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 Select Formation mechanism and control methods of acicular ferrite in HSLA steels: A review Shao Yi, Liu Chenxi, Yan Zesheng, Li Huijun, Liu Yongchang J. Mater. Sci. Technol.    2018, 34 (5): 737-744.   DOI: 10.1016/j.jmst.2017.11.020 Abstract   HTML PDF (5405KB)   High strength low alloy (HSLA) steels have been widely used in pipelines, power plant components, civil structures and so on, due to their outstanding mechanical properties as high strength and toughness, and excellent weldability. Multi-phase microstructures containing acicular ferrite or acicular ferrite dominated phase have been proved to possess good comprehensive properties in HSLA steels. This paper mainly focuses on the formation mechanisms and control methods of acicular ferrite in HSLA steels. Effect of austenitizing conditions, continuous cooling rate, and isothermal quenching time and temperature on acicular ferrite transformation was reviewed. Furthermore, the modified process to control the formation of multi-phase microstructures containing acicular ferrite, as intercritical heat treatments, step quenching treatments and thermo-mechanical controlled processing, was summarized. The favorable combination of mechanical properties can be achieved by these modified treatments.
 Select Macro to nanoscale deformation of transformation-induced plasticity steels: Impact of aluminum on the microstructure and deformation behavior Injeti V.S.Y., Li Z.C., Yu B., Misra R.D.K., Cai Z.H., Ding H. J. Mater. Sci. Technol.    2018, 34 (5): 745-755.   DOI: 10.1016/j.jmst.2017.11.011 Abstract   HTML PDF (6647KB)   This work aims to elucidate the impact of aluminum-content on microstructure and deformation mechanisms of transformation-induced plasticity (TRIP) steels through macroscale and nanoscale deformation experiments combined with post-mortem electron microscopy of the deformed region. The solid-state transformation-induced mechanical deformation varied with the Al contents, and influenced tensile strength-ductility combination. Steels with 2-4 wt% Al were characterized by TRIP effect. In contrast to 2Al-TRIP and 4Al-TRIP steels, twinning-induced plasticity (TWIP) was also observed in conjunction with strain-induced martensite in 6Al-TRIP steel. This behavior is attributed to the increase in stacking fault energy with the increase of Al content and stability of austenite, which depends on the local chemical variation. The study addresses the knowledge gap with regard to the effect of Al content on austenite stability in medium-Mn TRIP steels. This combination is expected to potentially enable cost-effective alloy design with high strength-high ductility condition.
 Select Microstructures and mechanical properties of friction stir welds on 9% Cr reduced activation ferritic/martensitic steel Zhang Chao, Cui Lei, Liu Yongchang, Liu Chenxi, Li Huijun J. Mater. Sci. Technol.    2018, 34 (5): 756-766.   DOI: 10.1016/j.jmst.2017.11.049 Abstract   HTML PDF (8907KB)   In this study, the microstructures and mechanical properties of 9%Cr reduced activation ferritic/martensitic (RAFM) steel friction stir welded joints were investigated. When a W-Re tool is used, the recommended welding parameters are 300 rpm rotational speed, 60 mm/min welding speed and 10 kn axial force. In stir zone (SZ), austenite dynamic recrystallization induced by plastic deformation and the high cooling rates lead to an obvious refinement of prior austenite grains and martensite laths. The microstructure in SZ contains lath martensite with high dislocation density, a lot of nano-sized MX and M3C phase particles, but almost no M23C6 precipitates. In thermal mechanically affect zone (TMAZ) and heat affect zone (HAZ), refinement of prior austenite and martensitic laths and partial dissolution of M23C6 precipitates are obtained at relatively low rotational speed. However, with the increase of heat input, coarsening of martensitic laths, prior austenite grains, and complete dissolution of M23C6 precipitates are achieved. Impact toughness of SZ at -20 °C is slightly lower than that of base material (BM), and exhibits a decreasing trend with the increase of rotational speed.
