to an initial SP structure, an initial LP structure resulted in a lower start temperature of martensite transformation (M_s) under identical press-hardening processes. This resulted in a higher dislocation density in martensite in the LP specimen, improving its YS and retaining more austenite, thereby enhancing the TRIP effect after press-hardening and improving work hardening. Thus, compared to an initial SP structure, the LP one resulted in higher resultant YS, UTS, and TE after press-hardening with a solution period of 20-30 s.

Quantitative Analysis of the Effect of Annealing Temperature on the Work-hardening Behavior of Medium-Mn Seel Based on in-situ High-energy X-ray Diffraction

2025-05-27 00:00:00.0

Due to the relatively low mechanical properties of the first generation of Advanced high-strength steels (AHSSs) and high cost and the associated production difficulties of the second generation of AHSSs, the majority of the research efforts are thus now shifting towards medium-Mn steels with 5-12 wt.% Mn, which provides an excellent strength-ductility combination yet at a reasonable production cost. The synchrotron-based high-energy X-ray diffraction (HE-XRD) technique has provided a new method for characterizing the microstructure evolution of metallic materials during loading. In the current work, the micromechanical and work hardening behavior of Fe-0.1C-10Mn-2Al steel deformation at room temperature at critical annealing temperatures of 625°C, 650°C, 675°C and 700°C was investigated by in-situ HE-XRD. The total elongation (TE) and the product of the ultimate tensile stress and total elongation (PSE) reached a maximum of 45.6% and 53% at the critical annealing temperature of 650°C, respectively. The transition kinetics of residual austenite (RA) were fitted by the Olson and Cohen (OC) models. According to the lattice strain and the corresponding volume fraction, the phase stress and flow stresses contributed by the constituent phases are obtained. The work hardening rate is decomposed into four influencing factors related to the TRIP effect and load distribution, namely the austenite corresponding force, the load distribution between austenite and martensite, the rate of martensite formation, and the load distribution between ferrite and austenite. The influence of each contributor on the work-hardening behavior was quantitatively analyzed and superimposed, and the calculated results were in good agreement with the experimental work-hardening rate obtained from the true stress-strain curve. Finally, it is found that the LB promotes the transformation of austenite to martensite, and the SFE of RA is highly correlated with the annealing temperature. A linear relationship between the volume fraction of austenite to martensite during LB propagation and the SFE of RA is unrivaled. These findings facilitate an in-depth understanding of the deformation behavior in medium-Mn steels.

Microstructure evolution behavior of Zr-1.0Sn-1.0Nb-0.3Fe alloy under Ar+ irradiation

2025-05-19 00:00:00.0

Zirconium alloys have been used as fuel cladding materials in water-cooled nuclear reactors for decades due to their low thermal neutron absorption cross-section, reasonable mechanical properties, good compatibility with fuels, and good corrosion resistance. During the operation, zirconium alloy cladding materials are subjected not only to the erosion and corrosion from high-temperature and high-pressure cooling water but also to intense neutron irradiation. When zirconium alloys are subjected to irradiation, structure defects including and type dislocation loops and voids or bubbles are often formed in matrix because of the displacement cascades. Neutron irradiation also causes second phase particles (SPPs) amorphization and elemental diffusion. Due to the long period and high cost of neutron irradiation experiments, the radioactivity of irradiated samples, and the effect of ion irradiation on the microstructure of zirconium alloys is similar to that of neutron irradiation, so ion irradiation is often used to simulate the damage introduced by neutron irradiation under laboratory conditions. In order to study the microstructure evolution behavior of zirconium alloys under ion irradiation, N36 (Zr-1.0Sn-1.0Nb-0.3Fe, mass fraction, %) alloy is irradiated with 1.8 MeV Ar+ at 300 ℃ to prepare samples with irradiation damage doses of 2, 5, 10, and 17 displacement per atom (dpa) respectively. Microstructures of the specimens before and after irradiation are characterized and analyzed by SEM and TEM. The results show that the alloy is completely recrystallized before irradiation, and SPPs are mainly hcp-Zr(Nb,Fe)2. With the increase of irradiation dose, the density of Ar bubbles near the damage peak increases first and then tends to be stable, but the size of Ar bubbles increases gradually. type dislocation loops are formed in the irradiation damage area of the alloy matrix. The density of type dislocation loops increases with the dose, and then tends to be stable. However, no type dislocation loops are detected. Zr(Nb,Fe)2 SPPs in the irradiation damage region are all amorphized, and the threshold of amorphization is lower than 0.4 dpa, and no obvious element diffusion phenomenon is found.

Effect of Ribbon Thickness on Magnetic Domain Structures and High-Frequency Magnetic Properties of a Fe_75.2Si₁₃B₈Cu₁Nb_2.8 Nanocrystalline Soft Magnetic Alloy #br#

2025-05-15 00:00:00.0

The development of third-generation semiconductors has increased power density in electronic devices while increasing demands for high-frequency performance of internal soft magnetic materials. Fe-based nanocrystalline alloys are among the most promising candidates for high-frequency applications owing to their excellent comprehensive soft magnetic properties, including high saturation magnetic flux density (B_s), high permeability, and low core loss (P_cm). However, further improvements in their high-frequency properties are required. This study prepared Fe_75.2Si₁₃B₈Cu₁Nb_2.8 nanocrystalline alloy ribbons with 15–23 μm thicknesses by adjusting the Cu wheel speed. The effects of ribbon thickness on the structure and static and high-frequency magnetic properties of the nanocrystalline alloys were investigated. Furthermore, the high-frequency magnetization mechanisms of nanocrystalline alloys with varying ribbon thicknesses were examined through magnetic domain structure characterization. Results indicate that all as-spun alloy ribbons exhibit an amorphous structure and transform into a similar amorphous/α-Fe nanocrystalline dual-phase structure after annealing at 843 K for 60 min, with average α-Fe grain sizes of 11.0–11.6 nm. The static magnetic properties of all ribbons are nearly identical, with B_s and coercivities of 1.35–1.36 T and 0.5–0.6 A/m, respectively. On the contrary, the high-frequency soft magnetic properties improve with decreasing ribbon thickness. The effective permeability (μ_e) of thinner ribbons remains stable and exhibits milder attenuation with increasing frequency. At 100 kHz and 1 MHz, the 15 μm ribbon has μ_e of 17000 and 5200, respectively, which are substantially higher than the values of 14000 and 2900 for the 23-μm ribbon. Moreover, the thinner ribbons demonstrate reduced P_cm. At 0.2 T/100 kHz and 0.2 T/500 kHz, the 15-μm ribbon shows P_cm of 67 and 811 W/kg, representing reductions of 38.0% and 41.9%, respectively, compared with the 23-μm ribbon. Loss separation analysis indicates that the reduced P_cm of the thin ribbon is primarily attributed to decreased eddy current loss and residual loss. The decreased ribbon thickness refines the magnetic domains, facilitating domain rotation at high frequencies and improving high-frequency permeability while reducing residual loss.

Slow Strain Rate Tensile Behavior of P92 Steel in Liquid Pb–Bi Eutectic Environment

2025-05-14 00:00:00.0

Liquid metal embrittlement (LME) refers to the phenomenon wherein the ductility of a solid metal markedly decreases upon contact with a liquid metal. This issue is a critical constraint in the development of lead-cooled fast reactors (LFRs). The commonly accepted mechanism involves the adsorption of liquid Pb–Bi atoms at the crack tips of solid metals, which alters the bonding interactions, lowers the critical stress for rupture, and ultimately causes embrittlement. 9Cr ferritic/martensitic steels are considered potential candidate materials for LFRs. In this study, the slow strain rate tensile behavior of P92 steel was investigated in liquid lead–bismuth eutectic (LBE), focusing on the effects of temperature (150–500 °C), strain rate (4 × 10⁻⁷–4 × 10⁻⁴ s⁻¹), and dissolved oxygen concentration (from 10⁻¹⁰ mass fraction to oxygen saturation). The results showed that no significant embrittlement occurred in oxygen-saturated conditions. In contrast, high embrittlement sensitivity was observed in oxygen-deficient LBE at low strain rates. At high oxygen concentrations, a protective oxide film forms on the surface, effectively isolating the solid metal from liquid LBE, thereby preventing embrittlement. However, this protective effect diminishes significantly if the integrity of the oxide film is compromised. High strain rates promote mechanical damage to the oxide layer, which facilitates LME. In oxygen-poor LBE, the protective oxide film barely forms, leading to direct exposure of the steel matrix to the liquid LBE. This exposure causes dissolution corrosion, where pits formed by metal dissolution act as crack initiation sites owing to localized stress concentration, promoting LME. Interestingly, excessively high strain rates can accelerate crack propagation to such an extent that the crack tip advances before sufficient adsorption of Pb/Bi atoms occurs. This rapid progression inhibits the embrittling action of the liquid metal, thereby reducing the LME effect under low dissolved oxygen conditions.

Effects of O and Fe Element Contents on the Microstructure and Mechanical Properties of TB9 Titanium Alloy Wire

2025-05-14 00:00:00.0

TB9 (Ti–3Al–8V–6Cr–4Mo–4Zr, mass fraction, %) titanium alloy wires are critical materials for aerospace fasteners, springs, and related components. However, systematic research on the effects contents of trace elements such as O and Fe on the microstructure and mechanical properties of TB9 wires remains insufficient, and effective control measures are lacking, often resulting in unstable properties of the fabricated parts. To address the abovementioned issues, this study investigates the effects of O and Fe contents on the microstructure and mechanical properties of TB9 alloy wires in four different states: solution-treated, solution-treated and aged, solution-treated followed by cold drawing, and cold drawn followed by aging. This investigation provides guidance for property optimization and stable process control of the alloy. The results show that O content affects the microstructure and properties of TB9 through mechanisms such as solid solution strengthening, grain refinement, modulation of α-phase precipitation, and interfacial segregation. In the solution-treated state, increasing O content increases alloy strength through solid solution strengthening and grain size reduction, with a slight decrease in ductility. Upon direct aging at 490 °C after solution treatment, α-phase precipitates uniformly in the grains, leading to peak strength. At an O content of 0.11%, an ultimate tensile strength of 1357 MPa is achieved with a favorable strength–ductility balance. However, when the O content increases to 0.14%, brittle fracture occurs. Cold-drawn deformation considerably increases the strength of all alloys. During subsequent aging, the α-phase precipitation temperature decreases considerably, resulting in substantially higher strength compared to solution-treated and aged alloys. The alloy containing 0.11%O achieves an ultimate tensile strength of 1453 MPa and improved ductility after aging at 520 °C. However, excessive O addition leads to embrittlement. In the TB9 alloy, minor variations in Fe content primarily affect properties through solid solution strengthening and grain refinement, but the influence is minimal. Therefore, an optimal O content below 0.11 wt.% is beneficial for achieving high-strength TB9 alloy wires, while excessive oxygen should be avoided in engineering applications owing to its embrittlement effect. Iron content, in the standard range, has a negligible impact on the microstructure and mechanical properties.

Revisit to Multiple Orientation Relationships of Mg₂Sn Precipitates in Mg Alloys

2025-05-09 00:00:00.0

The orientation relationships (ORs) between precipitates and a matrix are closely related to the precipitate morphology. A deep understanding of morphological crystallography forms the basis for controlling precipitate shapes and provides scientific guidance for improving the strength and plasticity of materials. The coexistence of multiple ORs has been observed in several precipitation systems, providing opportunities to manipulate precipitate morphology. The Mg–Sn-based alloy is a typical example of such a system. Previous studies have reported 13 ORs between Mg₂Sn and the matrix in this system. These studies explained and predicted these ORs using the preferential matching principle for primary planes. However, this traditional approach alone cannot fully explain why the number of observed ORs is considerably lower than the predicted number. This study systematically analyzes the previous experimental data to further explore the natural preference rules governing the primary planes. By applying lattice matching criteria that considers smaller mismatches and shorter matching vectors, several well-matched vector pairs are identified. However, no vector pair simultaneously exhibits the shortest length and minimal misfit. It is proposed that the presence of multiple well-matched vector pairs contributes to the coexistence of multiple ORs. A systematic analysis of the previous experimental results also reveals that preventing dislocations with long Burgers vectors is an important factor that further constrains the primary facets. A new analytical method focusing on the primary matching column—shared by primary and secondary facets—is introduced. The primary matching column must be parallel to the short Burgers vector of dislocations, unless it is aligned parallel to an invariant or near-invariant line. Based on the characteristics of the primary matching column, the observed ORs are classified, improving our understanding of ORs and explaining why most precipitates lie on the (0001)_α plane. These new findings provide insights into the coexistence of multiple ORs in other systems and contribute to a knowledge base for optimizing precipitate morphologies.

Effect of Transient Heat Treatment on the Interfacial Microstructure and Bonding Properties of Titanium/Steel Transit Joint

2025-05-08 00:00:00.0

During the secondary welding process of titanium–steel hybrid structures for ships, repeated thermal cycles inevitably change the interfacial microstructure and mechanical properties of explosive-welded transit joints. These changes directly affect the bonding strength of hybrid structures and navigational safety of ships. Herein, simulated transient heat treatment (STHT) was adopted to study the relationship among temperature, interfacial microstructure, and bonding properties of a titanium–steel hybrid structures during the secondary welding process. Results showed that when the STHT temperature was <600 °C, the interfacial microstructure remained stable. Meanwhile, at ≥700 °C, recrystallization occurred, causing grains near the interface to become coarse. Furthermore, the thickness of the intermetallic compound layer comprising FeTi and TiC increased rapidly. Unlike aluminum–steel hybrid structures, in which the interfacial bonding strength monotonically decreases with increasing temperature, the bonding and shear strengths of the titanium–steel hybrid structures initially increased and then decreased, with peaks appearing at 500 and 600 °C. The thermal effect at medium and low temperatures promoted atomic diffusion and stress release, thereby improving the interfacial bonding strength. At high temperatures, grain coarsening and intermetallic compound growth decreased the bonding strength of the transit joint. Overall, the critical threshold temperature at the interface of the titanium–steel hybrid structures is 600 °C.

Hot Corrosion Behavior of DZ411 Nickel-Based Superalloy NiCoCrAlY Coating at 950 °C

2025-05-07 00:00:00.0

NiCoCrAlY coatings offer good resistance to high-temperature oxidation and corrosion, making them an effective protective layer for gas turbine blades. However, in marine environments, these coatings are susceptible to hot corrosion damage, considerably affecting service lifespan and safety. With the development of modern gas turbines with high efficiency and power, the service temperatures of their blades have increased. An in-depth understanding of the hot corrosion mechanisms of NiCoCrAlY coatings above 900 °C is crucial for developing advanced gas turbine blade materials suitable for marine applications. This study investigates the hot corrosion kinetics using conventional gravimetric analysis while considering the quality effect of molten salt volatilization. XRD, SEM, and EDS were used to analyze the surface and interface microstructure, as well as the compositional characteristics of the NiCoCrAlY coating on the DZ411 alloy. These analyses were performed after exposure at 950 °C under Na₂SO₄ salt film and a mixed salt film comprising Na₂SO₄ and NaCl. Results demonstrated that after modifying conventional hot corrosion weight-loss kinetics to consider molten salt volatilization, the hot corrosion mass change becomes a weight gain. The weight gain rate was higher in mixed salt environments than in pure sulfate environments. The hot corrosion weight gain of the alloy is attributed to cyclic oxidation–sulfidation reactions in the coating. Cross-sectional analysis of the corrosion product layer revealed a similar structure under both salt conditions: an outer layer of Al₂O₃, a middle layer of porous oxides dominated by Al₂O₃ and Cr₂O₃, and an inner interdiffusion zone. In addition, Al₂S₃ and Cr₂S₃ precipitates were present at the coating–substrate interface. Furthermore, the presence of chloride salt in the mixed salt environment facilitated chlorination reactions, promoting the diffusion of sulfur and oxygen, and accelerating the degradation of the coating compared with the pure sulfate condition.

Corrosion behavior and mechanism of low alloy steel under low temperature freezing and thawing conditions

2025-05-06 00:00:00.0

This study investigates the corrosion behavior and mechanisms of low-alloy steels under low-temperature freezing–thawing conditions. With the increasing use of low-alloy steels in polar, cold, and marine regions, traditional corrosion research has not adequately addressed the phenomena occurring under alternating low temperatures and freezing–thawing cycles. Freezing–thawing cycles not only cause repeated freezing and melting of moisture on the steel surface but also affect the formation of corrosion product films, crack propagation, and the protective properties of the metal substrate. Addressing this research gap, the study of corrosion behavior of low-alloy steels under such conditions aims to provide a deeper understanding of the underlying mechanisms in extreme environments, thereby offering essential theoretical support for improving the reliability of steel applications in cold environments. An indoor simulation test was designed based on the annual temperature fluctuation characteristics of extreme cold environments, with controlled temperature differences and specific freezing and thawing limits. The corrosion behavior and mechanisms of low-alloy steel under freezing–thawing conditions were investigated through weight loss measurements, surface and cross-sectional morphology observation of corrosion products, compositional analysis of the corrosion products, and electrochemical testing. The results indicated that the highest corrosion rate occurred under freeze–thaw cycles at 10–0 °C, while the lowest rate was observed at −10− −20 °C. This indicates that temperature exerts a greater influence on the corrosion rate. Under all four freezing–thawing conditions, the corrosion product films exhibited cracking, providing pathways for corrosive ions to accelerate corrosion. The corrosion products primarily comprised α-FeOOH, β-FeOOH, γ-FeOOH, and Fe₃O₄, with the outer rust layer predominantly composed of γ-FeOOH and the inner rust layer mainly composed of α-FeOOH. The pitting diameter increased and then decreased with the extension of the testing cycle under all freezing–thawing conditions. Moreover, pitting exhibited a tendency for longitudinal development, reaching a maximum pitting depth of 37.9 μm. Furthermore, electrochemical processes persisted beneath the ice layer, with the lowest corrosion current density measured as 0.002 μA/cm².

