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CN 21-1315/TG
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Current Issue
 05 April 2019, Volume 35 Issue 4 Previous Issue    Next Issue
Orginal Article
 Select Low temperature annealing of metals with electrical wind force effects Daudi Waryoba, Zahabul Islam, Baoming Wang, Aman Haque J. Mater. Sci. Technol., 2019, 35 (4): 465-472.  DOI: 10.1016/j.jmst.2018.09.069 Conventional annealing is a slow, high temperature process that involves heating atoms uniformly, i.e., in both defective and crystalline regions. This study explores an electrical alternative for energy efficiency, where moderate current density is used to generate electron wind force that produces the same outcome as the thermal annealing process. We demonstrate this on a zirconium alloy using in-situ electron back scattered diffraction (EBSD) inside a scanning electron microscope (SEM) and juxtaposing the results with that from thermal annealing. Contrary to common belief that resistive heating is the dominant factor, we show that 5 × 104 A/cm2 current density can anneal the material in less than 15 min at only 135 °C. The resulting microstructure is essentially the same as that obtained with 600 °C processing for 360 min. We propose that unlike temperature, the electron wind force specifically targets the defective regions, which leads to unprecedented time and energy efficiency. This hypothesis was investigated with molecular dynamics simulation that implements mechanical equivalent of electron wind force to provide the atomistic insights on defect annihilation and grain growth.
 Select Dynamic deformation behavior and microstructural evolution during high-speed rolling of Mg alloy having non-basal texture Sang-Hoon Kim, Jeong Hun Lee, Chong Soo Lee, Jonghun Yoon, Sung Hyuk Park J. Mater. Sci. Technol., 2019, 35 (4): 473-482.  DOI: 10.1016/j.jmst.2018.10.010 The dynamic deformation behaviors and resultant microstructural variations during high-speed rolling (HSR) of a Mg alloy with a non-basal texture are investigated. To this end, AZ31 alloy samples in which the basal poles of most grains are predominantly aligned parallel to the transverse direction (TD) are subjected to hot rolling with different reductions at a rolling speed of 470 m/min. The initial grains with a TD texture are favorable for {10-12} twinning under compression along the normal direction (ND); as a result, {10-12} twins are extensively formed in the material during HSR, and this consequently results in a drastic evolution of texture from the TD texture to the ND texture and a reduction in the grain size. After the initial grains are completely twinned by the {10-12} twinning mechanism, {10-11} contraction twins and {10-11}-{10-12} double twins are formed in the {10-12} twinned grains by further deformation. Since the contraction twins and double twins have crystallographic orientations that are favorable for basal slip during HSR, dislocations easily accumulate in these twins and fine recrystallized grains nucleate in the twins to reduce the increased internal strain energy. Until a rolling reduction of 20%, {10-12} twinning is the main mechanism governing the microstructural change during HSR, and subsequently, the microstructural evolution is dominated by the formation of contraction twins and double twins and the dynamic recrystallization in these twins. With an increase in the rolling reduction, the average grain size and internal strain energy of the high-speed-rolled (HSRed) samples decrease and the basal texture evolves from the TD texture to the ND texture more effectively. As a result, the 80% HSRed sample, which is subjected to a large strain at a high strain rate in a single rolling pass, exhibits a fully recrystallized microstructure consisting of equiaxed fine grains and has an ND basal texture without a TD texture component.
 Select Effects of calcination temperature and Li+ ions doping on structure and upconversion luminescence properties of TiO2:Ho3+-Yb3+ nanocrystals Kaishun Zou, Guangzong Dong, Juncheng Liu, Boxu Xu, Danping Wang J. Mater. Sci. Technol., 2019, 35 (4): 483-490.  DOI: 10.1016/j.jmst.2018.10.018 Ho3+-Yb3+ co-doped and Ho3+-Yb3+-Li+ tri-doped TiO2 nanocrystals were prepared using the sol-gel method. Effects of the calcination temperature and Li+ ions doping on the structure and upconversion luminescence properties of Ho3+-Yb3+ co-doped TiO2 nanocrystals were investigated. The upconversion luminescence of nanocrystals was enhanced with the reduction of the crystal lattice symmetry and the crystallinity improvement of the matrix, which were facilitated by the calcination temperature change and Li+ ions doping. The lowest lattice symmetry and the best crystallinity of the matrix resulted in the maximum luminescence intensity.
 Select Predicting recrystallized grain size in friction stir processed 304L stainless steel M.P. Miles, T.W. Nelson, C. Gunter, F.C. Liu, L. Fourment, T. Mathis J. Mater. Sci. Technol., 2019, 35 (4): 491-498.  DOI: 10.1016/j.jmst.2018.10.021 A major dilemma faced in the nuclear industry is repair of stainless steel reactor components that have been exposed to neutron irradiation. When conventional fusion welding is used for repair, intergranular cracks develop in the heat-affected zone (HAZ). Friction stir processing (FSP), which operates at much lower peak temperatures than fusion welding, was studied as a crack repair method for irradiated 304L stainless steel. A numerical simulation of the FSP process in 304L was developed to predict temperatures and recrystallized grain size in the stir zone. The model employed an Eulerian finite element approach, where flow stresses for a large range of strain rates and temperatures inherent in FSP were used as input. Temperature predictions in three locations near the stir zone were accurate to within 4%, while prediction of welding power was accurate to within 5% of experimental measurements. The predicted recrystallized grain sizes ranged from 7.6 to 10.6 μm, while the experimentally measured grains sizes in the same locations ranged from 6.0 to 7.6 μm. The maximum error in predicted recrystallized grain size was about 39%, but the associated stir zone hardness from the predicted grain sizes was only different from the experiment by about 10%.