 Select Formation of face-centered cubic titanium in laser surface re-melted commercially pure titanium plate Li Zhuo, Cheng Xu, Li Jia, Wang Huaming J. Mater. Sci. Technol.    2018, 34 (5): 767-773.   DOI: 10.1016/j.jmst.2017.09.004 Abstract   HTML PDF (3353KB)   Micron-scale face-centered cubic titanium phase (named as δ phase) were noticed in the re-melted zone of laser surface re-melted commercially pure titanium plate. The morphology, sub-structure, orientation and distribution of δ phase were investigated by scanning electron microscopy, electron back-scattered diffraction and transmission electron microscopy. Three kind formation processes of δ phase were put forward based on the investigation. The first one is α′ → δ transformation which takes place in single α′ grains and leads to the orientation relationship {001}δ//{0001}α′ <110>δ//<11$\overline{2}$0>α′. The second one is β → α′ + δ transformation which takes place at α′/α′ interfaces and leads to the orientation relationship {001}δ//{1$\overline{1}$0}β 〈110〉δ//〈111〉β. The third one is another kind of β → α′ + δ transformation that takes place at α′/α′ interfaces and leads to the orientation relationship {1$\overline{1}$1}δ//{1$\overline{1}$0}β < 110 > δ// < 111 < β. It is believed that the transformations of δ phase are stress assistant ones and in the present investigation, the phase transformation stress of β → α′ transformation acts as the assistant driving force for the formation of δ phase.
 Select Effect of Cu content on microstructure and properties of Fe-16Cr-2.5Mo damping alloy Hu Xiaofeng, Du Yubin, Yan Desheng, Rong Lijian J. Mater. Sci. Technol.    2018, 34 (5): 774-781.   DOI: 10.1016/j.jmst.2017.05.007 Abstract   HTML PDF (4199KB)   Five Fe-16Cr-2.5Mo damping alloys with different Cu contents (0%, 0.25%, 0.5%, 1.0% and 2.0%) were prepared. The microstructure was observed by scanning transmission electron microscopy (STEM) and the damping behavior was measured by using a dynamic mechanical analyzer (DMA). The results show that the grain size of experimental alloy with (0.25-1.0%) Cu was refined compared with the 0Cu alloy. The Cu element is fully dissolved in the matrix and there are no Cu precipitates and carbides observed. Although the internal stress increases because of Cu addition, the damping capacity of the 0.5Cu and 1.0Cu alloys has been significantly improved. The reason of damping improvement is that the magnetic domain structure is strongly modified. Meanwhile, the strength was improved gradually due to the Cu solid solution strengthening and grain refining. In the 2.0Cu alloy, lots of Cu-riched particles appeared in the matrix. These Cu precipitates with 10-15 nm in size are spherical and homogeneously distributed, which strongly induce strength improvement through precipitation strengthening. On the contrary, the elongation and impact energy of the 2.0Cu alloy decrease sharply. In addition, lots of Cu precipitates will significantly decrease the damping capacity by hindering the mobility of domain walls.
 Select Influence of alloy element partitioning on strength of primary α phase in Ti-6Al-4V alloy Zeng L.R., Chen H.L., Li X., Lei L.M., Zhang G.P. J. Mater. Sci. Technol.    2018, 34 (5): 782-787.   DOI: 10.1016/j.jmst.2017.07.016 Abstract   HTML PDF (3247KB)   The partitioning effect of Al (α-phase stabilizer) and V elements (β-phase stabilizer) on strength of the primary α phases in the α/β Ti-6Al-4V alloy with the bimodal microstructure was investigated. It was found that partitioning of Al and V elements took place in the Ti-6Al-4V alloy during the recrystallization process, leading to the variation of the content of Al and V elements in the primary α phases with changing the volume fraction of the primary α phase. Nanoindentation tests reveal a general trend that the strength of the primary α phases increases with decreasing the volume fraction of the primary α phases, and such trend is independent on the loading direction relative to the c-axis of the α phase. The enhanced strength is attributed to the increase of the content of Al element in the primary α phase, but it is not dominated evidently by the change of the V content. The solid solution strengthening contributed from both the elastic strain introduced by the solute atoms and the variation of the density of states was estimated theoretically.
