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CN 21-1315/TG
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Current Issue
 01 August 2020, Volume 50 Issue 0 Previous Issue    Next Issue
Research Article
 Select Aqueous-solution-driven HfGdOx gate dielectrics for low-voltage-operated α-InGaZnO transistors and inverter circuits Yongchun Zhang, Gang He, Wenhao Wang, Bing Yang, Chong Zhang, Yufeng Xia J. Mater. Sci. Technol., 2020, 50 (0): 1-12.  DOI: 10.1016/j.jmst.2020.03.007 In this work, a non-toxic and environmentally friendly aqueous-solution-based method has been adopted to prepare gadolinium-doped hafnium oxide (HfO2) gate dielectric thin films. By adjusting the gadolinium (Gd) doping concentration, the oxygen vacancy content, band offset, interface trap density, and dielectric constant of HfGdOx (HGO) thin films have been optimized. Results have confirmed that HGO thin films with Gd doping ratio of 15 at.% have demonstrated appropriate dielectric constant of 27.1 and lower leakage current density of 5.8 × 10 -9 A cm -2. Amorphous indium-gallium-zinc oxide (α-IGZO) thin film transistors (TFTs) based on HGO thin film (Gd: 15 at.%) as gate dielectric layer have exhibited excellent electrical performance, such as larger saturated carrier mobility (μsat) of 20.1 cm 2 V -1 S -1, high on/off current ratio (Ion/Ioff) of ~10 8, smaller sub-threshold swing (SS) of 0.07 V decade -1, and a negligible threshold voltage shift (△VTH) of 0.08 V under positive bias stress (PBS) for 7200 s. To confirm its potential application in logic circuit, a resistor-loaded inverter based on HGO/α-IGZO TFTs has been constructed. A high voltage gain of 19.8 and stable full swing characteristics have been detected. As a result, it can be concluded that aqueous-solution-driven HGO dielectrics have potential application in high resolution flat panel displays and ultra-large-scale integrated logic circuits.
 Select Joining of SiO2f/SiO2 composite to Nb using Ag-Cu-In-Ti brazing alloys Bo Chen, Wen-Jiang Zou, Wen-Wen Li, Shi-Biao Wu, Hua-Ping Xiong, Xin Wu J. Mater. Sci. Technol., 2020, 50 (0): 13-20.  DOI: 10.1016/j.jmst.2019.08.002 Three compositions of Ag-Cu-In-Ti system brazing alloys were designed for joining SiO2f/SiO2 ceramic composite to Nb metal. The wettability of the three alloys on the composite was studied with the sessile drop method. The results showed that after heating at 1073 K for 30 min, they exhibited contact angles of 74°, 83° and 86°, respectively. The brazing alloys were fabricated into foils by rapid solidification technique. Among the three brazing filler alloys the joints brazed with AgCu-10In-5Ti at 1073 K for 10 min presented the maximum average shear strength of 30.9 MPa. During the brazing process the active element Ti diffused strongly from the filler alloy to the composite surface and a reaction layer with a thickness of 2-3 μm was formed. Sound metallurgical bonding was also achieved at the Nb side. The interface structure of the joint brazed with the AgCu-10In-5Ti alloy can be described as the following sequence: SiO2f/SiO2→SiO2+Cu3Ti3O→SiO2+TiO+Ti5Si4→Ag(s, s)+(Cu-Ti)→Nb. However, for the filler alloy with 4.0 wt% Ti content the joint strength was only 15.8 MPa.
 Select Effects of Cr addition on Charpy impact energy in austenitic 0.45C-24Mn-(0,3,6)Cr steels Seok Gyu Lee, Bohee Kim, Min Cheol Jo, Kyeong-Min Kim, Junghoon Lee, Jinho Bae, Byeong-Joo Lee, Seok Su Sohn, Sunghak Lee J. Mater. Sci. Technol., 2020, 50 (0): 21-30.  DOI: 10.1016/j.jmst.2019.12.032 Effects of Cr addition (0, 3, and 6 wt%) on Charpy impact properties of Fe-C-Mn-Cr-based steels were studied by conducting dynamic compression tests at room and cryogenic temperatures. At room temperature, deformation mechanisms of Charpy impacted specimens were observed as twinning induced plasticity (TWIP) without any transformation induced plasticity (TRIP) in all the steels. At cryogenic temperature, many twins were populated in the Cr-added steels, but, interestingly, fine ε-martensite was found in the 0Cr steel, satisfying the Shoji-Nishiyama (S–N) orientation relationship, {111}γ//{0002}ε and < 101 >γ//< $11\bar{2}0$ >ε. Even though the cryogenic-temperature staking fault energies (SFEs) of the three steel were situated in the TWIP regime, the martensitic transformation was induced by Mn- and Cr-segregated bands. In the 0Cr steel, SFEs of low-(Mn,Cr) bands lay between the TWIP and TRIP regimes which were sensitively affected by a small change of SFE. The dynamic compressive test results well showed the relation between segregation bands and the SFEs. Effects of Cr were known as not only increasing the SFE but also promoting the carbide precipitation. In order to identify the possibility of carbide formation, a precipitation kinetics simulation was conducted, and the predicted fractions of precipitated M23C6 were negligible, 0.4-1.1 × 10 -5, even at the low cooling rate of 10 °C/s.
