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 20 June 2019, Volume 35 Issue 6 Previous Issue    Next Issue
 Select Superb cryogenic strength of equiatomic CrCoNi derived from gradient hierarchical microstructure Bin Gan, Jeffrey M. Wheeler, Zhongnan Bi, Lin Liu, Jun Zhang, Hengzhi Fu J. Mater. Sci. Technol., 2019, 35 (6): 957-961.  DOI: 10.1016/j.jmst.2018.12.002 This work demonstrates the effectiveness of a cryogenic torsional pre-straining for significantly improving the cryogenic strength of an equiatomic CrCoNi alloy. The origin of this phenomenon is elucidated by various microstructural characterization tools, which shows that the sequential torsion and tension tests lead to the observed hierarchical microstructure through the activation of different twinning systems and stacking faults. This gives rise to the significant increase in the yield strength from 600 MPa to 1215 MPa, while the fracture strain changes from 68% to 48%. The current study reveals that the incorporation of nanotwins architecture by shear deformation may constitute a viable strategy to tune the mechanical performance and, in particular, to dramatically increase the strength while keeping a good ductility.
 Select Enhancement of strength and electrical conductivity for a dilute Al-Sc-Zr alloy via heat treatments and cold drawing Li Liu, Jian-Tang Jiang, Bo Zhang, Wen-Zhu Shao, Liang Zhen J. Mater. Sci. Technol., 2019, 35 (6): 962-971.  DOI: 10.1016/j.jmst.2018.12.023 Developing heat-resistant conductors with high strength and high electrical conductivity is a key issue in the electrical conductor industries, as the ever-increasing power transmission poses higher requirement on the thermal stability of electrical conductor wires. Dilute Al-Sc-Zr alloys are considered as promising candidates due to the excellent heat resistance and high electrical conductivity, but the low strength always limits their application on electrical wires. Yet, few efforts on process design have been made in dilute Al-Sc-Zr alloys to enhance the strength. Here, various kinds of processing paths via combination of cold drawing, ageing and/or annealing were conducted to improve the strength and electrical conductivity of a dilute Al-Sc-Zr alloy. Results show that enhanced strength and electrical conductivity were obtained after cold drawing + ageing or pre-ageing + cold drawing + annealing treatments processes. Optimal properties (194 MPa in ultimate tensile strength and 61% IACS in electrical conductivity) were obtained through cold drawing followed by ageing. Microstructure evolution which affects strength and electrical conductivity was systematically investigated using TEM and 3DAP. The enhanced strength was mainly attributed to the suitable interactions between strain strengthening and precipitation strengthening. The enhancement in electrical conductivity was caused by precipitation of solute atoms and recovery of defects. These results provide foundations for the processing design of Al-Sc-Zr conducting wires with good properties and push forward their potential application in heat resistant conductor industries.
 Select Microstructural evolution of aluminum alloy during friction stir welding under different tool rotation rates and cooling conditions X.H. Zeng, P. Xue, L.H. Wu, D.R. Ni, B.L. Xiao, K.S. Wang, Z.Y. Ma J. Mater. Sci. Technol., 2019, 35 (6): 972-981.  DOI: 10.1016/j.jmst.2018.12.024 The microstructural evolution during friction stir welding (FSW) has long been studied only using one single welding parameter. Conclusions were usually made based on the final microstructure observation and hence were one-sided. In this study, we used the “take-action” technique to freeze the microstructure of an Al-Mg-Si alloy during FSW, and then systematically investigated the microstructures along the material flow path under different tool rotation rates and cooling conditions. A universal characteristic of the microstructural evolution including four stages was identified, i.e. dynamic recovery (DRV), dislocation multiplication, new grain formation and grain growth. However, the dynamic recrystallization (DRX) mechanisms in FSW depended on the welding condition. For the air cooling condition, the DRX mechanisms were related to continuous DRX associated with subgrain rotation and geometric DRX at high and low rotation rates, respectively. Under the water cooling condition, we found a new DRX mechanism associated with the progressive lattice rotation resulting from the pinning of the second-phase particles. Based on the analyses of the influencing factors of grain refinement, it was clearly demonstrated that the delay of DRV and DRX was the efficient method to refine the grains during FSW. Besides, ultra-high strain rate and a short duration at high temperatures were the key factors to produce an ultrafine-grained material.
 Select Main α relaxation and slow β relaxation processes in a La30Ce30Al15Co25 metallic glass J.C. Qiao, Y.H. Chen, R. Casalini, J.M. Pelletier, Y. Yao J. Mater. Sci. Technol., 2019, 35 (6): 982-986.  DOI: 10.1016/j.jmst.2018.12.003 Dynamic relaxation processes are fundamental to understand the mechanical and physical properties of metallic glasses. In the current work, mechanical relaxations in a La30Ce30Al15Co25 bulk metallic glass were probed by dynamic mechanical analysis. In contrast to many metallic glasses, La30Ce30Al15Co25 metallic glass shows a pronounced slow β relaxation peak. Physical aging below the glass transition temperature Tg leads to an increase of the apparent activation energy and a decrease of the slow β relaxation magnitude. The correlation between the slow β relaxation and the main α relaxation is discussed.