 Select Microstructure, texture evolution and mechanical properties of cold rolled Ti-32.5Nb-6.8Zr-2.7Sn biomedical beta titanium alloy Lan Chunbo, Wu Yu, Guo Lili, Chen Huijuan, Chen Feng J. Mater. Sci. Technol.    2018, 34 (5): 788-792.   DOI: 10.1016/j.jmst.2017.04.017 Abstract   HTML PDF (1929KB)   Ti-32.5Nb-6.8Zr-2.7Sn (TNZS, wt%) alloy was produced by using vacuum arc melting method, followed by solution treatment and cold rolling with the area reductions of 50% and 90%. The effects of cold rolling on the microstructure, texture evolution and mechanical properties of the experimental alloy were investigated by optical microscopy, X-ray diffraction, transmission electron microscopy and universal material testing machine. The results showed that the grains of the alloy were elongated along rolling direction and stress-induced α″ martensite was not detected in the deformed samples. The plastic deformation mechanisms of the alloy were related to {112}〈111〉 type deformation twinning and dislocation slipping. Meanwhile, the transition from γ-fiber texture to α-fiber texture took place during cold rolling and a dominant {001}〈110〉α-fiber texture was obtained after 90% cold deformation. With the increase of cold deformation degree, the strength increased owing to the increase of microstrain, dislocation density and grain refinement, and the elastic modulus decreased owing to the increase of dislocation density as well as an enhanced intensity of {001}〈110〉α-fiber texture and a weakened intensity of {111}〈112〉γ-fiber texture. The 90% cold rolled alloy exhibited a great potential to become a new candidate for biomedical applications, since it possesses low elastic modulus (47.1 GPa), moderate strength (883 MPa) and high elastic admissible strain (1.87%), which are superior than those of Ti-6Al-4V alloy.
 Select Ferromagnetic element microalloying and clustering effects in high Bs Fe-based amorphous alloys Chen Pingbo, Liu Tao, Kong Fengyu, Wang Anding, Yu Chunyan, Wang Gang, Chang Chuntao, Wang Xinmin J. Mater. Sci. Technol.    2018, 34 (5): 793-798.   Abstract   HTML PDF (2427KB)   Fe83(Cox,Niy)(B11Si2P3C1)1-x,y/17 (x, y = 1-3) amorphous alloys with high saturation magnetic flux density (Bs) and excellent soft-magnetic properties were developed and then the microalloying and clustering effects were explored. The microalloying of Co and Ni improves the Bs from 1.65 T to 1.67-1.72 T and 1.66-1.68 T, respectively. The Ni-doped alloys exhibit better soft-magnetic properties, containing a low coercivity (Hc) of about 5.0 A/m and a high Effective permeability (μe) of (8-10) × 103, whereas the microalloying of Co leads to a deteriorative Hc of 5.0-13.0 A/m and a μe of (5-8) × 103. Moreover, microalloying of Ni can increase the ductile-brittle transition (DBT) temperature of the ribbons, while a totally opposite effect is found in the Co-doped alloys. The formation of dense α-Fe(Co,Ni) clusters during annealing process is used to explain the distinct effects of Co and Ni microalloying on the magnetic properties and bending toughness.
 Select Effect of Mo on microstructural characteristics and coarsening kinetics of γ' precipitates in Co-Al-W-Ta-Ti alloys Zhou H.J., Xue F., Chang H., Feng Q. J. Mater. Sci. Technol.    2018, 34 (5): 799-805.   DOI: 10.1016/j.jmst.2017.04.012 Abstract   HTML PDF (3074KB)   The solvus temperature, volume fraction, coarsening behavior of γ' precipitates and the partitioning behavior of alloying elements as well as lattice misfit of γ/γ' phases influence the creep behavior of Ni- and Co-base superalloys. However, few investigations about the microstructural characteristics and the coarsening behavior of γ' precipitates were reported in multicomponent novel Co-base superalloys during thermal exposure. Two alloys containing different contents of molybdenum and tungsten have been investigated to explore the effect of molybdenum on γ' solvus temperature, γ + γ' microstructure and γ' coarsening in Co-Al-W-Ta-Ti-base alloys. The results showed that the γ' solvus temperature decreases with the addition of Mo. Mo addition reduces the γ' volume fractions after aging above 1000 °C, but results in negligible influence on the γ' volume fractions aging at 900 °C. Meanwhile, γ' coarsening is controlled by diffusion in experimental alloys after aging at 900 °C and 1000 °C, and the kinetics of γ' growth in experimental alloys are consistent with the predictions of LSW theory.
 Select Intergrowth of C phase within μ phase in a Re-containing Ni-base single crystal superalloy Gao Shuang, Zhou Yizhou, Liu Zhi-Quan, Jin Tao J. Mater. Sci. Technol.    2018, 34 (5): 806-810.   DOI: 10.1016/j.jmst.2017.04.009 Abstract   HTML PDF (2689KB)   Precipitation of topologically close-packed phases in a Re-containing Ni-base single crystal superalloy was studied under elevated temperature by in situ transmission electron microscopy. Above 1150 °C, a new intergrowth structure called C phase was found within μ phase with a defined crystallographic orientation relationship. Elements mapping analysis reveals that C phase has a similar element composition as μ phase, but contains a lower level of Cr, W and Re. With increasing temperature, the proportion of C phase increases gradually. At 1250 °C, C phase becomes the dominant precipitate. It is demonstrated that C phase is more thermodynamically stable than μ phase above 1150 °C that leads to an intergrowth pathway from μ phase to C phase.
