Started in 1985 Semimonthly
ISSN 1005-0302
CN 21-1315/TG
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Effects of rare earth on microstructure and impact toughness of low alloy Cr-Mo-V steels for hydrogenation reactor vessels
Zhonghua Jiang, Pei Wang, Dianzhong Li, Yiyi Li
J. Mater. Sci. Technol.    2020, 45 (0): 1-14.   doi:10.1016/j.jmst.2019.03.012
Abstract113)   HTML4)    PDF (6541KB)(49)      

The effects of rare earth (RE) on the microstructure and impact toughness of low alloy Cr-Mo-V bainitic steels have been investigated where the steels have RE content of 0 to 0.048 wt.%. The results indicate that the normalized microstructures of the steels are typical granular bainite (GB) composed primarily of bainitic ferrite and martensite and/or austenite (M-A) constituents. The M-A constituents are transformed into ferrite and carbides and/or agglomerated carbides after tempering at 700 °C for 4 h. The addition of RE decreases the onset temperature of bainitic transformation and results in the formation of finer bainitic ferrite, and reduces the amount of carbon-rich M-A constituents. For the normalized and tempered samples, the ductile-to-brittle transition temperature (DBTT) decreases with increasing RE content to a critical value of 0.012 wt.%. Lower DBTT and higher upper shelf energy are attributed to the decreased effective grain size and lower amount of coarse agglomerated carbides from the decomposition of massive M-A constituents. However, the addition of RE in excess of 0.012 wt.% leads to a substantial increase in the volume fraction of large-sized inclusions, which are extremely detrimental to the impact toughness.

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Effects of ultrasonic assisted friction stir welding on flow behavior, microstructure and mechanical properties of 7N01-T4 aluminum alloy joints
Zhiqiang Zhang, Changshu He, Ying Li, Lei Yu, Su Zhao, Xiang Zhao
J. Mater. Sci. Technol.    2020, 43 (0): 1-13.   doi:10.1016/j.jmst.2019.12.007
Abstract96)   HTML5)    PDF (10012KB)(31)      

Conventional friction stir welding (FSW) and ultrasonic assisted friction stir welding (UAFSW) were employed to weld 6-mm thick 7N01-T4 aluminum alloy plates. Weld forming characteristics and material flow behavior in these two different welding processes were studied and compared. Ultrasonic vibration was applied directly on the weld in axial direction through the welding tool. Metal flow behavior, microstructure characteristics in the nugget zone (NZ) and evolution of the mechanical properties of naturally aged joints were studied. Results show that the ultrasonic vibration can significantly increase the welding speed of defect-free welded joint. At the rotation speed of 1200 rpm, the UAFSW can produce defect-free welded joints at a welding speed that is 50% higher than that of the conventional FSW. Ultrasonic vibrations can also improve surface quality of the joints and reduce axial force by 9%. Moreover, ultrasonic vibrations significantly increase the volume of the pin-driven zone (PDZ) and decrease the thickness of the transition zone (TZ). The number of subgrains and deformed grains resulting from the UAFSW is higher than that from the FSW. By increase the strain level and strain gradient in the NZ, the ultrasonic vibrations can refine the grains. Ultrasonic energy is the most at the top of the NZ, and gradually reduces along the thickness of the plate. The difference in strengths between the FSW and the UAFSW joints after post-weld natural aging (PWNA) is small. However, the elongation of the UAFSW is 8.8% higher than that of the FSW (PWNA for 4320 h). Fracture surface observation demonstrates that all the specimens fail by ductile fracture, and the fracture position of the UAFSW joint changes from HAZ (PWNA for 120 h) to NZ (PWNA for 720 and 4320 h).

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Cellular automaton modeling of austenite formation from ferrite plus pearlite microstructures during intercritical annealing of a C-Mn steel
Chunni Jia, Chengwu Zheng, Dianzhong Li
J. Mater. Sci. Technol.    2020, 47 (0): 1-9.   doi:10.1016/j.jmst.2020.02.002
Abstract92)   HTML5)    PDF (4092KB)(28)      

A mesoscopic cellular automaton model was developed to study the microstructure evolution and solute redistribution of austenization during intercritical annealing of a C-Mn steel. This model enables a depiction of three-stage kinetics of the transformation combined with the thermodynamic analysis: (1) the rapid austenite growth accompanied with pearlite degeneration until the pearlite dissolves completely; (2) the slower austenite growth into ferrite with a rate limiting factor of carbon diffusion in austenite; and (3) the slow austenite growth in control of the manganese diffusion until the final equilibrium reached for ferrite and austenite. The effect of the annealing temperature on the transformation kinetics and solute partition is also quantitatively rationalized using this model.