 Select Highly stable carbon-based perovskite solar cell with a record efficiency of over 18% via hole transport engineering Qian-Qian Chu, Bin Ding, Jun Peng, Heping Shen, Xiaolei Li, Yan Liu, Cheng-Xin Li, Chang-Jiu Li, Guan-Jun Yang, Thomas P. White, Kylie R. Catchpole J. Mater. Sci. Technol., 2019, 35 (6): 987-993.  DOI: 10.1016/j.jmst.2018.12.025 Carbon-based perovskite solar cells show great potential owing to their low-cost production and superior stability in air, compared to their counterparts using metal contacts. The photovoltaic performance of carbon-based PSCs, however, has been progressing slowly in spite of an impressive efficiency when they were first reported. One of the major obstacles is that the hole transport materials developed for state-of-the-art Au-based PSCs are not suitable for carbon-based PSCs. Here, we develop a low-temperature, solution-processed Poly(3-hexylthiophene-2,5-diyl) (P3HT)/graphene composite hole transport layer (HTL), that is compatible with paintable carbon-electrodes to produce state-of-the-art perovskite devices. Space-charge-limited-current measurements reveal that the as-prepared P3HT/graphene composite exhibits outstanding charge mobility and thermal tolerance, with hole mobility increasing from 8.3 × 10-3 cm2 V-1 s-1 (as-deposited) to 1.2 × 10-2 cm2 V-1 s-1 (after annealing at 100 °C) - two orders of magnitude larger than pure P3HT. The improved charge transport and extraction provided by the composite HTL provides a significant efficiency improvement compared to cells with a pure P3HT HTL. As a result, we report carbon-based solar cells with a record efficiency of 17.8% (certified by Newport); and the first perovskite cells to be certified under the stabilized testing protocol. The outstanding device stability is demonstrated by only 3% drop after storage in ambient conditions (humidity: ca. 50%) for 1680 h (non-encapsulated), and retention of ca. 89% of their original output under continuous 1-Sun illumination at room-temperature for 600 h (encapsulated) in a nitrogen environment.
 Select Surface modification with oxygen vacancy in Li-rich layered oxide Li1.2Mn0.54Ni0.13Co0.13O2 for lithium-ion batteries Bozhou Chen, Bangchuan Zhao, Jiafeng Zhou, Zhitang Fang, Yanan Huang, Xuebin Zhu, Yuping Sun J. Mater. Sci. Technol., 2019, 35 (6): 994-1002.  DOI: 10.1016/j.jmst.2018.12.021 A couple of layered Li-rich cathode materials Li1.2Mn0.54Ni0.13Co0.13O2 without any carbon modification are successfully synthesized by solvothermal and hydrothermal methods followed by a calcination process. The sample synthesized by the solvothermal method (S-NCM) possesses more homogenous microstructure, lower cation mixing degree and more oxygen vacancies on the surface, compared to the sample prepared by the hydrothermal method (H-NCM). The S-NCM sample exhibits much better cycling performance, higher discharge capacity and more excellent rate performance than H-NCM. At 0.2 C rate, the S-NCM sample delivers a much higher initial discharge capacity of 292.3 mAh g-1 and the capacity maintains 235 mAh g-1 after 150 cycles (80.4% retention), whereas the corresponding capacity values are only 269.2 and 108.5 mAh g-1 (40.3% retention) for the H-NCM sample. The S-NCM sample also shows the higher rate performance with discharge capacity of 118.3 mAh g-1 even at a high rate of 10 C, superior to that (46.5 mAh g-1) of the H-NCM sample. The superior electrochemical performance of the S-NCM sample can be ascribed to its well-ordered structure, much larger specific surface area and much more oxygen vacancies located on the surface.
 Select Deposition of FeCoNiCrMn high entropy alloy (HEA) coating via cold spraying Shuo Yin, Wenya Li, Bo Song, Xingchen Yan, Min Kuang, Yaxin Xu, Kui Wen, Rocco Lupoi J. Mater. Sci. Technol., 2019, 35 (6): 1003-1007.  DOI: 10.1016/j.jmst.2018.12.015 High entropy alloys (HEAs) are of great interest in the community of materials science and engineering due to their unique phase structure. They are constructed with five or more principal alloying elements in equimolar or near-equimolar ratio. Therefore, HEAs can derive their performance from multiple principal elements rather than a single element. In this work, solid-state cold spraying (CS) was applied for the first time to produce FeCoNiCrMn HEA coating. The experimental results confirm that CS can be used to produce a thick HEA coating with low porosity. As a low-temperature deposition process, CS completely retained the HEA phase structure in the coating without any phase transformation. The characterization also reveals that the grains in the CSed HEA coating had experienced significant refinement as compared to those in the as-received HEA powder due the occurrence of dynamic recrystallization at the highly deformed interparticle region. Due to the increased dislocation density and grain boundaries, CSed HEA coating was much harder than the as-received powder. The tribological study shows that the CSed FeCoNiCrMn HEA coating resulted in lower wear rate than laser cladded HEA coatings.