 Select Preliminary study of microstructure, mechanical properties and corrosion resistance of antibacterial Ti-15Zr-xCu alloy for dental application Sharafadeen Kunle Kolawole, Wang Hai, Shuyuan Zhang, Ziqing Sun, Muhammad Ali Siddiqui, Ihsan Ullah, Wei Song, Frank Witte, Ke Yang J. Mater. Sci. Technol., 2020, 50 (0): 31-43.  DOI: 10.1016/j.jmst.2020.03.003 Ti-15Zr-xCu (3 ≤ x ≤ 7, wt.%) novel antibacterial and antibiofilm alloys with competitive mechanical properties, biological responses and corrosion resistance were designed and fabricated. Annealing heat treatment on Ti-15Zr-7Cu (TZC-7A), after holding for 2 h at slightly above their beta transus temperature (BTT) ensured their tensile strength (UTS), yield strength (YS) and hardness (HRV) were improved by 31.2%, 20% and 12.3% respectively compared to the control without Cu, Ti-15Zr (T-15ZA). Although the 3 wt.% Cu alloy displayed the highest elongation (26%), the TZC-7A alloy also possessed a good ductility. Presence of evenly dispersed Ti2Cu and Zr2Cu Cu-rich intermetallic phases formed as interwoven and alternating lamellae within the α + β matrix as a result of Cu addition, as revealed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). These greatly contributed to their strengthening and bactericidal properties. Over 98% antibacterial effect against E. coli and S. aureus have been imparted, coupled with excellent biofilm inhibition. Potentiodynamic polarization curves showed that the TZC-7A alloy possessed higher corrosion resistance than commercially pure titanium, cp-Ti; contact angle test revealed enhanced hydrophilicity; while confocal laser scanning microscopy (CLSM) and cell counting kit (CCK-8) assays also displayed drastically lowered bacterial adhesion rate with comparatively no cytotoxicity. Cell attachment on all alloys was similar but the best spread was obtained on TZC-7A after 24 h. The developed alloy has good potential as an antibacterial implant material with combination of optimized properties.
 Select Probing the compound effect of spatially varying intrinsic defects and doping on mechanical properties of hybrid graphene monolayers Kritesh Kumar Gupta, Tanmoy Mukhopadhyay, Aditya Roy, Sudip Dey J. Mater. Sci. Technol., 2020, 50 (0): 44-58.  DOI: 10.1016/j.jmst.2020.03.004 Doping in pristine 2D materials brings about the advantage of modulating wide range of mechanical properties simultaneously. However, intrinsic defects (such as Stone-Wales and nanopore) in such hybrid materials are inevitable due to complex manufacturing and synthesis processes. Besides that, defects and irregularities can be intentionally induced in a pristine nanostructure for multi-synchronous modulation of various multi-functional properties. Whatever the case may be, in order to realistically analyse a doped graphene sheet, it is of utmost importance to investigate the compound effect of doping and defects in such 2D monolayers. Here we present a molecular dynamics based investigation for probing mechanical properties (such as Young’s modulus, post-elastic behaviour, failure strength and strain) of doped graphene (C 14 and Si) coupling the effect of inevitable defects. Spatial sensitivity of defect and doping are systematically analyzed considering different rational instances. The study reveals the effects of individual defects and doping along with their possible compounded influences on the failure stress, failure strain, Young’s modulus and constitutive relations beyond the elastic regime. Such detailed mechanical characterization under the practically relevant compound effects would allow us to access the viability of adopting doped graphene in various multifunctional nanoelectromechanical devices and systems in a realistic situation.