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Microstructure evolution, Cu segregation and tensile properties of CoCrFeNiCu high entropy alloy during directional solidification
Huiting Zheng, Ruirun Chen, Gang Qin, Xinzhong Li, Yanqing Su, Hongsheng Ding, Jingjie Guo, Hengzhi Fu
J. Mater. Sci. Technol.    2020, 38 (0): 19-27.   doi:10.1016/j.jmst.2019.08.019
Abstract53)   HTML5)    PDF (6650KB)(14)      

CoCrFeNiCu (equiatomic ratio) samples (ø 8 mm) were directionally solidified at different velocities (10, 30, 60 and 100 μm/s) to investigate the relationship between solidification velocity and microstructure formation, Cu micro-segregation as well as tensile properties. The results indicate that the morphology of the solid-liquid (S-L) interface evolves from convex to planar and then to concave with the increase of solidification velocity. Meanwhile, the primary and the secondary dendritic arm spacings decrease from 100 μm to 10 μm and from 20 μm to 5 μm, respectively. They are mainly influenced by the axial heat transfer and grain competition growth. During directional solidification, element Cu is repelled from the FCC phase and accumulates in the liquid owe to its positive mixing enthalpy with other elements. Tensile testing results show that the ultimate tensile strength (UTS) gradually increases from 400 MPa to 450 MPa, and the strain of the specimen prepared at the velocity of 60 μm/s is higher than those of others. The fracture mode of all specimens is the mixed fracture containing both ductile fracture and brittle fracture, in which ductile fracture plays a fundamental role. In addition, the brittle fracture is induced by Cu segregation. The improvement of UTS is resulted from columnar grain boundary strengthening.

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Inhibition effects of benzalkonium chloride on Chlorella vulgaris induced corrosion of carbon steel
Junlei Wang, Tiansui Zhang, Xinxin Zhang, Muhammed Asif, Lipei Jiang, Shuang Dong, Tingyue Gu, Hongfang Liu
J. Mater. Sci. Technol.    2020, 43 (0): 14-20.   doi:10.1016/j.jmst.2020.01.012
Abstract51)   HTML2)    PDF (2654KB)(19)      

In this work, a surfactant, benzalkonium chloride (BAC), was used to study its effects on both the growth of Chlorella vulgaris and the corrosion caused by its biofilm. Experimental results indicated that BAC at a low concentration of 3 mg/L suppressed C. vulgaris growth and achieved 81 % corrosion inhibition based on weight loss reduction. The inhibition effects increased when the BAC dosage was increased. At 30 mg/L, the corrosion inhibition increased to 95 %. Electrochemical results supported surface pitting analysis, weight loss results data and confirmed the corrosion inhibition.

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Microstructure and tensile properties of DD32 single crystal Ni-base superalloy repaired by laser metal forming
Shiwei Ci, Jingjing Liang, Jinguo Li, Yizhou Zhou, Xiaofeng Sun
J. Mater. Sci. Technol.    2020, 45 (0): 23-34.   doi:10.1016/j.jmst.2020.01.003
Abstract50)   HTML4)    PDF (7715KB)(20)      

In this work, the microstructure and tensile properties of DD32 single-crystal (SC) superalloy repaired by laser metal forming (LMF) using pulsed laser have been studied in detail. The microstructures of the deposited samples and the tensile-ruptured samples were characterized by optical microscopy (OM), transmission electron microscope (TEM) and scanning electron microscope (SEM). Due to high cooling rate, the primary dendrite spacing in the deposited area (17.2 μm) was apparently smaller than that in the substrate area (307 μm), and the carbides in the deposited samples were also smaller compared with that in the substrate area. The formation of (γ+γ′) eutectic in the initial layer of repaired SC was inhibited because of the high cooling rate. As the deposition proceeded, the cooling rate decreased, and the (γ+γ′) eutectic increased gradually. The (γ+γ′) eutectic at heat-affected zone (HAZ) in the molten pool dissolved partly because of the high temperature at HAZ, but there were still residual eutectics. Tensile test results showed that tensile behavior of repaired SC at different temperatures was closely related to the MC carbides, solidification porosity, γ′ phase, and (γ+γ′) eutectic. At moderate temperature, the samples tested fractured preferentially at the substrate area due to the fragmentation of the coarse MC carbide in the substrate area. At elevated temperature, the (γ+γ′) eutectic and solidification porosity in the deposited area became the source of cracks, which deteriorated the high-temperature properties and made the samples rupture at the deposited area preferentially.

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Large electric field-induced strain in the novel BNKTAN-BNBLTZ lead-free ceramics
Chao Wang, Qiang Li, Weiming Zhang, Huiqing Fan
J. Mater. Sci. Technol.    2020, 45 (0): 15-22.   doi:10.1016/j.jmst.2019.09.040
Abstract44)   HTML3)    PDF (5043KB)(9)      

(1-x)Bi0.5(Na0.82K0.18)0.5Ti0.96(Al0.5Nb0.5)0.04O3-xBi0.46Na0.46Ba0.5La0.02Ti0.97Zr0.03O3 lead-free ceramics (abbreviated as BNKTAN-100xBNBLTZ) was prepared by the conventional solid reaction. XRD patterns and EDS spectrums revealed that a stable solid solution had been formed between BNBLTZ and BNKTAN. With the introduction of BNBLTZ anti-ferroelectric content, BNKTAN relaxor ferroelectrics exhibited the excellent field-induced-strain for x = 0.04 corresponding to electric field-induced strain S ~ 0.505 % and normal strain d33* ~777 pm/V at 65 kV/cm. The large strain response was attributed to the emergence of PNRs in the relaxation process. Additionally, an excellent fatigue resistance performance was obtained within 105 cycles (S = 0.505 %-0.495 % and d33* = 777-758 pm/V, 65 kV/cm). It suggested that prepared ceramics had the great potential to strain sensor and actuators.