 Select Effect of cooling rate on microstructure, microsegregation and mechanical properties of cast Ni-based superalloy K417G Gong Li, Chen Bo, Zhang Long, Ma Yingche, Liu Kui J. Mater. Sci. Technol.    2018, 34 (5): 811-820.   DOI: 10.1016/j.jmst.2017.03.023 Abstract   HTML PDF (5648KB)   The effects of different solidification rates after pouring on the microstructures, microsegregation and mechanical properties of cast superalloy K417G were investigated. Scheil-model was applied to calculate the temperature range of solidification. The casting mould with different casting runners was designed to obtain three different cooling rates. The microstructures were observed and the microsegregation was investigated. Also, high temperature tensile test was performed at 900 °C and stress rupture test was performed at 950 °C with the stress of 235 MPa. The results showed that the secondary dendrite arm spacing, microsegregation, the size and volume fraction of γ′ phase and the size of γ/γ′ eutectic increased with decreasing cooling rate, but the volume fraction of γ/γ′ eutectic decreased. In the cooling rate range of 1.42 °C s-1-0.84 °C s-1, the cast micro-porosities and carbides varied little, while the volume fraction and size of γ′ phase and γ/γ′ eutectic played a decisive role on mechanical properties. The specimen with the slowest cooling rate of 0.84 °C s-1 showed the best comprehensive mechanical properties.
 Select Development of through-thickness texture gradient and persistence of shear-type textures during annealing of commercial purity aluminium sheet processed by accumulative roll-bonding Li Saiyi, Yang Liang, Qin Nan J. Mater. Sci. Technol.    2018, 34 (5): 821-831.   DOI: 10.1016/j.jmst.2017.04.019 Abstract   HTML PDF (9221KB)   Ultrafine-grained commercial-purity aluminum (AA1070) sheets produced by four cycles of accumulative roll-bonding (ARB) without lubrication are subjected to annealing treatments in the temperature range from 250 °C to 400 °C. Microstructures and microtextures in the surface and center regions of the ARBed and annealed sheets are measured by electron backscatter diffraction. The results show that annealing treatments at 325 °C or above lead to a reduction in the microstructure gradient but a significant through-thickness texture gradient different from that in the as-deformed state. The center region is featured by the development of a strong cube texture at the expense of rolling components. In the surface region, shear-type components are either enhanced or largely retained, showing a high persistency upon annealing. While the grain structures are restored predominantly by continuous recrystallization in the surface region, a mixture of continuous and discontinuous recrystallization is envisaged for the center region.
 Select Magnetization reversal of antiferromagnetically coupled perpendicular anisotropy films driven by current Zhao X.T., Zhao Y.Q., Liu W., Dai Z.M., Wang T.T., Zhao X.G., Zhang Z.D. J. Mater. Sci. Technol.    2018, 34 (5): 832-835.   DOI: 10.1016/j.jmst.2017.04.008 Abstract   HTML PDF (1389KB)   By inserting an ultrathin Pt layer at Co/Ru interface, we established antiferromagnetic coupling with out-of-plane magnetization in Co/Ru/Co film stacks fabricated by sputtering. To achieve configuration suitable for free layer, the magnetic properties of the stacks have been investigated by changing the thickness of Co, Ru and Pt layers using an orthogonal wedges technique. It is found that magnetic properties for upper Co layer thinner than 0.5 nm are sensitive to little change in Ru thickness. Improving continuity of upper Co layer by slightly increasing the thickness can effectively increase the squareness of minor loop. The switching magnetization of synthetic antiferromagnetic (SAF) structure is achieved by DC current under an in-plane static magnetic field of ± 500 Oe. This structure is very promising for free layer in spintronic application.