 Select Hydrogen embrittlement behavior of Inconel 718 alloy at room temperature Xinfeng Li, Jin Zhang, Eiji Akiyama, Qinqin Fu, Qizhen Li J. Mater. Sci. Technol., 2019, 35 (4): 499-502.  DOI: 10.1016/j.jmst.2018.10.002 Hydrogen embrittlement of Inconel 718 alloy was investigated. Multi-scale observation technique were employed, comprising slow strain rate tensile tests, scanning electron microscopy and transmission electron microscopy analysis. The results demonstrate that hydrogen charging deteriorates mechanical properties of the alloy. Inconel 718 alloy shows partial Portevin-Le Chatelier (PLC) effect at room temperature when hydrogen charging current density is 220 mA cm-2 and 590 mA cm-2. Moreover, plastic deformation features with dislocation cells are detected in hydrogen-induced brittle zone. Thus, it is concluded that dragging effect of hydrogen atoms on dislocations contributes to PLC effect.
 Select Effect of minor content of Gd on the mechanical and degradable properties of as-cast Mg-2Zn-xGd-0.5Zr alloys Junxiu Chen, Lili Tan, Xiaoming Yu, Ke Yang J. Mater. Sci. Technol., 2019, 35 (4): 503-511.  DOI: 10.1016/j.jmst.2018.10.022 RE-containing Mg alloys used as biodegradable medical implants exhibit good promising application due to their good mechanical properties and degradation resistance. In this work, effect of Gd on the microstructure, mechanical properties and biodegradation of as-cast Mg-2Zn-xGd-0.5Zr alloys was investigated. The results showed that there were mainly α-Mg, I-phase, W-phase and MgZn2 phase in Mg-Zn-Gd-Zr alloys. With increase of the Gd content, the strength of the alloys was enhanced due to the second phase strengthening and grain refinement. The degradation resistance of Mg-2Zn-0.5Zr alloy was increased by adding 0.5-1% Gd due to the uniformly distributed second phases which acted as a barrier to prevent the pitting corrosion. However, increasing Gd content to 2% reduced the degradation resistance of the alloy due to the galvanic corrosion between the matrix and the second phases. The good degradation resistance and mechanical properties of as-cast Mg-2Zn-1Gd-0.5Zr alloy makes it outstanding for biomaterial application.
 Select Microstructure evolution and interface structure of Al-40 wt% Si composites produced by high-energy ball milling Yuanyuan Chen, Zhangping Hu, Yifei Xu, Jiangyong Wang, Peter Schützendübe, Yuan Huang, Yongchang Liu, Zumin Wang J. Mater. Sci. Technol., 2019, 35 (4): 512-519.  DOI: 10.1016/j.jmst.2018.10.005 High silicon content Al-Si composites with a composition of Al-40 wt% Si were fabricated via a high-energy ball milling method. The microstructure evolution of Al-40 wt% Si milled powders and sintered composites has been thoroughly studied by scanning electron microscopy, X-ray diffraction, energy-dispersive spectrometry and high-resolution transmission electron microscopy. The mechanism of ball milling Al-40 wt% Si powders has been disclosed in detail: fracture mechanism dominating in the early stages, followed by the agglomeration mechanism, finally reaching the balance between the fragments and the agglomerates. It has been found that the average particle sizes of mixed Al-Si powders can be refined to the nanoscale, and the crystallite sizes of Al and Si have been reduced to 10 nm and 62 nm upon milling for 2 h-50 h, respectively. The finally formed Al-Si interfaces after ball milling for 50 h are well-cohesive. A dense and homogenous Al-40 wt% Si composite have been achieved by solid-state sintering at 550 °C. The results thus provide an effective support for producing bulk nanostructured Al-Si composites.
 Select Effect of aging treatment on the microstructures and mechanical properties evolution of 25Cr-20Ni austenitic stainless steel weldments with different Nb contents Xu Zhang, Dianzhong Li, Yiyi Li, Shanping Lu J. Mater. Sci. Technol., 2019, 35 (4): 520-529.  DOI: 10.1016/j.jmst.2018.10.017 The microstructure evolutions and the mechanical properties of the 25Cr-20Ni austenitic stainless steel weld metals with different Nb contents were investigated during the long term aging treatment at 700 °C. M23C6, Nb(C, N), α-Cr phase and Nb-nitride phase (Z phase) were observed in the microstructures of the aged weld metals. The results showed that the α-Cr phase precipitated in the interdendritic regions of the weld metals after being exposed to 700 °C for 500 h and the element Nb accelerated the precipitation of the α-Cr phase significantly. The density of the α-Cr phase decreased with the increase of the distance away from the primary Nb(C, N). Additionally, the α-Cr phase showed a crystallographic relationship with the austenitic matrix, [1$\bar{1}$$\bar{1}]α-Cr // [1\bar{1}0]γ and (01\bar{1})α-Cr // (\bar{1}$$\bar{1}$1)γ. It was observed that the Z phase precipitated in the periphery of the Nb(C, N) and may replace the Nb(C, N) after long term exposure to high temperature. The transformation of the Nb(C, N) into Z phase suggested that the Z phase had a higher stability than the Nb(C, N) particles at 700 °C for long term aging. The tensile strength of the Nb-bearing weld metal showed a continuous decrease at the initial stage of the aging treatment and then went up slightly with the prolonged aging time. However, the elongations and the impact energies of the weld metals decreased monotonously with the increase of the aging time.