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New FeNiCrMo(P, C, B) high-entropy bulk metallic glasses with unusual thermal stability and corrosion resistance
Yanhui Li, Siwen Wang, Xuewei Wang, Meiling Yin, Wei Zhang
J. Mater. Sci. Technol.    2020, 43 (0): 32-39.   doi:10.1016/j.jmst.2020.01.020
Abstract42)   HTML0)    PDF (5137KB)(16)      

New Fe20-35Ni20Cr20-30Mo5-15(P0.6C0.2B0.2)20 bulk metallic glasses with excellent thermal stability, strength, and corrosion resistance have been developed through the high-entropy alloy design strategy. The high-entropy bulk metallic glasses (HE-BMGs) possess larger supercooled liquid regions of ~69 K, higher crystallization onset temperatures of ~852 K, larger undercoolings of ~109 K, and more sluggish crystallization process upon heating than the conventional metallic glass benefited from the high mixing entropy effect. The HE-BMGs also exhibit ultrahigh strength of ~3.4 GPa, Vickers hardness of ~1107, and superior corrosion resistance in acids and NaCl solutions by formation of highly stable Cr- and Mo-enriched passive films. The new metal-metalloid HE-BMG system and exceptional properties give the alloys good promise for both scientific and engineering applications.

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Thickness-dependent mechanical properties of nacre in Cristaria plicata shell: Critical role of interfaces
S.M. Liang, H.M. Ji, X.W. Li
J. Mater. Sci. Technol.    2020, 44 (0): 1-8.   doi:10.1016/j.jmst.2019.10.039
Abstract41)   HTML2)    PDF (2901KB)(21)      

The thickness dependence of mechanical properties of nacre in Cristaria plicata shell was studied under three-point bending tests. The results show that the mechanical behavior of nacre exhibits a strong thickness dependence. The bending strength firstly increases with the increase of specimen thickness and then becomes roughly constant as the thickness reaches a certain value of ~2.5 mm. However, the mean value of work per unit volume increases constantly with increasing specimen thickness; meanwhile, the cracking mode changes from penetration into the platelets to deflection along the interfaces. The theoretical analyses indicate that the thickness-dependent mechanical properties of nacre are mainly caused by the variation in the number of inter-lamellar interfaces. The more the number of inter-lamellar interfaces is, the higher the strength and work of fracture of nacre under bending tests will be. However, as the number of inter-lamellar interfaces reaches a certain value (e.g., in the present specimen with 2.5 mm thickness), the strength tends to remain constant, while the work of fracture still increases. Therefore, the present research findings are expected to provide a valuable guidance for the interfacial design of nacre-like materials with high strength and toughness.

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The combined influence of grain size distribution and dislocation density on hardness of interstitial free steel
Wei Li, Martina Vittorietti, Geurt Jongbloed, Jilt Sietsma
J. Mater. Sci. Technol.    2020, 45 (0): 35-43.   doi:10.1016/j.jmst.2019.11.025
Abstract39)   HTML3)    PDF (2263KB)(10)      

Understanding the relationship between microstructure features and mechanical properties is of great significance for the improvement and specific adjustment of steel properties. The relationship between mean grain size and yield strength is established by the well-known Hall-Petch equation. But due to the complexity of the grain configuration within materials, considering only the mean value is unlikely to give a complete representation of the mechanical behavior. The classical Taylor equation is often used to account for the effect of dislocation density, but not thoroughly tested in combination with grain size influence. In the present study, systematic heat treatment routes and cold rolling followed by annealing are designed for interstitial free (IF) steel to achieve ferritic microstructures that not only vary in mean grain size, but also in grain size distribution and in dislocation density, a combination that is rarely studied in the literature. Optical microscopy is applied to determine the grain size distribution. The dislocation density is determined through XRD measurements. The hardness is analyzed on its relation with the mean grain size, as well as with the grain size distribution and the dislocation density. With the help of the variable selection tool LASSO, it is shown that dislocation density, mean grain size and kurtosis of grain size distribution are the three features which most strongly affect hardness of IF steel.

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Towards understanding twinning behavior near fracture surface in magnesium
Hao Li, Qinghui Zeng, Pengfei Yang, Qi Sun, Jianmin Wang, Jian Tu, Minhao Zhu
J. Mater. Sci. Technol.    2020, 43 (0): 230-237.   doi:10.1016/j.jmst.2020.01.007
Abstract38)   HTML0)    PDF (2762KB)(11)      

Deformation twin is one of the most important strain accommodation mechanisms and ultimately influences the mechanical properties for magnesium and its alloys. Especially, {10$\bar{1}$1} twin is usually thought to be closely related to the fracture or fatigue process of magnesium alloys. In the present work, the characteristics of microstructure near fracture region of deformed magnesium alloy have been investigated by a combination of electron back-scatter diffraction (EBSD) and transmission electron microscope (TEM). It has found that a large of deformation twins occur near fraction region, including {10$\bar{1}$2} and {10$\bar{1}$1} primary twins, {10$\bar{1}$1}-{10$\bar{1}$2} double twin and {10$\bar{1}$1}-{10$\bar{1}$2}-{10$\bar{1}$1}-{10$\bar{1}$2} quadruple twin. The actual boundaries of {10$\bar{1}$1} twins at atomic scale consist of {10$\bar{1}$1} coherent twinning boundaries (TBs) and parallel basal-pyramidal (BPy/PyB) planes. The tip of {10$\bar{1}$1} twin can even end up with BPy/PyB interfaces only. The experimental observations also reveal that when two {10$\bar{1}$1} twin variants sharing a common [11$\bar{2}$0] zone axis approach each other, the growth of one twin is usually hindered by the boundaries of the other twin. In addition, an apparent “crossing” phenomenon is also discovered when interaction of two {10$\bar{1}$1} twins takes place. According to these experimental observations, the possible underlying mechanisms behind such phenomena are proposed and discussed. These finding are expected to provide an insight into understanding the twinning behavior and the relationship between twin and fracture in magnesium and other materials with hexagonal structure.