 Select Unveiling the underlying mechanism of forming edge cracks upon high strain-rate rolling of magnesium alloy Biwu Zhu, Xiao Liu, Chao Xie, Jing Su, Pengcheng Guo, Changping Tang, Wenhui Liu J. Mater. Sci. Technol., 2020, 50 (0): 59-65.  DOI: 10.1016/j.jmst.2020.03.006 In the current study, high strain-rate rolling (≥10 s -1) has been successfully employed to produce Mg-3Al-1Zn alloy sheets to a high reduction of 82% with a fine grain structure in a single pass. The underlying mechanism of forming primary and secondary edge cracks has been investigated. It is found that dynamic recrystallization (DRX) induced by subgrains tends to blunt cracks, while twinning-induced DRX is mainly observed around sharp crack tips. The motion of emitted dislocations from blunted cracks is inhibited by the DRX grain boundaries. This, on one hand, increases local work hardening, and on the other hand, causes stress concentration along grain boundaries especially in the triple junctions leading to the formation of secondary cracks.
 Select Synthesis of monolithic carbon aerogels with high mechanical strength via ambient pressure drying without solvent exchange Zhi Yang, Jian Li, Xiaojing Xu, Shengyang Pang, Chenglong Hu, Penglei Guo, Sufang Tang, Hui-Ming Cheng J. Mater. Sci. Technol., 2020, 50 (0): 66-74.  DOI: 10.1016/j.jmst.2020.02.013 A simple, fast and cost-effective method for monolithic carbon aerogels (CAs) preparation was proposed through sol-gel polycondensation of resorcinol with formaldehyde in a basic aqueous solution followed by ambient pressure drying without solvent exchange, and carbonization. The microstructure and network strength of CAs were tailored by adjusting the catalyst concentration ([resorcinol]/[sodium carbonate] in the range of 300-2000), water content ([deionized water]/[resorcinol] equals to 17 and 24, respectively), and gelation temperature (Tgel in the range of 30-90°C). Resultantly, the CAs with a wide range of density (0.30-1.13 g/cm 3), high specific surface area (465-616 m 2/g), high compressive strength (6.5-147.4 MPa) and low thermal conductivity (0.065-0.120 W·m -1 K -1) were obtained in this work. Moreover, the large-sized CAs (100 × 100 × 20 mm 3) can also be prepared by this method since the formed robust skeleton network can resist shrinkage/collapse of pore structure and prevent cracking during drying. The improved mechanical strength and monolithic forming abilities could be mainly attributed to the uniform arrangement of carbon particles and pores, fine particle size, abundant network structure and enhanced particle neck.
 Select Influence of secondary phases of AlSi9Cu3 alloy on the plasma electrolytic oxidation coating formation process Ting Wu, Carsten Blawert, Mikhail L.Zheludkevich J. Mater. Sci. Technol., 2020, 50 (0): 75-85.  DOI: 10.1016/j.jmst.2019.12.031 To understand the formation and growth of plasma electrolytic oxidation (PEO) coatings in presence of different secondary phases in a substrate, PEO treatment was carried out on AlSi9Cu3 alloy at different treatment times ranging from 15?s to 480?s. The coating formation and evolution process was traced by surface and cross-sectional observation of the layers on the different phases of the alloy. The results demonstrated a sequential involvement of the different phases in the plasma discharges: firstly, discharges start on the α-Al matrix, then on the intermetallic Al2Cu and β-Al5FeSi phases at the same time and finally on the eutectic Si. The presence of intermetallic Al2Cu remarkably affects the initial dissolution, the deposition of conversion products and the ignition of discharges at the early stages of processing. Eutectic Si in the substrate exhibits the highest electrochemical stability at all stages and contributes in the beginning to a distinct coating morphology eventually. The resultant PEO coating tends to be uniform if processing times are longer and a double-layer structure appears in the coating.
 Select Controlling and adjusting the concentration distribution during solidification process using static magnetic fields Pinfang Jiang, Jiantao Wang, Long Hou, Yves Fautrelle, Xi Li J. Mater. Sci. Technol., 2020, 50 (0): 86-91.  DOI: 10.1016/j.jmst.2020.03.002 As concentration distribution changes have important effects on material structures and properties, controlling the concentration distribution is essential to alloy performance. The aim of the present work is to control and adjust the concentration distribution by the static magnetic field. It is found that the magnetic field disperses grain boundary segregation and causes the uniform distribution of concentration. Further, by the three-dimensional computed tomography (3D-CT) reconstruction, the flow distribution is seen and the effect mechanism of the magnetic field is revealed. The present work may clarify the ambiguous understanding on the effect of the static magnetic field on solidification process.