Recrystallization Behavior and Split Texture Evolution of Hot Compressed Mg–Gd–Y–Nd Alloy

2025-05-06 00:00:00.0

Alloying design and grain refinement strategies have emerged as promising synergistic approaches to overcoming the persistent strength–ductility trade-off that occurs during Mg alloy processing. However, the regulatory effect of plastic deformation on microstructural evolution requires further systematic investigation to synergistically optimize mechanical performance. In this study, multi-pass and single-pass hot compression tests were performed on Mg–Gd–Y–Nd alloys that had been rapidly solidified, and the key parameters for alloy design in big data research—dynamic recrystallization (DRX), geometrically necessary dislocation (GND) density, and texture—were analyzed using EBSD and Wulff net. The results show that a split texture in the transverse direction (TD) (in the Wulff net coordinate system, the peak distribution is in the ranges of 20° ≤ φ ≤ 40° and 50° ≤ θ ≤ 70°, where θ represents the radius, φ denotes the latitude) was formed during the multi-pass hot compression. The primary factors that affect the formation of TD split texture are the reduction in the number of grains having a 2° correlated misorientation angle, the increase in the fraction of soft-oriented grains, and the reorientation of grains into positions that are favorable for activating the prismatic <a> slip. In contrast, the results showed that a split texture in the extruding direction (ED) (in the Wulff net coordinate system, the peak distribution is in the ranges of 60° ≤ φ ≤ 70° and θ ≈ 80°) was formed during single-pass hot compression. The primary factor that affected the formation of the ED split texture was the need for a significant number of grains to reorient to positions that were favorable for activating the pyramidal <c + a> slip to coordinate the deformation of the c-axis during single-pass hot compression. The splitting texture of magnesium alloys enhances both strength and plasticity. The TD split texture involves orienting the c-axis of the grains toward the TD. When a load is applied in the TD direction, the deformation is primarily coordinated by the activation of the pyramidal <c + a> slip (with a high critical resolved shear stress (CRSS)), which is beneficial for enhancing strength. When a load is applied in the non-TD directions (i.e., the ED and normal direction (ND)), the deformation is coordinated by the prismatic <a> slip and basal <a> slip (with a lower CRSS than the pyramidal <c + a> slip), which is beneficial for enhancing plasticity. With an ED split texture, the c-axis of the grains are oriented toward the ED. When a load is applied in the ED direction, the deformation is primarily coordinated by the activation of the pyramidal <c + a> slip, which is beneficial for enhancing strength. When a load is applied in the non-ED directions (i.e., the TD and ND directions), the deformation is coordinated by the prismatic <a> slip and basal <a> slip, which is beneficial for enhancing plasticity. The recrystallized grain fraction during multi-pass hot compression was 16% and was accompanied by a higher GND density (4.59 × 10¹³ m⁻²) than during single-pass hot compression (4.20 × 10¹³ m⁻²), as well as a significant weakening of the mixed crystal structure. This outcome was primarily attributed to the alternating occurrences of continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) during multi-pass hot compression. In contrast, during single-pass hot compression, the recrystallized grain fraction was 14% and larger micron-sized substructures were retained (which led to the formation of the mixed crystal structure). This outcome was primarily attributed to the dominance of DDRX during single-pass hot compression.

Research on the Deformation Mechanism in the Stress Platform Region of Nickel-Titanium Shape Memory Alloys

2025-04-28 00:00:00.0

The shape memory effect in shape memory alloys primarily arises from their unique stress plateau characteristics. However, owing to the complexity of multiple mechanisms, quantitatively analyzing the contributions of different deformation mechanisms to strain during the stress plateau stage remains challenging. This study systematically investigates the deformation evolution law of nickel–titanium shape memory alloys during the stress plateau stage by combining in situ dynamic transmission electron microscopy observations with statistical analysis of deformation defect structures in macroscopic deformed samples. A quantitative method for calculating the contributions of various deformation mechanisms to strain is developed. The results show that prior to the onset of the stress plateau, the strain in nickel–titanium alloys primarily results from the thickening of intrinsic secondary twins. Upon entering the stress plateau region, detwinning of <011> Ⅱ-type twins becomes the primary deformation mechanism, contributing approximately 62% to the plastic deformation. With further increase in the strain, the participation of nano (001)-type secondary twins and dislocation slip gradually increases. When the strain reaches 6%, the contribution ratios of detwinning, nano (001)-type secondary twinning, and dislocation slip to plastic deformation were 69.3%, 19.3%, and 11.4%, respectively. Thus, detwinning remains the primary deformation mechanism, although the role of nano (001)-type secondary twinning is also crucial.

Design and Fabrication of Bamboo-fiber Like TiB/Ti Composites with Strengthening and Toughening Mechanisms

2025-04-23 00:00:00.0

Natural bamboo fiber possesses excellent mechanical properties due to its unique microstructure, potentially enabling the configurational design of reinforcement in metal matrix composites. Meanwhile, discontinuously reinforced titanium matrix composites are high-strength materials with low ductility. Inspired by the intricate structure of biomimetic bamboo fiber with high strength and toughness, TiB/Ti composites reinforced with a bamboo-fiber-like structure were successfully fabricated in this study. Comparing the mechanical properties of the fiber-like structure with those of their uniform counterpart, the fiber-like structure of TiB whiskers was important in mechanical behaviors, and fracture mechanisms of the composites. For the same volume fraction, the fiber-like-structure-reinforced TiB/Ti composites maintained the high tensile strength of its uniform counterpart but extended elongation to 19.1%. Tensile strength was primarily maintained by leveraging the high load-bearing efficiency of the fiber-like TiB distribution and reinforcement-induced fine-grained strengthening. Meanwhile, the continuous matrix region synergistically improved the strain hardening rate of the composites, enhancing their plastic deformation ability. Moreover, cracks were effectively passivated and deflected during the synergistic deformation process of the fiber-like structure to improve the toughness of the composites. This work reveals the potential of employing a non-uniform reinforcement distribution strategy to synergistically improve the strength and ductility/toughness of titanium matrix composites.

KEYWORDS titanium matrix compo

Research Progress on Corrosion and Irradiation Performance of Accident Tolerant Fuel Cladding FeCrAl Alloy

2025-04-22 00:00:00.0

The Fukushima nuclear accident in Japan highlighted the susceptibility of zirconium alloy fuel claddings. Exposure to high temperatures caused the cladding to react rapidly with steam, resulting in the release of hydrogen and subsequent explosions. These events led to significant radioactive leakage, posing substantial risks to public and environmental safety. To mitigate these risks, accident-tolerant fuel claddings have been proposed as a replacement for traditional zirconium alloy claddings. In recent years, research has focused on developing accident-resistant cladding materials domestically and internationally. FeCrAl alloys have emerged as key materials for medium- and long-term fuel cladding solutions because of their exceptional resistance to high-temperature steam corrosion and superior performance under irradiation. Significant progress has been made in the screening of alloy compositions, the optimization of processing methods, and the evaluation of service performance. Full-scale FeCrAl tubes have progressed to the critical stage of neutron irradiation tests. This paper comprehensively reviews nuclear-grade FeCrAl alloys and evaluates their performance under normal and accident conditions. This review examines the uniform and stress-corrosion behavior of FeCrAl alloys in high-temperature water, the factors influencing these properties, and their oxidation performance in high-temperature steam. The paper also summarizes the impact of irradiation on the alloy’s microstructure and mechanical properties, highlighting the role of irradiation in accelerating uniform and stress corrosion. This analysis provides significant insights into the service performance of FeCrAl alloys and offers valuable knowledge to support their continued optimization as enhanced, accident-tolerant fuel cladding materials.

Significantly Improved High-Temperature Wear Resistance of 414N Hard-Faced Layer by Microalloying and Its Mechanism

2025-04-16 00:00:00.0

Based on 414N hard-faced layer, the microalloyed MA414N wire with niobium (Nb), titanium (Ti), and vanadium (V) was designed and used as novel hard-faced layer by build-up welding in this work. High-temperature wear experiments were conducted to compare the wear resistances of 414N and MA414N. It was found that the wear volume of MA414N was remarkable lower than that of 414N at the temperature range of 500 °C to 700 °C. Above all, the wear volume of MA414N is only 22% of the 414N at 700 ℃, clearly showing a significant improvement in high-temperature wear resistance. The high-temperature wear methods are oxidation wear, abrasive wear, and fatigue wear for the 414N hard-faced layer, and its wear rate increased with elevating temperature. However, it was found that the major method is oxidation wear for the MA414N, with a small number of abrasive wear and fatigue wear, and the wear rate decreased with elevating temperature. Based on SEM/EDX, TEM, and XRD analyses, it was concluded that the dispersively precipitated nanosized (Nb, Ti, V)(C, N) particles introduced a self-strengthening effect in MA414N during high-temperature wear process, leading to a significantly improved wear resistance. Secondly, microalloying inhibited the precipitation of intergranular (Cr, Fe)23C6, which not only reduced the risk of intergranular crack initiation and carbide spalling, but also promoted the formation of dense hexagonal (Fe, Cr)?O? on wear surface with strong protection. Furthermore, microalloying reduced the content of austenite in MA414N during wear process, which also had certain benefits to the improvement of high-temperature wear resistance. According to the above-mentioned results, two models were put forward to explain the high-temperature wear mechanism of 414N and MA414N, respectively.

Chengcong Huang Lu-Ning Wang — 2025-04-16 00:00:00.0

Zinc-based alloys are emerging as the next generation of degradable metals due to their favorable degradation rates. Additive manufacturing offers the capability to produce personalized structures, making it a promising method for generating biomedical metal components. However, during laser powder bed fusion (LPBF), zinc-based alloys can experience significant evaporation because of their low melting point, which often results in poor surface quality. Optimizing powder morphology and particle size presents a potential strategy to enhance forming quality. In this study, two Zn-1Mn-0.4Mg (mass fraction, %) alloy powders with particle sizes of 15–53 μm and 30–75 μm were produced using electrode induction gas atomization (EIGA) and the plasma rotating electrode process (PREP), respectively. Bulk alloys were fabricated using LPBF, and the processing windows for both powders were compared to investigate the effects of powder morphology and particle size on the microstructural and mechanical properties. The results indicated that PREP powders, which exhibited better sphericity and larger particle size, had a broader and more stable LPBF processing window. This led to bulk samples with higher density and smaller grain sizes compared to those prepared with EIGA powders. Under optimal processing parameters (P = 50 W, V = 600 mm/s), both bulk sample types displayed similar tensile yield strength (210 MPa) and ultimate tensile strength (285 MPa). However, the compressive yield strength of the EIGA + LPBF samples ((288.2 ± 4.3) MPa) was slightly higher than that of the PREP + LPBF samples ((258.5 ± 3.5) MPa). By contrast, the PREP + LPBF samples demonstrated significantly greater tensile elongation (21.8% ± 2.3%) compared to the EIGA + LPBF samples (3.3% ± 0.7%). This enhanced ductility was primarily attributed to the more homogeneous thermal conductivity and strain distribution in PREP powders during the LPBF process. These findings offer valuable insights into raw material selection, microstructural control, and mechanical property optimization for LPBF-processed low-melting-point zinc-based alloys.

Effect of oxygen on the microstructure and mechanical properties of Zr-based bulk metallic glass composites by additive manufacturing

2025-04-15 00:00:00.0

The additive manufacturing technology of Zr-based bulk metallic glass composites demonstrates unique advantages in the field of precision device manufacturing, owing to its capability to overcome the forming constraints of complex components imposed by traditional processing methods. However, the inevitable presence of oxygen impurities in the powder and forming atmosphere, may have an adverse impact on the microstructure and properties of the formed parts. In this study, two different processes were selected to prepare Zr₄₈Cu_46.25Al₄Ag₁Sn_0.75 alloy powders. The powders were formed by laser powder bed fusion (L-PBF) under different vacuum conditions. The influence of different powder states and oxygen contents in the L-PBF forming atmosphere on the microstructure and mechanical properties of the alloys was studied. Results indicated that the oxygen content of the alloy prepared by vacuum induction gas atomization (VIGA) was as high as 1700 ´ 10^-6. The crystallization of the VIGA powders was serious after L-PBF formation under a low vacuum atmosphere of 500 ´ 10^-6, and a large amount of brittle intermetallic compounds (AlZr₂, etc.) were precipitated. When the L-PBF formation of the VIGA powders was conducted in a high vacuum atmosphere below 100 ´ 10^-6, the crystallization behavior and precipitation of brittle intermetallic compounds were slightly suppressed. However, a brittle fracture still occurred during compression deformation. The powders prepared by the plasma rotating electrode process (PREP) had a low oxygen content (approximately 570 ´ 10^-6). When the L-PBF formation of the PREP powders was conducted in a high-vacuum atmosphere below 100 ´ 10^-6, the glass-forming ability of the alloy was high. The crystalline phase that forms in the heat-affected zone during crystallization was the B2-CuZr phase, whereas the precipitation of brittle intermetallic compounds was basically suppressed. When the energy density of the L-PBF formation was 34 J/mm³, the compressive strength of the alloy was 1973 MPa, whereas the compressive strain remained about 6%.

Effect of Cr on the Isothermal Decomposition Behavior of Austenite in Low-Carbon Ti Bearing Microalloyed Steel

2025-04-15 00:00:00.0

Ferrite precipitation-strengthened steel characterized by a large amount of nano-carbide precipitation has been widely used in the automotive industry to meet its growing energy-reduction requirements. Because of its high strength and excellent tensile flange properties, it has always been of great interest to researchers. Among the topics of interest, the composition of nanoscale carbides and their precipitation behavior have always been a popular subject area of research. In recent years, studies have confirmed that the addition of Cr is beneficial to the formation of nanoscale complex carbide in ferrite and can refine the size of and increase the number density of precipitates, thereby enhancing the strength of the ferrite. The precipitation behavior of carbides is closely related to the phase transformation of ferrite. However, there is controversy about how Cr affects the kinetics of the ferrite phase transition. This study investigated the effect of Cr on the isothermal decomposition behavior of austenite in low-carbon Ti-bearing microalloyed steel using experimental and numerical simulation methods. The driving forces behind the transformation of ferrite and pearlite, as well as the growth kinetics of ferrite, were calculated using Thermo-Calc software for thermodynamics. The reasons for Cr promoting the formation of pearlite were elucidated through first-principles calculations. The isothermal experimental results indicate that Cr element inhibits the nucleation and growth of ferrite and promotes the decomposition of austenite into pearlite. Thermodynamic calculations show that Cr reduces the driving force behind the γ-to-α phase transition, which results in an increase in the critical nucleation radius (r*) and critical nucleation work (ΔG*). Moreover, Cr significantly reduces the diffusion coefficient of C in austenite and increases austenite’s diffusion activation energy (Q_c). The combined effect of both factors leads to the slow nucleation of ferrite. During the growth of ferrite, its phase transformation under negligible-partitioned local equilibrium (NPLE) mode is controlled by C diffusion. Cr reduces the diffusion coefficient of C in austenite, thereby decreasing the growth rate of ferrite. Under partitioned local equilibrium (PLE) mode, the phase transformation is controlled by the diffusion of the alloying elements. Because the diffusion coefficients (D_i) of element i follow the order D_Cr ＜ D_Mn ＜ D_Si, the growth rate of ferrite is slow. Furthermore, Cr element can significantly reduce the Gibbs free energy of the cementite, thereby increasing the driving force of the transformation of pearlite. According to first-principle calculations, Cr atoms tend to substitute for Fe and Mn atoms in cementite, a (Fe, Mn)₃C alloy. Particularly for the Fe atoms, they can markedly decrease the formation energy and promote the precipitation of cementite, thereby facilitating the transformation to pearlite.

Effect of various seawater pressures on dynamics microstructure evolution and failure mechanism of copper alloys for ship propellers
Effect of Various Seawater Pressures on Dynamics Microstructure Quantitative Analysis and Failure Mechanism of Copper Alloys for Ship Propellers

2025-04-09 00:00:00.0

The microstructure evolution and stress-coupling failure of copper alloys are the primary factors contributing to their accelerated degradation in deep-sea environments, particularly under seawater pressure. In this study, high-manganese aluminum bronze (MAB), a material commonly used in ship propellers, is selected as the research subject. The atomic structure model of MAB alloy with aluminum atomic surface segregation and the strain-coupling model under varying seawater pressures are established through a combination of simulation and experimentation. The failure mechanism under different pressures is further examined, and the corrosion rate is predicted. As seawater pressure increases from 0.1 MPa to 10.0 MPa, the alloy’s dislocation density and strain level progressively rise. When the pressure exceeds 6.0 MPa, these increases become more pronounced. The dislocation density of the alloy’s (111) crystal surface increases by 6.08 × 10⁻³ nm⁻², while the elastic micro-strain level rises by 0.79%. Experimental results indicate that the weight loss rate of copper alloys escalates with increasing pressure. Additionally, the morphology of the corrosion products transitions from a spot-like to a lamellar structure. Density functional theory calculations reveal that seawater pressure significantly affects the stability of the alloy structure. When seawater pressure surpasses the critical threshold of 6.0 MPa, the copper vacancy formation energy and migration–dissolution activation barrier decrease substantially, leading to a notable increase in the dissolution rate. The molecular dynamics method is employed to investigate the adsorption behavior of Cl⁻ on the alloy surface under deep-sea pressure and its influence on corrosion. The results demonstrate that increasing pressure enhances the chemical adsorption of Cl⁻. However, at 10.0 MPa, the saturated adsorption of Cl⁻ on the surface does not significantly affect the dissolution rate.