 Select Design of near-α Ti alloys via a cluster formula approach and their high-temperature oxidation resistance Beibei Jiang, Donghui Wen, Qing Wang, Jinda Che, Chuang Dong, Peter K. Liaw, Fen Xu, Lixian Sun J. Mater. Sci. Technol., 2019, 35 (6): 1008-1016.  DOI: 10.1016/j.jmst.2018.12.013 The multi-component composition characteristics of high-temperature near-α Ti alloys were investigated in the present work by means of a cluster formula approach. The uniform cluster formula [CN12 cluster](glue atom)3 for the hexagonal close-packed α solid solution was first obtained based on the Friedel oscillation theory, with a total atom number in the formula of Z = 16. Then it was analyzed that the Z values in the cluster composition formulas of typical near-α Ti alloys are within the range of Z = 16.00 $\widetilde{1}$6.30, being perfectly consistent with the ideal Z = 16. Based on it, a series of new alloys with Z = 16 and with Nb/Ta substitution for Mo in Ti1100 alloy were designed, suction-cast into φ 6 mm rods, and then heat-treated with solid solution and aging. It was found that the alloy with co-addition of Mo, Ta and Nb has a high strength and good ductility at both room and high temperatures. More importantly, the additions of Nb and Ta can contribute to the formation of continuous and compact Al2O3 scales, resulting in an obvious improvement of oxidation resistances at both 923 K and 1073 K. The effects of Mo, Ta and Nb on the oxidation behaviors of the designed alloys at 1073 K were further discussed.
 Select Effect of Cu addition on microstructures and tensile properties of high-pressure die-casting Al-5.5Mg-0.7Mn alloy Lingyang Yuan, Liming Peng, Jingyu Han, Baoliang Liu, Yujuan Wu, Juan Chen J. Mater. Sci. Technol., 2019, 35 (6): 1017-1026.  DOI: 10.1016/j.jmst.2018.11.024 In this study, Cu was added into the high-pressure die-casting Al-5.5Mg-0.7Mn (wt%) alloy to improve the tensile properties. The effects of Cu addition on the microstructures, mechanical properties of the Al-5.5Mg-0.7Mn alloys under both as-cast and T5 treatment conditions have been investigated. Additions of 0.5 wt%, 0.8 wt% and 1.5 wt% Cu can lead to the formation of irregular-shaped Al2CuMg particles distributed along the grain boundaries in the as-cast alloys. Furthermore, the rest of Cu can dissolve into the matrixes. The lath-shaped Al2CuMg precipitates with a size of 15-20 nm × 2-4 nm were generated in the T5-treated Al-5.5Mg-0.7Mn-xCu (x = 0.5, 0.8, 1.5 wt%) alloys. The room temperature tensile and yield strengths of alloys increase with increasing the content of Cu. Increasing Cu content results in more Al2CuMg phase formation along the grain boundaries, which causes more cracks during tensile deformation and lower ductility. Al-5.5Mg-0.7Mn-0.8Cu alloy exhibits excellent comprehensive tensile properties under both as-cast and T5-treated conditions. The yield strength of 179 MPa, the ultimate tensile strength of 303 MPa and the elongation of 8.7% were achieved in the as-cast Al-5.5Mg-0.7Mn-0.8Cu alloy, while the yield strength significantly was improved to 198 MPa after T5 treatment.
 Select Interfacial bonding mechanism in Al/coated steel dissimilar refill friction stir spot welds Z. Shen, Y. Dingd, J. Chen, B. Shalch Amirkhiz, J.Z. Wen, L. Fu, A.P. Gerlich J. Mater. Sci. Technol., 2019, 35 (6): 1027-1038.  DOI: 10.1016/j.jmst.2019.01.001 Defect-free dissimilar Al/zinc coated steel and Al/AlSi coated steel welds were successfully fabricated by refill friction stir spot welding. However, Al alloy and uncoated steel could not be welded under the same welding condition. Al-Zn eutectic layer formed at the Al/zinc coated steel interface showed non-uniformity in thickness and nanoscale intermetallic (IMC) produced was discontinuous. The bonding formation between the Al-Zn layer and the surrounding materials was attributed to a liquid/solid reaction mechanism. Bonding formation at Al alloy and AlSi coated steel interface was attributed to a solid/solid reaction mechanism, as the joining process did not involve with melting of base metals or AlSi coating materials. Kissing bond formed at the weld boundary acted as a crack initiation and propagation site, and the present study showed that weld strength of Al 5754/AlSi coated steel was greatly influenced by properties of original IMC layer.
 Select Refining constitutive relation by integration of finite element simulations and Gleeble experiments D.J. Yu, D.S. Xu, H. Wang, Z.B. Zhao, G.Z. Wei, R. Yang J. Mater. Sci. Technol., 2019, 35 (6): 1039-1043.  DOI: 10.1016/j.jmst.2018.12.026 Thermo-mechanical coupled finite element calculations were carried out to simulate the Gleeble compression of the samples of a titanium alloy (Ti60), and the results are analyzed and compared with the actual compression tests conducted on a Gleeble 3800 thermo-mechanical simulator. The changes in temperature, stress and strain distribution in the samples and the source of error on the constitutive relations from Gleeble hot compression test were analyzed in detail. Both simulations and experiments showed that the temperature distribution in the specimen is not uniform during hot compression, resulting in significant deformation inhomogeneity and non-ignorable error in the flow stress strain relation, invalidating the uniform strain assumption commonly assumed when extracting the constitutive relation from Gleeble tests. Based on the finite element simulations with iterative corrections, we propose a scheme to refine the constitutive relations from Gleeble tests.