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A strategy for designing stable nanocrystalline alloys by thermo-kinetic synergy
H.R. Peng, B.S Liu, F. Liu
J. Mater. Sci. Technol.    2020, 43 (0): 21-31.   doi:10.1016/j.jmst.2019.11.006
Abstract38)   HTML1)    PDF (3752KB)(14)      

Aiming to design stable nanocrystalline (NC) materials, so far, it has been proposed to construct nanostructure stability maps in terms of thermodynamic parameters, while kinetic stabilization has seldom been considered, despite the synergy of thermodynamics and kinetics. Consequently, the thermodynamically stabilized NC materials may be easily subjected to grain growth at high temperatures due to the weakly kinetic stabilization. Starting from the thermo-kinetic synergy, a stabilization criterion is proposed as a function of intrinsic solute parameters (e.g. the activation energy for bulk diffusion and the segregation enthalpy), intrinsic solvent parameters (e.g. the intrinsic activation energy for GB migration and the GB energy) and processing parameters (e.g. the grain size, the temperature and the solute concentration). Using first-principles calculations for a series of combinations between fifty-one substitutional alloying atoms as solute atoms and Fe atom as fixed solvent atom, it is shown that the thermal stability neither simply increases with increasing the segregation enthalpy as expected by thermodynamic stabilization, nor monotonically increases with increasing the activation energy for bulk diffusion as described by kinetic stabilization. By combination of thermodynamic and kinetic contributions, the current stabilization criterion evaluates quantitatively the thermal stability, thus permitting convenient comparisons among NC materials involved by various combinations of the solute atoms, the solvent atoms, or the processing conditions. Validity of this thermo-kinetic stabilization criterion has been tested by current experiment results of Fe-Y alloy and previously published data of Fe-Ni, Fe-Cr, Fe-Zr and Fe-Ag alloys, etc., which opens a new window for designing NC materials with sufficiently high thermal stability and sufficiently small grain size.

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Evaluation of the inhibition behavior of carbon dots on carbon steel in HCl and NaCl solutions
Yuwei Ye, Zilong Jiang, Yangjun Zou, Hao Chen, Shengda Guo, Qiumin Yang, Liyong Chen
J. Mater. Sci. Technol.    2020, 43 (0): 144-153.   doi:10.1016/j.jmst.2020.01.025
Abstract37)   HTML0)    PDF (5061KB)(11)      

An eco-friendly and effective corrosion inhibitor (N-CDs) was acquired by hydrothermal method in methacrylic acid and ethyl(methyl)amine precursors. Afterwards, the weight loss and electrochemistry measurement were chosen to appraise the corrosion inhibition behavior of as-prepared N-CDs for Q235 steel in Cl- contained solutions. The change rules of EIS and Tafel data displayed that the as-prepared N-CDs revealed a high-efficiency protection for steel in all test environments. Meanwhile, the inhibition efficiency of steel reached up to 93.93 % (1 M HCl) and 88.96 % (3.5 wt% NaCl) at 200 mg/L of N-CDs. Furthermore, the N-CDs could form the adsorption film on steel surface to avoid the strong attack of Cl-. By analysis, the adsorption mechanism of as-prepared N-CDs on steel surface was physicochemical interaction, which strictly complied with the Langmuir adsorption model in both solutions.

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Microstructure and mechanical properties of (TiZrNbTaMo)C high-entropy ceramic
Kai Wang, Lei Chen, Chenguang Xu, Wen Zhang, Zhanguo Liu, Yujin Wang, Jiahu Ouyang, Xinghong Zhang, Yudong Fu, Yu Zhou
J. Mater. Sci. Technol.    2020, 39 (0): 99-105.   doi:10.1016/j.jmst.2019.07.056
Abstract36)   HTML0)    PDF (3451KB)(82)      

A high-entropy (TiZrNbTaMo)C ceramic has been successfully fabricated by hot pressing the newly-synthesized quinary carbide powder to investigate its microstructure and mechanical properties. The carbothermal reduction process of equimolar quinary metallic oxides at 1500 ℃ for 1 h generates a carbide powder mixture, which consists mainly of TaC- and ZrC-based solid solutions. The as-synthesized powder was then sintered to form a single-phase high-entropy ceramic by a two-step hot pressing at 1850 ℃ for 1 h and 2100 ℃ for 0.5 h, respectively. The high-entropy ceramic exhibits a fine grain size of about 8.8 μm, a high compositional uniformity and a high relative density of 98.6% by adding Mo as the strategic main component. The measured nanohardness values of (TiZrNbTaMo)C ceramic are 25.3 GPa at 9.8 N and 31.3 GPa at 100 mN, respectively, which are clearly higher than those of other available high-entropy carbide ceramics.