Multi-Scale Fatigue Crack Propagation Prediction Based on the Dual Drive of Random Forest Algorithm and Data Augmentation Strategy

2025-04-08 00:00:00.0

Data-driven methods based on machine learning have been employed to predict fatigue crack propagation. However, existing studies have largely overlooked the multi-scale nature of this process. Relying solely on macroscopic data for long-crack prediction often fails to capture the complete crack growth process, potentially resulting in non-conservative predictions. Moreover, purely data-driven models often lack interpretability, exhibit limited generalization capabilities, and struggle to adhere to physical laws. The challenge of integrating modeling with reasonable interpretations, driven by both data and mechanisms, remains a significant issue for researchers. In this study, we selected 304 austenitic stainless steel as the research object. To identify the algorithm with the best predictive performance, firstly, the fatigue crack propagation prediction capabilities of three algorithms were compared: K-nearest neighbor regression (KNN), support vector machine regression (SVR), and random forest regression (RF). The most effective algorithm and implemented data augmentation strategies based on three fatigue crack propagation models were selected, namely, long cracks, short cracks, and multi-scale, to enhance prediction accuracy. Finally, using a dual-drive model framework that incorporated the data augmentation strategy, predictions of multi-scale fatigue crack propagation under different loads were conducted. Compared to the SVR and KNN algorithms, the results indicated that the RF algorithm had a lower RMSE value and higher coefficient of determination (R²), making it more suitable for predicting fatigue crack propagation. Under a load of 370 MPa, the prediction accuracy for the training set ranked in the order of RF > KNN > SVR. By contrast, the accuracy for the test set was in the order of RF > SVR > KNN. The multi-scale fatigue crack propagation model effectively captured the entire process of crack growth, whereas the long- and short-crack models accurately represented only parts of it. Data enhancement based on the multi-scale model demonstrated significantly better results than the other two models, with increases in accuracy of 25.76% and 71.74% for the training and test sets, respectively. The dual-drive model based on the RF algorithm and data enhancement strategy exhibited strong generalization capabilities. Under a load of 350 MPa, the RMSE values for the training and test sets were 0.046 and 0.111, respectively, and R² reached 0.995 and 0.961. Under a load of 330 MPa, the RMSE values for the training and test sets were 0.171 and 0.081, respectively, and the R² reached 0.911 and 0.721. Finally, compared with the pure data-driven model based on the RF algorithm, the predictions from the dual-drive model were found to be significantly more accurate.

Effects of Hot Isostatic Pressing on the Helium Ion Irradiation Performance of Additively Manufactured FeCrAl Alloys

2025-04-03 00:00:00.0

FeCrAl alloys are regarded as highly promising candidates for accident-tolerant fuel cladding. This study investigates the influence of hot isostatic pressing (HIP) on the microstructural evolution and mechanical properties of additively manufactured FeCrAl alloys subjected to He ion irradiation. Irradiation experiments were conducted on both the as-printed and HIP-treated conditions of additively manufactured FeCrAl alloys at 350 °C and 450 °C, using 400 keV He ions with a fluence of 1.02 × 101? ions/cm2. The formation and evolution of irradiation-induced defects, including He bubbles and dislocation loops, as well as the corresponding changes in mechanical properties, were systematically characterized and analyzed using transmission electron microscopy (TEM) and nanoindentation techniques. The results indicate that, following HIP treatment, the size of dislocation loops and He bubbles in FeCrAl alloys increased, while the density of He bubbles decreased, compared to the pristine as-printed state. Nanoindentation measurements further revealed that the irradiation hardening rate of the HIP-treated FeCrAl alloy (52%) is significantly higher than that of the as-printed alloy (41%) under identical irradiation conditions. The DBH model calculations confirmed that He bubbles and dislocation loops in the irradiated samples are the primary contributors to irradiation-induced hardening. The analysis suggests that the high density of dislocation lines in the as-printed state facilitates the effective absorption of He atoms and interstitials, which in turn reduces the density and size of dislocation loops and suppresses the growth of He bubbles. The microstructural modifications induced by HIP have been found to reduce the irradiation resistance of the additively manufactured FeCrAl alloys to some extent. HIP is commonly used to enhance the mechanical properties of additively manufactured alloys. However, this process eliminates pre-existing point defect sinks in the alloy, which, to some extent, reduces the material's irradiation resistance.

Hot corrosion behavior of corrosion-resistant alloys during the glass-ceramics solidification of simulated high level salt slags

2025-04-02 00:00:00.0

In the glass-ceramic solidification of waste salt slag reaction environment contains both strongly corrosive molten chlorides and molten glass binders, investigating the corrosion behavior of alloys in this environment is crucial for the selection of materials used in the ceramic curing of waste salt slag reaction canister and composition optimization. This study focuses on the corrosion resistance behavior and mechanism of three typical corrosion-resistant alloys, Inconel 690, Inconel 625 and 310S, during the glass-ceramic solidification process of waste salt slag. The results show that selective diffusion of Cr occurs in all three alloys during the corrosion process, generating a large number of Kirkendall cavities within the alloy matrix. The presence of molten chlorides reacts with Cr2O3 as well as with elemental Cr, further exacerbating the process of Cr dealloying. The corrosion products of Inconel 690 in this environment are predominantly Cr2O3, and the alloy shows the lowest dimension loss, but also the most severe internal corrosion caused by selective diffusion of Cr due to its highest Cr content. For Inconel 625, the diffusion of Cr in the alloy is significantly inhibited and the degree of internal corrosion is less severe due to the lattice distortion caused by the solid solution of Mo and Nb in the alloy, which hinders lattice diffusion. However, as the final oxidation product of Mo, MoO3 has a low melting point and is volatile, leading to matrix fracture and oxide layer shedding and a resultant increased dimension loss of Inconel 625. The base element of 310S, namely Fe, is prone to react with chlorine and oxygen to produce Fe2O3, and the large participation of the base element in the reaction leads to a high dimension loss of the alloy, which is particularly noticeable in the atmosphere region environment rich in oxygen and chlorine vapour.

Effect of Heat Treatment on Flame Retardant Property of GH4169 Alloy Fabricated by Selective Laser Melting

2025-03-31 00:00:00.0

With the advancement of high-thrust liquid rocket engine toward higher chamber pressure and specific impulse, the turbine pump system of the gas generator is being increasingly exposed to extreme high-temperature and high-pressure oxygen-enriched environments. This exacerbates the risk of metal–oxidation combustion failure of superalloy components. The production of high-performance and complex-structured superalloy parts via selective laser melting technology has emerged as a key research area in the manufacture of critical aerospace components. Therefore, the investigation of the oxygen-enriched combustion mechanism of high-temperature alloy materials fabricated using the selective laser melting technology is of great practical significance. This study investigated the effect of heat treatment on the flame retardancy of GH4169 alloy produced via selective laser melting using a self-developed experimental equipment designed for oxygen-enriched combustion testing of metal materials. A high-speed camera was used to observe and record the combustion process. The microstructure and combustion morphology of the alloy were analyzed using OM, SEM and EDS. The as-deposited sample exhibited a typical fish-scale molten pool morphology, with numerous low-melting-point Laves phases precipitating in the interdendritic regions and grain boundaries. Following heat treatment at 980 °C, most Laves phases dissolved, and needle-like and short rod-like δ–Ni₃Nb phases precipitated at the grain boundaries and in the interdendritic regions. Upon increasing the solution temperature to 1080 °C, the volume of precipitated phases was significantly reduced. Consequently, only a small number of nano-sized Laves particles were formed within the grains and a certain amount of blocky carbides was located at the grain boundaries. Combustion resistance testing revealed that the as-deposited sample exhibited the poorest resistance, whereas samples heat-treated at 1080 °C demonstrated the highest resistance. Further, microstructural analysis confirmed that the combustion behavior was closely related to the type and volume of the precipitates within the matrix. Laves phases with high Nb content melted early in the combustion, thereby accelerating the reaction owing to the high combustion heat of Nb. The δ–Ni₃Nb phases formed after heat treatment at 980 °C also promoted combustion. Thus, these findings suggest that eliminating the low-melting-point phases and controlling the types of precipitated phases are key strategies for enhancing the anti-combustion properties of GH4169 alloy.

Phase constitution, microstructure and properties of Ti-doped SmFe12-based nanocrystalline permanent magnetic alloys

2025-03-31 00:00:00.0

With the increasing demand for rare-earth permanent magnetic materials and the over-exploitation of rare-earth resources, ThMn12-type SmFe12-based alloys have attracted extensive attention from researchers. However, the SmFe12 phase has poor structure stability and would decompose to form α-Fe phase at room temperature, drastically reducing the coercivity of the alloy. The difficulty in developing bulk SmFe12-based alloys with high coercivity limits their applications as permanent magnetic materials. In this study, amorphous powder of SmFe12-based alloy was prepared by optimizing the Ti doping amount using the high-energy ball milling method, and nanocrystalline SmFe12-based sintered magnets with a high proportion of the SmFe12 phase were obtained by rapid hot-pressing sintering to crystallize the amorphous powder. The grain size and phase constitution of the alloys were regulated by optimizing the heat treatment process, and the coercivity of the alloys was improved. The method of synergistically tailoring the phase constitution and microstructure of SmFe12-based alloys by alloying, nanocrystallization and heat treatment proposed in this study provides a new approach for the development of high-coercivity SmFe12-based magnets.

Assessment of Corrosion Kinetics of Aluminum-Manganese Bronzes in Deep-Sea Dynamic Environments and Failure Tendency Prediction

2025-03-21 00:00:00.0

The corrosion behavior and failure dynamics of MAB alloy in deep sea flow environment were studied by combining experiment and simulation. The results indicate that there is a critical flow velocity (4 m/s) at which the alloy corrosion rate sharply increases during flow-induced corrosion. The shear stress generated by seawater flow induces atomic-level axial tensile strain on the (111) crystal facet of the alloy, significantly lowering the migration-dissolution activation energy barrier of Cu atoms, leading to a noticeable shift in corrosion potential towards more negative values. Under the combined effects of flow velocity and pressure, the alloy surface transitions from the original “flow trace” pattern to continuous pits, resulting in a marked increase in corrosion rate. Based on density functional theory (DFT), a microstructural model of the copper alloy under combined seawater flow and pressure was developed. The calculated corrosion rate constants aligned closely with the experimental data, enabling effective prediction of corrosion tendencies in dynamic deep-sea environments. Response surface methodology variance analysis (ANOVA) revealed that flow velocity, dissolved oxygen, pressure, and temperature influence the corrosion rate of the alloy in descending order, with the interaction between flow velocity and pressure being the most significant. The final corrosion prediction model had an R2 value of 94.59%, demonstrating its high accuracy.

Study on the formation mechanism of{10-12}twins and the selection rule of martensitic variants in TC4 laser surface remelting layer

2025-03-21 00:00:00.0

Laser surface remelting (LSR) technology can effectively improve the mechanical properties of TC4 alloy, but the effect of LSR process on the microstructure and subsequent tensile deformation mechanism is still unclear. In this paper, LSR treatment was applied on the front and back of full martensitic TC4 sample, and then tensile test was carried out. Systematic characterization was performed by transmission electron microscopy (TEM) and electron back scattering diffractometer (EBSD), to further explore the microstructure evolution during tensile process, particularly, the formation and distribution of {10-12} twins. The results show that the residual stress is induced significantly by rapid cooling during the LSR process, which not only promotes the preferential formation of specific martensite variants, but also reduces the critical shear stress required for twinning . Simultaneously, the evaporation of Al element during laser treatment reduces the c/a ratio of TC4 alloy, weakens the anisotropy of the crystal lattice, and further promotes the formation of {10-12}twins. These twins are mainly concentrated in several martensite variants with specific orientation, which shows an obvious tendency of preferential formation under the rapid cooling condition.

Industrial Experiment and Numerical Simulation of Inclusion Removal During Ladle Holding Period

2025-03-18 00:00:00.0

In high-quality steel production, the final stage of secondary refining typically involves a ladle holding period to facilitate the flotation and removal of inclusions. This improves the cleanliness of the molten steel. This study investigated the evolution of inclusion number density and area fraction during the ladle holding period by conducting industrial experiments on composition adjustment via the sealed argon bubbling refining process of SPHC low-carbon steel. The results indicated that inclusions would be removed by floating during the ladle holding period. The inclusion number density and area fraction decreased from 52.8 #/mm² and 279 × 10^–6 at the start of the holding period to 22.1 #/mm² and 148 × 10^–6 after 15 min, respectively. To clarify the movement and removal of inclusions, a numerical model was developed based on a discrete phase model, incorporating multiphase flow and heat transfer during the ladle holding period. The calculations revealed the following conclusions: (1) inclusions smaller than 10 μm primarily followed the molten steel’s fluid flow, with removal rates of 51.6% and 66.7% after 900 and 1800 s, respectively; (2) inclusions with a diameter of 100 μm were influenced by the molten steel’s fluid flow and their buoyancy, achieving removal rates of 70.2% and 89.2% after 900 and 1800 s, respectively; (3) inclusions with a diameter of 1000 μm primarily underwent flotation, reaching an approximately 100% removal rate after 120 s. This study quantitatively examined the relationship between inclusion removal rates, particle sizes, and refining time during the ladle holding period, thereby providing theoretical guidance for the industrial production of high-quality steel.

Effect of Ultrasonic Power on the Microstructure and Shear Strength of W90/Sn/Mg Joint

2025-03-17 00:00:00.0

Tungsten alloy, known for its high hardness, thermal stability, and excellent nuclear radiation shielding performance, is a critical material for radiation shielding applications. It is widely used in aerospace, national defense, and medical fields. However, tungsten alloy has drawbacks such as high density and processing difficulty, which limit its applicability in high-efficiency, miniaturized, and lightweight designs, particularly in nuclear medical treatment and nuclear-powered flight devices. A double-layered W/Mg structure is expected to serve as a next-generation nuclear radiation shielding material. Due to the significant differences in melting temperatures and coefficients of thermal expansion between W and Mg, conventional soldering methods are ineffective for joining them. In this study, a W90 tungsten heavy alloy and AZ31B magnesium alloy were successfully bonded using an ultrasonic-assisted soldering method with pure Sn. The bonding temperature was 250 °C, and the ultrasonic duration was 4 s. The interfacial microstructure of the W90/Sn/Mg joint under varying ultrasonic power levels was analyzed using scanning electron microscopy and energy dispersive spectroscopy. Additionally, the shear strength of the joint was tested to assess the impact of ultrasonic power on interfacial bonding and mechanical performance. The results indicated that effective bonding formed at the W90/Sn interface, with Ni₃Sn₄ compounds appearing between the (Ni, Fe) matrix and Sn. The seam consisted of a β-Sn matrix phase and Mg₂Sn compounds, with an Mg₂Sn layer forming on Mg/Sn interface. As ultrasonic power increased, joint width decreased while the Mg₂Sn layer thickness increased. The shear strength of the joint initially increased with ultrasonic power but later decreased. At ultrasonic power levels of 100 and 150 W, the joint strength reached a maximum of 10.5 MPa, with failure occurring at the W90/Sn interface. When ultrasonic power increased to 200 W, joint strength declined, and fracture occurred at the Mg₂Sn layer. The acoustic pressure distribution of liquid Sn during the ultrasonic-assisted soldering process was simulated. During a single ultrasonic cycle, the acoustic pressure of liquid Sn oscillated periodically from negative to positive and back to negative values. The maximum acoustic pressure was observed at the center of the liquid Sn, gradually decreasing toward the edges. The cavitation effect, driven by collapsing bubbles, generated high temperatures, high pressures, micro-jets, and shock waves, as explained through theoretical calculations. Based on the Keller–Miksis equation, as ultrasonic power increased, the ratio of the cavitation bubble radius to its initial radius (R(t)/R₀) increased from 16.3 to 47.7, with collapse velocities ranging from 3262 to 6985 m/s. According to the Noltingk–Neppiras theory, as acoustic pressure increased, cavitation-induced temperatures rose from 14614 to 24989 K, while pressure increased from 1.08 × 10⁵ MPa to 9.45 × 10⁵ MPa. The generated temperature and pressure were sufficient to break the Mg alloy’s oxide film and promote interfacial reactions.

High-Throughput Preparation and Corrosion Performance of Yb–Gd Modified Si Bond Coat

2025-02-28 00:00:00.0

Silicon carbide fiber-reinforced silicon carbide ceramic matrix composites (SiC_f/SiC CMCs) are considered strategic thermal, structural materials for advanced aircraft engines due to their lightweight, high specific strength, and excellent high-temperature capability. In harsh combustion environments, environmental barrier coatings (EBCs) are required to protect SiC_f/SiC CMC from water vapor corrosion and low-melting-point oxide corrosion, ensuring the long-term serviceability of SiC_f/SiC components. The development of next-generation high thrust-to-weight ratio aero engines has imposed more stringent requirements on the service life and temperature resistance of EBCs. Typical EBCs consist of a ceramic topcoat and a Si bond coat. However, the water vapor corrosion and oxidation of Si are essential factors contributing to EBC failure during service. Doping rare earth elements into the Si bond coat is expected to enhance its high-temperature capability and corrosion resistance, making it a promising strategy for developing advanced bond coat materials. For rare earth element-modified Si bond coat materials, composition variations fundamentally influence performance changes, including phase composition. However, the composition-property relationship of rare earth-modified Si bond coats remains unclear. In this study, Yb–Gd modified Si bond coat material chips were prepared using multi-target magnetron sputtering for co-deposition and were characterized using a high-throughput method. Transformation trends were identified by analyzing the phase composition and micromorphological changes in different Yb–Gd silicide compositions under extreme conditions (1100 and 1300 °C in an air atmosphere and 1300 °C in a water vapor-air atmosphere). It was found that the primary oxidation products of the Yb–Gd–Si ternary rare earth (RE) silicide at high temperatures are RE_9.33(SiO₄)₆O₂ and RE₂SiO₅ (where RE = Gd, Yb). RE₂SiO₅ undergoes a phase transformation from X1 to X2 as oxidation temperature increases. The morphology of Yb–Gd–Si silicide varies with composition after high-temperature water vapor corrosion. The corrosion mechanism involves initial oxidation, followed by the dissipation of Si and RE in the form of Si(OH)₄ and RE(OH)₃ under water vapor flow, where different silicon contents lead to distinct corrosion morphologies. A high-throughput performance database for RE silicide was established, enabling the regulation of component combinations to influence the phase composition of the bond coat material. This approach facilitates the development of new bond coat material with high melting points for extreme conditions.