 Select Phase-field simulation for the evolution of solid/liquid interface front in directional solidification process Yuhong Zhao, Bing Zhang, Hua Hou, Weipeng Chen, Meng Wang J. Mater. Sci. Technol., 2019, 35 (6): 1044-1052.  DOI: 10.1016/j.jmst.2018.12.009 In this study, the phase field method was used to study the multi-controlling factors of dendrite growth in directional solidification. The effects of temperature gradient, propelling velocity, thermal disturbance and growth orientation angle on the growth morphology of the dendritic growth in the solid/liquid interface were discussed. It is found that the redistribution of solute leads to multilevel cavity and multilevel fusion to form multistage solute segregation, and the increase of temperature gradient and propelling velocity can accelerate the dendrite growth of directional solidification, and also make the second dendrites more developed, which reduces the primary distance and the solute segregation. When the temperature gradient is large, the solid-liquid interface will move forward in a flat interface mode, and the thermal disturbance does not affect the steady state behavior of the directionally solidified dendrite tip. It only promotes the generation and growth of the second dendrites and forms the asymmetric dendrite. Meanwhile, it is found that the inclined dendrite is at a disadvantage in the competitive growth compared to the normal dendrite, and generally it will disappear. When the inclination angle is large, the initial primary dendrite may be eliminated by its secondary or third dendrite.
 Select Achieving strength-ductility synergy in cold spray additively manufactured Al/B4C composites through a hybrid post-deposition treatment Naeem ul Haq Tariq, Lawrence Gyansah, Xiang Qiu, Chunni Jia, Hasan Bin Awais, Chengwu Zheng, Hao Du, Jiqiang Wang, Tianying Xiong J. Mater. Sci. Technol., 2019, 35 (6): 1053-1063.  DOI: 10.1016/j.jmst.2018.12.022 Cold spray additive manufacturing (CSAM) provides a potential solid state manufacturing route to fabricate variety of aluminum matrix composites (AMCs) with reduced possibility of undesired chemical reactions and residual thermal stresses. This study presents a hybrid (i.e. hot compression + hot rolling) post-deposition treatment to reinvigorate the mechanical properties of cold spray additively manufactured Al/B4C composites. The as-deposited samples were initially subjected to 30% thickness reduction via hot compression treatment at ~500 °C followed by a hot rolling treatment with 40% thickness reduction in 2 passes. Electron backscatter diffraction (EBSD) and high resolution transmission electron microscopy (HRTEM) results revealed that after hybrid post-deposition treatment (involving 70% accumulative thickness reduction), the aluminum grains in the matrix were extensively refined due to simultaneous operation of continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX). Furthermore, interfacial defects were remarkably reduced while the nature of Al/Al and Al/B4C interfacial bonding was changed from sheer mechanical interlocking to metallurgical bonding which facilitated efficient transference of applied load to uniformly dispersed bimodal B4C particles. As a result, ultimate tensile strength (UTS) and elongation (EL) of the as-deposited sample were simultaneously improved from ~37 to 185 MPa and ~0.3% to 6.2%, respectively.
 Select Tailoring the secondary phases and mechanical properties of ODS steel by heat treatment Qian Zhao, Zongqing Ma, Liming Yu, Huijun Li, Chenxi Liu, Chong Li, Yongchang Liu J. Mater. Sci. Technol., 2019, 35 (6): 1064-1073.  DOI: 10.1016/j.jmst.2018.12.008 The oxide dispersion strengthened (ODS) steel with the nominal composition of Fe-14Cr-2W-0.3Ti-0.2V-0.07Ta-0.3Y2O3 (wt%) was fabricated by mechanical alloying and hot isostatic pressing (HIP). In order to optimize the relative volume fraction of secondary phases, the as-HIPed ODS steel was annealed at 800 °C, 1000 °C, 1200 °C for 5 h, respectively. The microstructures and different secondary phases of the as-HIPed and annealed ODS samples were identified by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The tensile properties of all the ODS steels at room temperature were also investigated. The results indicate that annealing is an effective way to control the microstructure and the integral secondary phases. The annealing process promotes the dissolution of M23C6 particles, thus promoting the precipitation of TiC. No obvious coarsening of Y2Ti2O7 nanoparticles can be observed during annealing. The tensile results indicate that the annealed ODS sample with the optimized secondary phases and high density possesses the best mechanical properties.
 Select Effects of Y and Zn additions on electrical conductivity and electromagnetic shielding effectiveness of Mg-Y-Zn alloys Lizi Liu, Xianhua Chen, Jingfeng Wang, Liying Qiao, Shangyu Gao, Kai Song, Chaoyue Zhao, Xiaofang Liu, Di Zhao, Fusheng Pan J. Mater. Sci. Technol., 2019, 35 (6): 1074-1080.  DOI: 10.1016/j.jmst.2018.12.010 Microstructure, electrical conductivity, and electromagnetic interference (EMI) shielding effectiveness (SE) of cast Mg-xZn-yY (x = 2-5, y = 1-10) alloys were systematically investigated to understand the effects of Zn and Y additions on electrical conductivity and electromagnetic shielding effectiveness of the alloys. Experimental results indicate that the electrical conductivity and SE of the Mg-xZn-yY alloys decrease with Y/Zn ratio. Electrical conductivity is the main factor that affects the electromagnetic shielding properties and the variation tendency of electromagnetic shielding properties of the Mg-xZn-yY alloys is consistent with conductivity. Valence of Y and Zn atoms, configuration of extranuclear electron and volumetric difference are main reasons for the variations in the electrical conductivity. A high density of second phase and the formation of semi-continuous network structure can also improve the SE value at high frequencies.