 Select Crystal structure of Cr4AlB4: A new MAB phase compound discovered in Cr-Al-B system Haiming Zhang, Fu-zhi Dai, Huimin Xiang, Zhili Zhang, Yanchun Zhou J. Mater. Sci. Technol., 2019, 35 (4): 530-534.  DOI: 10.1016/j.jmst.2018.10.006 In this communication, the crystal structure of Cr4AlB4, a new MAB phase compound (where M is a transition metal, A is Al or Si, B is boron) discovered in Cr-Al-B system is reported. This new MAB phase was synthesized from a mixture of CrB and Al powders at 1000 °C and its crystal structure was determined by a combination of X-ray diffraction, first-principles calculations and energy dispersive X-ray spectroscopy (EDS). Cr4AlB4 crystallizes in an orthorhombic structure with Immm space group. The lattice constants are a = 2.9343(6) ?, b = 18.8911(0) ?, c = 2.9733(7) ?, and the atomic positions are Cr1 at 4g (0, 0.2936(5), 0), Cr2 at 4h (0.5, 0.5859(7), 0), Al at 2b (0, 0.5, 0.5), B1 at 4h (0, 0.3839(8), 0.5) and B2 at 4g (0.5, 0.6646(2), 0.5).
 Select Recent advances in biodegradation controls over Mg alloys for bone fracture management: A review Ming-Shi Song, Rong-Chang Zeng, Yun-Fei Ding, Rachel W. Li, Mark Easton, Ivan Cole, Nick Birbilis, Xiao-Bo Chen J. Mater. Sci. Technol., 2019, 35 (4): 535-544.  DOI: 10.1016/j.jmst.2018.10.008 Magnesium (Mg) alloys possess comparable physical and mechanical properties to bone, making them an outstanding candidate of implant materials for bone fracture treatment. In addition to the excellent biocompatibility, and bioactivity, the engagement of Mg alloys is key for a number of biological functionalities in the human body. The unique biodegradation nature of Mg alloy implants implies that it may not require a secondary removal procedure when the expected supporting tasks accomplish, as they may simply and safely “disappear” over time. Nonetheless, the demonstrated drawback of potentially rapid degradation, is an issue that must be addressed appropriately for Mg implants and is consequently given unique attention in this review article. Herein, the critical criteria and the state-of-the-art strategies for controlling the degradation process of Mg alloys are reported. Furthermore, future developments of biodegradable Mg and its alloys systems with satisfactory specifications for clinical trials and deployment, are discussed. This review aims to provide information to materials scientists and clinical practitioners in the context of developing practical biodegradable Mg alloys.
 Select Multi-scale study of ductility-dip cracking in nickel-based alloy dissimilar metal weld Yifeng Li, Jianqiu Wang, En-Hou Han, Wenbo Wu, Hannu H?nninen J. Mater. Sci. Technol., 2019, 35 (4): 545-559.  DOI: 10.1016/j.jmst.2018.10.023 A ductility-dip-cracking (DDC)-concentrated zone (DCZ) in a width of about 3 mm was observed adjacent to the AISI 316 L/52 Mw fusion boundary (FB) in 52 Mw. The morphology, microstructure, mechanical and thermal properties and corrosion behavior in simulated primary water of DDC/DCZ were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), 3D X-ray tomography (XRT), 3D atom probe (3DAP), slow strain rate tensile (SSRT) testing and thermal dilatometry. The results indicate that DDCs are random-shaped and disc-like cavities with corrugated structure of inner surface and are parallel in groups along straight high-angle boundaries of columnar grains, ranging from micrometers to millimeters in size. Large-size M23C6 carbides dominate on the grain boundaries rather than MC (M=Nb, Ti), and thus the bonding effect of carbides is absent for the straight grain boundaries. The impurity segregation of O is confirmed for the inner surfaces of DDC. The oxide film formed on the inner surface of DDC (about 50 nm) is approximately twice as thick as that on the matrix (about 25 nm) in simulated primary water. The yield strength, tensile strength and elongation to fracture of 52 Mw-DCZ (400 MPa, 450 MPa and 20 %, respectively) are lower than those of 52 Mw-MZ (460 MPa, 550 MPa and 28 %, respectively). The intrinsic high-restraint weld structure, the additional stress/strain caused by the thermal expansion difference between AISI 316 L and 52 Mw as well as the detrimental carbide precipitation and the resulting grain boundary structure all add up to cause the occurrence of DCZ in the dissimilar metal weld.
