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 Microstructure and Fracture Behavior of 316L Austenitic Stainless Steel Produced by Selective Laser Melting
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 15 May 2020, Volume 45 Issue 0 Previous Issue
Research Article
 Select 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 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.
 Select 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 (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.
 Select 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 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.
 Select 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 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.
Letter
 Select 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 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.
Research Article
 Select Microstructure and composition evolution of a single-crystal superalloy caused by elements interdiffusion with an overlay NiCrAlY coating on oxidation Lanlan Yang, Minghui Chen, Jinlong Wang, Yanxin Qiao, Pingyi Guo, Shenglong Zhu, Fuhui Wang J. Mater. Sci. Technol.. 2020, 45 (0): 49-58.   DOI: 10.1016/j.jmst.2019.11.017 MCrAlY (M=Ni and/or Co) overlay coating is widely used as a protective coating against high temperature oxidation and corrosion. However, due to its big difference in chemical composition with the underlying superalloy, elements interdiffusion occurs inevitably. One of the direct results is the formation of interdiffusion zone (IDZ) and secondary reaction zone (SRZ) with a high density of fine topological closed-packed phases (TCPs), weakening dramatically the mechanical properties of the alloy substrate. It is by now the main problem of modern high-temperature metallic coatings, but there are still hardly any reports studying the formation, growth and transformation of IDZ and SRZ in deep, as well as the precipitation of TCPs. In this work, a typical NiCrAlY coating is deposited by arc ion plating on a single-crystal superalloy N5. Elements interdiffusion between them and its relationship on microstructure were clarified. Cr rather than Al from the coating diffuses into the alloy at high temperatures and segregates immediately beneath their interface, contributing largely to the formation of IDZ. Simultaneously, diffusion of Ni from the deep alloy to IDZ leads to the formation and continuous expansion of SRZ.
 Select Diffusion bonding of AlCoCrFeNi2.1 eutectic high entropy alloy to TiAl alloy Peng Li, Shuai Wang, Yueqing Xia, Xiaohu Hao, Honggang Dong J. Mater. Sci. Technol.. 2020, 45 (0): 59-69.   DOI: 10.1016/j.jmst.2019.10.041 High entropy alloys have special microstructure and remarkable properties. To explore their potential engineering application in high temperature structures, the microstructure evolution of bonding interface, the elemental diffusion behavior and mechanical property of the diffusion bonded joint between AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA) and TiAl alloy were investigated. Four reaction layers (rodlike B2 phase, Al(Co, Ni)2Ti, τ3-Al3NiTi2 + TiAl, τ3-Al3NiTi2 + TiAl + Ti3Al) formed in the diffusion zone near FCC phase of EHEA, but three layers (Al(Co, Ni)2Ti, τ3-Al3NiTi2 + TiAl, τ3-Al3NiTi2 + TiAl + Ti3Al) formed near B2 phase. Al and Ni controlled the reaction diffusion of EHEA and TiAl alloy, coarsened the acicular precipitated B2 phase and turned TiAl phase into Al(Co, Ni)2Ti and τ3-Al3NiTi2 phases. All these reaction layers grew in a parabolic manner as a function of bonding temperature. Rodlike B2 phase has the lowest growth activation energy of 125.2 kJ/mol, and the growth activation energy of τ3-Al3NiTi2 + TiAl layer near B2 phase is much lower than that near FCC phase. The penetration phenomenon and convex structure formed in the diffusion zone, which resulted in interlocking effect and enhanced the strength of resultant joints. The highest shear strength of 449 MPa was achieved at 950 °C. And the brittle fracture generally initiated at the interface between Al(Co, Ni)2Ti and τ3-Al3NiTi2 + TiAl layers.