 Select Synthesis of Bi2S3/carbon nanocomposites as anode materials for lithium-ion batteries Jin Bai, Xiao Chen, Emilia Olsson, Huimin Wu, Shiquan Wang, Qiong Cai, Chuanqi Feng J. Mater. Sci. Technol., 2020, 50 (0): 92-102.  DOI: 10.1016/j.jmst.2020.01.045 Metal sulfides such as Bismuth sulfide (Bi2S3) hold immense potential to be promoted as anode materials for lithium-ion batteries (LIBs), owing to their high theoretical gravimetric and volumetric capacities. However, the poor electrical conductivity and volume expansion during cycling hinder the practical applications of Bi2S3. In this work, we used pyrrole and glucose as carbon source to design the surface carbon coating on the surface of Bi2S3 particles, to improve the structural stability of Bi2S3. Two composite materials were synthesized - Bi2S3 coated with nitrogen doped carbon (Bi2S3@NC), and Bi2S3 coated with carbon (Bi2S3@C). When used as anode active materials, both Bi2S3@NC and Bi2S3@C showed improved performance compared to Bi2S3, which confirms surface carbon coating as an effective and scalable way for the modification of Bi2S3 material. The electrode based on Bi2S3@NC materials demonstrated higher performance than that of Bi2S3@C, with an initial discharge capacity of 1126.5?mA?h/g, good cycling stability (500?mA?h/g after 200 cycles at 200?mA/g) and excellent rate capability. Finally, Li storage and migration mechanisms in Bi2S3 are revealed using first principle density functional theory calculations.
 Select Achieving a strong polypropylene/aluminum alloy friction spot joint via a surface laser processing pretreatment S.C. Han, L.H. Wu, C.Y. Jiang, N. Li, C.L. Jia, P. Xue, H. Zhang, H.B. Zhao, D.R. Ni, B.L. Xiao, Z.Y. Ma J. Mater. Sci. Technol., 2020, 50 (0): 103-114.  DOI: 10.1016/j.jmst.2020.02.035 Strong metal/non-polar plastic dissimilar joints are highly demanded for the lightweight design in many fields, which, however, are rather challenging to achieve directly via welding. In this study, we designed a laser processing pretreatment on the Al alloy to create a deep porous Al surface structure, which was successfully joined to the polypropylene (PP) via friction spot welding. A maximum joint strength of 29 MPa was achieved, the same as that of the base PP (i.e. the joint efficiency reached 100%), much larger than ever reported. The joining mechanism of the Al alloy and the PP was mainly attributed to the large mechanical interlocking effect between the laser processed Al porous structure and the re-solidified PP and the formation of chemical bond at the interface. The deep porous Al surface structure modified by laser processing largely changed the Al-PP reaction feature. The evidence of the C—O—Al chemical bond was first time found at the non-polar plastic/Al joint interface, which was the reaction result between the oxide on the Al alloy surface and thermal oxidization products of the PP during welding. This study provides a new way for enhancing metal-plastic joints via surface laser treatment techniques.
 Select A data-driven framework to predict the morphology of interfacial Cu6Sn5 IMC in SAC/Cu system during laser soldering Anil Kunwar, Lili An, Jiahui Liu, Shengyan Shang, Peter Råback, Haitao Ma, Xueguan Song J. Mater. Sci. Technol., 2020, 50 (0): 115-127.  DOI: 10.1016/j.jmst.2019.12.036 A data-driven approach combining together the experimental laser soldering, finite element analysis and machine learning, has been utilized to predict the morphology of interfacial intermetallic compound (IMC) in Sn-xAg-yCu/Cu (SAC/Cu) system. Six types of SAC solders with varying weight proportion of Ag and Cu, have been processed with fiber laser at different magnitudes of power (30-50 W) and scan speed (10-240 mm/min), and the resultant IMC morphologies characterized through scanning electron microscope are categorized as prismatic and scalloped ones. For the different alloy composition and laser parameters, finite element method (FEM) is employed to compute the transient distribution of temperature at the interface of solder and substrates. The FEM-generated datasets are supplied to a neural network that predicts the IMC morphology through the quantified values of temperature dependent Jackson parameter (αJ). The numerical value of αJ predicted from neural network is validated with experimental IMC morphologies. The critical scan speed for the morphology transition between prismatic and scalloped IMC is estimated for each solder composition at a given power. Sn-0.7Cu having the largest critical scan speed at 30 W and Sn-3.5Ag alloy having the largest critical scan speed at input power values of 40 W and 50 W, thus possessing the greatest likelihood of forming prismatic interfacial IMC during laser soldering, can be inferred as most suitable SAC solders in applications exposed to shear loads.