 Select A novel direct reduction method to synthesize ordered Fe-Pt alloy nanoparticles Q. Zheng, Z.R. Zhang, J. Du, L.L. Lin, W.X. Xia, J. Zhang, B.R. Bian, J.P. Liu J. Mater. Sci. Technol., 2019, 35 (4): 560-567.  DOI: 10.1016/j.jmst.2018.09.036 In this work, a direct green solid-phase reduction method for the fabrication of large yield of ordered phase Fe-Pt alloy nanoparticles was reported, in which inorganic salts were used as metal precursors and H2-containing atmosphere was used as reducer. Utilizing this method, the composition and chemical ordered phase, such as L12-Fe3Pt, L12-FePt3, and L10-FePt phases can be easily achieved by one step reaction. The synthesized nanoparticles have clean surface because no organic precursors, no organic solutions or organic surfactants/ligands were used. Their magnetic performance and the formation mechanism of Fe-Pt alloy nanoparticles were also investigated. This strategy can be applied to synthesize many other types of alloy nanoparticles with desired composition and necessary crystal structure, which can be used for a variety of practical applications, such as in magnetism and catalyst research fields.
 Select Evolution of the microstructure and solute distribution of Sn-10wt% Bi alloys during electromagnetic field-assisted directional solidification Zhe Shen, Minghu Peng, Dongsheng Zhu, Tianxiang Zheng, Yunbo Zhong, Weili Ren, Chuanjun Li, Weidong Xuan, Zhongming Ren J. Mater. Sci. Technol., 2019, 35 (4): 568-577.  DOI: 10.1016/j.jmst.2018.09.037 The effects of forced flows at different velocities on microstructure and solute distribution during the directional solidification of Sn-10 wt% Bi alloys under a simultaneous imposition of a transverse static magnetic field (TSMF) and an external direct current (DC) have been investigated experimentally and numerically. The experimental results show that the solid-liquid interface will gradually become sloping with the increase of the forced flow velocity when the thermoelectric magnetic convection (TEMC) dominates the forced flow at solidification front. However, the interface will gradually become planar as the flow velocity further increases when the electromagnetic convection (EMC) dominates the forced flow. Moreover, when the flow velocity gradually increases, the primary dendrite spacing decreases from 384 to 105 μm accordingly. The simulation results show that the solute distribution at the two sides of the sample can be significantly changed by the forced flow at solidification front. The rejected solute will be unidirectionally transported to one side of the sample along the TEMC (a low-velocity forced flow), thereby causing the formation of a sloping interface. However, the rejected solute will be returned back along the EMC (a higher-velocity force flow), which results in a planar interface. Furthermore, the solute content at the two sides of the sample under the forced flows at different velocities was measured. The results are in good agreement with the simulation results, which shows that the solute content difference between the two sides of the sample reaches the maximum when a 0.5 T TSMF is applied, while the solute content difference decreases to zero with a simultaneous application of a 0.5 T TSMF and a 1.6 × 105 A/m2 external DC.
 Select Strengthening FCC-CoCrFeMnNi high entropy alloys by Mo addition Gang Qin, Ruirun Chen, Huiting Zheng, Hongze Fang, Liang Wang, Yanqing Su, Jingjie Guo, Hengzhi Fu J. Mater. Sci. Technol., 2019, 35 (4): 578-583.  DOI: 10.1016/j.jmst.2018.10.009 In order to strengthen the face-centered-cubic (FCC) type CoCrFeMnNi high entropy alloys (HEAs), different contents of Mo (0-16 at.%, similarly hereinafter) were alloyed. Phase evolution, microstructure, mechanical properties and related mechanism of these HEAs were systematically studied. The results show that sigma phase is appeared with addition of Mo, and the volume fraction of it increases gradually from 0 to 66% with increasing Mo content. It is found that Mo is enriched in sigma phase, which indicates that Mo element is beneficial to form sigma phase. Compressive testing shows that the yield strength of the alloys increases gradually from 216 to 765 MPa, while the fracture strain decreases from 50% (no fracture) to 19% with increasing of Mo. The alloy exhibits the best compressive performance when Mo content reaches 11%, the yield strength, fracture strength and fracture strain are 547 MPa, 2672 MPa and 44% respectively. The increased volume fraction of sigma phase plays an important role in improving the compressive strength of (CoCrFeMnNi)100-xMox HEAs.
 Select General trends in surface stability and oxygen adsorption behavior of transition metal diborides (TMB2) Wei Sun, Fuzhi Dai, Huimin Xiang, Jiachen Liu, Yanchun Zhou J. Mater. Sci. Technol., 2019, 35 (4): 584-590.  DOI: 10.1016/j.jmst.2018.10.012 The potential applications of transition metal diborides (TMB2) in extreme environments are particularly attractive but still blocked by some intrinsic properties such as poor resistances to thermal shock and oxidation. Since surface plays a key role during grain growth and oxygen adsorption, an insight into the surface properties of TMB2 is essential for understanding the materials performance and accelerating the development of ultra-high temperature ceramics. By employing two-region modeling method, the stability and oxygen adsorption behavior of TMB2 surfaces were investigated by first-principles calculations based on density functional theory. The effects of valance electron concentration on the surface stability and oxygen adsorption were studied and the general trends were summarized. After analyzing the anisotropy in surface stability and oxygen adsorption, the observed grain morphology of TMB2 were well explained, and it was also predicted that YB2, HfB2 and TaB2 may have better initial oxidation resistance than ZrB2.