 Select Multi-stimuli-triggered and self-repairable fluorocarbon organic coatings with urea-formaldehyde microcapsules filled with fluorosilane Paul C. Uzoma, Fuchun Liu, En-Hou Han J. Mater. Sci. Technol.. 2020, 45 (0): 70-83.   DOI: 10.1016/j.jmst.2019.11.022 For the purpose of overcoming the lack of durability problems associated with superhydrophobic surfaces which hitherto has limited their use; we prepared multi-stimuli wettability response coatings using a mixture of fluorocarbon resin and urea-formaldehyde microcapsules filled with fluorosilane via interfacial polymerization process. The microcapsules are of good thermal stability and can be triggered to release their core content on exposure to atmospheric conditions resulting in the increase in the water contact angle from 97° to 151°. The prepared coatings gave good adhesion strength, and also showed an increase in the hydrophobic property after undergoing scratch, solvent and UV accelerated aging test. In addition, they offered good self-healing of the hydrophobic property after an initial loss due to alkaline immersion and oxygen plasma etching. The electrochemical measurements revealed a remarkable impedance recovery and suppression of corrosion activities, suggesting them to be a potential candidate material for corrosion protection.
 Select Direct electrospinned La2O3 nanowires decorated with metal particles: Novel 1 D adsorbents for rapid removal of dyes in wastewater Yan Xing, Jing Cheng, Jian Wu, Mengfei Zhang, Xing-ao Li, Wei Pan J. Mater. Sci. Technol.. 2020, 45 (0): 84-91.   DOI: 10.1016/j.jmst.2019.11.030 Inorganic nanocomposites have attracted continuous attention as low cost and eco-friendly adsorbents. However, low adsorption rate and poor cycle performance limit their further application. Herein, lanthanum oxide (LO) ceramic nanowires decorated with nickel nanoparticles (NiNPs) have been synthesized by a facile electrospinning method. The decomposition of polymer precursor left high pore volume in the resultant ceramic nanowires, giving the nanowires a high surface area. The NiNPs-LO nanowires can remove various dye contaminants with adsorption rate constant higher than most adsorbents. More importantly, the NiNPs-LO nanowires can be separated and reused conveniently with stable cyclic performance, avoiding not only energy consumption but also secondary pollution. The adsorption performance fits well with pseudo-second-order and Langmuir isothermal model, indicating a monolayer adsorption process. Combined with structural and FT-IR analysis, the excellent adsorption ability is mainly ascribed to the electron interaction between the composite interfaces and contaminates under the assistance of high surface area and porosity. This work provides an ideal recyclable adsorbent for the ultra-fast water purification.
 Select Prediction on temperature dependent elastic constants of “soft” metal Al by AIMD and QHA Haijun Zhang, Chenhui Li, Philippe Djemia, Rui Yang, Qingmiao Hu J. Mater. Sci. Technol.. 2020, 45 (0): 92-97.   DOI: 10.1016/j.jmst.2019.11.029 First-principles methods based on density functional theory (DFT) are nowadays routinely applied to calculate the elastic constants of materials at temperature of 0 K. Nevertheless, the first-principles calculations of elastic constants at finite temperature are not straightforward. In the present work, the feasibility of the ab initio molecular dynamic (AIMD) method in calculations of the temperature dependent elastic constants of relatively “soft” metals, taking face centered cubic (FCC) aluminum (Al) as example, is explored. The AIMD calculations are performed with carefully selected strain tensors and strain magnitude. In parallel with the AIMD calculations, first-principles calculations with the quasiharmonic approximation (QHA) are performed as well. We show that all three independent elastic constant components (C11, C12 and C44) of Al from both the AIMD and QHA calculations decrease with increasing temperature T, in good agreement with those from experimental measurements. Our work allows us to quantify the individual contributions of the volume expansion, lattice vibration (excluding those contributed to the volume expansion), and electronic temperature effects to the temperature induced variation of the elastic constants. For Al with stable FCC crystal structure, the volume expansion effect contributes the major part (about 75%~80%) in the temperature induced variation of the elastic constants. The contribution of the lattice vibration is minor (about 20%~25%) while the electronic temperature effect is negligible. Although the elastic constants soften with increasing temperature, FCC Al satisfies the Born elastic stability criteria with temperature up to the experimental melting point.