 Select Hydrogen-assisted fracture features of a high strength ferrite-pearlite steel Yuefeng Jiang, Bo Zhang, Dongying Wang, Yu Zhou, Jianqiu Wang, En-Hou Han, Wei Kea J. Mater. Sci. Technol., 2019, 35 (6): 1081-1087.  DOI: 10.1016/j.jmst.2018.12.019 Up to now, the exact reason of hydrogen-induced fracture for ferrite-pearlite (FP) steel is still not fully understood. This study presents detail observations of the feature beneath the fracture surface with the aim to reveal the hydrogen-induced cracking initiation and propagation processes. Slow strain rate tensile (SSRT) testing shows that the FP steel is sensitive to hydrogen embrittlement (HE). Focused ion beam (FIB) was used to prepare samples for TEM observations after HE fracture. The corresponding fractographic morphologies of hydrogen charged specimen exhibit intergranular (IG) and quasi-cleavage (QC) fracture feature. Pearlite colony, ferrite/pearlite (F/P) boundary and the adjacent ferrite matrix are found to be responsible for the initial HE fracture and the subsequent propagation. With increasing of the stress intensity factor, fracture mode is found to change from mixed IG and QC to entire QC feature which only occurs at the ferrite matrix. No crack is observed at the ferrite/cementite (F/C) interface. This may be mainly due to the limited pearlite lamella size and relatively low interface energy.
 Select Corrosion resistance of in-situ growth of nano-sized Mg(OH)2 on micro-arc oxidized magnesium alloy AZ31—Influence of EDTA Chang-Yang Li, Xiao-Li Fan, Rong-Chang Zeng, Lan-Yue Cui, Shuo-Qi Li, Fen Zhang, Qing-Kun He, M. Bobby Kannan, , Dong-Chu Chen, Shao-Kang Guan J. Mater. Sci. Technol., 2019, 35 (6): 1088-1098.  DOI: 10.1016/j.jmst.2019.01.006 One of the major obstacles for the clinical use of biodegradable magnesium (Mg)-based materials is their high corrosion rate. Micro-arc oxidation (MAO) coatings on Mg alloys provide mild corrosion protection owing to their porous structure. Hence, in this study a dense Mg(OH)2 film was fabricated on MAO-coated Mg alloy AZ31 in an alkaline electrolyte containing ethylenediamine tetraacetic acid disodium (EDTA-2Na) to reinforce the protection. Surface morphology, chemical composition and growth process of the MAO/Mg(OH)2 hybrid coating were examined using field-emission scanning electron microscopy, energy dispersive X-ray spectrometer, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared spectrophotometer. Corrosion resistance of the coatings was evaluated via potentiodynamic polarization curves and hydrogen evolution tests. Results manifested that the Mg(OH)2 coating possesses a porous nano-sized structure and completely seals the micro-pores and micro-cracks of the MAO coating. The intermetallic compound of AlMn phase in the substrate plays a key role in the growth of Mg(OH)2 film. The current density of Mg(OH)2-MAO composite coating decreases three orders of magnitude in comparison with that of bare substrate, indicating excellent corrosion resistance. The Mg(OH)2-MAO composite coating is beneficial to the formation of calcium phosphate corrosion products on the surface of Mg alloy AZ31, demonstrating a great promise for orthopaedic applications.
 Select Characterization of high-pressure die-cast hypereutectic Al-Si alloys based on microstructural distribution and fracture morphology X.Y. Jiao, J. Wang, C.F. Liu, Z.P. Guo, G.D. Tong, S.L. Ma, Y. Bi, Y.F. Zhang, S.M. Xiong J. Mater. Sci. Technol., 2019, 35 (6): 1099-1107.  DOI: 10.1016/j.jmst.2018.12.005 The fracture behavior of high-pressure die-cast hypereutectic (HPDC) Al-Si alloys was investigated using a high-resolution laboratory CT and synchrotron X-ray tomography with a particular focus on the influence of HPDC microstructure. Results showed that microstructure of the alloy was mainly comprised of primary silicon particles (PSPs), Al dendrites, Cu-rich phases and pores. Most of the coarse PSPs, Cu-rich phases and pores were located in the center of the specimen. The rapid solidification of HPDC led to a heterogeneous microstructural feature. Elemental Cu was enriched in the frontiers of solid-liquid interface, causing the formation of large size dendritic arms. The pores were formed in the interdendrites which endured high stress intensity under high applied stress. Microcracks were originated from pores and further connected Cu-rich phases causing intergranular fracture. PSPs worked as obstacles causing piling-up dislocations in the phase interface. In the regions where large size of PSPs enriched in, PSPs ruptured rather than debonded from matrix, indicating transgranular fractures of PSPs. Microcracks originated around pores and PSPs tended to converge on the main cracks to decrease the energy required for crack propagation.