Effects of Nitrogen Content on the Microstructure and Mechanical Properties of an Ultralow Carbon Austenitic Stainless Steel for Application in the Nuclear Industry

2025-02-13 00:00:00.0

Under certain harsh conditions in the nuclear industry, austenitic stainless steels need to resistant the intergranular corrosion and also have high strength and toughness. However, the conventional stainless steels such as 310 and 316 steels cannot meet the service requirements. In this work, it focused on the effects of different nitrogen content on microstructure and mechanical properties for an ultra-low carbon austenitic stainless steel in both hot-rolled and solid-solution-treated conditions, which are expect to provide an important reference for a suitable nitrogen content added in the steel. It reveals that the steels were the single austenitic phase and without any δ ferrite when the nitrogen content increased from 0 to 0.5% by XRD analyses. With the increase in nitrogen content, the average grain size of steels decreased gradually. It indicates that the addition of nitrogen can significantly refine the grain size, especially in the solid-solution-treated conditions. However, when the nitrogen content reached about 0.3%, the precipitates began to formed. When the nitrogen content further increasing to about 0.5%, abundant nitrides proved Cr2N and CrN nitrides with relatively larger size were found. The results of mechanical properties indicate that with the increase in nitrogen content, despite of the slight decrease in elongation, the Vickers-hardness, the ultimate tensile strength and yield strength were significantly improved. When the nitrogen content was about 0.3%, the ultimate tensile strength, yield strength and elongation of the solid-solution-treated steel were 798 MPa, 388 MPa and 63.3%, respectively, exhibiting an excellent balance of strength and ductility. The improvement of the strength was mainly derived from the solid solution strengthening of nitrogen, fine-grain strengthening and small portion of precipitation strengthening. Therefore, based on the balance of mechanical properties, it suggests that the nitrogen content should not be higher than about 0.3% in the ultra-low carbon austenitic stainless steel in this work.

Design of additively manufactured Ni-based superalloys based on solid solution elements

2025-01-20 00:00:00.0

Cracking and insufficient strength at high temperatures are major challenges in the manufacturing of additive-manufactured nickel-based superalloys, but can be effectively solved by a composition design based on solid solution elements. In this study, the amounts of γ′ phases, cracks, lattice mismatch, and topological close-packed (TCP) phase in additively manufactured Ni-based superalloys were investigated through thermodynamic calculations, optical microscopy, X-ray diffraction analysis, scanning and transmission electron microscopies, and tensile property tests. The preliminarily optimized ZGH-10 alloy exhibited a good microstructure and excellent tensile properties, with a solid solution strength and crack area percentage of 236 MPa and 1.3 ´ 10⁻⁴%, respectively. The lattice mismatch of the ZGH-10 alloy (-0.26%) contributes to square γ' phases. After thermal exposure to 1000 °C for 500 h, no TCP precipitation appears in the ZGH-10 alloy . At 25℃, 760℃, and 1,000℃, the ZGH-10 alloy delivers tensile s

Effect of Mn/N Ratio on the Microstructure and Mechanical Properties of Multi-Pass Welding HAZ of 22% Cr Low Nickel Type Duplex Stainless Steel

2024-12-23 00:00:00.0

Variations in the Mn/N ratio greatly affect the formation of reformed austenite and precipitation and two-phase ratio in duplex stainless steel (DSS) during the welding thermal cycle. Therefore, investigating the influence of the Mn/N ratio on the multipass welding heat-affected zone (HAZ) microstructure is beneficial for enhancing the comprehensive mechanical properties of the HAZ of thick low-nickel-type DSS plates. This study examined the effect of the Mn/N ratio on the microstructure evolution and mechanical properties of the multipass-welded HAZ of low-nickel-type DSS in comparison with the solid solution state and 2205 DSS. The higher nitrogen content (3.77 Mn/N ratio) resulted in substantial partial transformed austenite formation with a high volume fraction of 58.7%, yielding a tensile strength of 836 MPa and an elongation of 39.5%, indicating its high strength and plasticity. As the Mn/N ratio increases to 17.80, an increase in the Widmanstätten austenite (WA) amount caused cellular Cr₂N formation at the δ/γ phase interfaces and dislocation walls in ferrite for HAZ, considerably decreasing elongation compared to the solid solution state. For DSS with a high Mn/N ratio of 65.91, the grain boundary austenite (GBA) amount initially increased and then decreased, while the intragranular austenite (IGA) and WA amounts increased in HAZ, inhibiting Cr₂N precipitation and reducing ductility and strength. Compared with the 2205 DSS, the fracture surfaces of DSS with lower Mn/N ratios exhibited brittle fracture. The combined strengthening effect of the high-nitrogen solid solution and Cr₂N precipitation on the austenite phase increased the hardness difference between the two phases in the HAZ of the 3.77 Mn/N ratio. In addition, the formation of some large (Cr, Mn) O inclusions promoted crack initiation to some extent, lowering the impact energy to 24.2 J. High Mn/N ratio addition led to more GBA and WA segmented ferrite refinement, as well as the formation of small amounts of IGA, which hindered crack propagation. Moreover, the precipitation of a small amount of the σ phase increased the hardness of ferrite, reduced the hardness difference between the two phases, and increased the impact energy to 134.8 J.

Prediction of mechanical properties of biodegradable zinc alloys based on machine learning

GUO Chuanping, SHI Chenchen, LIU Peng, GAO Dongfang, ZHAO Yangyang, QIAO Yang — 2024-12-17 00:00:00.0

Recent studies indicate that zinc alloys are preferred in biodegradable metal materials owing to their unique biodegradability and biocompatibility. However, their mechanical properties are relatively insufficient; thus, it is crucial to design zinc alloys that meet the mechanical performance implantation standards required for application in biomedicine. The traditional alloy design method depends on experience and trial and error for low efficiency and high cost. In recent years, the rapid development of artificial intelligence has provided new tools and methods for material science. Machine learning (ML), a subset of artificial intelligence, offers new ideas for material design and prediction. This study obtained the mechanical property data of the Zn Mg Mn alloy through experimental investigation and literature review. A performance-oriented ML model was used to predict the compressive yield strength (CYS) and hardness of the Zn Mg Mn alloy, considering various element types and contents and different alloy preparation processes. Furthermore, the influence of the element types and contents on the microstructure and macroscopic mechanical properties of the material was explored. This model used an existing dataset and compared it with seven different ML algorithms to determine that the k-nearest neighbor algorithm has the best predictive ability, exceeding 90%. To further validate the accuracy of the model prediction, a random method was used to select data beyond the dataset for comparative analysis with the model results. Simultaneously, the Shapley Additive exPlanations was applied to quantitatively examine the correlation between the two alloying elements, the preparation process, the CYS, and the hardness in the Zn Mg Mn alloy. The Mg element was determined to significantly impact the alloy’s CYS and hardness. Finally, the influence of individual elements on the mechanical properties of the materials was analyzed through microstructure characterization; the results showed that the formation of new phases (Mg₂Zn₁₁ and MgZn₁₃) due to adding these elements significantly affected the mechanical properties. Based on the research results, this study proposed a composition ratio range for the Zn Mg Mn alloy to satisfy the mechanical performance standards required for medical implant materials. When Mg is between 2.25% and 2.50% (mass fraction, the same below) and Mn is between 2.50% and 3.50%, the CYS and hardness of the alloy comply with the implant standards.

Slip Transfer in Accumulative Roll Bonding Cu/Nb Multilayer Composites

YANG Ran SONG Shaojie LIU Feilong SHEN Ximei SONG Kexing LIU Feng — 2024-12-17 00:00:00.0

Niobium-based alloys are commonly used as superconductors in particle accelerators and fusion tokamaks. However, magnets made of these alloys experience considerable radiation damage, particularly from helium transmutation products in nuclear reactors, which tend to aggregate at grain (GBs) and phase (PBs) boundaries. This aggregation severely degrades the material’s performance. Furthermore, niobium is highly prone to oxidation at high temperatures, further restricting its applications in extreme environments. Recent studies have demonstrated that Cu/Nb multilayer composites fabricated through accumulative roll bonding (ARB) exhibit high yield strength, acceptable ductility, and excellent radiation resistance, making them highly promising for nuclear industry applications. In Cu/Nb multilayer composites with face-centered cubic/body-centered cubic structures prepared via ARB, interfacial instability and strain concentration can occur during deformation due to the high three-dimensional incompatibility of heterophase interfaces. In this study, Cu/Nb polycrystalline multilayer composites were prepared using ARB. In situ tensile tests were conducted using a scanning electron microscope to investigate the slip transfer and blocking behaviors at the GBs and PBs. These behaviors were studied by observing the slip trace alignment and surface morphology continuity. Slip transfer behavior in Cu/Nb multilayer materials was elucidated through statistical analysis of the Luster–Morris parameter m' = cosψcosκ and residual Burgers vector Δb = |b_s2 − b_s1|, where ψ and κ represent the angles between the slip plane normal directions and the slip directions, respectively, and b_s2 and b_s1 are the unit Burgers vectors of the slip systems on either side of the interface. In the Cu layer, slip transfer occurs at the GBs when m′ exceeds 0.77 and Δb is less than 0.029. In the Nb layer, slip transfer occurs when m′ exceeds 0.81 and Δb is less than 0.250. For the Cu/Nb PBs, slip transfer occurs when m′ exceeds 0.93 and Δb is less than 0.170. Notably, the minimum m′ and maximum Δb values for slip transfer at Cu GBs are lower than those at Nb GBs, indicating that slip transfer is more likely to occur at GBs in the Cu layer. The minimum m' value for slip transfer at Cu/Nb PBs is higher than that at both Cu and Nb GBs, whereas the maximum Δb value lies between the two types of GBs. This phenomenon can be attributed to the more complex structure, higher interface energy, and lower shear strength of Cu/Nb PBs than GBs. To achieve slip transfer across fcc/bcc PBs, a larger resolved shear stress is thermodynamically required, and kinetically, the slip systems on both sides of the PB must be closely aligned, resulting in a higher m' threshold and a centered Δb threshold.

MICROMECHANICS MODELLING ON HETEROSTRUCTURE DEFORMATION-INDUCED EFFECT IN BIMODAL STRUCTURE COPPER

2024-12-12 00:00:00.0

Owing to their unique hetero-deformation induced strengthening and strain hardening (HDI) effect, bimodal structure alloys can exhibit excellent strength–ductility synergy. However, the roles of the HDI mechanism remain incompletely understood. This study establishes a micromechanical model of the bimodal structure of pure Cu with 230 nm fine grains and 3 μm coarse grains as an example and estimates the HDI effects on the mechanical properties of Cu. This model is based on the Mori–Tanaka mean field method. The bimodal structure comprises a particulate inclusion phase of coarse grains (soft zone) and a hard matrix phase of fine grains (hard zone). Combining plastic strain gradient and dislocation theories, the HDI effect is proposed as a three-stage process of soft zone hardening, hard zone softening, and stress repartitioning, thereby providing quantitative assessments of the HDI effects on the overall mechanical behaviors of bimodal copper. Based on the experimental observations, the forward stress effect is considered to result from softening via the disentanglement and annihilation of entangled dislocations and from hardening caused by shear band formations. The model predictions favorably agree with the existing experimental data. The role of the forward stress effect in the mechanical response is apparently enhanced when the hard zone (230 nm grains) completely encloses the soft zone (3 μm grains). Moreover, increasing the volume fraction of the soft zone decreases the overall strength and reduces the ductility of Cu. At soft zone fractions of 25%–35%, the strength and ductility were optimally balanced with a tensile strength of 417–420 MPa and an elongation of 28.2%–29.8%. The ultimate tensile strength and elongation were 7% lower and 10-fold higher, respectively, than those of unimodal 230-nm-grained Cu and almost doubled and halved, respectively, from those of unimodal 3-μm-grained Cu.

Studies on Texture Evolution and Deformation Modes for Zr-4 during Stamping: Experiments and Modeling

2024-11-15 00:00:00.0

Stamping experiments with typical feature were performed in order to quantitatively evaluate the formability. The microstructure and texture features for the component after stamping were measured for various locations along longitudinal section by electron backscattered diffraction (EBSD) method. The selection of the deformation modes was then studied with in-grain misorientation axes method (IGMA). In conjunction with finite element method (FEM) simulation, a visco-plastic self-consistent (VPSC) model was used to predict the texture evolution, quantitatively analyze the activation of deformation modes and clarify the relationship between the texture feature change and slip/twin activities. The results show that crack occurred around punch radius. The position located around punch radius follows a plane strain path, while the position located around slope wall and die radius follows a uniaxial tension path. Prismatic slip dominated the deformation for each loading strain path. Pyramidal slip with the combination of tensile twin was activated to coordinate the stamping deformation around die radius, which lead to the formation of new texture component along rolling direction (RD). The increasing basal texture strength around punch radius was attributed to basal slip which acts as the secondary dominating slip system. Prismatic slip with the coordination of basal slip promote the texture type changing from double-peak to basal texture.

Co and Si co-doping strategy for tuning magnetism and mechanical properties of Ni-Mn-Ti alloy

2024-11-13 00:00:00.0

The novel all-d-metal Ni-Mn-Ti Heusler alloy exhibits excellent elastocaloric effect and mechanical properties, but loses the significant magnetocaloric effect found in conventional Ni-Mn-based Heusler alloys due to the weak magnetism of both austenite and martensite. In this study, the Co and Si co-doping strategy to tune martensitic transformation, magnetic and mechanical properties of Ni-Mn-Ti alloy was investigated by the first-principles calculations. The results show that the doped Si atom tends to occupy the Mn sublattice, and the co-doped Co and Si atoms exhibit an aggregate distribution tendency in the alloys. The increase in Co content causes the austenite to change from antiferromagnetic to ferromagnetic state, whereas the martensite remains in antiferromagnetic state. This situation will lead to a magneto-structural coupling transition at higher Co content range, which is crucial to achieve significant magnetocaloric effects. The electronic density of electronic states (DOS) of the alloys was used to elucidate the physical mechanism of the observed phenomena. These findings provide a theoretical basis for the design of solid-state phase change materials with low thermal hysteresis and significant magnetocaloric effects, which will help to promote the practical application of solid-state refrigeration technology.

Effects of grain size and Schmid factor difference on fatigue cracking at twin boundaries in CrCoNi medium-entropy alloy

2024-11-12 00:00:00.0

The fatigue cracking behaviors at twin boundaries (TBs) in a CrCoNi medium-entropy alloy with two different grain sizes were systematically studied with the slipping morphology method under low-cycle fatigue tests. The results reveal that irrespective of the grain size, the transition from slip band (SB) cracking to TB cracking varies with the increase in the difference of Schmid factors (DSF) between the matrix and the twin. The propensity for TB cracking becomes increasingly facilitated as the DSF escalates. Moreover, the magnitude of the required DSF for TB cracking is influenced by the grain size. The required DSF for TB cracking diminishes with increasing grain size. As the grain size increases, even the minimal DSF can lead to significant pilling-up of dislocations near the TBs, exacerbating the damage and rendering these boundaries favorable sites for fatigue cracking.

Microstructures, Microhardness and Corrosion Properties of Nanocrystalline 304 Stainless Steel Plate

2024-09-06 00:00:00.0

The microhardness of nanocrystalline 304 stainless steel plate (NSSP-304) produced by severe rolling technology and its counterpart of the conventional polycrystalline 304 stainless steel (CPSS-304) were investigated on the three planes perpendicular to each other. The microstructures of NSSP-304 and CPSS-304 were characterized using an X-ray diffractometer (XRD), a transmission electron microscope (TEM) and an electron backscatter diffraction (EBSD). The microhardness values on the three planes of NSSP-304 are larger than those of CPSS-304, which has nothing to do with martensite phase. The microhardness distributions on the three planes of NSSP-304 are more uniform than those of CPSS-304. The microhardness value of NSSP-304 on the rolling plane is about 40 HV larger than those on its other two planes, which is related to its weak texture ({110}<211>). Despite the weak texture of NSSP-304, the corrosion rates of NSSP-304 vary with corrosion time within the narrower ranges than those of CPSS-304 during potentiostatic polarization in 0.5 mol/L HCl solution at room temperature and in 6%FeCl3 (mass fraction, %) solution (35℃). The corrosion rates of NSSP-304 are smaller than those of CPSS-304 in the two kinds of solutions. The pitting corrosion resistances of NSSP-304 are larger than those of CPSS-304 in the two kinds of aqueous solutions. These results demonstrate that the texture, the high microhardness, the twins and high-density dislocation of NSSSP-304 did not degrade its uniform and pitting corrosion resistances even though NSSP-304 suffered from severe deformation during its production process (the total deformation larger than 70%). Compared to CPSS-304, the higher microhardness, the improved uniform and pitting corrosion resistances of NSSP-304 are attributed to its different valence electron configurations (the larger binding energies of valence electrons, the larger weights of valence electrons at higher energy levels, the smaller weights of valence electrons at lower levels and the larger work function) owing to its grain refinement, dislocation accumulation, deformation twins and larger fraction of LAGBs grains.

Effect of hot pressing temperature on microstructure and tensile properties of SiC/Al-Zn-Mg-Cu composites

2019-05-20 00:00:00.0

Particulate reinforced aluminum matrix composites have been widely used in industrial fields. In general, high strength aluminium alloys, such as 2024Al are employed to produce stronger composites. However, the composites with high strength Al-Zn-Mg-Cu alloys as the matrices are paid relative attentions. Therefore, the corresponding optimization for fabrication parameters has not been well understood. In the present work, SiC particles with volume fraction of 15% reinforced Al-7.5Zn-2.8Mg-1.7Cu (wt.%) composites were fabricated using powder metallurgy (PM) technique at hot pressing temperatures of 500, 520, 540 and 560 °C. TEM, EPMA and tensile test were used to study the effect of hot pressing temperature on the microstructure and tensile properties of SiC/Al-Zn-Mg-Cu composites. The measured densities indicated that all the composites were completely condensed, no pores were discovered. Undissolved phase containing Mg and Cu segregated in matrix of the composites hot pressed at 500 and 520 °C, resulting in instable tensile properties. With increasing hot pressing temperature to 540 °C, Mg and Cu were uniformly distributed in the composites which exhibited the stable tensile properties. With further increasing temperature to 560 °C, Mg segregated around SiC particles due to interface reaction. In this case, the thickness of SiC/Al interface layers increased significantly compared to the other composites, resulting in the reduction of tensile strength. HAADF-STEM and EDS analyses showed that the interface compounds were oxide of Mg and coarse MgZn2 phase.