 Select In situ selective laser gas nitriding for composite TiN/Ti-6Al-4V fabrication via laser powder bed fusion P.A. Morton, H.C. Taylor, L.E. Murr, O.G. Delgado, C.A. Terrazas, R.B. Wicker J. Mater. Sci. Technol.. 2020, 45 (0): 98-107.   DOI: 10.1016/j.jmst.2019.11.009 Laser-assisted gas nitriding of selective Ti-6Al-4 V surfaces has been achieved during laser powder bed fusion fabrication by exchanging the argon build gas environment with nitrogen. Systematic variation of processing parameters allowed microdendritic TiN surface coatings to be formed having thicknesses ranging from a few tens of microns to several hundred microns, with TiN dendrite microstructure volume fractions ranging from 0.6 to 0.75; and corresponding Vickers microindentation hardness values ranging from ~ 7.5 GPa-9.5 GPa. Embedded TiN hard layers ranging from 50 μm to 150 μm thick were also fabricated in the laser-beam additively manufactured Ti-6Al-4 V alloy producing prototype, hybrid, planar composites having alternating, ductile Ti-6Al-4 V layers with a hardness of ~ 4.5 GPa and a stiff, TiN layer with a hardness of ~8.5 GPa. The results demonstrate prospects for fabricating novel, additively manufactured components having selective, hard, wear and corrosion resistant coatings along with periodic, planar or complex metal matrix composite regimes exhibiting superior toughness and related mechanical properties.
 Select Effects of oxide stringers on the β-phase depletion behaviour in thermally sprayed CoNiCrAlY coatings during isothermal oxidation H. Chen, A. Rushworth J. Mater. Sci. Technol.. 2020, 45 (0): 108-116.   DOI: 10.1016/j.jmst.2019.11.018 This paper details an investigation of the effects of oxide stringers on the β-phase depletion behaviour in thermally sprayed CoNiCrAlY coatings. Vacuum Plasma Sprayed (VPS) CoNiCrAlY coatings, which are free of oxide stringers, are used as the reference materials in comparison with High-Velocity Oxy-Fuel (HVOF) sprayed CoNiCrAlY coatings during isothermal oxidation at 1100 °C. An outer layer of spinel oxides and an inner layer of alumina are formed in the as-sprayed coatings, while only a single alumina scale is found in the heat-treated coatings. Less β-phase depletion occurred in the HVOF coatings than in the VPS coatings. It was found that the β phases tend to coalesce at the oxide stringers in the HVOF coatings, which is likely due to the internal oxide particles and stringers acting as short diffusion barriers to tie up the β phase and inhibit the β-phase depletion.
 Select Ultrahigh strength Mg-Y-Ni alloys obtained by regulating second phases S.Z. Wu, X.G. Qiao, M.Y. Zheng J. Mater. Sci. Technol.. 2020, 45 (0): 117-124.   DOI: 10.1016/j.jmst.2019.11.026 Mg-Y-Ni alloys with different second phases were designed by changing Y/Ni atomic ratio from 1.5 to 0.5. The microstructure and mechanical properties of as-cast and as-extruded alloys were investigated. The as-cast Mg-Y-Ni alloy with Y/Ni ratio of 1.5 is composed of α-Mg and long period stacking ordered (LPSO) phase. When Y/Ni ratio is equal to 1, nanoscale lamellar γ' phase and eutectic Mg2Ni phase are formed in addition to LPSO phase. As Y/Ni ratio decreases further, the amount of eutectic Mg2Ni phase increases, while the amount of LPSO phase decreases. After extrusion, the LPSO and γ' phases are distributed along the extrusion direction, while eutectic Mg2Ni phase is broken and dispersed in the as-extruded alloys. LPSO phase and Mg2Ni phase in the alloys promote dynamic recrystallization (DRX) during extrusion, while γ' phase inhibits DRX. Consequently, the Mg96Y2Ni2 (at.%) alloy with LPSO phase and γ' phase as the main second phases shows the strongest basal texture after extrusion. The tensile yield strength of the as-extruded Mg-Y-Ni alloys increases first and then decreases with decreasing Y/Ni ratio. The as-extruded Mg96Y2Ni2 (at.%) alloy with Y/Ni = 1 exhibits excellent mechanical properties with tensile yield strength of 465 MPa, ultimate tensile strength of 510 MPa and elongation to failure of 7.2%, which is attributed to the synergistic effect of bulk LPSO phase and nanoscale γ' phase.