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Microstructure evolutions and interfacial bonding behavior of Ni-based superalloys during solid state plastic deformation bonding
Jian Yang Zhang, Bin Xu, Naeemul Haq Tariq, MingYue Sun, DianZhong Li, Yi Yi Li
J. Mater. Sci. Technol.    2020, 46 (0): 1-11.   doi:10.1016/j.jmst.2019.11.015
Abstract36)   HTML3)    PDF (4996KB)(18)      

As an advanced solid state bonding process, plastic deformation bonding (PDB) is a highly reliable metallurgical joining method that produces significant plastic deformation at the bonding interface of welded joints through thermo-mechanical coupling. In this study, PDB behavior of IN718 superalloy was systematically investigated by performing a series of isothermal compression tests at various processing conditions. It was revealed that new grains evolved in the bonding area through discontinuous dynamic recrystallization (DDRX) at 1000-1150 °C. Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) results revealed that the bonding of joints is related with interfacial grain boundary (IGB) bulging process, which is considered as a nucleation process of DRXed grain under different deformation environments. During recrystallization process, the bonded interface moved due to strain-induced boundary migration (SIBM) process. Stored energy difference (caused by accumulation of dislocations at the bonding interface) was the dominant factor for SIBM during DRX. The mechanical properties of the bonded joints were dependent upon the recrystallized microstructure and SIBM ensued during PDB.

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Mechanical properties and corrosion behavior of selective laser melted 316L stainless steel after different heat treatment processes
Decheng Kong, Chaofang Dong, Xiaoqing Ni, Liang Zhang, Jizheng Yao, Cheng Man, Xuequn Cheng, Kui Xiao, Xiaogang Li
J. Mater. Sci. Technol.    2019, 35 (7): 1499-1507.   doi:10.1016/j.jmst.2019.03.003
Abstract35)   HTML5)    PDF (6531KB)(28)      

Irregular grains, high interfacial stresses and anisotropic properties widely exist in 3D-printed metallic materials, and this paper investigated the effects of heat treatment on the microstructural, mechanical and corrosion properties of 316 L stainless steel fabricated by selective laser melting. Sub-grains and low-angle boundaries exist in the as-received selective laser melted (SLMed) 316 L stainless steel. After heat treatment at 1050 °C, the sub-grains and low-angle boundaries changed slightly, and the stress state and strength decreased to some extent due to the decrease of dislocation density. After heat treatment at 1200 °C, the grains became uniform, and the dislocation cells vanished, which led to a sharp decline in the hardness and strength. However, the ductility was improved after recrystallization heat treatment. The passive film thickness and corrosion potential of the SLMed 316 L stainless steel decreased after heat treatment, and the pitting potential also decreased due to the accelerated transition from metastable to steady-state pitting; this accelerated transition was caused by the presence of weak passive films at the enlarged pores after heat treatment, especially for an adequate solid solution treatment.

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Microstructure and properties of Ti-6Al-4V fabricated by low-power pulsed laser directed energy deposition
Hua Tan, Mengle Guo, Adam T. Clare, Xin Lin, Jing Chen, Weidong Huang
J. Mater. Sci. Technol.    2019, 35 (9): 2027-2037.   doi:10.1016/j.jmst.2019.05.008
Abstract35)   HTML5)    PDF (6960KB)(55)      

Thin-wall structures of Ti-6Al-4V were fabricated by low-power pulsed laser directed energy deposition. During deposition, consistent with prior reports, columnar grains were observed which grew from the bottom toward the top of melt pool tail. This resulted in a microstructure mainly composed of long and thin prior epitaxial β columnar grains (average width ≈200 μm). A periodic pattern in epitaxial growth of grains was observed, which was shown to depend upon laser traverse direction. Utilizing this, a novel means was proposed to determine accurately the fusion boundary of each deposited layer by inspection of the periodic wave patterns. As a result it was applied to investigate the influence of thermal cycling on microstructure evolution. Results showed that acicular martensite, α' phase, and a small amount of Widmanst?tten, α laths, gradually converted to elongated acicular α and a large fraction of Widmanst?tten α laths under layer-wise thermal cycling. Tensile tests showed that the yield strength, ultimate tensile strength and elongation of Ti-6Al-4V thin wall in the build direction were 9.1%, 17.3% and 42% higher respectively than those typically observed in forged solids of the same alloy. It also showed the yield strength and ultimate tensile strength of the transverse tensile samples both were $\widetilde{1}$3.3% higher than those from the build direction due to the strengthening effect of a large number of vertical β grain boundaries, but the elongation was 69.7% lower than that of the build direction due to the uneven grain deformation of β grains.

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Tensile ductility and deformation mechanisms of a nanotwinned 316L austenitic stainless steel
Y.Z. Zhang, J.J. Wang, N.R. Tao
J. Mater. Sci. Technol.    2020, 36 (0): 65-69.   doi:10.1016/j.jmst.2019.02.008
Abstract34)   HTML0)    PDF (1627KB)(15)      

A nanotwinned 316L austenitic stainless steel was prepared by means of surface mechanical grinding treatment. After recovery annealing, the density of dislocations decreases obviously while the average twin/matrix lamella thickness still keeps in the nanometer scale. The annealed nanotwinned sample exhibits a high tensile yield strength of 771 MPa and a considerate uniform elongation of 8%. TEM observations showed that accommodating more dislocations and secondary twinning inside the nanotwins contribute to the enhanced ductility and work hardening rate of the annealed nanotwinned sample.