Effect of Solution Temperature on Tensile Deformation Behavior of Mn-N Bearing Duplex Stainless Steel

2018-10-24 00:00:00.0

EFFECT OF AUSTENITIZATION TEMPERATURE ON THE DRY SLIDING WEAR RESISTANCE OF A MEDIUM CARBON QUENCHING AND PARTITIONING STEEL

2017-07-24 00:00:00.0

Effect of retained austenite on the wear property of martensitic steel is still in controversial by now. Selecting traditional quenching and tempering (Q&T) sample with identical composition Fe-0.4C-1.5Mn-1.5Si as reference, the dry sliding wear property of quenching and partitioning (Q&P) samples with different austenitization temperatures was studied. The results show that the volume faction of retained austenite in the Q&P samples with full austenitization at 860 ?C or 1000 ?C respectively is nearly same (~14.37 vol.% in the former and ~13.79 vol.% in the later). The corresponding carbon concentration in retained austenite is relatively high (1.37 wt.% in the former and 1.38 wt.% in the later). Therefore, its mechanical stability is very strong. Under the conditions of constant low loading (50 N) and slide speed (40 mm/s), it is not easy to induce martensitic transformation. Which results in the low friction and wear resistance of these two kinds of samples. The slight better in wear resistance of samples with low austenitization temperature can be attributed to the microstructural refinement. When the austenitization temperature was reduced to 800 ?C, the critical Q&P samples were obtained. Microstructure analysis indicates there exist the highest volume fraction of retained austenite (~22.28 vol.%) plus a small amount of ferrite (~6.75 vol.%) in martensitic matrix, which results in the lowest microhardness among present four kinds of samples. However, the mechanical stability of these retained austenites is weak due to low carbon concentration (~1.06 wt.%). The obvious martensitic transformation occurred accompanying sliding wear, contributing to extra hardening and providing additional compressive stress on the touching surface caused by volume expansion. Therefore, the critical Q&P samples with austenitization temperature at 800 ?C exhibit the best wear resistance among present four kinds of samples. Based on experimental results, it is true that it is the weak mechanical stability, but not the amount only, of retained austenite in martensitic steel, plays critical role in improving wear resistance by additional hardening from martensitic transformation.

EXPERIMENTAL AND FINITE ELEMENT SIMULATION OF MILLING POCESS FOR γ-TiAl intermetallics

2017-02-17 00:00:00.0

In this paper, a meso-model of Gamma -TiAl intermetallic was developed using ABAQUS finite element software. The surface morphology and edge fracture mechanism of different material models were analyzed, and the effects of cutting parameters on the surface roughness and size of edge fracture were investigated. The results indicate that the cracks and pits occur between the lamellar and lamellar with different material properties. At the same time, due to the low ductility of gamma -TiAl intermetallic, the negative shear angle begins to form at the exit of the workpiece, then the edge fracture is formed. In addition, both for the surface roughness and the size of edge fracture, the experimental curves are slightly higher than the simulated curves obtained by the hexagonal lamellar, and smaller than those obtained by the rectangular lamellar. With the increasing of the cutting depth, the surface roughness and the size of edge fracture increase gradually, on the contrary, the cutting speed has a small effect. Therefore, in order to obtain a fine surface quality during machining of gamma TiAl intermetallic, the cutting speed can be adopted as high as possible, but not the cutting depth.

STUDY ON ATOMIC-SCALE STRENGTHENING MECHANISM OF TRANSITION-METAL NITRIDE MNx FILMS (M=Ti, Zr, Hf) WITHIN WIDE COMPOSITION RANGES

2016-07-21 00:00:00.0

Transition-metal nitrides have long attracted considerable attention among researchers and ubiquitous applications in various fields due to their renowned mechanical properties. However almost all the discussions of the strengthening mechanism were on conventional meso scale. For further understanding on the atomic-scale strengthening mechanism of transition-metal nitrides, three groups of MNx (M=Ti, Zr, Hf) films with different nitrogen contents were synthesized on the Si (100) substrates by magnetic filtering arc ion plating. The morphologies and thickness of the as-deposited films were characterized by FESEM; the microstructures and the residual stresses were characterized by XRD; the XPS and Nanoindenter were used to measure the chemical states and hardness (also the elastic modulus) of as-deposited films respectively. The results show that all three groups MNx films perform the B1-NaCl single-phase structure within the large composition ranges. The preferred orientation, thickness, grain size and residual stress of the MNx films with different nitrogen contents were not changed so much. While the nanohardness and elastic modulus of MNx both first increased and then decreased with the nitrogen content rise, and the peak values all exist when x near to 0.82. The strengthening mechanism was discussed and the decisive factor of composition dependent hardness enhancement was found from the atomic-scale chemical bonding states and electronic structure in this work, rather than the conventional meso-scale factors, such as preferred orientation, gain size and residual stress.

Influence of Substrate Properties on Deposition Behaviour of 316L Stainless Steel Powder

2016-07-11 00:00:00.0

EFFECT OF Re AND W ON THE RECRYSTALLIZATION OF AS-CAST Ni-BASED SINGLE CRYSTAL SUPERALLOYS

2016-01-27 00:00:00.0

Ni-based single crystal (SX) superalloys have been used as blades in aero-space industry and land-based applications due to their excellent high-temperature properties. However, residual strain is introduced into as-cast SX superalloy blades during the manufacturing process, such as casting, grinding or shot peening, and so on. Recrystallization (RX) occurs easily during subsequent high temperature heat treatment. In previous work, it is believed that RX has detrimental effect on the mechanical properties of SX superalloy. Furthermore, in order to improve the mechanical properties, more and more refractory elements, such as W, Re, Mo, Ta, are added into SX superalloys. However, so far, few reports about the effect of refractory elements on the RX in as-cast SX superalloys have been available. In the present paper, the effect of Re and W on the RX behavior of as-cast Ni-based SX superalloy was studied. Single crystal superalloys with different Re and W were annealed from 1230 oC to 1330 oC after indented using Brinell hardness tester. It is found that RX grains formed at the surface under indentation and grew preferentially along the dendritic cores. Subsequent growth of RX was impeded by the residual coarse g'and (g+g') eutectics in the interdendritic regions. The volume fraction of (g+g') eutectics, the ?g' solvus temperature are increased with the addition of Re and W, which are resulted in the increase of the threshold of RX temperature. For all SX superalloys studied in this paper, RX area increases with the increasing of annealing temperature due to the dissolution of g' and (g+g') eutectics. At the same annealing temperature, in comparison to Re, W shows more tendencies to inhibit RX growth. Additionally, SX superalloy contained both Re and W has smallest RX area in the present experiments.

EFFECT OF COILING TEMPERATURE ON MICROSTRUCTURE AND MECHNICAL PROPERTIES OF Ti-V-Mo COMPLEX MICROALLOYED ULTRA-HIGH STRENGTH STEEL

2016-01-11 00:00:00.0

Among various hardening factors of steels, precipitation hardening has the least embrittlement vector value except grain refinement hardening. Giving full play to the precipitation hardening of microalloyed carbonitrides, which is an important aspect in the development of microalloyed high strength steels. Recently, the research on precipitation behavior and development of microalloyed high strength steels is mainly focused on these relatively simple microalloyed steel including single V, single Ti, Ti-V and Ti-Mo microalloyed steels, while paid less attention on complex microalloyed steels such as Ti-V-Mo steels. Therefore, it is expected to provide a theoretical basis and a practical significance for the development of Ti-V-Mo microalloyed high strength steel. Various hardening increments at different coiling temperatures were calculated. Meanwhile, the effect of coiling temperatures on yield strength and the influence of MC particles on uniform elongation were discussed by means of OM, EBSD, XRD and Physical-chemical phase analysis. The results show that the experiment steel has the best mechanical properties with ultimate tensile strength of 1134 MPa，yield strength of 1080 MPa, elongation of 13.2％ and uniform elongation of 6.8％ at coiling temperature of 600 ℃. The precipitation hardening increment was high to about 444～480 MPa due to about 72.6 wt％ of total precipitates with a size of ＜10 nm. In addition, precipitation hardening and grain refinement hardening are the main mechanisms to improve the strength of the experiment steel, while the variation in precipitation hardening increment causes a significent difference in yield strength. With the coiling temperature increases from 500 ℃ to 600 ℃, the ultimate tensile strength and yield strength increase continuously, but the uniform elongation increases slowly instead of decreasing, which is mainly attributed from an increase of precipitation hardening increment.

FRECKLE FORMATION IN COMPLEX CASTINGS OF SUPERALLOYS

2016-01-11 00:00:00.0

The casting shape effect on the freckle formation in directionally solidified superalloys was investigated, revealing some new characters about the freckle occurrence. It was observed that the ceramic core can lead to the freckling inside the castings. The freckles are preferably formed on the edge instead of the smooth surfaces of the castings. The freckle formation is promoted on the contracting sections but suppressed by expanding ones. These phenomena are contrast to the today’s freckling knowledge and could be explained by using the proposed coanda effect of liquid flow in the mushy zone, leading to a better understanding of freckle formation.

PLASTIC DEFORMATION BEHAVIOR OF DIRECTIONALLY SOLIDIFIED U720Li SUPERALLOY AT ELEVATED TEMPERATURE

2016-01-11 00:00:00.0

U720Li, a kind of precipitation type nickel-based superalloy, shows excellent mechanical properties at elevated temperature, which is also known as the difficult-to-deform alloy because of the high-alloying. To solve its deformation problem, new methods would be developed to enlarge the temperature deforming window and improve its plasticity. The hot compression deformation behaviors of directionally solidified and equiaxed grain U720Li alloys were studied by the MMS-300 testing system, as well as the dynamic recrystallization nucleation and growth mechanisms during the hot deformation were discussed. The microstructural characteristics of the alloy under different deformation conditions were examined using OM, SEM and EBSD. The results show that the deforming resistances of both directionally solidified and equiaxed grain U720Li alloys decrease with the increasing of deforming temperature. When the angle ? between the compression deforming direction and dendrite growth direction is 90°, the deforming resistance of directionally solidified U720Li alloy would be lower. With this direction, the coordination deformation between the dendrites becomes better and no crack can be found after deformation, which indicates that the deforming ability is best along ????90° and it can be considered as the optimal deforming direction for directionally solidified U720Li alloy. Compared with equiaxed grain alloy, directionally solidified U720Li alloy performs higher deformation ability and more homogenous microstructures. During the deformation of directionally solidified U720Li alloy, bulging nucleation of grain boundary migration and dislocation pile-up induced nucleation are found as the main mechanism for the nucleation of dynamic recrystallization. In addition, the deformation activation energy of directionally solidified U720Li alloy is 766 kJ/mol, which is 482 kJ/mol lower than that of equiaxed grain alloy, indicating the directional solidified U720Li alloy exhibits better hot-working plasticity.

EFFECTS OF TOPOLOGICALLY CLOSE PACKED ? PHASE ON MICROSTRUCTURE AND PROPERTIES IN POWDER METALLURGY Ni-BASED SUPERALLOY WITH MICROELEMENT Hf

2016-01-11 00:00:00.0

The precipitation kinetics and morphology of topologically close-packed μ phase in FGH97 powder metallurgy superalloy with 0-0.89 % microelement Hf and the effect of μ phase on the mechanical properties of FGH97 powder metallurgy superalloy billet with 0.30 % Hf has been investigated. The results showed that μ phase precipitated obviously in the alloys with 0.30 % and 0.89 % Hf after long-term aging at 750-900℃, the amount and size of μ phase increased as the aging temperature, aging time and Hf content increasing. μ phase mainly precipitated in grains as strip and flake shapes. After long-term aging at 550-650℃, no μ phase precipitated in FGH97 PM superalloy billet with 0.30 % Hf and the tensile properties and stress-rupture properties at high temperature were not decreased, which showed excellent microstructure stability. After long term aging at 750℃, precipitated μ phase had little effect on tensile strength at high temperature, however, the tensile ductility and high temperature stress rupture life reduced, and the stress rupture ductility increased by about 20%. In this paper, the precipitation behavior of μ phase, the redistribution of elements in γ solid solution and the FGH97 PM superalloy fracture morphology characteristics have been discussed in detail. The mechanism of the brittle and ductile dual effect of μ phase on the mechanical properties has been explained theoretically. The methods of controlling and avoiding excessive μ phase precipitation which leaded to performance deterioration have been proposed.

PREPARATION OF Ti1-xAlxN COATING IN CUTTING TITANIUM ALLOY AND ITS CUTTING PERFORMANCE

2016-01-05 00:00:00.0

High-strength lightweight titanium alloy structural materials have been widely used in aerospace and other industry. However, the titanium is hard to machine due to its characteristics of low thermal conductivity, high chemical affinity, and low elastic modulus. Coating tools provide a solution to overcome the problem of cutting titanium alloy. Ti1-xAlxN coating is one of the most popular candidates in cutting titanium alloy. However, the cutting performance and wear mechanism of the sputtering Ti1-xAlxN coating should be studied further in order to meet the demands of cutting titanium alloy. In this work, Ti1-xAlxN coatings with different Al contents have been prepared by magnetron sputtering. Microstructure and mechanical properties of the coatings were examined by XRD, SEM, EDX and Nanoindenter. Results show that the coatings is a single fcc structure with a (111) preferredorientation when x is in the range of 0.50~0.58. When the Al content is 0.63, the hexagonal AlN is formed in the coating and the hardness declines. In addition, the surface particle size of Ti1-xAlxN coatings increases and the coating density decreases with increasing the Al content. The results of titanium cutting experiment indicate that the tool wear is mainly adhesive wear and chipping. The cutting performances of Ti0.50Al0.50N coated tool is slightly better than uncoated tool and are much better than those of Ti0.42Al0.58 and Ti0.37Al0.63N coated tools at a lower cutting speed (65 m/min). The cutting performance of Ti0.50Al0.50N coated tool is the best at a higher cutting speed of 100 m/min and is four times larger than that of uncoated tool. The excellent cutting performance of Ti0.50Al0.50N coating is mainly due to its high surface density and high hardness, which lead to the formation of regular and dense built-up edge during titanium cutting. Therefore, Ti0.50Al0.50N coating with a (111) preferredorientation, dense surface and relatively low Al content is recommended in high speed turning titanium.

EFFECTS OF IN718 GRAIN SIZE ON ULTRASONIC BACKSCATTING SIGNALS AND ITS NONDESTRUCTIVE EVALUATION METHOD

2015-12-18 00:00:00.0

Superalloy In718 enjoys wide application in such crucial parts as turbine engine disks due to high strength, great toughness and corrosion resistance in different temperature environment. Since the mechanical properties of superalloy In718 are greatly influenced by the grain size, a nondestructive detection method is studied in order to determine the grain size quickly and effectively. In this research, superalloy In718 test samples of different grain sizes were produced and the empirical mode decomposition (EMD) method was employed to find the characteristics of the time-frequency domain of the ultrasonic backscattering signals. Then the effects of the grain size over the intrinsic mode function (IMF) of different frequency bands was analyzed to seek the relations between the power and the grain size of the IMF signals. The original backscattering signals and IMF1 (the first IMF) signals barely respond to the change of the grain size because of their wide frequency bandwidths; the distribution of the frequency domain of the IMF2 signals is centralized and the amplitude of the peak frequency increases with the grain size, and the correlation coefficient between the power and the grain size is 0.995, much higher than that of other modes. This method eliminates the components irrelative to the grain size and takes the IMF2 components which fully reflect the intensity of the grain scattering as the characteristic signals of the grain size evaluation to build an ultrasonic backscattering EMD model evaluating the grain size of superalloy In718. The actual measurement results of the grain size show that the sensitivity of this method is 3.74 times the traditional backscattering method; the evaluation errors over the two verification test samples are -3.72% and 2.87%, apparently more accurate than the ultrasonic velocity method; compared with the attenuation method, this method requires no information of the thickness so that the evaluation results are independent of the thickness measuring error; compared with the metallographic method, this method is more efficient and requires no damage on the components to be evaluated.

INFLUENCE OF SOLID-STATE PHASE TRANSFORMATION ON RESIDU-AL STRESS IN P92 STEEL WELDING JOINT

2015-12-14 00:00:00.0

Microstructure and welding residual stresses in ferritic heat-resistant steels such as P92 have been considered as the most important factors in the structural integrity and life assessment of power plant weldments. Applying computational tools to predict microstructure and residual stress distribution in practical welded structures is a preferable way to create safer, more reliable and lower cost structures. In this study, the effects of volume change, yield strength variation and transformation induced plasticity (TRIP) on the generation of residual stresses in P92 steel welded joints were investigated experimentally and numerically. Optical mi-croscope and Vickers hardness tester were used to characterize the microstructure and hardness of the weldments. The hole-drilling strain-gage method was employed to determine the residual stress distribution across the weldments. Based on SYSWELD software, a thermal-metallurgical-mechanical finite element method was developed to simulate welding temperature filed and residual stress distribution in P92 steel joints. Firstly, numerical experiments of Satoh test were carried out to clarify the influence of solid-state phase transfor-mation on the formation of residual stresses. The simulation results show that the volume change and the yield stress variation have a great effect on the magnitude and distribution profiles of residual stresses in the FZ and HAZ, and even alter the sign of the stresses, while TRIP have a relaxation effect on the tendency of stress variation during phase transformation. Secondly, a finite element model (FEM) was established to calculate the welding residual stress distribution in a single-pass bead-on P92 steel joint. In the FEM, three main constituent phases (austenite, as-quenched martensite and tempered martensite) in P92 steel were taken into account. Finally, the simulation results of welding residual stress were compared with the ex-periments obtained by hole-drilling method. The numerical simulation results are generally in a good agreement with the measured data.