 Select Designing new work-hardenable ductile Ti-based multilayered bulk metallic glass composites with ex-situ and in-situ hybrid strategy Shifeng Lin, Zhengwang Zhu, Shaofan Ge, Long Zhang, Dingming Liu, Yanxin Zhuang, Huameng Fu, Hong Li, Aimin Wang, Haifeng Zhang J. Mater. Sci. Technol., 2020, 50 (0): 128-138.  DOI: 10.1016/j.jmst.2019.12.037 To overcome the trade-off between the devisable microstructure and the excellent tensile ductility of bulk metallic glass composites (BMGCs), a novel ex-situ and in-situ hybrid strategy is successfully proposed to design a series of the work-hardenable ductile Ti-based multilayered BMGCs (ML-BMGCs). The as-prepared ML-BMGCs, consisting of α-phases, β-phases and amorphous phases, exhibit a controllable multilayered structure of the Ti layers and the amorphous layers with alternative distribution. The size and volume fraction of the crystalline phases are tuned by Nb microalloying. It is found that the ML-BMGCs possess a suitable size and volume fraction of the crystalline phases when Nb microalloying content are 5% (at.) or 8% (at.), and they obtain an optimum combination of the specific strength of 243 MPa g kg 1 or 216 MPa g kg -1, and tensile plasticity of 4.33%±0.1% or 5.10%±0.1%. The deformation mechanism of the as-prepared ML-BMGCs during tension is also revealed. The ex-situ Ti layers and in-situ dendrites together effectively serve as absorbers to suppress the propagation of shear bands and multiply shear bands. And the deformation of ex-situ α-Ti phases by dislocation slip and the transformation from in-situ metastable β-Ti phase to orthorhombic α"-Ti during tension impart significant work-hardening capability to the ML-BMGCs. The present study provides a guidance of developing novel high-performance BMGCs with a controllable microstructure.
 Select One-step preparation of porous aminated-silica nanoparticles and their antibacterial drug delivery applications Yongren Wu, Shun Chen, Yang Liu, Zhiwei Lu, Shaokun Song, Yang Zhang, Chuanxi Xiong, Lijie Dong J. Mater. Sci. Technol., 2020, 50 (0): 139-146.  DOI: 10.1016/j.jmst.2019.12.015 Porous functionalized silica nanoparticles have attracted the interest of researchers as they are excellent carriers for antibacterial drug delivery applications. In this work, porous aminated-silica nanoparticles (SiO2-NH2 NPs) were prepared via one-step approach through the ammonia-catalyzed hydrolysis of tetraethylorthosilicate (TEOS) and (3-aminopropyl) triethoxysilane (APTES) in a mixed water-ethanol system. The obtained SiO2-NH2 NPs displayed a spherical morphology and relatively uniform size distribution, while the morphology and structure of SiO2-NH2 NPs were mainly determined by the order of the reagents added and the pH value of the solution. After characterization, the results showed that there were a large number of -NH2 groups on the surface of porous SiO2-NH2 NPs and that the porous SiO2-NH2 NPs had a large surface area of 476 m2 g -1 with an average pore width of 4.3 nm. Through an absorbing-releasing experiment and bacterial test, those SiO2-NH2 NPs were found to exhibit efficient absorption and release of drugs as well as a pH-dependent release pattern of epirubicin-loaded SiO2-NH2 NPs. Meanwhile, SiO2-NH2@capsaicin NPs exhibited antibacterial properties. Those porous SiO2-NH2 NPs could be a candidate for drug delivery for antibacterial applications owing to their tailored porous structure and high surface area.
 Select A hybrid artificial photosynthesis system with molecular catalysts covalently linked onto TiO2 as electron relay for efficient photocatalytic hydrogen evolution Jie Zhang, Gehong Zhang, Jing Zhang J. Mater. Sci. Technol., 2020, 50 (0): 147-152.  DOI: 10.1016/j.jmst.2019.12.028 Efficient charge carrier transfer from light harvesters to catalysts greatly determines the photocatalytic activity in an artificial photosynthesis (AP) system for solar hydrogen evolution. In this study, an AP system composed of xanthene dye as light harvester and cobaloxime molecular complex as catalyst, with TiO2 as electron relay, was designed for photocatalytic hydrogen evolution under visible light (λ > 420 nm). It was demonstrated that with cobaloxime molecule covalently linked onto the TiO2 electron relay, the resulting hybrid AP system exhibited much increased photocatalytic activity as compared to that without TiO2. The greatly increased photocatalytic activity should be due to the efficient electron transfer from xanthene dye as light harvester and cobaloxime molecular complex as catalyst, shuttled by the TiO2 electron relay, for the following water reduction reaction. The present study demonstrates a facile and feasible strategy to guide the design of high performance AP systems through the electron relay shuttled and promoted charge transfer process.