 Select Analysis of local crystallographic orientation in an annealed Ti60 billet Z.B. Zhao, Z. Liu, Q.J. Wang, J.R. Liu, R. Yang J. Mater. Sci. Technol., 2019, 35 (4): 591-595.  DOI: 10.1016/j.jmst.2018.10.014 A heterogeneous microstructure in terms of local orientation distribution is often found in near-α titanium alloys. The presence of large regions with similar crystallographic orientation, called ‘macrozones’, could drastically decrease the fatigue performance of titanium alloys. The present work reports on the crystallographic orientation of a near-α titanium alloy, Ti60, billet after annealing in an α+β phase field. The texture was found to be weak, and no obvious macrozone was found in our measured zone where the variant selection is suppressed. Meanwhile, in-depth electron backscattered diffraction (EBSD) analysis was applied to evaluate the final microstructure, and the mechanisms by which they formed were analyzed.
 Select Effect of chloride ion on corrosion behavior of low carbon steel in 0.1 M NaHCO3 solution with different dissolved oxygen concentrations Fang Xue, Xin Wei, Junhua Dong, Changgang Wang, Wei Ke J. Mater. Sci. Technol., 2019, 35 (4): 596-603.  DOI: 10.1016/j.jmst.2018.10.001 The corrosion behavior of low carbon steel in 0.1 M NaHCO3 + 0.1 M NaCl solution with different dissolved oxygen concentrations was investigated with gravimetric tests and electrochemical measurements. Results show that the corrosion mass loss of steel is remarkably increased with the addition of chloride ion. In the initial stage, the carbon steel tends to active dissolution due to the dissolution effect of chloride ion on the oxide film; as a result, the corrosion potential maintains as low values. With the immersion time going by, the corrosion potential of the steel is promoted as a result of the accumulation of the corrosion products. However, the rust layer is loose and porous due to the deteriorating effect of chloride ion, which decreases the reduction resistance of oxygen. Meanwhile, the porous rust layer could be repaired by the depolarization of oxygen. Under the synergistic effect of chloride ion and oxygen, the corrosion of the steel is accelerated during the repeated process of dissolution and reparation of the oxide film.
 Select Performance optimization of dye-sensitized solar cells by gradient-ascent architecture of SiO2@Au@TiO2 microspheres embedded with Au nanoparticles Mingyue Li, Na Yuan, Yiwen Tang, Ling Pei, Yongdan Zhu, Jiaxian Liu, Lihua Bai, Meiya Li J. Mater. Sci. Technol., 2019, 35 (4): 604-609.  DOI: 10.1016/j.jmst.2018.09.030 Highly homogeneous, well dispersed SiO2@Au@TiO2 (SAT) microspheres decorated with Au nanoparticles (AuNPs) were prepared and incorporated into the photoanode with an optimized concentration gradient-ascent. The effects of SAT microspheres and the gradient-ascent architecture on the light absorption and the photoelectric conversion efficiency (PCE) of the dye-sensitized solar cells (DSSCs) were investigated. Studies indicate that the introduction of SAT microspheres and the gradient-ascent architecture in the photoanode significantly enhance the light scattering and harvesting capability of the photoanode. The DSSC with the optimized SAT gradient-ascent photoanode has the maximum short circuit current density (Jsc) of 17.7 mA cm-2 and PCE of 7.75%, remarkably higher than those of the conventional DSSC by 23.7% and 28.0%, respectively. This significantly enhancement of the performance of the DSSC can be attributed to the excellent light reflection/scattering of SAT, the localized surface plasma resonance (LSPR) effect of AuNPs within the microspheres, and the gradient-ascent architecture of SAT microspheres inside the photoanode. This study demonstrates that the tri-synergies of the scattering of SAT microspheres, the LSPR of AuNPs and the gradient-ascent architecture can effectively improve the PCE of DSSC.
 Select Creation of passivated Nb/N p-n co-doped ZnO nanoparticles and their enhanced photocatalytic performance under visible light illumination Shuang Gao, Weiyi Yang, Jun Xiao, Bo Li, Qi Li J. Mater. Sci. Technol., 2019, 35 (4): 610-614.  DOI: 10.1016/j.jmst.2018.09.056 Passivated niobium/nitrogen (Nb-N) p-n co-doped zinc oxide nanoparticles were created by a simple precipitation process with in-situ self-formed NaCl “cage” to confine the nanoparticle growth followed by the heat treatment in a flow of ammonia gas. Enhanced optical absorbance into the visible light region was observed in the Nb/N co-doped ZnO nanoparticle photocatalyst due to the Nb/N co-doping effect. It demonstrated a largely enhanced photocatalytic performance in the disinfection of Escherichia coli bacteria under visible light illumination, which could be attributed to the passivated co-doping of Nb-N to suppress the photogenerated charge carrier recombination on dopants. This robust approach for passivated p-n co-doping may also be applied to other material systems for a wide range of technical applications.