EFFECTS OF AL AND SI ON THE MECHANICAL PROPERTIES AND CORROSION RESISTANCE IN LIQUID LEAD–BISMUTH EUTECTICOF 9CR2WVTA FERRITIC/MARTENSITIC STEEL

2015-12-09 00:00:00.0

9Cr2WVTa steel is one kind of reduced activation ferritic/martensitic (RAFM) steels, which are considered as the candidate structural materials for the accelerator driven subcritical system (ADS). Effects of Al and Si on the microstructure, tensile properties, impact toughness and corrosion behavior in liquid lead–bismuth eutectic (LBE) of 9Cr2WVTa steels were investigated. The results showed that the addition of Al and Si promoted the formation of δ-ferrite, and Al was a much stronger ferrite stabilizer than Si. The presence of δ-ferrite significantly degraded the impact toughness of 9Cr2WVTa steels. M23C6 carbides were observed to precipitate at the δ-ferrite grain boundaries, and stress concentrations were created at the carbide/matrix interface, resulting in the intergranular cracking after deformation. Static corrosion tests were conducted in oxygen-saturated LBE at 550 ℃ for 5000 h to study the effects of Al and Si on the corrosion behaviors in LBE. It is shown that the addition of Al and Si improved the corrosion resistance in LBE due to that appreciable enrichments of Al and Si in inner oxide layer increased the compactness of oxide layer and reduced the diffusion rates of alloy elements and oxygen atoms.

EFFECT OF SOLID SOLUTION TREATMENT ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF HOT-PRESSED CoCrW ALLOYS

2015-12-01 00:00:00.0

ABSTRACT: Nowadays, cobalt-based alloys have been employed in the fields requiring high temperature, corrosion and wear resistance, such as nuclear, aerospace and gas turbine industries. However the cast cobalt-based alloys have relatively high brittleness, and low toughness. So powder metallurgy is used to mold CoCrW alloys. Although their toughness is improved, powder metallurgy CoCrW alloys are still too fragile in some situation. In order to improve the mechanical properties of powder metallurgy CoCrW alloys, solid solution treatment was taken in this work. CoCrW alloys prepared by hot pressing process were solid soluted and the influences on microstructure of the alloys were investigated using SEM, XRD and TEM. Mechanical properties changes of the solutioned CoCrW alloys were studied by hardness test, room-temperature uniaxial tensile and ball-on-disc reciprocating wear test. The results indicate that both the as-hot pressed and solutioned CoCrW alloys consist of the same phases: M23C6, M12C, CrCo intermetallic andγ-Co matrix. The content of M23C6 and prior particle boundaries decrease remarkably after solution treatment, and the ductility and wear resistance of CoCrW alloys are improved. Both the hardness and the tensile strength at room temperature of CoCrW alloys increase first and then decrease with increasing solution temperature and time.

EFFECT OF IRON-RICH PHASE PARTICLES WITH DIFFERENT CONCENTRATIONS ON THE BENDABILITY OF Al-Mg-Si-Cu SERIESS ALLOYS

2015-11-24 00:00:00.0

The influence of iron-rich phase particle with different contents on the bendability of the Al-Mg-Si-Cu alloys was investigated by means of bending and tensile tests, OM, SEM and TEM characterization. The results reveal that, with the increase of iron-rich phase particle content, the bendability of the alloy sheets in the longitudinal and transverse directions is quite different, and the outer surface of the alloy sheets after bending of 180° along the two directions becomes much rough, especially along the transverse direction. When the iron-rich phase concentration increases to the medium level (0.2wt% Fe), the quality of outer surface after bending is very good. But with further increasing iron-rich phase to the high level (0.5wt% Fe), micro cracks can be produced after bending along the two directions. Although increasing iron-rich phase concentration does not give a great effect on the elongation of the alloys in the two directions, yet, according to the tensile fracture and microstructure in the slid surface of the specimen after bending or tension test, the roughening of outer surface of the alloy sheet without ion-rich phase is closely related with the formation of shear bands, while for the alloy sheet with high concentration of ion-rich phases, the formation of micro cracks after bending is mainly related with the size, morphology and distribution of coarse iron-rich phases. In addition, based on the quantitative relationship between iron-rich phase concentration and bendability of the alloy sheets, the models of outer surface roughening and micro cracks forming during bending were also put forward in this paper.

INVESTIGATION ON MECHANICAL AND STRESS CORROSION CRACKING PROPERTIES OF WEAKNESS ZONE IN FRICTION STIR WELDED 2219-T8 ALUMINUM ALLOY

2015-11-09 00:00:00.0

Aluminum alloy 2219 (AA2219) is widely used in the aerospace industry, and friction stir welding (FSW) is an ideal method to join it. The ultimate tensile strength of a FSW AA2219-T8 joint can be as high as 344 MPa which is significantly higher than that welded by other methods such as gas tungsten arc welding. However, the thermo-mechanically affected zone (TMAZ) in the FSW joints of AA2219-T6/T8 is a weakness zone of mechanical property and is susceptible to stress corrosion cracking (SCC), but the reasons are not been well understood. In this work, the mechanical and electrochemical properties of different zones in AA2219-T8 joints obtained by the FSW method were studied. The welding thermal cycles during welding were measured using an array of type K thermocouples. During the tensile process of the joints, digital image correlation (DIC) technique and high speed video technique were employed to investigate the deformational behavior and fracture pathway of the TMAZ, respectively. A microcell method was used to study the micro-electrochemical characteristics of the joints with and without stress. The results showed that the minimum strength located at a position where the weighted strengthening effects of both thermal cycles and stir action were the weakest. The DIC results revealed that the deformation concentrated mainly in the TMAZ during the tensile tests. However, due to the different restraints from the nugget zone (NZ) led to a large strain in the root side than that in the crown side. This made the root side susceptible to cracks initiation. In situ tensile testing indicated that cracks occurred only in the TMAZ at 190 MPa, indicating that the protective surface films in the TMAZ were more prone to crack than those in other zones of the joint. This led the TMAZ to be the weakest zone to pitting corrosion in an aggressive environment. Once pits generate in the TMAZ, the local stress will concentrate near the tip of the pitting, resulting in failure.

CRACK INITIATION AND PROPAGATION AROUND HOLES OF NI-BASED ALLOY DURING THERMAL FATIGUE CYCLE

2015-09-01 00:00:00.0

Ni-base single crystal (SX) superalloys are widely used for production of blades in gas turbines and aircraft engines for their superior mechanical performance at high temperatures. To obtain high cooling efficiency, most of the SX blades consist of thin wall with cooling holes. However, thermal fatigue cracks are usually observed in blades with this kind of structures. Thus, it must be valuable to investigate the crack initiation and propagation around a hole during thermal fatigue tests in a SX superalloy. In the present work a second generation SX Ni-based superalloy were used. Plate specimens that parallel to directional solidification (DS) direction and along (100) or (110) planes were prepared. A hole with diameter of 0.5 mm were drilled vertical to the surface in the middle of the plate by electro-discharge machining (EDM). Thermal fatigue tests were performed between room temperature and 1100 ℃. Effect of crystal orientation on the crack initiation and propagation were investigated and the reasons were analyzed. It was found that a thin recast layer were produced around holes of EDM drilled. The thickness of the recast layer was 15 um in the maximum. Crystal orientation has great effect on the crack initiation sites and propagation kinetics. After 80 cyc thermal fatigue tests, in (110) specimens cracks initiated at the edge of the holes that vertical to the DS direction, then grew quickly and propagated along directions about 45° from the DS direction. After 200 cyc tests, cracks developed to more than 2 mm in length. While in (100) specimens no cracks could be observed even after 200 cyc thermal fatigue tests. This difference was mainly due to the combined effects of different thermal stress caused by the anisotropy of single crystals and of the microstructure characteristics.

SOLID-STATE REACTION BETWEEN SOLID ALLOY DEOXIDIZED BY Mn AND Si AND MnO-SiO2-FeO OXIDE DURING HEAT TREATMENT AT 1473K

In order to control physicochemical characteristics of inclusions in steel through appropriate heat treatment process, solid-state reaction between solid alloy deoxidized by Mn and Si and MnO-SiO2-FeO oxide during heat treatment was studied. Using confocal scanning laser Microscope (CSLM) and high temperature induction furnace, the reaction between the Fe-Mn-Si alloy and MnO-SiO2-FeO oxide during heat treatment at 1473K and its influence on the compositions and phases in the alloy and oxide were investigated by diffusion couple method. A suitable method for pre-melting oxide and producing diffusion couple of Fe-Mn-Si alloy and MnO-SiO2-FeO oxide was proposed to obtain good contact between them. After that, the diffusion couple sample with Ti foil for reducing oxygen partial pressure and bulk alloy containing the same compositions was sealed in a quartz tube for carrying out subsequent heat treatment experiment. In addition, equilibrium compositions and phases of the oxide and alloy during solidification and the solid-state reaction mechanism between them were analyzed and discussed. Quantitative analysis of each element in alloy and oxide was calibrated by standard sample before analysis. Result showed that solid-state reaction and element diffusion between the Fe-Mn-Si alloy and MnO-SiO2-FeO oxide were observed which indicated that the alloy and oxide in the diffusion couple was not equilibrated at 1473K, even though the liquid phases of them were equilibrated at 1873K. The activity of FeO in MnO-SiO2-FeO oxide decreased with the decrease of temperature and excess oxygen diffused from oxide to alloy. Mn and Si contents in the alloy were consumed by the chemical reaction and some MnO-SiO2 particles in the alloy near the interface were generated. As the heat treatment time increased from 10 to 50 h, the widths of particle precipitation zone (PPZ) and manganese depleted zone (MDZ) increased from 79 and 120 to 138 μm and 120 μm, respectively. During the heat treatment, the width of MDZ was always greater than that of PPZ. Moreover, Due to the separation of the FeO, pure Fe particles were formed in the oxide. The MnO and FeO contents in the oxide increased and decreased respectively with the increase of the heat treatment time.

AL2O3 NANOPARTICLE AND NIAL REINFORCED FE-BASED ODS ALLOYS SYNTHESIZED BY THERMITE REACTION

MICROSTRUCTURE AND PROPERTIES OF WELDING JOINT OF A NEW CORROSION RESISTANT NI-BASED ALLOY

Industrial wastewater showed the characteristics of high concentration, complex composition and difficult to degrade. Supercritical water oxidation (SCWO) gained extensive attention and application in wastewater treatment. This method of wastewater treatment to be carried out in the high temperature, high pressure, strong corrosion and oxidation conditions. Thus, the corrosion resistance of the materials using the treatment equipment should be performance excellent. Especially the preheater or reactor piping material, the problem is more outstanding. A new corrosion resistant Ni-based alloy used in supercritical water oxidation environment has been investigated. The microstructure and fracture morphologies of the welding joint are observed and analyzed by OM, SEM and EDS, and the microhardness, tensile strength and other mechanical properties are tested as well. The results indicated that the welding seam of the alloy welding joint can be categorized into cast structure. The microstructure of fusion zone has no welding defect, and the heat affect zone (HAZ) has no grain coarsening phenomenon. The 5 level grain size of the alloy is appropriate. The Vickers-hardness values of the alloy welding seam are less than the matrix. However, as the number of isometric crystals increased, the Vickers-hardness values of welding remelting zone become bigger. Because of including W, Mo, Al ,Ti in the alloy, X-2# alloy welding joint has good high-temperature strength and thermal stability Due to the tensile strength of welding joints in the new alloys is lower than the parent materials, the welding seam could be the weakest link. The tensile tests of room temperature and high temperature are both tenacity fractures, and the fracture mechanism is mixed with normal fault and shear fault.

CHARACTERIZATION OF CU-RICH AND NIAL-RICH PHASES IN MARTENSITE AND RETAINED AUSTENITE OF PRECIPITATION STRENGTHENING STEEL

Precipitation strengthening plays an important role to improve the mechanics properties of steel, and Cu-rich and NiAl-rich phases are two kinds of common precipitates. The present work aimed to study the characteristic of strengthening phases in austenite, martensite, and their phase boundaries in precipitation strengthening steel by atom probe tomography (APT). The hot rolled samples were aged at 500℃ for 1h after solution treatment at 900℃ for 2h. The results show that Cu-rich phase and NiAl-rich phase co-precipitate at phase boundaries and martensite, while no precipitates exist in retained austenite, and there is a precipitation free zone in martensite near phase boundaries. The equivalent radius, spacing, and the concentration of strengthening phases at phase boundaries are larger than that in martensite. There is a separated tendency between Cu-rich phase and NiAl-rich phase, and the separated tendency at phase boundaries is larger than that in martensite. The different behaviors of strengthening phases at phase boundaries from that in martensite are caused by the high density of defects at the phase boundaries, which promote the growth rate of precipitates.

THE STUDY OF CREEP BEHAVIOR ON TWO HR3C HEAT RESISTANT STEELS BASED ON STRESS RELAXATION TESTS

Rupture life is a main property for a material using at high-temperature condition. Usually, the rupture life is gained from creep rupture test. As creep and stress relaxation are two main behaviors for a material served in high-temperature environment, researchers are trying to work out the interrelationship through which one of the two behaviors can be deduced from the other one. Recently, a number of researchers have taken stress relaxation test to replace creep rupture test on studying the creep behavior, and furthermore predicting the rupture life. Applied to evaluate the creep behavior, the stress relaxation test is proved to be superior to the traditional creep rupture test for its short time, small at damage, abundant of information and so on. In this work, the stress relaxation test was used to analyze the creep behavior of two HR3C heat resistant steels with different grain sizes. Additionally, considering the change of microstructure during serve period, the aged HR3C steel was used to compare with as-received HR3C steel for studying the aging effects on the creep behavior. Furthermore, the creep behavior was correlated to their microstructure characteristics. The result was shown that the creep behaviors of two HR3C heat resistant steels varied significantly in spite of their similarity in chemical composition. The coarse grained HR3C steel had lower creep rate, larger stress exponent, greater activation energy, and higher creep resistance than that of fine grained HR3C steel for both as-received one and aged one. The long-term aging processing brought two HR3C steels with damaging effects on microstructure, which lead to an aged HR3C steel with increased creep rate, lowered stress exponent and activation energy, and reduced creep resistance. And the damaging effects on the coarse grained HR3C steel were larger than that on fine grained HR3C steel, which means the coarse grained HR3C steel had much more stable creep resistance than that of fine grained HR3C steel.

INFLUENCE OF Ti/Al RATIOS ON γ′ COARSENING BEHAVIOR AND MECHANICAL PROPERTIES OF GH984G DURING LONG-TERM AGING

2014-08-19 00:00:00.0

In this research, the coarsening behavior of γ′ precipitates and the effects of Ti/Al ratios to tensile properties after long-term aging in GH984G were investigated. It was suggested that the coarsening of sphere γ′ precipitates growed faster with the aging temperature increased from 700℃ to 800℃. The growth kinetics of the γ′ precipitates were explained by Lifshitz-Slyozov-Wagner’s theory of element diffusion controlled growth at 700℃ and 750℃ during long-term aging. However, the coarsening of γ′ precipitates at 800℃ was contrary to Lifshitz-Slyozov-Wagner’s theory. The γ′ precipitates with high Ti/Al growed fast as the aging time less than 3000h, whereas it slowed down as aging time prolonged. Moreover, the coarsening of γ′ precipitates was fast at 800℃, whereas it slowed down as the aging temperature decreased to 700℃. Ti/Al had no effect on the alloy of standard heat treatment and 700 ℃ tensile properties after long time aging. Therefore, the γ′ precipitates of GH984G alloy possessed excellent stability of microstructure at 700℃, and it could control the γ′ precipitates growth further by adjusting Ti/Al ratio.

Crack initiation and propagation behaviors of high Nb-containing TiAl alloy in fatigue-creep interaction

2014-08-13 00:00:00.0

TiAl-based alloys appear as potential competitors to steels and superalloys applied in aerospace and automotive industries due to their low density, high specific strength and stiffness and good oxidation resistance at elevated temperatures. As a new generation of TiAl-based alloys, the high Nb-containing TiAl alloys have become a promising high temperature structural material due to their better high temperature strength and oxidation resistance than ordinary TiAl alloys. TiAl-based alloy components such as low pressure turbine blade and compressor impeller often serve in near steady conditions for a duration of time once peak operating conditions are achieved at high temperature. The components suffer not only from rapidly induced damage from start-up and shutdown cycles, but also from creep damage under sustained loading periods. Moreover, the possible interaction damage between fatigue and creep must be considered. Thus, the study of fatigue-creep interaction for TiAl-based alloys is of great practical importance. Large numbers of researches were focused on the fatigue or creep properties of TiAl-based alloys, however, the fatigue-creep interaction behavior was rarely reported. Therefore, the crack initiation and propagation behavior of a nearly lamellar Ti-45Al-8Nb-0.2W-0.2B-0.1Y alloy in fatigue-creep interaction was observed at 750 ?C. The cyclic loading tests were carried out using a mini servo-hydraulic fatigue machine in a scanning electronic microscope (SEM) chamber. The entire process of crack initiation and propagation was observed. The load cycling was trapezoidal by applying a dwell time at the maximum tension stress. The results were indicated that micro-cracks mainly occurred at internal grain boundaries in the form of creep void or fatigue micro-crack. The micro-cracks firstly extended along the grain boundary by absorbing the creep voids or the stress concentration around crack tips, then connected with each other forming a longer crack. As the crack was frustrated by grain boundaries of other orientations, the crack began to grow in the thickness direction. Meanwhile, the micro-cracks perpendicular to loading direction emerged. Eventually, the frustrated cracks interconnected resulting in fracture. Compared to the in situ SEM observations in fatigue deformation, the dwell time resulted in the increase of probability of grain boundary crack initiation and the changes of crack propagation path. Thus, the fracture mode transform from transcrystalline to intercrystalline and the fatigue lifetime significantly decreased. The model of the crack initiation and propagation behaviors of high Nb-containing TiAl alloys in fatigue-creep interaction was presented in this paper.