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High-performance single-wall carbon nanotube transparent conductive films
Song Jiang, Peng-Xiang Hou, Chang Liu, Hui-Ming Cheng
J. Mater. Sci. Technol.    2019, 35 (11): 2447-2462.   doi:10.1016/j.jmst.2019.07.011
Abstract34)   HTML3)    PDF (6524KB)(22)      

A single-wall carbon nanotube (SWCNT) has superior optical, electrical, and mechanical properties due to its unique structure and is therefore expected to be able to form flexible high-performance transparent conductive films (TCFs). However, the optoelectronic performance of these films needs to be improved to meet the requirements of many devices. The electrical resistivity of SWCNT TCFs is mainly determined by the intrinsic resistivity of individual SWCNTs and their junction resistance in networks. We analyze these key factors and focus on the optimization of SWCNTs and their networks, which include the diameter, length, crystallinity and electrical type of the SWCNTs, and the bundle size and interconnects in networks, as well as chemical doping and microgrid design. We conclude that isolated/small-bundle, heavily doped metallic or semiconducting SWCNTs with a large diameter, long length and high crystallinity are necessary to fabricate high-performance SWCNT TCFs. A simple, controllable way to construct macroscopic SWCNT networks with Y-type connections, welded junctions or microgrid design is important in achieving a low resistivity. Finally, some insights into the key challenges in the manufacture and use of SWCNT TCFs and their prospects are presented, hoping to shed light on promoting the practical application of SWCNT TCFs in future flexible and stretchable optoelectronics.

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Nitrogen-doped graphite encapsulated Fe/Fe3C nanoparticles and carbon black for enhanced performance towards oxygen reduction
Zhu Jie, Xiong Zewei, Zheng Jiming, Luo Zhihong, Zhu Guangbin, Xiao Chao, Meng Zhengbing, Li Yibing, KunLuo
J. Mater. Sci. Technol.    2019, 35 (11): 2543-2551.   doi:10.1016/j.jmst.2019.07.008
Abstract34)   HTML8)    PDF (3279KB)(22)      

Non-noble metal (NNM) catalysts have recently attracted intensive interest for their high catalytic performance towards oxygen reduction reaction (ORR) at low cost. Herein, a novel NNM catalyst was synthesized by the simple pyrolysis of carbon black, urea and a Fe-containing precursor, which exhibits excellent ORR catalytic activity, superior durability and methanol tolerance versus the Pt/C catalyst in both alkaline and acidic solutions. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) characterizations demonstrate that the product is a nitrogen-doped hybrid of graphite encapsulated Fe/Fe3C nanoparticles and carbon black. X-ray photoelectron spectrum (XPS) and electrochemical analyses indicate that the catalytic performance and chemical stability correlate closely with a nitrogen-rich layer on the Fe/Fe3C nanoparticle after pyrolysis with presence of urea, leading to the same four-electron pathway towards ORR as the Pt/C catalyst. The hybrid is prospective to be an efficient ORR electrocatalyst for direct methanol fuel cells with high catalytic performance at low cost.

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Theoretical prediction on thermal and mechanical properties of high entropy (Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)C by deep learning potential
Fu-Zhi Dai, Bo Wen, Yinjie Sun, Huimin Xiang, Yanchun Zhou
J. Mater. Sci. Technol.    2020, 43 (0): 168-174.   doi:10.1016/j.jmst.2020.01.005
Abstract34)   HTML0)    PDF (1983KB)(16)      

High entropy materials (HEMs, e.g. high entropy alloys, high entropy ceramics) have gained increasing interests due to the possibility that they can provide challenge properties unattainable by traditional materials. Though a large number of HEMs have emerged, there is still in lack of theoretical predictions and simulations on HEMs, which is probably caused by the chemical complexity of HEMs. In this work, we demonstrate that the machine learning potentials developed in recent years can overcome the complexity of HEMs, and serve as powerful theoretical tools to simulate HEMs. A deep learning potential (DLP) for high entropy (Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)C is fitted with the prediction error in energy and force being 9.4 meV/atom and 217 meV/Å, respectively. The reliability and generality of the DLP are affirmed, since it can accurately predict lattice parameters and elastic constants of mono-phase carbides TMC (TM = Ti, Zr, Hf, Nb and Ta). Lattice constants (increase from 4.5707 Å to 4.6727 Å), thermal expansion coefficients (increase from 7.85×10-6 K-1 to 10.58×10-6 K-1), phonon thermal conductivities (decrease from 2.02 W·m-1·K-1 to 0.95 W·m-1·K-1), and elastic properties of high entropy (Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)C in temperature ranging from 0 °C to 2400 °C are predicted by molecular dynamics simulations. The predicted room temperature properties agree well with experimental measurements, indicating the high accuracy of the DLP. With introducing of machine learning potentials, many problems that are intractable by traditional methods can be handled now. It is hopeful that deep insight into HEMs can be obtained in the future by such powerful methods.

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Additive manufacturing of high-strength CrMnFeCoNi high-entropy alloys-based composites with WC addition
Jinfeng Li, Shuo Xiang, Hengwei Luan, Abdukadir Amar, Xue Liu, Siyuan Lu, Yangyang Zeng, Guomin Le, Xiaoying Wang, Fengsheng Qu, Chunli Jiang, Guannan Yang
J. Mater. Sci. Technol.    2019, 35 (11): 2430-2434.   doi:10.1016/j.jmst.2019.05.062
Abstract33)   HTML18)    PDF (2482KB)(47)      

Laser melting deposition with WC addition has been developed to fabricate high-strength CrMnFeCoNi-based high-entropy alloys-based composites. By this technique, a microstructure of compact refined equiaxed grains can be achieved, and the tensile strength can be remarkably improved. The sample with 5 wt% WC addition shows a promising mechanical performance with a tensile strength of 800 MPa and an elongation of 37%. The improvement in mechanical property may be attributed to the formation of Cr23C6 reinforcement precipitates, which could promote the heterogeneous nucleation of grains and hinder the propagation of slip bands.