 Select Effect of TiN diffusion barrier on elements interdiffusion behavior of Ni/GH3535 system in LiF-NaF-KF molten salt at 700 ℃ Chengxu Wang, Wei Chen, Minghui Chen, Demin Chen, Ke Yang, Fuhui Wang J. Mater. Sci. Technol.. 2020, 45 (0): 125-132.   DOI: 10.1016/j.jmst.2019.11.023 A TiN interlayer with high electrical conductivity was prepared between the GH3535 alloy and the Ni coating as a diffusion barrier to elements interdiffusion with the goal of increasing the corrosion resistance of GH3535 alloy in molten FLiNaK salt at 700 ℃. Results indicated that Ni coating could be directly electroplated on the TiN coated GH3535 alloy without extra conductive transition layer. TiN layer showed excellent thermal and chemical stabilities at elevated temperature in this molten salt system, without phase decomposition. The Ni/TiN composite coating was stable enough to resist corrosion in LiF-NaF-KF molten salt at 700 ℃. Elements interdiffusion between the substrate and Ni coating could be effectively inhibited and the corrosion resistance of the alloy was greatly enhanced. Besides, the TiN interlayer remained continuous and well adhered to the Ni coating as well as the substrate after corrosion test.
 Select Analysis of abnormal texture formation and strengthening mechanism in an extruded Mg-Gd-Y-Zn-Zr alloy Xueze Jin, Wenchen Xu, Zhongze Yang, Can Yuan, Debin Shan, Bugang Teng, Bo Cheng Jin J. Mater. Sci. Technol.. 2020, 45 (0): 133-145.   DOI: 10.1016/j.jmst.2019.11.021 A Mg-Gd-Y-Zn-Zr magnesium alloy with different initial states was extruded under different extrusion parameters. The effect of solution treatment and extrusion parameters on the microstructure, texture and mechanical properties were analyzed in detail, and the abnormal texture formation and strengthening mechanism was revealed. When extruded at low temperature and small extrusion ratio, the bimodal microstructure consisting of fine dynamically recrystallized grains and coarse deformed grains occurred both in the as-cast alloy and solution-treated alloy. When the extrusion temperature and extrusion ratio were increased, the amount and size of dynamically recrystallized grains increased and the grain size of the solution-treated alloy showed higher growth rate. Furthermore, an abnormal texture with <0001> parallel with extrusion direction developed due to the occurrence of non-basal slip and continuous dynamic recrystallization. This could be enhanced by solution treatment, high temperature, and large extrusion ratio. Both the as-cast alloy and solution-treated alloy exhibited the highest tensile strength after extrusion at 300 °C with an extrusion ratio of 9. Grain refinement was the main strengthening mechanism utilized in both the as-cast alloy and the solution-treated alloy. Work hardening played an important role in the sample extruded at low temperature and small extrusion ratio, with the highest contribution of about 33 MPa after extrusion at 300 °C with an extrusion ratio of 9. Texture strengthening contributed more in the sample extruded at high temperature and large extrusion ratio, but no more than 24.1 MPa. Solution strengthening was another strengthening mechanism in the extruded as-cast alloy, especially at high temperature and large extrusion ratio (no more than 9 MPa).
 Select Microstructural features of biomedical cobalt-chromium-molybdenum (CoCrMo) alloy from powder bed fusion to aging heat treatment Haoqing Li, Ming Wang, Dianjun Lou, Weilong Xia, Xiaoying Fang J. Mater. Sci. Technol.. 2020, 45 (0): 146-156.   DOI: 10.1016/j.jmst.2019.11.031 The design freedom of powder bed fusion process selective laser melting (SLM) enables flexibility to manufacture customized, geometrically complex medical implants directly from the CAD models. Co-based alloys have adequate wear and corrosion resistance, fatigue strength, and biocompatibility, which enables the alloys to be widely used in medical devices. This work aims to investigate the evolution of microstructures and their influence on tribological property of CoCrMo alloy processed by SLM and aging heat treatment. The results showed that very weak <110> texture along the building direction and microsegregation along cellular boundaries were produced. The presence of high residual stress and fine cellular dendrite structure has a pronounced hardening effect on the as-SLM and aging-treated alloys at moderate temperatures. Furthermore, the hexagonal ε phase transformed from the γ matrix during SLM became significant after subsequent aging at moderate temperatures, which further increased the nanohardness and scratch resistance. High temperature (1150 °C) heating caused homogenized recrystallization microstructure free of residual stress and ε phase, which sharply decreased the hardness and scratch resistance. The material parallel to the building direction exhibited improved tribological property in both SLMed and aging-treated alloy than that of the material perpendicular to the building direction. The anisotropy in frictional performance may be considered when designing CoCrMo dental implants using laser additive manufacturing.