 Select Minimizing serrated flow in Al-Mg alloys by electroplasticity Hui Xu, Xuebing Liu, Di Zhang, Xinfang Zhang J. Mater. Sci. Technol., 2019, 35 (6): 1108-1112.  DOI: 10.1016/j.jmst.2018.12.007 Serrated flow under the influence of electroplasticity in Al-Mg alloys is investigated within the dynamic strain aging interpretations of the Portevin-Le Chatelier effect. The stability of plastic region is extended by the pulsed electric current with the increase of critical strain and the extension of waiting time. Meanwhile, the dislocation density, affecting the plastic instabilities due to the interaction between solute atoms and mobile dislocations, is greatly reduced, which plays a dominant role in the suppression of serrated flow.
 Select Effect of electropulsing treatment on static recrystallization behavior of cold-rolled magnesium alloy ZK60 with different reductions Huijun Guo, Xun Zeng, Jianfeng Fan, Hua Zhang, Qiang Zhang, Weiguo Li, Hongbiao Dong, Bingshe Xu J. Mater. Sci. Technol., 2019, 35 (6): 1113-1120.  DOI: 10.1016/j.jmst.2018.11.008 Effect of electropulsing treatment (EPT) on recrystallization behavior of cold-rolled Mg alloy ZK60 strips was investigated. It was found that EPT significantly improved nucleation rate and migration ability of grain boundaries, leading to accelerated recrystallization of the deformed metals at relatively low temperature. After the recrystallization induced by EPT, the average grain size of 20% rolling reduction samples decreased from 113 μm to around 10 μm, meanwhile the typical basal-type texture of the cold-rolled sample was weakened. EPT was normally accompanied with a thermal and an athermal effects. The athermal effect played a dominated role in increasing nucleation rate, while the thermal effect promoted grain growth. A fewer recrystallized grains originated along the grain boundaries in the 10% reduction samples, while most of the recrystallization took place inside the twins in the 20% reduction samples.
 Select Solvent-free microwave-assisted synthesis of tenorite nanoparticle-decorated multi-walled carbon nanotubes Jennifer A. Rudd, Cathren E. Gowenlock, Virginia Gomez, Ewa Kazimierska, Abdullah M. Al-Enizi, Enrico Andreoli, Andrew R. Barron J. Mater. Sci. Technol., 2019, 35 (6): 1121-1127.  DOI: 10.1016/j.jmst.2019.01.002 Copper-decorated carbon nanotubes (CNTs) have important applications as precursors for ultra-conductive copper wires. Tenorite-decorated CNTs (CuO-CNTs) are ideal candidates and are currently developed using laborious processes. For this reason, we have developed a facile and scalable method for the synthesis of CuO-CNTs from copper acetate. It was found that the optimal loading of copper acetate onto the CNTs was 23.1 wt% and that three 1-minute microwave treatments were sufficient for the decomposition of copper acetate to copper oxide. The loading of copper oxide onto the nanotubes was confirmed using X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and thermogravimetric analysis. The materials were characterised using X-ray diffraction and scanning electron microscopy.
 Select Anisotropic corrosion resistance of TiC reinforced Ni-based composites fabricated by selective laser melting Hongmei Zhang, Dongdong Gu, Lixia Xi, Han Zhang, Mujian Xia, Chenglong Ma J. Mater. Sci. Technol., 2019, 35 (6): 1128-1136.  DOI: 10.1016/j.jmst.2018.12.020 Electrochemical measurements on three planes of TiC/Inconel 718 composites fabricated by selective laser melting (SLM) were performed to study the corrosion property. The results showed that the YZ-plane with dense and fine columnar structures possessed high microhardness and superior corrosion resistance in 3.5 wt% NaCl solution. For the XZ-plane, a decreased anti-corrosion property was observed owing to its inhomogeneous microstructures. While the XY-plane with large irregular pores and clustered ring-like structures was more susceptible to corrosion compared with the other two planes. Comparative analysis suggested that the anisotropic corrosion behaviors were significantly dependent on the surface defects, microstructure characteristics and added reinforcements.
 Select Data-driven evaluation of fatigue performance of additive manufactured parts using miniature specimens H.Y. Wan, G.F. Chen, C.P. Li, X.B. Qi, G.P. Zhang J. Mater. Sci. Technol., 2019, 35 (6): 1137-1146.  DOI: 10.1016/j.jmst.2018.12.011 This overview firstly introduces the state-of-the-art research progress in length scale-related fatigue performance of conventionally-fabricated metals evaluated by miniature specimens. Some key factors for size effects sensitive to microstructures including the specimen thickness, grain size and a ratio between them are highlighted to summarize some general rules for size effects. Then, ongoing research progress and new challenges in evaluating the fatigue performance of additive manufactured parts controlled by location-specific defects, microstructure heterogeneities as well as mechanical anisotropy using miniature specimen testing technique are discussed and addressed. Finally, a potential roadmap to establish a data-driven evaluation platform based on a large number of miniature specimen-based experiment data, theoretical computations and the ‘big data’ analysis with machine learning is proposed. It is expected that this overview would provide a novel strategy for the realistic evaluation and fast qualification of fatigue properties of additive manufactured parts we have been facing to.