 Select Microstructural evolution and stress state related to mechanical properties of electron beam melted Ti-6Al-4V alloy modified by laser shock peening Liang Lan, Xinyuan Jin, Shuang Gao, Bo He, Yonghua Rong J. Mater. Sci. Technol., 2020, 50 (0): 153-161.  DOI: 10.1016/j.jmst.2019.11.039 This work characterizes microstructural evolutions of electron beam melted (EBM) Ti-6Al-4V alloy modified via laser shock peening (LSP). The depth stress distribution and tensile properties of EBM Ti-6Al-4V alloy were measured before and after LSP. The results indicate that microstructure consists of β phase with 7.2% ± 0.4% vol.% and balance α lamellar in EBM sample, and the α lamella was refined into nano-equiaxed grains and submicro-equiaxed grains after LSP. The dominant refinement mechanism is revealed during LSP. Stacking faults were found in the LSP-treated sample, and their corresponding planes were determined as (0001) basal plane, (10$\bar{1}$0) prismatic plane, and (10$\bar{1}$$\bar{1}$) pyramidal plane obtained by high resolution transmission electron microscopy. The subgrains and high-angle grains formed during dynamic recrystallization were identified by selected area electron diffraction pattern. The LSP treatment produces a significantly residual compressive stress approximately -380 MPa with the depth of compressive stress layer reaching 450 μm. Strength and elongation of the EBM sample were significantly increased after LSP. The strength and ductility enhancements are attributed to compressive stress, grain refinement and grain gradient distribution of α phase.
 Select Isothermal γ → ε phase transformation behavior in a Co-Cr-Mo alloy depending on thermal history during electron beam powder-bed additive manufacturing Yufan Zhao, Yuichiro Koizumi, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba J. Mater. Sci. Technol., 2020, 50 (0): 162-170.  DOI: 10.1016/j.jmst.2019.11.040 Powder bed fusion with electron beam (PBF-EB), allows Co-Cr-Mo (CCM) implants with patient-customization to be fabricated with high quality and complex geometry. However, the variability in the properties of PBF-EB-built CCM alloy, mainly due to the lack of understanding of the mechanisms that govern microstructural heterogeneity, brings limitations in extensive application. In this study, the microstructural heterogeneity regarding the γ-fcc → ε-hcp phase transformation was characterized. The phase transformation during PBF-EB was analyzed depending on the thermal history that was elucidated by the numerical simulation. It revealed that isothermal γ → ε transformation occurred during the fabrication. Importantly, the difference in γ/ε phase distribution was a result of the thermal history determining which method phase transformation was taking place, which can be influenced by the PBF-EB process parameters. In the sample with a low energy input ((${{E}_{\text{area}}}$ = 2.6 J/mm 2), the martensitic transformation was dominant. As the building height increased from the bottom, the ε phase fraction decreased. On the other hand, in the sample with a higher energy input ((${{E}_{\text{area}}}$ = 4.4 J/mm 2), the ε phase formed via diffusional-massive transformation and only appeared in a short range of the lower part away from the bottom.
 Select Resistance modulation in Ge2Sb2Te5 Jitendra K. Behera, WeiJie Wang, Xilin Zhou, Shan Guan, Wu Weikang, Yang A. Shengyuan, Robert E. Simpson J. Mater. Sci. Technol., 2020, 50 (0): 171-177.  DOI: 10.1016/j.jmst.2020.03.016 Chalcogenide based phase change random access memory (PCRAM) holds great promise for high speed and large data storage applications. This memory is scalable, requires a low switching energy, has a high endurance, has fast switching speed, and is nonvolatile. However, decreasing the switching time whilst increasing the cycle endurance is a key challenge for this technology to replace dynamic random access memory. Here we demonstrate high speed and high endurance ultrafast transient switching in the SET state of a prototypical phase change memory cell. Volatile switching is modeled by electron-phonon and lattice scattering on short timescales and charge carrier excitation on long timescales. This volatile switching in phase change materials enables the design of hybrid memory modulators and ultrafast logic circuits.