 Select Ultrasonic excitation induced nanocrystallization and toughening of Zr46.75Cu46.75Al6.5 bulk metallic glass W. Zhai, L.H. Nie, X.D. Hui, Y. Xiao, T. Wang, B. Wei J. Mater. Sci. Technol.. 2020, 45 (0): 157-161.   DOI: 10.1016/j.jmst.2019.10.035 Intensive power ultrasound is introduced to Zr46.75Cu46.75Al6.5 bulk metallic glass (BMG) as an easy-procurable, non-destructive physical method to modulate its atomic rearrangement and shear deformation behavior. The microstructure after ultrasonic excitation with amplitude about 15 μm in 20 kHz for 2 h is characterized by large amount of Cu10Zr7 nanocrystals with size of 20-50 nm embedded in the glass matrix. This leads to a sharp increase in the critical stress for the first pop-in event of shear banding, and thus simultaneously improves both compressive plasticity and yield strength. Our findings provide a novel approach for overcoming the strength-ductility trade-off dilemma.
 Select Galvanic and asymmetry effects on the local electrochemical behavior of the 2098-T351 alloy welded by friction stir welding Mariana X. Milagre, Uyime Donatus, Naga V. Mogili, Rejane Maria P. Silva, Bárbara Victória G. de Viveiros, Victor F. Pereira, Renato A. Antunes, Caruline S.C. Machado, João Victor S. Araujo, Isolda Costa J. Mater. Sci. Technol.. 2020, 45 (0): 162-175.   DOI: 10.1016/j.jmst.2019.11.016 Scanning electrochemical microscopy (SECM) and scanning vibrating electrode technique (SVET) were used to investigate the electrochemical behaviour of the top surface of the 2098-T351 alloy welded by friction stir welding (FSW). The SVET technique was efficient in identifying the cathodic and anodic weld regions. The welding joint (WJ), which comprises the thermomechanically affected zone (TMAZ) and the stir zone (SZ), was cathodic relative to the heated affected zone (HAZ) and the base metal (BM). The reactivities of the welding joint at the advancing side (AS) and the retreating side (RS) were analyzed and compared using SECM technique in the competition mode by monitoring the dissolved oxygen as a redox mediator in 0.005 mol L-1 NaCl solution. The RS was more electrochemically active than the AS, and these results were correlated with the microstructural features of the welded alloy.
 Select Electroless-hydrothermal construction of nickel bridged nickel sulfide@mesoporous carbon nitride hybrids for highly efficient noble metal-free photocatalytic H2 production Lu Zhang, Yuanyuan Cui, Fengli Yang, Quan Zhang, Juhua Zhang, Mengting Cao, Wei-Lin Dai J. Mater. Sci. Technol.. 2020, 45 (0): 176-186.   DOI: 10.1016/j.jmst.2019.11.020 Metallic Ni bridged NiS@mesoporous carbon nitride hybrids were for the first time fabricated through a one-pot electroless-assisted hydrothermal method. The intimate Ni bridge between the interface of mesoporous carbon nitride and NiS was confirmed by HRTEM and in-depth XPS analysis using an Ar+-cluster sputtering gun and a possible mechanism was put forward to elucidate the formation process of the unique structure. Without adding any noble metals as cocatalysts, the optimized catalyst 10% NiS/m-CN-160-12 showed a H2 evolution rate of 1419 μmol·g-1·h-1, which is about 34 and 14 fold higher than that of bare mesoporous carbon nitride and NiS, respectively. The dramatically enhanced photocatalytic performance was mainly ascribed to the synergistic effect of NiS cocatalyst loading and the formation of metallic Ni between the interface of mesoporous carbon nitride and NiS, which served as a charge-transfer bridge to facilitate the transfer and separation of photo-induced electron-hole pairs.