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High strength and ductility Mg-8Gd-3Y-0.5Zr alloy with bimodal structure and nano-precipitates
Xiaoxiao Wei, Li Jin, Fenghua Wang, Jing Li, Nan Ye, Zhenyan Zhang, Jie Dong
J. Mater. Sci. Technol.    2020, 44 (0): 19-23.   doi:10.1016/j.jmst.2019.10.024
Abstract33)   HTML4)    PDF (2424KB)(13)      

To resolve the strength-ductility trade-off problem for high-strength Mg alloys, we prepared a high performance Mg-8Gd-3Y-0.5 Zr (wt%) alloy with yield strength of 371 MPa, ultimate tensile strength of 419 MPa and elongation of 15.8%. The processing route involves extrusion, pre-deformation and aging, which leads to a bimodal structure and nano-precipitates. Back-stress originated from the deformation-incompatibility in the bimodal-structure alloy can improve ductility. In addition, dislocation density in coarse grains increased during the pre-deformation strain of 2%, and the dislocations in coarse grains can promote the formation of chain-like nano-precipitates during aging treatment. The chain-like nano-precipitates can act as barriers for dislocations slip and the existing mobile dislocations enable good ductility.

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Temperature-gradient induced microstructure evolution in heat-affected zone of electron beam welded Ti-6Al-4V titanium alloy
Shilin Zhang, Yingjie Ma, Sensen Huang, Sabry S. Youssef, Min Qi, Hao Wang, Jianke Qiu, Jiafeng Lei, Rui Yang
J. Mater. Sci. Technol.    2019, 35 (8): 1681-1690.   doi:10.1016/j.jmst.2019.04.004
Abstract33)   HTML2)    PDF (6735KB)(34)      

The heat-affected zone (HAZ) of electron beam welded (EBW) joint normally undergoes a unique heat-treating process consisting of rapid temperature rising and dropping stages, resulting in temperature-gradient in HAZ as a function of the distance to fusion zone (FZ). In the current work, microstructure, elements distribution and crystallographic orientation of three parts (near base material (BM) zone, mid-HAZ and near-FZ) in the HAZ of Ti-6Al-4V alloy were systematically investigated. The microstructure observation revealed that the microstructural variation from near-BM to near-FZ included the reduction of primary α (αp) grains, the increase of transformed β structure (βt) and the formation of various α structures. The rim-α, dendritic α and abnormal secondary α (αs) colonies formed in the mid-HAZ, while the “ghost” structures grew in the near-FZ respectively. The electron probe microanalyzer (EPMA) and electron back-scattered diffraction (EBSD) technologies were employed to evaluate the elements diffusion and texture evolution during the unique thermal process of welding. The formation of the various α structures in the HAZ were discussed based on the EPMA and EBSD results. Finally, the nanoindentation hardness of “ghost” structures was presented and compared with nearby βt regions.

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Development of gradient microstructure in the lattice structure of AlSi10Mg alloy fabricated by selective laser melting
Mulin Liu, Naoki Takata, Asuka Suzuki, Makoto Kobashi
J. Mater. Sci. Technol.    2020, 36 (0): 106-117.   doi:10.1016/j.jmst.2019.06.015
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To identify the microstructural features of the lattice structures of Al alloys built via the selective laser melting (SLM) process, AlSi10Mg alloy with a body-centered cubic (BCC)-type lattice structure was prepared. Characteristic microstructures comprising melt pools with several columnar α-Al phases with <001 > orientations along the elongation direction and surrounded by eutectic Si particles were observed at all portions of the built lattice structure. In the node portions of the lattice structure, a gradient microstructure (continuous change in microstructure) was observed. The columnar α-Al phases were observed near the top surface of the node portion, whereas they became coarser and more equiaxed near the bottom surface, resulting in softening localized near the bottom surface. In the strut portions of the lattice structure, the columnar α-Al phases were elongated along the inclined direction of struts. This trend was more prevalent near the bottom surface. The α-Al phases became coarser and more equiaxed near the bottom surface as well. The aforementioned results were the basis of a discussion of the development of the gradient microstructure in lattice-structured Al alloys during the SLM process in terms of thermal conductivities at the boundaries between the manufactured (locally melted and rapidly solidified) portions and adjacent (unmelted) alloy powder.

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Intrinsic two-way shape memory effect in a Ni-Mn-Sn metamagnetic shape memory microwire
Zhen Chen, Daoyong Cong, Yin Zhang, Xiaoming Sun, Runguang Li, Shaohui Li, Zhi Yang, Chao Song, Yuxian Cao, Yang Ren, Yandong Wang
J. Mater. Sci. Technol.    2020, 45 (0): 44-48.   doi:10.1016/j.jmst.2019.10.042
Abstract32)   HTML3)    PDF (2180KB)(19)      

An intrinsic two-way shape memory effect with a fully recoverable strain of 1.0 % was achieved in an as-prepared Ni50Mn37.5Sn12.5 metamagnetic shape memory microwire fabricated by Taylor-Ulitovsky method. This two-way shape memory effect is mainly owing to the internal stress caused by the retained martensite in austenite matrix, as revealed by transmission electron microscopy observations and high-energy X-ray diffraction experiments. After superelastic training for 30 loading/unloading cycles at room temperature, the amount of retained martensite increased and the recoverable strain of two-way shape memory effect increased significantly to 2.2 %. Furthermore, a giant recoverable strain of 11.2 % was attained under a bias stress of 300 MPa in the trained microwire. These properties confer this microwire great potential for micro-actuation applications.