 Select Growth kinetics of MgH2 nanocrystallites prepared by ball milling Caiqin Zhou, Yayu Peng, Qingan Zhang J. Mater. Sci. Technol., 2020, 50 (0): 178-183.  DOI: 10.1016/j.jmst.2020.01.063 MgH2 is one of promising hydrogen storage materials due to its high hydrogen capacity of 7.6 wt%. However, MgH2 nanocrystallites easily grow up during hydrogen absorption―desorption cycling, leading to deterioration of hydrogen storage properties. To clarify the growth kinetics of MgH2 crystallites, the growth characteristics of MgH2 nanocrystallites are investigated in this work. The growth exponents of MgH2 nanocrystallites in pure MgH2 and MgH2―10 wt% Pr3Al11 samples are evaluated to be n = 5 and n = 6, respectively. Meanwhile, their activation energies for crystallite growth are also determined to be 109.2 and 144.2 kJ/mol, respectively. The increase of growth exponent and rise of activation energy for crystallite growth in MgH2―10 wt% Pr3Al11 composite are ascribed to the presence of nano-sized Pr3Al11 phase.
 Select Orange-red, green, and blue fluorescence carbon dots for white light emitting diodes Ling Jin, Li Zhang, Lei Yang, Xingrong Wu, Cheng Zhang, Kang Wei, Lifang He, Xinya Han, Hongbin Qiao, Abdullah M. Asiri, Khalid A. Alamry, Kui Zhang J. Mater. Sci. Technol., 2020, 50 (0): 184-191.  DOI: 10.1016/j.jmst.2020.03.020 Fluorescent carbon dots (CDs) are admirable nanomaterials due to their superior optical properties and low cost. However, regulation of the emission colors in CDs for lighting device still remains a challenge. In this work, we developed a green hydrothermal method to obtain three emission colors of CDs using l-tyrosine (for blue CDs), o-phenylendiamine (for green CDs) and l-tyrosine/o-phenylendiamine mixture (for orange-red CDs). These CDs possess good water dispersibility, strong emission with high quantum yields, and excellent photostability. Furthermore, the resulting CDs were dispersed in polyvinyl alcohol (PVA) matrix to yield solid state films, where the self-quenching effect in solid state was effectively avoided. A full-color light-emitting diode was fabricated by packing the CDs-PVA composites on the top of a UV-chip, which have superior potential applications in CDs-based solid-state lighting system.
 Select Achieving work hardening by forming boundaries on the nanoscale in a Ti-based metallic glass matrix composite Jing Fan, Wei Rao, Junwei Qiao, P.K. Liaw, Daniel Şopu, Daniel Kiener, Jürgen Eckert, Guozheng Kang, Yucheng Wu J. Mater. Sci. Technol., 2020, 50 (0): 192-203.  DOI: 10.1016/j.jmst.2020.02.036 Achieving work hardening in metallic glass matrix composites (MGMCs) is the key to the extensive use of these attractive materials in structural and functional applications. In this study, we investigated the formation of nanoscale boundaries resulted from the interaction between matrix and dendrites, which favors the work-hardening deformation in an in-situ Ti41Zr32Ni6Ta7Be14 MGMC with β-Ti dendrites in a glassy matrix at room temperature. The microstructures of samples after tension were observed by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The work-hardening mechanism of the present composites involves: (1) appearance of dense dislocation walls (DDWs), (2) proliferation of shear bands, (3) formation of boundaries on the nanoscale, and (4) interactions between hard and soft phases. A theoretical model combined with experimental data reveals the deformation mechanisms in the present work, proving that the in-situ dendrites with outstanding hardening ability in the glass matrix can provide the homogeneous deformation under tensile loading at room temperature.
 Select Duality of the fatigue behavior and failure mechanism in notched specimens of Ti-7Mo-3Nb-3Cr-3Al alloy Zhihong Wu, Hongchao Kou, Nana Chen, Mengqi Zhang, Ke Hua, Jiangkun Fan, Bin Tang, Jinshan Li J. Mater. Sci. Technol., 2020, 50 (0): 204-214.  DOI: 10.1016/j.jmst.2020.01.060 An interesting phenomenon of dual S-N fatigue behavior is investigated in a metastable β titanium alloy, Ti-7Mo-3Nb-3Cr-3Al notched cylindrical specimens with an elastic stress concentration factor of Kt = 3. Fractographic studies revealed all specimens, and irrespective of lifetime, failed from the specimen surface because of stress concentration occurs at the notch root. Typically, the short-life-distribution is usually associated with surface-failure-without-facets and the long-life-distribution generally occurs due to surface-failure-with-facets. This competing failure leads to increasing the variability in fatigue lifetime and further facilitates the difficulty in prediction of fatigue lifetime. Crack-initiation area characterization was conducted by using mechanical grinding, focused ion beam milling and subsequent electron back-scattered diffraction (EBSD) analysis of the 2D section across faceted grains. Results show that the αp particles (especially the elongated αp particles) well-oriented for basal slip activation is a preferential fatigue-critical microstructural configuration. Additionally, the β+αs matrix has a higher KAM value than the αp particles in fatigued microstructures and significant dislocation activity in the form of dislocation tangles is observed in αp boundaries.