 Select Ice-templated porous tungsten and tungsten carbide inspired by natural wood Yuan Zhang, Guoqi Tan, Da Jiao, Jian Zhang, Shaogang Wang, Feng Liu, Zengqian Liu, Longchao Zhuo, Zhefeng Zhang, Sylvain Deville, Robert O. Ritchie J. Mater. Sci. Technol.. 2020, 45 (0): 187-197.   DOI: 10.1016/j.jmst.2019.10.021 The structures of tungsten and tungsten carbide scaffolds play a key role in determining the properties of their infiltrated composites for multifunctional applications. However, it is challenging to construct and control the architectures by means of self-assembly in W/WC systems because of their large densities. Here we present the development of unidirectionally porous architectures, with high porosities exceeding 65 vol.%, for W and WC scaffolds which in many respects reproduce the design motif of natural wood using a direct ice-templating technique. This was achieved by adjusting the viscosities of suspensions to retard sedimentation during freezing. The processing, structural characteristics and mechanical properties of the resulting scaffolds were investigated with the correlations between them explored. Quantitative relationships were established to describe their strengths based on the mechanics of cellular solids by taking into account both inter- and intra-lamellar pores. The fracture mechanisms were also identified, especially in light of the porosity. This study extends the effectiveness of the ice-templating technique for systems with large densities or particle sizes. It further provides preforms for developing new nature-inspired multifunctional materials, as represented by W/WC-Cu composites.
 Select Impact of hydrogen microalloying on the mechanical behavior of Zr-bearing metallic glasses: A molecular dynamics study Binbin Wang, Liangshun Luo, Fuyu Dong, Liang Wang, Hongying Wang, Fuxin Wang, Lei Luo, Baoxian Su, Yanqing Su, Jingjie Guo, Hengzhi Fu J. Mater. Sci. Technol.. 2020, 45 (0): 198-206.   DOI: 10.1016/j.jmst.2019.11.027 Comparative studies on Zr35Cu65 and Zr65Cu35 amorphous systems were performed using molecular dynamic simulations to explore whether their hydrogenated mechanical behavior depends on the content of hydride-forming elements. Although both of them present an increased strength and ductility after hydrogen microalloying, we observe the improved mechanical behavior for Zr35Cu65 is more pronounced than that for Zr65Cu35. In these two samples, the distribution of configurational potential energy and flexibility volume respectively follows a similar H-induced variation tendency; all of the hydrogenated alloys not just have more stable atoms with smaller flexibility volume, but possess a larger fraction of readily activated atoms. However, the atomic-scale details, based on the local “gradient atomic packing structure” model, indicate minor additions of hydrogen can promote more “soft spots” along with more strengthened “backbones” in the low-Zr alloy than that in the high-Zr sample, which endows the former with much higher strength and deformability after hydrogen microalloying. We regard this finding as a further step forward to distilling the tell-tale metrics of the H-dependent mechanical behavior observed in Zr-based metallic glasses.
 Select Microstructure and dislocation structure evolution during creep life of Ni-based single crystal superalloys Hao Yu, Wei Xu, Sybrand van der Zwaag J. Mater. Sci. Technol.. 2020, 45 (0): 207-214.   DOI: 10.1016/j.jmst.2019.11.028 The high performance of Ni single crystal superalloys during high temperature low stress creep service, is intrinsically determined by the combined effects of microstructural evolution and the dislocation behaviour. In the field of the evolution of dislocation network, two main recovery mechanism based on dislocation migration dominate the process. One is superdislocations shearing into γ’ rafts through a two-superpartials-assisted approach. Another is the compact dislocations migrating along γ/γ′ interface. These two mechanisms are similarly climb-rate-controlled process. In this work, a model for the minimum creep rate based on thermodynamic and kinetic calculations and using an existing detailed dislocation dynamics model has been built by taking the dislocation migration behaviours as well as the rafted microstructure into consideration, which can well reproduce the ([100] tensile) creep properties of existing Ni superalloy grades, without the need to make the dislocation parameter values composition dependent.