 Select In-situ synthesis of TiO2 nanostructures on Ti foil for enhanced and stable photocatalytic performance Ke Wang, Baodan Liu, Jing Li, Xiaoyuan Liu, Yang Zhou, Xinglai Zhang, Xiaoguo Bi, Xin Jiang J. Mater. Sci. Technol., 2019, 35 (4): 615-622.  DOI: 10.1016/j.jmst.2018.09.053 TiO2 nanostructures with strong interfacial adhesion and diverse morphologies have been in-situ grown on Ti foil substrate through a multiple-step method based on conventional plasma electrolytic oxidation (PEO) technology, hydrothermal reaction and ion exchange process. The PEO process is critical to the formation of TiO2 seeding layer for the nucleation of Na2Ti3O7 and H2Ti3O7 mediates that are strongly attached to the Ti foil. An ion exchange reaction can finally lead to the formation of H2Ti3O7 nanostructures with diverse morphologies and the calcination process can turn the H2Ti3O7 nanostructures into TiO2 nanostructures with enhanced crystallinity. The morphology of the TiO2 nanostructures including nanoparticles (NP), nanowhiskers (NWK), nanowires (NW) and nanosheets (NS) can be easily tailored by controlling the NaOH concentration and reaction time during hydrothermal process. The morphology, composition and optical properties of TiO2 photocatalysts were analyzed using scanning electron microscope (SEM), X-ray diffraction (XRD), photoluminescence (PL) spectroscopy and UV-vis absorption spectrum. Photocatalytic tests indicate that the TiO2 nanosheets calcined at 500 °C show good crystallization and the best capability of decomposing organic pollutants. The decoration of Ag cocatalyst can further improve the photocatalytic performance of the TiO2 nanosheets as a result of the enhanced charger separation efficiency. Cyclic photocatalytic test using TiO2 nanostructures grown on Ti foil substrate demonstrates the superior stability in the photodegradation of organic pollutant, suggesting the promising potential of in-situ growth technology for industrial application.
 Select Wear behavior of light-weight and high strength Fe-Mn-Ni-Al matrix self-lubricating steels Liuliu Han, Kun Li, Cheng Qian, Jingwen Qiu, Chengshang Zhou, Yong Liu J. Mater. Sci. Technol., 2019, 35 (4): 623-630.  DOI: 10.1016/j.jmst.2018.09.070 The good combination of mechanical and tribological properties for self-lubricating materials is crucial. In this work, novel self-lubricating Fe-16.4Mn-4.8Ni-9.9Al-xC (wt%) steels containing graphite phase were fabricated using mechanical alloying and spark plasma sintering. The compositions of the steels were designed by using thermodynamic calculation, and the effect of carbon addition on the microstructure was further investigated. The steel possesses high hardness of 621 HV, high yield strength of 1437 MPa and good fracture toughness at room temperature. The yield strengths are still above 600 MPa at 600 °C. The tribological behavior and mechanical properties from room temperature to 800 °C were studied, and the wear mechanisms at elevated temperatures were discussed. The steel has a stable friction coefficient of 0.4 and wear rate in a magnitude of 10-6 mm3/N·m below 600 °C. The good tribological properties of the steels were mainly attributed to the high hardness, lubrication of graphite and stable surface oxide layer.
 Select Toward a better understanding of microbiologically influenced corrosion caused by sulfate reducing bacteria Tingyue Gu, Ru Jia, Tuba Unsal, Dake Xu J. Mater. Sci. Technol., 2019, 35 (4): 631-636.  DOI: 10.1016/j.jmst.2018.10.026 Sulfate reducing bacteria (SRB) are often the culprits of microbiologically influenced corrosion (MIC) in anoxic environments because sulfate is a ubiquitous oxidant. MIC of carbon steel caused by SRB is the most intensively investigated topic in MIC because of its practical importance. It is also because biogenic sulfides complicate mechanistic SRB MIC studies, making SRB MIC of carbon steel is a long-lasting topic that has generated considerable confusions. It is expedient to think that biogenic H2S secreted by SRB acidifies the broth because it is an acid gas. However, this is not true because endogenous H2S gets its H+ from organic carbon oxidation and the fluid itself in the first place rather than an external source. Many people believe that biogenic H2S is responsible for SRB MIC of carbon steel. However, in recent years, well designed mechanistic studies provided evidence that contradicts this misconception. Experimental data have shown that cathodic electron harvest by an SRB biofilm from elemental iron via extracellular electron transfer (EET) for energy production by SRB is the primary cause. It has been demonstrated that when a mature SRB biofilm is subjected to carbon source starvation, it switches to elemental iron as an electron source and becomes more corrosive. It is anticipated that manipulations of EET related genes will provide genetic-level evidence to support the biocathode theory in the future. This kind of new advances will likely lead to new gene probes or transcriptomics tools for detecting corrosive SRB strains that possess high EET capabilities.