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High entropy (Yb0.25Y0.25Lu0.25Er0.25)2SiO5 with strong anisotropy in thermal expansion
Heng Chen, Huimin Xiang, Fu-Zhi Dai, Jiachen Liu, Yanchun Zhou
J. Mater. Sci. Technol.    2020, 36 (0): 134-139.   doi:10.1016/j.jmst.2019.07.022
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A novel high entropy (HE) rare earth monosilicate (Yb0.25Y0.25Lu0.25Er0.25)2SiO5 was synthesized by solid-state reaction method. X-ray diffraction and scanning electron microscopy analysis indicate that a single solid solution is formed with homogeneous distribution of rare-earth elements. HE (Yb0.25Y0.25Lu0.25Er0.25)2SiO5 exhibits excellent phase stability and anisotropy in thermal expansion. The coefficients of thermal expansion (CTEs) in three crystallographic directions are: αa = (2.57 ± 0.07) ×10-6 K-1, αb = (8.07 ± 0.13) ×10-6 K-1, αc = (9.98 ± 0.10) ×10-6 K-1. The strong anisotropy in thermal expansion is favorable in minimizing the coating/substrate mismatch if preferred orientation of HE (Yb0.25Y0.25Lu0.25Er0.25)2SiO5 is controlled on either metal or ceramic substrate.

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Facile fabrication of core-shell Ni3Se2/Ni nanofoams composites for lithium ion battery anodes
Wang Zhongren, Gao Quanbin, Lv Peng, Li Xiuwan, Wang Xinghui, Qu Baihua
J. Mater. Sci. Technol.    2020, 38 (0): 119-124.   doi:10.1016/j.jmst.2019.08.021
Abstract31)   HTML0)    PDF (2131KB)(15)      

Due to the highly porous structure, large specific surface area, and 3D interconnected metal conductive network, nanoporous metal foams have attracted a lot of attention in the field of energy conversion and storage, especially lithium-ion batteries, which are ideal for current collectors. In this work, we develop a facile approach to fabricate core-shell Ni3Se2/Ni nanofoams composites. The Ni3Se2/Ni composites make full use of both the advantages of metal conductive network and core-shell structure, resulting in a high capacity and superior rate performance. In addition, the composites can be directly converted into electrode by a simple mechanical compression, which is more convenient than traditional casting method. What’s more, this material and its structure can be extended to other devices in the field of energy conversion and storage.

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Hydrophobic epoxy resin coating with ionic liquid conversion pretreatment on magnesium alloy for promoting corrosion resistance
Liting Guo, Changdong Gu, Jie Feng, Yongbin Guo, Yuan Jin, Jiangping Tu
J. Mater. Sci. Technol.    2020, 37 (0): 9-18.   doi:10.1016/j.jmst.2019.06.024
Abstract31)   HTML0)    PDF (5801KB)(14)      

A hydrophobic epoxy resin coating with an environmental-friendly deep eutectic solvent (DES)-based conversion pretreatment was proposed to enhance the corrosion resistance of magnesium alloys. The hydrophobic epoxy resin coatings on the AZ31B magnesium alloy with and without the DES-based conversion pretreatment were thoroughly compared. It is found that the DES-based conversion film on the AZ31B magnesium alloy is mainly composed of MgH2, MgO and MgCO3. Furthermore, the conversion film possesses porous structure, which provides more anchor points for the following epoxy resin coating. However, without the DES-conversion pretreatment, the epoxy resin is difficult to be attached on the substrate during the dip-coating process. The double layered hybrid coating system promotes the corrosion resistance of the magnesium alloys significantly, which can be ascribed to the unique architecture and component including the hydrophobicity of the surface layer, the dense and interlocked epoxy resin, and the corrosion resistant DES-based conversion pretreatment.

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A new physical simulation tool to predict the interface of dissimilar aluminum to steel welds performed by friction melt bonding
T. Sapanathan, N. Jimenez-Mena, I. Sabirov, M.A. Monclús, J.M. Molina-Aldareguía, P. Xia, L. Zhao, A. Simar
J. Mater. Sci. Technol.    2019, 35 (9): 2048-2057.   doi:10.1016/j.jmst.2019.05.004
Abstract31)   HTML3)    PDF (3069KB)(38)      

Optimization of the intermetallic layer thickness and the suppression of interfacial defects are key elements to improve the load bearing capacity of dissimilar joints. However, till date we do not have a systematic tool to investigate the dissimilar joints and the intermetallic properties produced by a welding condition. Friction Melt Bonding (FMB) is a recently developed technique for joining dissimilar metals that also does not exempt to these challenges. The FMB of DP980 and Al6061-T6 was investigated using a new physical simulation tool, based on Gleeble thermo-mechanical simulator, to understand the effect of individual parameter on the intermetallic formation. The proposed method demonstrates its capability in reproducing the intermetallic characteristics, including the thickness of intermetallic bonding layer, the morphology and texture of its constituents (Fe2Al5 and Fe4Al13), as well as their nanohardness and reduced modulus. The advantages of physical simulation tool can enable novel developing routes for the development of dissimilar metal joining processes and facilitate to reach the requiring load bearing capacity of the joints.

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