 Select Finite element analysis of the effect of interlayer on interfacial stress transfer in layered graphene nanocomposites C.C. Roach, Y.C. Lu J. Mater. Sci. Technol., 2019, 35 (6): 1147-1152.  DOI: 10.1016/j.jmst.2018.12.012 Understanding the roles of interlayers in reinforcement efficiencies by layered graphene is very important in order to produce strong and light graphene based nanocomposites. The present paper uses the finite element method to evaluate the interfacial strain transfers and reinforcement efficiencies in layered graphene-polymer composites. Results indicate that the presence of compliant interlayers in layered graphene plays significant roles in the transfers of strain/stress from matrix to graphene and subsequently the reinforcement effectiveness of layered graphene. In general, the magnitude of shear strain transferred onto the rigid graphene decreases as the thickness of the interlayer increases. This trend becomes insignificant as the graphene becomes sufficiently large (s>25,000). The shear strain at the interface of graphene-matrix is also greatly influenced by the interlayer modulus. A stiffer interlayer would result in a higher shear strain transferred on the graphene. The performance of the interlayers is further affected by the property of the composite and the architecture of the layered graphene stack. If a composite contains more graphene phase, the efficiency of reinforcement by a layered graphene becomes improved. If a graphene stack contains more interlayers, the effectiveness of reinforcement at the edges of the graphene becomes negatively affected.
 Select Microstructure, precipitates and mechanical properties of powder bed fused inconel 718 before and after heat treatment Le Zhou, Abhishek Mehta, Brandon McWilliams, Kyu Cho, Yongho Sohn J. Mater. Sci. Technol., 2019, 35 (6): 1153-1164.  DOI: 10.1016/j.jmst.2018.12.006 IN718 alloy was fabricated by laser powder bed fusion (PBF) for examination of microstructure, precipitates and mechanical properties in the as-built state and after different heat treatments. The as-built alloy had a characteristic fine cellular-dendritic microstructure with Nb, Mo and Ti segregated along the interdendritic region and cellular boundary. The as-built alloys were then subjected to solution heat treatment (SHT) at 980 °C or 1065 °C for 1 h. SHT at 980 °C led to the formation of δ-phase in the interdendritic region or cellular boundary. The segregation was completely removed by the SHT at 1065 °C, but recrystallization was observed, and the carbides decorated along the grain boundaries. The as-built alloy and alloys with SHT at 980 °C and 1065 °C were two-step aged, which consisted of annealing at 720 °C for 8 h followed by annealing at 620 °C for 8 h. Transmission electron microscopy revealed the precipitation of γ' and γ” in all alloys after two-step aging, but the amount and uniformity of distribution varied. The Vickers hardness of the PBF IN718 alloy increased from 296 HV to 467 HV after direct aging. The hardness decreased to 267 HV and 235 HV after SHT at 980 °C and 1065 °C, respectively, but increased to 458 HV and 477 HV followed by aging. The evolution of Young’s modulus after heat treatment exhibited similar trend to that of hardness. The highest hardness was observed for IN718 after SHT at 1065 °C and two-step aging due to precipitation with greater amount and uniform distribution.
 Select Microstructural and textural evolutions in multilayered Ti/Cu composites processed by accumulative roll bonding Shuang Jiang, Ru Lin Peng, Nan Jia, Xiang Zhao, Liang Zuo J. Mater. Sci. Technol., 2019, 35 (6): 1165-1174.  DOI: 10.1016/j.jmst.2018.12.018 Ti/Cu multilayered composites were fabricated via accumulative roll bonding (ARB). During co-deformation of the constituent metals, the hard Ti layers necked preferentially and then fragmented with the development of shear bands. Transmission electron microscopy showed that with increasing ARB cycles, grains in Ti were significantly refined even though dynamic recrystallization has occurred. For Cu the significant grain refinement was only found within the shear banded region when the composite was processed after five ARB cycles. Due to the diffusion of Cu atoms into Ti at the heterophase interfaces, amorphization with a width less than 10 nm was identified even in the composite processed by one cycle. At higher ARB cycles, the width of amorphous region increased and intermetallic compounds CuTi appeared from the region. The lattice defects introduced at the heterophase interfaces under roll bonding was responsible for the formation of the nano-scaled compounds. X-ray diffraction showed that an abnormal {11$\bar{2}$0} fiber texture was developed in Ti layers, while significant brass-type textures were developed in Cu layers. Some orientations along the {11$\bar{2}$0} fiber favored the prismatic < a> slip for Ti. Tensile tests revealed the elevated strength without a substantial sacrifice of ductility in the composites during ARB. The unique mechanical properties were attributed to the significantly refined grains in individual metals, the good bonding between the constituent metals, as well as the development of an abnormal {11$\bar{2}$0} fiber texture in Ti layers.