 Select Synthesis and magnetic properties of shuriken-like nickel nanoparticles Liu Shengqing, Mei Jiaming, Zhang Chen, Zhang Jianchao, Shi Rongrong J. Mater. Sci. Technol.    2018, 34 (5): 836-841.   DOI: 10.1016/j.jmst.2017.04.006 Abstract   HTML PDF (2081KB)   Shuriken-like nickel nanoparticles were successfully synthesized by a thermal decomposition method at 200 °C with Nickel(II) acetylacetonate (Ni(acac)2) as the precursor and oleylamine (OAm) as the solvent and reductant, respectively. The phase structures, morphologies and sizes, and magnetic properties of the as-synthesized nickel products were characterized in detail by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission-scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and vibrating sample magnetometer (VSM). Some key reaction parameters, such as the reaction time, reaction temperature and surfactants, have important influence on the morphology of the final products. XRD pattern indicated that the products are well-crystallized face-centered cubic (fcc) nickel phase. SEM images demonstrated that the nickel nanoparticles are shuriken-like morphology with average size around 150 nm. The mechanism of shuriken-like Ni nanoparticles (NPs) is proposed. The magnetic hysteresis loops of shuriken-like and spherical nickel products illustrated the ferromagnetic nature at 300 K, indicating its potential applications in magnetic storage.
 Select Manganese ferrite nanoparticles with different concentrations: Preparation and magnetism Chen Zheng, Sun Xiao, Ding Zongling, Ma Yongqing J. Mater. Sci. Technol.    2018, 34 (5): 842-847.   DOI: 10.1016/j.jmst.2017.04.002 Abstract   HTML PDF (2164KB)   MnFe2O4 nanoparticles were synthesized by thermal decomposition of a metal-organic salt in organic solvent with a high boiling point. Some nanoparticles exhibited the triangular shape which has not been observed before as far as we know, while some nanoparticles formed the aggregates with different sizes and shapes. The strength of interparticle dipolar interaction was changed by diluting MnFe2O4 nanoparticles in the SiO2 matrix with different concentrations. The strong dipolar interaction has been suggested to enhance the blocking temperature (TB) and suppress the remanence (Mr) to saturation (Ms) magnetization ratio (Mr/Ms) for the spherical-like cobalt ferrite nanoparticles in many previous reports. However, Mr/Ms and TB of MnFe2O4 nanoparticles reported herein mainly depend on the size and shape of the nanoparticles and their aggregates.
 Select A new scale for optimized cryogenic magnetocaloric effect in ErAl2@Al2O3 nanocapsules Wei F., Ma S., Yang L., Feng Y., Wang J.Z., Hua A., Zhao X.G., Geng D.Y., Zhang Z.D. J. Mater. Sci. Technol.    2018, 34 (5): 848-854.   DOI: 10.1016/j.jmst.2017.04.003 Abstract   HTML PDF (2365KB)   The ErAl2@Al2O3 nanocapsules with ErAl2 core and Al2O3 shell were synthesized by modified arc-charge technique. The typical core-shell structure of the nanocapsules was confirmed by high resolution transmission electron microscopy and X-ray photoelectron spectroscopy. Transmission electron microscopy analysis shows the irregular sphere of the nanocapules with an average diameter of 26 nm. Magnetic investigation revealed the Curie temperature of ErAl2@Al2O3 nanocapsules at 20 K and the typical superparamagnetic behavior between blocking temperature and Curie temperature. Based on the blocking temperature and average diameter, the magnetocrystalline anisotropy constant of ErAl2@Al2O3 nanocapsules was estimated to illustrate the magnetic contribution to the -ΔSM. The large -ΔSM of 14.25 J/(kg K) was obtained under 50 kOe at 5 K. A vital parameter β was introduced in the present work to scale the optimized magnetic characteristics and the optimized mechanism was discussed in detail according to classical superparamagnetic theory. The results demonstrate that the optimal -ΔSM will be obtained when the magnetic parameter β is close to the theoretical coefficient.