 Select Multiphase-field modelling of concurrent grain growth and coarsening in complex multicomponent systems P.G. Kubendran Amos, Ramanathan Perumal, Michael Selzer, Britta Nestler J. Mater. Sci. Technol.. 2020, 45 (0): 215-229.   DOI: 10.1016/j.jmst.2019.11.011 Phase-field modelling of microstructural evolution in polycrystalline systems with phase-associated grains has largely been confined to continuum-field models. In this study, a multiphase-field approach, with a provision for introducing grain boundary and interphase diffusion, is extended to analyse concurrent grain growth and coarsening in multicomponent polycrystalline microstructures with chemically-distinct grains. The effect of the number of phases and components on the kinetics of evolution is investigated by considering binary and ternary systems of duplex and triplex microstructures, along with a single phase system. It is realised that the mere increase in the number of phases minimises the rate of concurrent grain growth and coarsening. However, the effect of components is substantially dependent on the respective kinetic coefficients. This work unravels that the disparity in the influence of phases and components is primarily due to the corresponding change introduced in the transformation mechanism. While the raise in number of phases convolutes the diffusion paths, the increase in number of component effects the rate of evolution through the interdiffusion, which introduces interdependency in the diffusing chemical-species. Additionally, the role of phase-fractions on the transformation rate of triplex microstructure is studied, and Correspondingly, the interplay of interface- and diffusion-governed evolution is elucidated. A representative evolution of three-dimensional triplex microstructure with equal phase-fraction is comparatively analysed with the evolution of corresponding two-dimensional setup.
 Select Precipitation on stacking faults in Mg-9.8wt%Sn alloy C.Q. Liu, C. He, H.W. Chen, J.F. Nie J. Mater. Sci. Technol.. 2020, 45 (0): 230-240.   DOI: 10.1016/j.jmst.2019.11.024 In this work, we have systematically investigated precipitation of β′-Mg3Sn phase on intrinsic stacking faults I1 and I2 in a Mg-9.8 wt%Sn alloy using aberration-corrected scanning transmission electron microscopy. All observed I1 faults are generated by the dissociation of c + a perfect dislocations and bounded by Frank partial dislocations having a Shockley component. Precipitation of β′ on I1 involves a shear of 1/3 < 01 $\bar{1}$ 0>α, similar to its formation directly from the α-Mg matrix. The β′ phase often nucleates at one end of an I1 fault due to the interaction between shear strain fields of β′ and the Shockley component of the Frank partial at that end, and subsequently grows towards the other end of the fault. When the β′ reaches to the other end, the Shockley partial bounding the lengthening end of the β′ reacts with the Frank partial bounding the fault, generating an a perfect dislocation that can glide away from the precipitate and the fault. The observed I2 faults are generated by the dissociation of a perfect dislocations and bounded by Shockley partials. Precipitation of β′ on I2 does not need a shear of 1/3 < 01 $\bar{1}$ 0>α, since the pre-existing I2 fault already provides an ABCA four-layer structure of β′. Thickening of the β′ that has already formed on the I2 involves the successive occurrence of three crystallographically equivalent shears of 1/3 < 01 $\bar{1}$ 0>α on every second (0002)α plane of the α-Mg matrix. Although this thickening mechanism is similar to that of the β′ formed directly from the α-Mg matrix, an a perfect dislocation will be produced when the β′ is thickened to eight layers, and it can again glide away from the precipitate and the fault.
 Select Facilely prepared layer-by-layer graphene membrane-based pressure sensor with high sensitivity and stability for smart wearable devices Tao Liu, Caizhen Zhu, Wei Wu, Kai-Ning Liao, Xianjing Gong, Qijun Sun, Robert K.Y. Li J. Mater. Sci. Technol.. 2020, 45 (0): 241-247.   DOI: 10.1016/j.jmst.2019.11.014 With the prosperous development of artificial intelligence, medical diagnosis and electronic skins, wearable electronic devices have drawn much attention in our daily life. Flexible pressure sensors based on carbon materials with ultrahigh sensitivity, especially in a large pressure range regime are highly required in wearable applications. In this work, graphene membrane with a layer-by-layer structure has been successfully fabricated via a facile self-assembly and air-drying (SAAD) method. In the SAAD process, air-drying the self-assembled graphene hydrogels contributes to the uniform and compact layer structure in the obtained membranes. Owing to the excellent mechanical and electrical properties of graphene, the pressure sensor constructed by several layers of membranes exhibits high sensitivity (52.36 kPa-1) and repeatability (short response and recovery time) in the loading pressure range of 0-50 kPa. Compared with most reported graphene-related pressure sensors, our device shows better sensitivity and wider applied pressure range. What’s more, we demonstrate it shows desired results in wearable applications for pulse monitoring, breathing detection as well as different intense motion recording such as walk, run and squat. It’s hoped that the facilely prepared layer-by-layer graphene membrane-based pressure sensors will have more potential to be used for smart wearable devices in the future.
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