ELECTROCHEMICAL CORROSION BEHAVIOR OF PCB-HASL IN NaHSO3/Na2SO3 SOLUTION

2014-08-12 00:00:00.0

With the innovation of electronic technology, integration and miniaturization become the future developing direction of PCB. Meanwhile, the corrosion problems of PCB also stand out more clearly, and even trace amounts of corrosion products will have a serious impact on the reliability of PCB. Under the actual condition for use, like sulfur-containing industrial environment, due to the diurnal temperature variations or/and the temperature field fluctuations for PCB itself, condensation phenomenon is likely to occur. Furthermore, as a result of the moisture absorption effect of granular deposit or supersaturated humidity, a layer of electrolyte solution will be formed on the surface of PCB, causing electrochemical corrosion. In this work, electrochemical impedance spectroscopy and scanning Kelvin probe techniques were used to study the corrosion behavior and mechanism of hot air solder leveling printed circuit boards (PCB-HASL) in a simulated electrolyte 0.1 mol/L NaHSO3 and 0.1 mol/L NaHSO3/Na2SO3 solutions with different pH values, and the influences of immersion time and pH value on the change of corrosion mechanism were discussed. Meanwhile, with the aids of OM, SEM combined with EDS, the nucleation and propagation processes of corrosion products on the surface of PCB-HASL were observed and analyzed. SEM and EDS results showed that the corrosion behavior of PCB-HASL in acid simulation solution was similar to pitting corrosion, and the corrosion pits were in a state of accelerated expansion at the early immersion stage. The corrosion products mainly consisted of oxides and sulfates of Sn. EIS and SKP analysis indicated that the PCB-HASL surface could be activated by NaHSO3 solution and pitting nucleation process only occurred at the early immersion stage. In the neutral or alkaline solution system of NaHSO3/Na2SO3, pitting corrosion couldn’t occur, and the transmission of the electrolyte to the electrode interface through the oxide film is the control step of the corrosion reaction.

MICROSTRUCTURE, MECHANICAL PROPERTIES AND ANALYSIS OF WORK HARDENING BEHAVIOR OF 1300MPa 0.14C-2.72Mn-1.3Si STEEL

2014-07-28 00:00:00.0

Multiphase microstructure which contains ferrite, lath martensite, tempered martensite and a specific proportion of retained austenite with chemistry content of Mn between low Mn and medium Mn(0.14C-1.4Si-2.72Mn) which belong to C-Si-Mn series was produced using continuous annealing simulator. By means of dilatometric simulation、OM、SEM、TEM、EBSD、XRD, microstructures of the steels in different heat treatment stages were characterized. The results illustrate that the tested steel sheet gained good comprehensive properties with yield strength 672 MPa, tensile strength upto 1333 MPa, total elongation A50 13% after annealing at 800 ℃, which can be explained by the refined microstructure, appropriate proportion of phases and a specific proportion of retained austenite. This article has been deeply analyzed the work hardening behavior and discussed the change of instantaneous work hardening rate. The multi-stage work hardening behavior were studied by modified C-J analysis, showing the influence of f/d of martensite and fraction of ferrite on it. The analysis shows the high instantaneous work hardening rate which is helpful for the improvement of strength, plasticity and toughness can be attributed to the proportion, morphology and distribution of ferrite and martensite, and also due to the coordination of each phase and combination action of each factor.

2014-07-23 00:00:00.0

PREPARATION AND OXIDATION BEHAVIOR OF A SINGLE PHASE PTAL2 COATING

2014-07-07 00:00:00.0

Pt-modified aluminide coating has been attracted great attention due to its advantage of the integrated property in resisting both high temperature oxidation and hot corrosion. By the presence of Pt, the spallation possibility of oxide scale and the detrimental effect of S may be restrained at a very low level. Besides, Pt could promote α-Al2O3 formation and stabilize β-NiAl phase. Thus Pt-modified aluminide (PtAl) coating has been widely used in some essential applications requiring reliability and extended service life. There are mainly PtAl2, β-(Ni,Pt)Al and γ/γ′-NiPtAl phases existing inside PtAl coating. In the current study, a single phase PtAl2 coating was prepared on a Ni-based K38G superalloy through pulse-electroplating of Pt and pack aluminization under step-by-step heating mode. At 1100 °C, the isothermal oxidation behavior of the single phase PtAl2 coating was evaluated by thermogravimetric analysis (TGA). Cyclic oxidation test of the PtAl2 coating was performed within a vertical muffle furnace at the same temperature. The results indicate that the singular PtAl2 coating possesses quite good isothermal oxidation resistance; however its resistance against cyclic oxidation is very poor. The cyclic stress induced by repeated heating and cooling has caused visible detachment of PtAl2 coating layer, which the spallation of PtAl2 in further would lead to a premature failure of the whole coating system. Partial spallation of PtAl2 layer, including undesirable consumption of Al inside β-NiAl nearby the spallation act the main reason responsible for the failure. Accordingly, it is not appropriate to apply single phase PtAl2 coating in the high temperature services involving stress and load. The degradation mechanism of the singular PtAl2 coating is investigated by discussing the stress generated from cyclic heating and cooling.

HOT DEFORMATION BEHAVIOR OF BLADES STEEL 10Cr12Ni3Mo2VN FOR ULTRA-SUPERCRITICAL UNITS

2014-07-07 00:00:00.0

10Cr12Ni3Mo2VN steel is mainly made by forging and usually used to make last stage blades of ultra supercritical unit, demanding strict standards of microstructure property because of its hard service environment, so it is necessary to do deep research on its hot deformation behavior. The hot deformation behavior of 10Cr12Ni3Mo2VN steel was investigated through high temperature compression tests on the Gleeble-1500 thermal-mechanical simulator at 850~1200 ℃ and strain rate range of 0.01~10 s-1. The results show that dynamic recrystallization becomes more prone to happen and recrystallized grain size increases with increasing temperature and decreasing strain rate. Isometric crystal and mixed structure appeared after compressed 60% at 1200 ℃ with high and low strain rates respectively. A new method of establishing the hot deformation hyperbolic sine constitutive equation by Levenberg-Marquardt algorithm was proposed. Parameters of the constitutive equations established by traditional linear fitting and Levenberg-Marquardt algorithm have a similar value, and both of the constitutive equations have a high prediction precision, so the method of establishing constitutive equation by Levenberg-Marquardt algorithm is credible.However, Levenberg-Marquardt algorithm can get all parameters at the same time with fewer and simpler steps compared to traditional linear fitting. In addition, the values of critical strain for dynamic recrystallization initiation were determined from the work hardening rate-strain curves and a model related to Zener-Hollomon parameter for predicting critical and peak strain under different deformation paraments was established.

2014-07-04 00:00:00.0

MICROSTRUCTURES AND LOW-CYCLE FATIGUE BEHAVIOR OF Al-9.0%Si-4.0%Cu- 0.4%Mg(-0.3%Sc) ALLOY

2014-06-20 00:00:00.0

In order to determine the influence of rare earth element Sc on the low-cycle fatigue behavior of casting Al-9.0%Si-4.0%Cu-0.4%Mg alloy with T6 treated state, the comparison in the low-cycle fatigue behavior was performed for the Al-9.0%Si-4.0%Cu-0.4%Mg and Al-9.0%Si-4.0%Cu-0.4%Mg-0.3%Sc alloys. The results show that at the low total strain amplitude, the Al-9.0%Si-4.0%Cu-0.4%Mg alloy exhibits the cyclic strain hardening during whole fatigue deformation, while the Al-9.0%Si-4.0%Cu-0.4%Mg(-0.3%Sc) alloy exhibits the cyclic strain hardening in the initial stage of fatigue deformation and then the stable cyclic stress response in the later stage of fatigue deformation. At the higher total strain amplitudes, the Al-9.0%Si-4.0%Cu-0.4%Mg(-0.3%Sc) alloys exhibit the cyclic strain hardening. The addition of Sc can effectively enhance the cyclic deformation resistance and prolong the fatigue lives of theAl-9.0%Si-4.0%Cu-0.4%Mg alloy with T6 treated state. At the lower total strain amplitudes, the cyclic deformation mechanism of the Al-9.0%Si-4.0%Cu-0.4%Mg(-0.3%Sc) alloys with T6 treated state is the plane slip, while at the higher total strain amplitudes, the cyclic deformation mechanism becomes the wavy slip.

THE SIMULATION AND EXPERIMENTAL STUDIES ON GRAIN SELECTION BEHAVIOR OF SINGLE CRYSTAL SUPERALLOYPARTⅠ. STARTER BLOCK OF GRAIN SELECTOR

2013-09-04 00:00:00.0

The rapid development of advanced aero-engine and industry gas turbine requires high performance of single crystal (SX) blade. Spiral selector is very important to produce SX blade, which includes starter block and spiral part. In this research, grain density changing and orientation deviating as the height of grain growth (the distance between section studied and the undersurface of the sample) increasing were studied by the experiment and simulation, and the designing rules for the starter block were given out. EBSD orientation mapping technology was used to get grains’ morphology and orientation. Mathematical and physical models were built for the directional solidification process. Adopting CA-FD method, the 3D macro temperature field of solidification process was calculated as well as grain growth. The properties of grains competitive growth and evolution process during directional solidification in starter block were analyzed based on macro and micro modeling results, and rules for grains competitive growth was explained, which provided theoretical supports for designing starter block.

Cu56Hf27Ti17 BULK METALLIC GLASS WITH HIGH FRACTURE TOUGHNESS

As a sort of quasi-brittle materials, fracture toughness of bulk metallic glasses (BMGs) is of paramount importance for their engineering application. Among the BMG families, Cu-based BMGs are of interest due to their low cost, high strength and less brittleness. As indicated in previous work, the Cu49Hf42Al9 BMG exhibits a good combination of toughness and glass forming ability (GFA). Moreover, toughness of BMG significantly depends on alloy composition. In the Zr-Cu-Al system, it was suggested that increasing the Al content in the alloy does not favor to the plasticity of the glass. Then, it is expected that Al-free Cu-Hf-Ti BMGs may be tougher than the Cu49Hf42Al9 BMG. In addition, notched cylindrical samples were used for the toughness assessment in previous investigation, which probably introduce an overestimation in toughness and difficulty to compare with archival data of engineering materials. To obtain the glassy plate samples for toughness measurements to meet the ASTM E399 requirement, alloys with robust GFA is necessary. In this work, the composition dependence of GFA for ternary Cu-Hf-Ti alloys was revisited. The alloys with the optimal GFA are located around the Cu56Hf27Ti17 and Cu57Hf27Ti16. Critical diameter to form the BMG rods was determined to be 5 mm. Then, the Cu56Hf27Ti17 BMG plates of 2.5 mm in thickness can be fabricated as the specimens for toughness tests. Using single-edge notched specimen for three-point bending test, notch toughness (KQ) of Cu56Hf27Ti17 BMG was determined to be 92±10 MPa?√m. It is nearly doubled with respect to the Cu49Hf42Al9 BMG (KQ=56±9 MPa?√m). It means that the Cu56Hf27Ti17 BMG is the toughest among currently-available Cu-based BMGs. High toughness of Cu56Hf27Ti17 BMG also correlates with its moderate Poisson’s ratio (ν=0.361) and low shear modulus (G=38.6 GPa). The enhanced toughness of Cu56Hf27Ti17 BMG is associated with the extended plastic zone size at the notch tip with the proliferation of shear banding events. The fact that the Cu56Hf27Ti17 superior to Cu49Hf42Al9 BMG in toughness seems supporting that Al element has an unfavorable effect on toughness of Cu-Zr/Hf-based BMGs.

MICROSTRUCTURE, MECHANICAL PROPERTIES AND STRENGTHENING MECHANISMS OF A Cu BEARING LOW-CARBON STEEL TREATED BY Q&P PROCESS

A low carbon steel containing Cu addition was treated by Q&P process using a CAS-200 Continuous Annealing Simulator. The microstructure of the steel was characterized by means of SEM, EBSD, XRD and TEM and its mechanical properties were investigated by tensile testing at room temperature. Cu-rich precipitates formed during the Q&P process were observed as spherical particles in martensitic laths and are 9 to 20 nanometers in diameter. According to Orowan mechanism, those fine particles may have a contribution to the yield strength of the steel about 134 MPa. Also observed are three different morphologies of retained austenite phase in the test steel, i.e. thin film-like, fine granular and blocky, formed at different locations. The test steel has a good comprehensive mechanical properties, of which the product of tensile strength and elongation, the tensile strength and the total elongation are as high as 21.2 GPa?%, 1326 MPa and 16%, respectively. The excellent combined properties can be attributed to the effect of transformation induced plasticity (TRIP) caused by the retained austenite.

MICROSTRUCTURE AND PHASE TRANSFORMATION OF Ni43Co7Mn41Sn9 HIGH TEMPERATURE SHAPE MEMORY ALLOY RIBBON

NiCoMnSn shape memory alloy (SMA) is expected to be a promising high temperature SMA. However, the brittleness has become a big obstacle for its practical application. It is known that, grain refining is effective in improving the ductility of a specific metallic alloy. The aim of this paper is to investigate the effect of melt-spinning on grain refinement and martensitic transformation and provide a guideline for the development of NiCoMnSn SMA. Ni43Co7Mn41Sn9 high temperature SMA ribbon was prepared by a single-roll melt-spinning method. The microstructure and martensitic transformation were investigated by means of OM, SEM, TEM, XRD and DSC, respectively. The experimental results showed that, the ribbon had a chemical composition close to the master alloy and exhibited a thermoelastic martensitic transformation at about 160 oC. The grains in the as-spun ribbon, ranging from 2 μm to 18 μm, were remarkably refined compared with the master alloy. In the as-prepared ribbon, most of the columnar grains grew along the direction vertical to the ribbon plane. At room temperature, non-modulated martensite (tetragonal structure) consisting of twin substructure is determined in the ribbon after relieving the internal stress. Transformation temperatures were lowered by 30 oC after heat treatment at 400 o C for 1h and then kept nearly constant with the increase of heat treatment temperatures.

A phase field study for kinetic scaling of grain coarsening in polycrystalline system containing second-phase particles

The kinetic scaling of the grain coarsening in the polycrystalline system containing the dispersive second-phase particles were studied by Phase field method. The obtained results showed that the increase in the volume fraction of second-phase particles enhanced the growth resistance of grain, resulting in the remarkable deviation of the relationship between the average grain radius Ra and the time t from the non-linear relationship t=ARam+B. The kinetic exponent m also increased with the increasing volume fraction of second-phase particles. No matter whether the second-phase particles existed or not in the system studied, the scaling rule had been satisfied at the late stage of grain coarsening. The increase in the volume fraction of the second-phase particles would cause the decrease in the peak value of structure factor profile. When the value of the wave vector k increases to a certain value, the structure factor curve of the studied system was essentially coincident. With the increase in the volume fraction of second-phase particles, The peak values of scaling function decreased and the peak width became wider. According to structure factor and scaling function, it was known that with the increase in the volume fraction of second-phase particles, the interaction among grains weakens and the grain size would become more uniform during the grain coarsening.

THE RESEARCH OF THE RELATIONSHIP BETWEEN PHASE TRANSITION PROCESS AND THE PROPERTIES IN MAGNETOCALORIC Mn1.2Fe0.8P0.76Ge0.24 COMPOUND

2013-05-22 00:00:00.0

MnFePGe compound has drawn tremendous attention due to its good potential as an alternative to vapor-compression techniques around room temperature. In this article, Mn1.2Fe0.8P0.76Ge0.24 compound was prepared by mechanical milling and subsequent spark plasma sintering (SPS) technique, its microstructure was investigated by SEM, meanwhile the relationship between its phase transition and the properties was investigated by Neutron diffraction, SQUID, DSC and XRD. The results show that the microstructure of Mn1.2Fe0.8P0.76Ge0.24 compound is compact, and the compound possess a hexagonal Fe2P-type crystal structure. Applied magnetic field or temperature change can induce the transformation between paramagnetic phase and ferromagnetic phase. When the applied magnetic field increased or temperature reduced, paramagnetic phase transformed to ferromagnetic phase and caused the magnetic entropy change becoming larger. Research shows that the magnetic entropy change of Mn1.2Fe0.8P0.76 Ge0.24 compound is directly corresponding to the percentage of the phase transition.

First principles investigations of alloying effects on oxidation resistance ofγ-TiAl

Qing-Miao Hu — 2013-03-05 00:00:00.0

The oxidation energies of Al2O3 and TiO2 containing different transition metal alloying elements are calculated by using a first-principles plane-wave pseudoptential method. We show that almost all the alloying elements increase the oxidation energies of Al2O3 and TiO2, i.e., destabilize both Al2O3 and TiO2. Comparing the oxidation energy of Al2O3 and TiO2, we find that, W, Mo, Re, Nb, etc., decrease significantly the stability of Al2O3 relatively to that of TiO2, indicating that these alloying elements may hamper efficiently the inner oxidation of Al in the γ-TiAl matrix so as to increase the high temperature oxidation resistant of γ-TiAl.

INFLUENCE OF THE STATE OF MICROSTRUCTURES OF EUTECTOID STEEL ON ROOM TEMPERATURE WORK-HARDENING BEHAVIOR

Long-fei Li SUN Zuqing — 2013-01-08 00:00:00.0

Steels with ultrafine (α+θ) duplex structure, consisting of ferrite matrix (α) with average grain size of about 1 μm and dispersed cementite particles (θ), have been investigated widely in recent years for making better the work-hardening capability of ultrafine-grained steels. In fact, the ratio of yield strength to tensile strength for plain carbon steels with ultrafine (α+θ) duplex structure is commonly larger than 0.85. For structural material, the low ratio of yield strength to tensile strength is beneficial to absorb external energy and delay the occurrence of destruction. However, the ratio of yield strength to tensile strength is still relatively high for steels with ultrafine (α+θ) duplex structure to act as the structural material. Namely, the work-hardening capability of ultrafine (α+θ) duplex steel needs further improving. It could be feasible for improving the work-hardening capability of ultrafine (α+θ) duplex steel to change the form, size and distribution of the cementite. Therefore, it is necessary to investigate the work-hardening behavior of steel with different cementite states. In the present research, four different microstructures of eutectoid steel were obtained by different thermo-mechanical treatments, i.e., lamellar pearlite, spheroidized pearlite, ultrafine (α+θ) duplex structure and fine-grained (α+θ) duplex structure. The effect of different microstructures on the room-temperature work-hardening behavior of the eutectoid steel was analyzed using room temperature tensile tests, SEM and TEM. The results indicated that the work-hardening characters of lamellar pearlite, which initial work hardening rate is large but decreases quickly with strain, have direct relationship with its large tensile strength, small yield ratio and low uniform elongation. Although the initial work hardening rate of the three ferrite/cementite particles duplex structures is lower, its decreases much slowly with strain comparing with that of lamellar pearlite. Therefore, three types of ferrite/cementite particles duplex structures demonstrate good plastic deformation capability. In comparison with spheroidized pearlite, ultrafine (α+θ) duplex structure and fine-grained (α+θ) duplex structure demonstrate better balance between strength and plasticity due to microstructure refinement.