 Select Amino-functionalized magnetic bacterial cellulose/activated carbon composite for Pb2+ and methyl orange sorption from aqueous solution Huang Xiaogui, Zhan Xiaozhang, Wen Cuilian, FengXu, Luo Lijin J. Mater. Sci. Technol.    2018, 34 (5): 855-863.   DOI: 10.1016/j.jmst.2017.03.013 Abstract   HTML PDF (2365KB)   A new nanostructured amino-functionalized magnetic bacterial cellulose/activated carbon (BC/AC) composite bioadsorbent (AMBCAC) was prepared for removal of Pb2+ and methyl orange (MO) from aqueous solution. The results demonstrated that the equilibrium adsorption capacity (qe) for Pb2+ obviously increases by 2.14 times after introduction of amino groups, the optimum pH for Pb2+ and MO adsorption was 5.0 and 3.0, respectively, and the qe of AMBCAC was 161.78 mg g-1 for Pb2+ and 83.26 mg g-1 for MO under the optimal conditions in this investigation. The kinetics and adsorption isotherm data of the sorption process were well fitted by pseudo-second-order kinetic model and Langmuir isotherm respectively. The thermodynamic results (the Gibbs free energy change ΔG < 0, the enthalpy change ΔH > 0, the entropy change ΔS > 0) implied that the adsorption process of Pb2+ and MO was feasible, endothermic and spontaneous in nature. These results support that the AMBCAC composite developed in this work can provide a cheap and efficient way for easy removal of both Pb2+ and MO as a promising adsorbent candidate for wastewater treatment.
 Select Orientation and strain rate dependent tensile behavior of single crystal titanium nanowires by molecular dynamics simulations Chang Le, Zhou Chang-Yu, Liu Hong-Xi, Li Jian, He Xiao-Hua J. Mater. Sci. Technol.    2018, 34 (5): 864-877.   DOI: 10.1016/j.jmst.2017.03.011 Abstract   HTML PDF (7326KB)   Molecular dynamics simulation was employed to study the tensile behavior of single crystal titanium nanowires (NWs) with [11$\overline{2}$0], [$\overline{1}$100] and [0001] orientations at different strain rates from 108 s-1 to 1011 s-1. When strain rates are above 1010 s-1, the state transformation from HCP structure to amorphous state leads to super plasticity of Ti NWs, which is similar to FCC NWs. When strain rates are below 1010 s-1, deformation mechanisms of Ti NWs show strong dependence on orientation. For [11$\overline{2}$0] orientated NW, {10$\overline{1}$1} compression twins (CTs) and the frequently activated transformation between CTs and deformation faults lead to higher plasticity than the other two orientated NWs. Besides, tensile deformation process along [11$\overline{2}$0] orientation is insensitive to strain rate. For [$\overline{1}$100] orientated NW, prismatic slip is the main deformation mode at 108 s-1. As the strain rate increases, more types of dislocations are activated during plastic deformation process. For [0001] orientated NW, {10$\overline{1}$2} extension twinning is the main deformation mechanism, inducing the yield stress of [0001] orientated NW, which has the highest strain rate sensitivity. The number of initial nucleated twins increases while the saturation twin volume fraction decreases nonlinearly with increasing strain rate.
 Select Refill friction stir spot welding of 5083-O aluminum alloy Xu Zhiwu, Li Zhengwei, Ji Shude, Zhang Liguo J. Mater. Sci. Technol.    2018, 34 (5): 878-885.   DOI: 10.1016/j.jmst.2017.02.011 Abstract   HTML PDF (3927KB)   In this work, refill friction stir spot welding (RFSSW) was used to weld 2 mm-thick 5083-O alloy. The Box-Behnken experimental design was used to investigate the effect of welding parameters on the joint lap shear property. Results showed that a surface indentation of 0.3 mm effectively eliminated the welding defects. Microhardness of the stir zone (SZ) was higher than that of the base material (BM) and the hardness decreased with increasing the heat input during welding. The optimum failure load of 7.72 kN was obtained when using rotating speed of 2300 rpm, plunge depth of 2.4 mm and refilling time of 3.5 s. Three fracture modes were obtained during the lap shear test and all were affected by the hook defect.
 Select Synthesis and properties of magnetic multi-walled carbon nanotubes loaded with Fe4N nanoparticles Hu Yang, Jiang Runjian, Zhang Jingbao, Zhang Chengsong, Cui Guodong J. Mater. Sci. Technol.    2018, 34 (5): 886-890.   DOI: 10.1016/j.jmst.2017.02.007 Abstract   HTML PDF (1393KB)   High saturation magnetization (>90 emu/g) multi-walled carbon nanotubes (MWCNTs) and Fe4N nanoparticles composite were successfully synthesized by gas nitriding at 550 °C. Almost all Fe4N nanoparticles were evenly distributed inside the carbon nanotubes and formed a special composite microstructure. This composite microstructure shows excellent soft magnetic property, structural stability, and chemical stability at room temperature. The investigations of electromagnetic and microwave absorption performances indicate that microwave absorbing capacity of low frequency band of MWCNTs were greatly improved by addition of Fe4N nanoparticles.