 Select Passivity breakdown on 436 ferritic stainless steel in solutions containing chloride Jiaming Wang, Shengsheng Qian, Yanhui Li, Digby D. Macdonald, Yiming Jiang, Jin Li J. Mater. Sci. Technol., 2019, 35 (4): 637-643.  DOI: 10.1016/j.jmst.2018.10.030 Passivity breakdown on 436 ferritic stainless steel (FSS) has been investigated in solutions containing different concentrations of chloride at 25 °C and interpreted in terms of the point defect model (PDM). The measured near-normal distributions of passivity breakdown potentials for 436 FSS under experimental conditions are in good agreement with the calculated results according to the PDM. The linear dependence of breakdown potential on the square root of potential scanning rate, which was described by the PDM, provides the estimation of the critical concentration of condensed vacancies at the metal/film interface, which leads to the passivity breakdown. This value is in good agreement with that calculated from the microstructure properties of the alloy substrate and the barrier layer of the passive film. This study demonstrates the validity of the PDM in describing the passivity breakdown on 436 FSS in NaCl solutions.
 Select Nitrogen-doped amorphous carbon coated mesocarbon microbeads as excellent high rate Li storage anode materials Zhimin Zou, Chunhai Jiang J. Mater. Sci. Technol., 2019, 35 (4): 644-650.  DOI: 10.1016/j.jmst.2018.10.016 A composite anode material consisting of a stable inner core of mesocarbon microbeads and a porous nitrogen-doped amorphous carbon shell active for lithium storage is prepared. The thin birnessite MnO2 nanosheets hydrothermally deposited on mesocarbon microbeads are in situ replaced by polypyrrole, which is then transformed to nitrogen-doped amorphous carbon layer by calcination in nitrogen atmosphere. The surface modified mesocarbon microbeads exhibit average discharge capacities of 444 and 103 mA h g-1 at the current densities of 0.1 and 3 A g-1, respectively, obvious higher than the corresponding values of the bare sample, 371 and 60 mA h g-1. Moreover, the composite anode maintains a discharge capacity of 306 mA h g-1 after 500 cycles at 1 A g-1, suggesting an excellent cycle stability. It is believed that the nitrogen-doped amorphous carbon layer has provided additional lithium storage capacity and stabilized the structure integrity of mesocarbon microbeads. This work demonstrates that the capacity and rate performance of commercial graphitic carbons can be much improved by simply introducing a nitrogen-doped carbon coating layer active for Li storage, making them attractive for high power Li-ion batteries.
 Select Electrochemical corrosion behavior of 2A02 Al alloy under an accelerated simulation marine atmospheric environment Min Cao, Li Liu, Zhongfen Yu, Lei Fan, Ying Li, Fuhui Wang J. Mater. Sci. Technol., 2019, 35 (4): 651-659.  DOI: 10.1016/j.jmst.2018.09.060 The corrosion behavior of 2A02 Al alloy under simulated marine atmospheric environment has been studied using mass-gain, scanning electron microscope/energy dispersive spectroscopy (SEM/EDS), laser scanning confocal microscopy, X-ray diffraction spectroscopy and localized electrochemical methods. The results demonstrate that the relationship between the corrosion induced mass-gain and the corrosion time is in accordance with the power rule. The mass-gain increases gradually during the corrosion time, while the corrosion rate decreases. With ongoing of the corrosion, corrosion products film changed from a porous to a compact structure. The various spectroscopic data show that the corrosion products films composed mainly of Al(OH)3, Al2O3 and AlCl3. The electrochemical corrosion behavior of the 2A02 Al alloy was studied by electrochemical impedance spectroscopy (EIS).
 Select Interaction of {11$\bar{2}$2} twin variants in hexagonal close-packed titanium Xiaocui Li, Jingwei Li, Bo Zhou, Mingchao Yu, Manling Sui J. Mater. Sci. Technol., 2019, 35 (4): 660-666.  DOI: 10.1016/j.jmst.2018.09.049 As multiple {11$\bar{2}$ 2} twin variants are often formed during deformation in hexagonal close-packed (hcp) titanium, the twin-twin interaction structure has a profound influence on mechanical properties. In this paper, the twin-twin interaction structures of the {11$\bar{2}$2} contraction twin in cold-rolled commercial purity titanium were studied by using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Formation of the {11$\bar{2}$2} twin variants was found to deviate the rank of Schmid factor, and the non-Schmid behavior was explained by the high-angle grain boundary nucleation mechanism. All the observed twin-twin pairs manifested a quilted-looking structure, which consists of the incoming twins being arrested by the obstacle twins. Furthermore, the quilted-looking {11$\bar{2}$2} twin-twin boundary was revealed by TEM and high resolution TEM observations. De-twinning, lattice rotation and curved twin boundary were observed in the obstacle twin due to the twin-twin reaction with the impinging twin. A twin-twin interaction mechanism for the {11$\bar{2}$2} twin variants was proposed in terms of the dislocation dissociation, which will enrich the understanding for the propagation of twins and twinning-induced hardening in hcp metals and alloys.