 Select Composition and phase structure dependence of mechanical and magnetic properties for AlCoCuFeNix high entropy alloys Cong Liu, Wenyi Peng, C.S. Jiang, Hongmin Guo, Jun Tao, Xiaohua Deng, Zhaoxia Chen J. Mater. Sci. Technol., 2019, 35 (6): 1175-1183.  DOI: 10.1016/j.jmst.2018.12.014 In this study, the effects of composition and phase constitution on the mechanical properties and magnetic performance of AlCoCuFeNix (x = 0.5, 0.8, 1.0, 1.5, 2.0, 3.0 in molar ratio) high entropy alloys (HEAs) were investigated. The results show that Ni element could lead to the evolution from face centered cubic (FCC), body centered cubic (BCC) and ordered BCC coexisting phase structure to a single FCC phase. The change of phase constitution enhances the plasticity but reduces the hardness and strength. One of the interesting points is the excellent soft magnetic properties of AlCoCuFeNix HEAs. Soft magnetic performance is dependent on composition and phase transition. AlCoCuFeNi1.5 alloy, achieving a better balance of mechanical and magnetic properties, could be applied as structure materials and soft magnetic materials (SMMs). High Curie temperature (>900 K) and strong phase stability below 1350 K of AlCoCuFeNi0.5 alloy confirm its practicability in a high-temperature environment. Atomic size difference (δ) is utilized as the critical parameter to explain the lattice strain and phase transformation induced by Ni addition.
 Select Enhancement of oxygen evolution reaction activity and durability of Ba0.5Sr0.5Co0.8Fe0.2O3-δ by CO2 thermal treatment Fengli Liang, Ziqiong Yang, Haipeng Deng, Jaka Sunarso, Lili Yang, Junkui Mao J. Mater. Sci. Technol., 2019, 35 (6): 1184-1191.  DOI: 10.1016/j.jmst.2019.01.005 This work demonstrates that in situ formation of carbonate layer on the surface of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) obtained by exposure to CO2 during heating between 500 °C and 700 °C can provide enhanced oxygen evolution reaction (OER) performance and durability in an alkaline solution relative to the original BSCF. Three temperatures, i.e., 500 °C, 600 °C, and 700 °C were chosen to perform the CO2 thermal treatment, resulting into BSCF-500, BSCF-600, and BSCF-700 samples. The OER was enhanced in the order of BSCF-500 < BSCF-700 < BSCF-600. BSCF-600 showed the best OER performance, i.e., a low overpotential of 0.36 V required to attain 10 mA cm-2 current density as well as a mass activity of 74.14 $Ag_{cat}^{-1}$ and a specific activity of 5.04 mA $Ag_{cat}^{-2}$ at an overpotential of 0.4 V. The OER performance durability of BSCF-600 was highlighted by its ability to maintain a stable potential of around 1.61 V vs. RHE (RHE: reversible hydrogen electrode) when charged at a constant current density of 10 mA cm-2 throughout the 800 min continuous chronopotentiometry test. The enhanced OER performance for BSCF-600 relative to the original BSCF is attributed to three factors: (i) higher electrochemically active surface area; (ii) faster charge transfer rate and higher electrical conductivity; and (iii) modified oxidation state of cobalt ions. The formation of thin carbonate layer in BSCF-600 appears to suppress the durability issue observed in BSCF.
 Select Caution in building a Burgers circuit for studying secondary dislocations Wen-Sheng Xu, Wen-Zheng Zhang J. Mater. Sci. Technol., 2019, 35 (6): 1192-1197.  DOI: 10.1016/j.jmst.2019.01.004 As the Burgers vector of a secondary dislocation may not conform to the translation vectors in the periodic pattern in a high-resolution transmission electron microscope (HRTEM) image, a Burgers circuit directly constructed according to an HRTEM image may render a wrong Burgers vector. The HRTEM images of the habit plane (HP) of δ precipitate in an Inconel 718 alloy were re-examined by using different Burgers circuits, as an example. Evidence is found for predicted secondary dislocation associated with a down step. The Burgers vector of the dislocation is 1/6[11$\bar{2}$]γ/1/3[00$\bar{1}$]δ, determined with the Burgers circuit built on the reference of the displacement shift completely lattice (DSCL). It is consistent with both the calculated and the measured result of the major defects in the habit plane. Different Burgers vectors due to different Burgers circuits were explained quantitatively. The present study has a general implication for the determination of the Burgers vectors of secondary interfacial dislocations in other systems.
 Select Hot deformation behavior and workability characteristic of a fine-grained Mg-8Sn-2Zn-2Al alloy with processing map Weili Cheng, Yang Bai, Shichao Ma, Lifei Wang, Hongxia Wang, Hui Yu J. Mater. Sci. Technol., 2019, 35 (6): 1198-1209.  DOI: 10.1016/j.jmst.2018.12.001 The hot deformation behavior of a fine-grained Mg-8Sn-2Zn-2Al (TZA822, in wt%) alloy was investigated in the temperature range of 150-350 °C and the strain rate of 0.01-10 s-1 employing thermomechanical simulator. In most of the cases, the material showed typical dynamic recrystallization (DRX) features i.e., a signal peak value followed by a gradual decrease or to reach a steady state. The work hardening rate was found to increase with decreasing temperature and increasing strain rate, while strain rates had great effects on work hardening behavior. Meanwhile, the constitutive analysis indicated that cross-slip of dislocations was likely to be the dominant deformation mechanism. In addition, the processing map at the strain of 0.1-0.7 showed two stability domains with high power dissipation efficiencies and the optimum hot working parameters for the studied alloy was determined to be 350 °C/0.01 s-1 and 350 °C/10 s-1, at which continuous DRX (CDRX) and discontinuous DRX (DDRX) as main softening mechanism. The instability regions occurred at 200-250 °C/10 s-1 and the main flow instability mechanism was twinning and/or flow localization bands, which were prone to induce cracks and caused in-consistent mechanical properties of the alloy.
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