MICROSTRUCTURE EVOLUTION OF DIRECTIONALLY SOLIDIFIED Al-12%Ni HYPEREUTECTIC ALLOY

2012-12-25 00:00:00.0

The Al-12%Ni hypereutectic alloy (mass fraction) from pure Ni and Al (99.9%) was induction melted and directional solidification with constant growth rates ranging from 1μm/s to 100μm/s and abrupt change of growth were carried out in a Bridgman-type furnace. After solidification, the samples were quickly quenched into liquid Ga–In–Sn alloy to preserve the microstructure. The microstructures of the samples were observed with OM and SEM. The result of experiments with constant growth rate indicates that at the growth rate of 1μm/s, after experiencing a certain growth distance, the primary Al3Ni phase is disappeared and the coupled growth of eutectic can be obtained. The morphology of Al3Ni phase is faceted when it is the leading phase at growth rate ranges from 2μm/s to 100μm/s. With increasing growth rates, the morphology of Al3Ni does not change from planar to cellular, and then to dendritic as the solid solution phases. Otherwise, facted to non-facted transition can be observed on Al3Ni phase. The result of experiments with abrupt change of growth rate indicates that the initial microstructure before abrupt change of growth rate determines the microstructure after abrupt change of growth rate. Only if there exists no coarse primary Al3Ni phase before abrupt change of growth rate can entirely coupled eutectic structure be obtained at relatively higher growth rates. After abrupt change of growth rate, the growth of primary Al3Ni phase was suppressed and the coupled eutectic can keep growth without any coarse primary phases. With the increase of values of abrupt change ratio, the lamellar spacing of eutectic decreased. The elongation rate of Al-12wt.%Ni alloy can be greatly accelerated by the abrupt change of growth rate during directional solidification.

Reach on pitting Corrosion Behavior of Copper in the solution with HCO3-、SO42-and Cl-

2012-12-25 00:00:00.0

The strategy for disposal of high-level radioactive waste in china is to enclose the spent nuclear fuel in sealed metal canisters which are embedded in bentonite clay hundreds meters down in the bed-rock. The choice of container material depends largely on the redox conditions and the aqueous environment of the repository. One of the choices for the fabrication of waste canisters is copper, because it is thermodynamically stable under the saline, anoxic conditions over the large majority of the container lifetime. However, in the early aerobic phase of the geological disposal the corrosion of copper could take place, and the corrosion behavior of copper would be influenced by the complex chemical conditions of groundwater markedly. Pitting corrosion of copper often take place in power plants or air-conditioning condensate water. The corrosion environment usually contains HCO3- SO42-and Cl-. In the early stage of geological disposal, if the aerobic water with HCO3- SO42-and Cl- immersion repository, the pitting corrosion of copper may occur. Some researchers believed that SO42-and Cl- would promote the occurrence of pitting corrosion of copper, and HCO3- will lead to surface passivation and inhibit pitting. Dong Junhua considered that in the solution with HCO3- and SO42-, HCO3-could firstly promote and then inhibit pitting. In order to study Cu pitting thermodynamic behavior in the groundwater, pitting corrosion of copper in the three-ion hybrid system has been researched in the solution with [HCO3-]=0.08mol/L and Cl-/SO42-, by the means of cyclic polarization test and SEM microscopy. The results showed that in the mixed solution of [HCO3-]=0.08mol/L ,SO42-/Cl- , SO42- and Cl- could promote the anodic dissolution of copper electrode, Cl- could reduce the corrosion potential of Cu to enhance its electrochemical activity. In the area picture of pitting sensitivity of Cu, the pitting critical concentration of Cl- was 0.02mol/L.When [Cl-] was low, pitting susceptibility of Cu was not significantly effected by SO42-; when [Cl-] was in the center, SO42- played a significant inhibitory effect on Cu pitting; when [Cl-] was high, SO42- played a reducing role in the first and then a increasing role in pitting susceptibility of Cu. Regardless of the concentration of SO42-, Cl- could promote the pitting of Cu. In this system, the self-catalytic effect of Cu pitting was sensitivity to SO42- and Cl-.

EFFECT OF Bi ADDITION ON THE CORROSION RESISTANCE OF Zr-1Nb ALLOY IN DEIONIZED WATER AT 360 ℃/18.6 MPa

2012-11-05 00:00:00.0

The effect of Bi contents on the corrosion resistance of Zr-1Nb-xBi (x =0.05%---0.3%, mass fraction) was investigated in deionized water at 360 ℃/18.6 MPa by autoclave tests. The results show that, in comparison to the corrosion resistance of Zr-1Nb alloy, the corrosion resistance can be improved by adding Bi into the Zr-1Nb alloy, and the more the Bi content is, the better the corrosion resistance is. TEM and EDS analyses on the microstructures of the alloys show that there are two types of second phase particles (SPPs), including ZrNbFe and β-Nb; The Bi contents have little effect on the type, size and amount of SPPs; 0.3% Bi can be completely dissolved in α–Zr matrix and has no influence on the solution content of Nb in α–Zr matrix. From the fracture and inner surface morphology of oxide films observed by SEM, it can be seen that the Bi dissolved in the α-Zr could noticeably slow down the microstructural evolution of oxide film, including the propagation of micro-cracks and the transformation from columnar grains to equiaxed grains in the oxide film. It can be concluded that increasing the content of Bi dissolved in α-Zr could improve the corrosion resistance of Zr-1Nb-xBi alloys.

Strain effect on transport properties of Nb3Sn multifilament strands of internal tin route for ITER

2016-08-18 00:00:00.0

Superconducting properties of Nb3Sn strands are much affected by the strain state of strands. More attention is concentrated on the influence of axial strain and bending test on the performance of Nb3Sn strands and cables for practical applications. Nb3Sn strands fabricated by internal tin process were measured in the axial strain ranged from -0.9% (compressive) to +0.6% (tensile) at 4.2K, 12T for ITER use. The different strain sensitivity of Jc to the strain state was elaborated for the strands. The degradation of the superconducting critical current density and the n value due to strain were analyzed.

Acta Metall Sin-Forthcoming Articles

Interaction Between Coarse and Fine Grains in the Inhomogeneous Microstructure of Stainless Steel Weld and Its Influence Mechanism on Residual Stress Distribution

Influence of Equiaxed Dendrite Movement and Columnar-to-Equiaxed Transition on Macrosegregation Formation in Fe-0.45wt.%C Steel Ingots

Fast Prediction of Flow Fields in a Slab Continuous Casting Mold by an Attention-Gated Dual-Head U-Net

Machine Learning-Driven Compositional Design of Refractory High Entropy Alloys with Synergistically Optimized Strength and Ductility

Effect of He+ Irradiation on the Microstructural Evolution and Properties of the Diffusion-Strengthened Copper Alloy Cu–Y2O3

Microstructural Inheritance Behavior and Mechanical Property Control of TA18 Alloy From Ingot to Tube Blank#br#

Effects of Electrically Assisted Forming on the Microstructure and Mechanical Properties of 7050 Aluminum Alloy Thin-Walled High-Ribbed Forgings

Deformation Behavior of Mg–Zn–Y–Sn Alloy Containing Long-Period Stacking Ordered Phase Under High Strain Rates

High-temperature Microstructural Stability of High-Si Austenitic Steels for Lead-Cooled Fast Reactor Fasteners

Research Progress and Future Prospect on New High-Speed Extruded Magnesium Alloys

Microstructure and Mechanical Properties of Laser Additively Fabricated TiBw/Ti65 Composite After Shot Peening

Formation Mechanism and Processing of Solute Clusters in Mg Alloys

Achieving Enhanced Electrical Contact Performance of Ag/Ti₃AlC₂ with Novel Spherical Micro/Nano Composite Reinforcing Phase

Effect of Tempering Time on the Hydrogen-Induced Delayed Fracture Behavior of Medium-Carbon Cr–Ni–Mo–V Type High-Strength Bolt Steel

Research Status and Development Trend of Precipitation Phase Regulation and Strengthening Mechanism in Ultra-High Strength Steel

Molecular Dynamics Study on the Nb Regulation Mechanism in the Primary Irradiation Damage of Zr-Nb Alloys

In Situ Study on Tensile Deformation and Damage of Particle-Reinforced Aluminum Matrix Composites Based on High Spatiotemporal Resolution Synchrotron Radiation CT

Progress in Exploring Plastic Deformation Mechanisms of Pure Tungsten

Microstructure control and energy storage mechanism of porous Cu-Al-Mn alloy for ultra-high near-elastic energy storage

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Research Progress and Trends in Characterization Techniques for Ultra-Fine Stainless Steel Wires: A Perspective Review

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Effects of Long-Term Thermal Aging at 700 °C up to 10000 h on Mechanical Properties and Microstructure of 14Cr-ODS Steel With the Addition of Al and Si

Preparation of NiCrBSi–ZrB2 Composite Powder and Coating and Their Hot Corrosion Behavior in NaCl–KCl–Na2SO4 Molten Salt

Prediction Method for High- and Low-Cycle Fatigue Life of Welded Joints Considering Multiscale Residual Stresses Relaxation

First-Principles Study on the Effects of Rare-Earth Er Doping on the Structural, Mechanical, Electronic and Dislocation Properties of (Ti0.25Ta0.25Hf0.25Nb0.25)2Zr2O7 High-Entropy Ceramics

Deformation Mechanism and Multiaxial Constitutive Modeling Method of High-Temperature Tension–Torsion Fatigue in FGH96 Superalloy

Influence of Strain Rate on the Dynamic Failure Behavior of Q345 Steel Welded Joints in Ultra-High Voltage Transformer Tanks

Influence of Al Doping on Thermoelectric Properties of CuInTe2

Effect of Heat Treatment on Interfacial Microstructure of Inertial Friction Welded Joints for GH4065A/IN718 Dissimilar Superalloys

Optimization Design of Mechanical Properties for Bucket Teeth Cast Steel Based on Machine Learning

Superplastic deformation behavior of an advanced Ti750S high-temperature titanium alloy sheet

Microstructural Evolution and Strengthening Mechanisms of Cu-Ni-Si/1010 Steel Bimetallic Composites via Direct Annealing and Cold Rolling + Annealing

Formation Mechanism of Heterogeneous Microstructure and Strengthening–Toughening Mechanisms in High-Strength, High-Toughness AZ91 Magnesium Alloy

Effect of Heat Treatment on Microstructural Evolution and Mechanical Properties of Inertial Friction Welded Joints for FGH96 Superalloy

Study on high-temperature creep and phase evolution of the Fe-Ni-Cr-W-Al alloy ethylene pyrolysis furnace tubes

Exploring the Influence of Experimental Conditions on Indentation Relaxation Behavior of a Heat-resistant Steel Based on Finite Element Method

Orientation Relationship Between Cold-Rolling Shear Band and Matrix Grain in Grain-Oriented Silicon Steel: Crystal Plasticity Calculation and Experiment

Research on Optimization of Laser Powder Bed Fusion Process for 304L Stainless Steel Based on Machine Learning and Multi-Objective Optimization

Microstructural Evolution and Slip Mechanisms in TC4 Titanium Alloy During Cyclic Deformation

Tuning local chemical order and multi-properties in high-entropy alloys via interstitial filling by non-metallic small atoms: a percolation-theory perspective

Multi-Scale Fatigue Crack Propagation Prediction of 304 Austenitic Stainless Steel Based on Physics-Informed Neural Network

Microstructure and High Temperature Oxidation Behavior of Silicide-Boride Composite Coatings on the Surface of Mo

Effect of Deep Cryogenic Treatment on Microstructure and Mechanical Properties of 18Ni(200) Maraging Steel

Study on ordering process of interfacial intermetallic compounds and strengthening mechanism of QAl10-5-5/TC6 bimetals

High-Temperature Oxidation Behavior of Novel Null-Matrix Alloy#br#

A Review on the Evolution Mechanism and Regulation Strategy of Strengthening Phases in Laser Powder Bed Fusion Nickel-Based Superalloys

Effects of Cold-Rolled Pre-Deformation on Microstructures and Mechanical Properties of 2219

Study on the Effect of Lattice Distortion Induced by SrZrO3 Doping on the Structure and Energy Storage Properties of Bi0.5Na0.5TiO3 Lead-free Ferroelectric Ceramics

Microstructure and Mechanical Properties of AC-CMT Arc Welded 7075Al Alloy Joint

Texture Controlling and Superelastic Tension-Compression Asymmetry of NiTi Alloy via Laser Powder Bed Fusion

Research Progress of the Niobium-stabilized Austenitic Stainless Steels for Generation IV Nuclear Reactors

Phase Transformation Behavior During Multi-Step Heat Treatment Processes in High-Carbon Chromium Bearing Steel

Molten Pool Characteristics and Inclusions Behaviors in Ni-Based Superalloy During Vacuum Arc Remelting

Correlation Between Large-Area Magnetic Domain Morphology and Saturation Magnetic Flux Density/Iron Loss in High-Permeability Grain-Oriented Silicon Steels

Dynamic mechanical response and spallation behavior of 60 steel under shock loading

Effect of Rare Earth Treatment on the Friction and Wear Property of Bainite/Martensite Bearing Steel

Active modulation of eutectic growth and creep mechanism of Co35Ti35Nb30 alloy under electrostatic levitation condition

DECOUPING HYDROGEN DIFFUSION PARAMETERS OF SUB-LAYERS IN ALUMINOSILICONIZED STEEL: INTEGRATED EXPERIMENTAL AND MODELING

Current-Carrying Friction Properties and Mechanism of Cu–Ti2AlC Composite Coating

Effect of Lanthanum on the Solidification behavior of GH4151 Superalloy

Effect of tempering temperature on stress corrosion resistance of F22M Steel for Blowout Preventer in Ultra-deep Well

TMechanism of Carbides in Promoting Stray Grain Formation at the Bottom of Laser Remelting Pools in DD32 Superalloy

Effect of Process Parameters and Post-Treatment on Room-Temperature Tensile Properties of GH3536 Superalloy Fabricated by Laser Powder Bed Fusion#br#

Phase-Field–Lattice Boltzmann Simulation on the Effect of Shrinkage Flow on Solute Dendritic Growth

Effect of alloying elements on phase interface behavior in Co-Ti based superalloys

Hierarchical Lamellar Heterostructured Metallic Materials

Microstructural Characteristics and Properties of Antibacterial Low-Alloyed Mg–Ag–In Alloys

Microstructural Evolution and Mechanical Property Enhancement in SiC-Reinforced AA7075 Aluminum Matrix Composites Processed via Laser Powder Bed Fusion Additive Manufacturing

Influences of High-Entropy Alloy Particles on the Microstructure and Mechanical Properties of Selective Laser Melted Al12Si Alloy during Solution Treatment

Micro- to Nanoscale Mechanical Behavior of Irradiation Hardening in Zr-Sn-Nb Alloys Under Ion Irradiation

Microstructure and Mechanical Properties of Mg-5Zn-0.5Ca Alloy Controlled by Ti Microslices

Microstructural Characteristics and Electrochemical Behavior of a Mg-0.1Sn Anode for Magnesium-ion Batteries

Microstructural Evolution and Dynamic Failure Mechanism of B10 Cu-Ni Alloy Under Multiple Stress Coupling in Flowing Seawater#br#

Molecular Dynamics Simulations of Tensile Deformation of Cu/Ta Nano-Bilayer Films and the Effect of Al and W Atoms Doping on the Deformation#br#

The Effect of Cumulative Deformation on the Recrystallization Mechanisms of F316 Austenitic Stainless Steel

Influence of Capsule Wall Thickness on Preparation and Mechanical Properties of Powder Metallurgy-Hot Isostatic Pressed Titanium Alloy Fan Impeller

Regulation of Heat Treatment and Synergistic Mechanism of Strength–Ductility in Precipitation-Strengthened High-entropy Alloy Fabricated by Laser Powder Bed Fusion#br#

Effect of Mo Content on the Microstructure and Mechanical Properties of 1000 MPa Grade High-Strength Steel Weld Metal

Effect of He⁺ Irradiation on the Microstructural Evolution and Properties of the Diffusion-Strengthened Copper Alloy Cu–Y₂O₃

Preparation of NiCrBSi–ZrB₂ Composite Powder and Coating and Their Hot Corrosion Behavior in NaCl–KCl–Na₂SO₄ Molten Salt

First-Principles Study on the Effects of Rare-Earth Er Doping on the Structural, Mechanical, Electronic and Dislocation Properties of (Ti_0.25Ta_0.25Hf_0.25Nb_0.25)₂Zr₂O₇ High-Entropy Ceramics

Influence of Al Doping on Thermoelectric Properties of CuInTe₂

Active modulation of eutectic growth and creep mechanism of Co₃₅Ti₃₅Nb₃₀ alloy under electrostatic levitation condition

Current-Carrying Friction Properties and Mechanism of Cu–Ti₂AlC Composite Coating

Site Occupation of Alloying Elements in α₂ Phase in High-Temperature Titanium Alloys: A First-Principles Study

lEffect of Ga Microalloying on Structure and Soft Magnetic Properties of a Fe_81.5Si₄B₁₃Cu_1.5 Nanocrystalline Alloy

Effect of Ribbon Thickness on Magnetic Domain Structures and High-Frequency Magnetic Properties of a Fe_75.2Si₁₃B₈Cu₁Nb_2.8 Nanocrystalline Soft Magnetic Alloy #br#

Revisit to Multiple Orientation Relationships of Mg₂Sn Precipitates in Mg Alloys

Effect of various seawater pressures on dynamics microstructure evolution and failure mechanism of copper alloys for ship propellers
Effect of Various Seawater Pressures on Dynamics Microstructure Quantitative Analysis and Failure Mechanism of Copper Alloys for Ship Propellers