 Select Fabrication of flower-like mesoporous TiO2 hierarchical spheres with ordered stratified structure as an anode for lithium-ion batteries Yujie Zheng, Bingjie Liu, Pei Cao, Hui Huang, Jing Zhang, Guowei Zhou J. Mater. Sci. Technol., 2019, 35 (4): 667-673.  DOI: 10.1016/j.jmst.2018.10.028 In this study, flower-like mesoporous TiO2 hierarchical spheres (FMTHSs) with ordered stratified structure and TiO2 nanoparticles (TNPs) were synthesized via a facile solvothermal route and an etching reaction. Multilamellar vesicles (MTSVs) and unilamellar TiO2/SiO2 vesicles (UTSVs) were prepared using cetyltrimethylammonium bromide and didodecyldimethylammonium bromide as structure-directing agents under different solvothermal conditions. FMTHSs and TNPs were obtained from the etching reactions of MTSVs and UTSVs, respectively, in an alkaline system. FMTHSs display flower-like, ordered stratified structures on each petal. The thickness of the ordered stratified structure is approximately 3-6 nm, and the number of layers is approximately 2-4. The FMTHSs2 electrode exhibits the first discharge capacity of 212.4 mA h g-1 at 0.2 C, which is higher than that of TNPs electrode (167.6 mA h g-1). The discharge specific capacity of FMTHSs2 electrode after 200 cycles at 1 C is 105.9 mA h g-1, which is higher than that of TNPs electrode (52.2 mA h g-1) after the same number of cycles. The outstanding performance of FMTHSs2 electrode is attributed to the advantages of FMTHSs. In particular, their own stratified structure can provide additional active sites for reactions. The hierarchical structure can provide short diffusion length for Li+, large electrolyte-electrode contact area, and superior accommodation of the strain of Li+ intercalation/deintercalation.
 Select Ionic liquids for electrochemical energy storage devices applications Huan Liu, Haijun Yu J. Mater. Sci. Technol., 2019, 35 (4): 674-686.  DOI: 10.1016/j.jmst.2018.10.007 Ionic liquids, defined here as room-temperature molten salts, composed mainly of organic cations and (in)organic anions ions that may undergo almost unlimited structural variations with melting points below 100 °C. They offer a unique series of physical and chemical properties that make them extreme important candidates for several energy applications, especially for clean and sustainable energy storage and conversion materials and devices. Ionic liquids exhibit high thermal and electrochemical stability coupled with low volatility, create the possibility of designing appropriate electrolytes for different type batteries and supercapacitors. Herein, varieties of ionic liquids applications are reviewed on their utilization as electrolytes for Li-ion batteries, Na-ion batteries, Li-O2(air) batteries, Li-Sulfur (Li-S) batteries, supercapacitors and as precursors to prepare and modify the electrode materials, meanwhile, some important research results in recent years are specially introduced, and the perspective on novel application of ionic liquids is also discussed.
 Select A modified θ projection model for constant load creep curves-II. Application of creep life prediction Chao Fu, Yadong Chen, Xiaofei Yuan, Sammy Tin, Stoichko Antonov, Koichi Yagi, Qiang Feng J. Mater. Sci. Technol., 2019, 35 (4): 687-694.  DOI: 10.1016/j.jmst.2018.09.035 To minimize the deviation of the predicted creep curves obtained under constant load conditions by the original θ projection model, a new modified version that can be expressed by ε=θ11-e-θ2t+θ3eθ4eθ5εt-1, was derived and experimentally validated in our last study. In the present study, the predictive capability of the modified θ projection model was investigated by comparing the simulated and experimentally determined creep curves of K465 and DZ125 superalloys over a range of temperatures and stresses. Furthermore, the linear relationship between creep temperature and initial stress was extended to the 5-parameter model. The results indicated that the modified model could be used as a creep life prediction method, as it described the creep curve shape and resulted in predictions that fall within a specified error interval. Meanwhile, this modified model provides a more accurate way of describing creep curves under constant load conditions. The limitations and future direction of the modified model were also discussed. In addition, this modified θ projection model shows great potential for the evaluation and assessment of the service safety of structural materials used in components governed by creep deformation.
 Select Electrochemical property of multi-layer anode supported solid oxide fuel cell fabricated through sequential tape-casting and co-firing Xiaoyang Chen, Weijie Ni, Xiaojia Du, Zaihong Sun, Tenglong Zhu, Qin Zhong, Minfang Han J. Mater. Sci. Technol., 2019, 35 (4): 695-701.  DOI: 10.1016/j.jmst.2018.10.015 In this work, a multi-layer anode supported solid oxide fuel cell (SOFC) is designed and successfully prepared through sequential tape casting and co-firing. The single cell is consisted of NiO-3YSZ (3YSZ: 3 mol.% yttria doped zirconia) anode support, NiO-8YSZ (8YSZ: 8 mol.% yttria stabilized zirconia) anode functional layer, dense 8YSZ electrolyte layer, and porous 3YSZ cathode scaffold layer with infiltrated La0.6Sr0.4Co0.2Fe0.8O3-δ cathode. The clear interfaces and good contacts between each layer, without element inter-diffusion being observed, suggest that this sequential tape casting and co-firing is a feasible and successful route for anode supported single cell fabrication. This cell exhibits remarkable high open circuit voltage of 1.097 V at 800°C under room temperature humidified hydrogen, with highly dense and gastight electrolyte layer. It provides a power density of 360 mW/cm2 under operation voltage of 0.75 V at 800°C and a stable operation of ～110 h at 750°C under current density of -300 mA/cm2. Furthermore, this cell also presents encouraging electrochemical responses under various anode hydrogen partial pressures and maintains high power output at low fuel concentrations.
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