 Select A review of high-strength nanolaminates and evaluation of their properties Mohammad Nasim, Yuncang Li, Ming Wen, Cuie Wen J. Mater. Sci. Technol., 2020, 50 (0): 215-244.  DOI: 10.1016/j.jmst.2020.03.011 Nanolaminates are composed of nanoscale-thick alternating layers of different materials and their properties are dependent on the individual layers, the layer thickness and the interfaces between the layers. Nanolaminates composed of cubic crystal structured metals are usually ductile compared to nanolaminates containing hexagonal crystal structured metals. Mechanical properties such as strength and hardness of nanolaminates increase with a decrease in individual layer thickness down to a few nanometers and they become independent when the thickness of individual layers is less than a couple of nanometers. This review provides a detailed analysis of the effects of individual layer thickness and the interface structures on the strength and the strengthening mechanisms of nanolaminates, their ductility and fracture behavior in terms of structural variations including grain morphologies, nanotwins, amorphous phases and crystal structures of the layers. The principles for designing nanolaminates with exceptionally high mechanical and physical properties and their fabrication are also highlighted. Some contradictory issues such as strengthening mechanisms, elastic modulus dependency on individual layer thickness and the effect of a thin amorphous layer on the strength are discussed. This review also provides future research directions in designing the high-strength nanolaminates that will facilitate practical engineering applications through analyzing up-to-date research efforts.
 Select Effects of Rare Earth elements on microstructure evolution and mechanical properties of 718H pre-hardened mold steel Hanghang Liu, Paixian Fu, Hongwei Liu, Yanfei Cao, Chen Sun, Ningyu Du, Dianzhong Li J. Mater. Sci. Technol., 2020, 50 (0): 245-256.  DOI: 10.1016/j.jmst.2019.12.035 718H Pre-hardened mold steels with different Rare Earth (RE) contents were prepared to investigate the influence of RE on microstructure evolution and mechanical properties through a series of experiments and theoretical analysis. The results indicated that the toal oxygen (T.O) content decreased from 15 ppm to 6 ppm with 0.022 wt% RE addition, which is attributed to the active chemical properties of RE elements. For test steels, RE additions of 0.012 wt% and 0.022 wt% were significantly effective for refining inclusions by eliminating 11.5% large-sized inclusions with diameter exceeding 10 μm compared with that of 0RE steel. RE addition contributed to modify irregular MnS and Al2O3 inclusions into ellipsoidal RE-inclusions (RE2O3, RES, RE2O2S and REAlO3). The purification of molten steel and the modification of inclusions by RE treatment have significant effects on improvement of the fatigue crack growth tests (FCG) inhibition ability and impact energy as well as the isotropy. However, excessive addition of RE elements (0.07 wt%) seriously reduced the impact energy, ultimate tensile strength and FCG inhibition ability due to rapidly increase of the volume fraction of large-sized inclusions. In addition to the inclusions formed by RE treatment, trace solid solution RE atoms improve the stability of undercooled austenite, resulting in the transformation region of bainite and perlite of 0.07RE steel shifting to the bottom right and prolonging the incubation period compared with that of 0RE steel.
 Select Normal and abnormal grain growth in magnesium: Experimental observations and simulations Risheng Pei, Sandra Korte-Kerzel, Talal Al-Samman J. Mater. Sci. Technol., 2020, 50 (0): 257-270.  DOI: 10.1016/j.jmst.2020.01.014 Commercial purity as-cast magnesium was hot rolled and subsequently annealed at different temperatures in order to investigate its grain growth behavior and link it to the texture evolution. Annealing at an intermediate temperature of 220 °C gave rise to abnormal grain growth with a few grains reaching a grain diameter 10 times larger than the mean. Increasing the annealing temperature to 350 °C yielded normal grain growth. Both types of grain growth revealed a strengthening of the (0001) <$11\bar{2}0$> texture component. It is hypothesized that a dislocation density gradient after recrystallization grants (0001) <$11\bar{2}0$> grains a size advantage during early stages of growth. The type of growth will be, however, determined by the mobility of the present grain boundaries and triple junction drag, which are strongly dependent on the annealing temperature. The above hypothesis of the interplay between these parameters was explored through curvature- and residual dislocation-density-gradient-driven grain growth simulations using a formerly developed level-set approach. The simulation outcome suggests that application of such a modeling approach in microstructure studies of magnesium can provide valuable new insights into the problem of grain growth and associated texture evolution.
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