 Select First-Principles Study on the Stability and Electronic Properties of Bi-Doped Sr3Ti2O7 Lu Yanli, Liu Fang, Li Xiang, Gao Feng, Chen Zheng J. Mater. Sci. Technol.    2018, 34 (5): 891-898.   DOI: 10.1016/j.jmst.2016.11.023 Abstract   HTML PDF (2662KB)   The electronic and crystal structural properties of Bi-doped Sr3Ti2O7 are studied using the first principles density functional theory (DFT) based on pseudopotentials basis and plane-wave method. Our results show that the formation energy of Bi doping in Site-1 and Site-2 of Sr3Ti2O7 increases with increasing doping concentration. And at the same doping concentration, the formation energy of Bi doping in Site-2 is lower than that in Site-1. The undoped Sr3Ti2O7 is found to be an insulator and its Fermi level stays at the top of the valence band. While the Fermi level of the Bi-doped Sr3Ti2O7 moves into the bottom of conduction band, the system undergoes an insulator-to-metal phase transition. Furthermore, our calculation results demonstrated that the Fermi level of the Bi-doped Sr3Ti2O7 goes deeper into the bottom of conduction band with increasing doping concentration.
 Select Friction stir based welding and processing technologies - processes, parameters, microstructures and applications: A review Padhy G.K., Wu C.S., Gao S. J. Mater. Sci. Technol.    2018, 34 (1): 1-38.   DOI: 10.1016/j.jmst.2017.11.029 Abstract   HTML PDF (15049KB)   Friction stir welding (FSW) has achieved remarkable success in the joining and processing of aluminium alloys and other softer structural alloys. Conventional FSW, however, has not been entirely successful in the joining, processing and manufacturing of different desired materials essential to meet the sophisticated green globe requirements. Through the efforts of improving the process and transferring the existing friction stir knowledge base to other advanced applications, several friction stir based daughter technologies have emerged over the timeline. A few among these technologies are well developed while others are under the process of emergence. Beginning with a broad classification of the scattered frictions stir based technologies into two categories, welding and processing, it appears now time to know, compile and review these to enable their rapid access for reference and academia. In this review article, the friction stir based technologies classified under the category of welding are those applied for joining of materials while the remnant are labeled as friction stir processing (FSP) technologies. This review article presents an overview of four general aspects of both the developed and the developing friction stir based technologies, their associated process parameters, metallurgical features of their products and their feasibility and application to various materials. The lesser known and emerging technologies have been emphasized.
 Select Correlation between microstructures and mechanical properties of high-speed friction stir welded aluminum hollow extrusions subjected to axial forces Liu Xiangqian, Liu Huijie, Wang Tianhao, Wang Xiangguo, Yang Si J. Mater. Sci. Technol.    2018, 34 (1): 102-111.   DOI: 10.1016/j.jmst.2017.11.015 Abstract   HTML PDF (6094KB)   The AA6005A-T6 aluminum hollow extrusions were friction stir welded at a high welding speed of 2000 mm/min and various axial forces. The results show that the nugget zone (NZ) is characterized by fine equiaxed grains, in which a low density of equilibrium phase β is observed. The grains in the thermo-mechanically affected zone (TMAZ) are elongated, and the highest density of dislocations and a low density of β' precipitates can be found in grains. The heat affected zone (HAZ) only experiences a low thermal cycle, and a high density of β” precipitates and a low density of β' precipitates remain in the coarsened grains. The microhardness evolutions in the NZ, TMAZ and HAZ are governed by the grain refinement and dislocation strengthening, the dislocation and precipitation strengthening, and the precipitation and solid solution strengthening, respectively. When increasing the axial force, the changing trend of one strengthening mechanism is contrary to the other in each zone, and the microhardness increases in different zones. As a result, the tensile strength roughly increases with raising the axial force, and all joints show good tensile properties as the high welding speed inhibits the coarsening and dissolution of strengthening precipitates significantly.
 Select Fabrication of large-bulk ultrafine grained 6061 aluminum alloy by rolling and low-heat-input friction stir welding Liu C.Y., Qu B., Xue P., Ma Z.Y., Luo K., Ma M.Z., Liu R.P. J. Mater. Sci. Technol.    2018, 34 (1): 112-118.   DOI: 10.1016/j.jmst.2017.02.008 Abstract   HTML PDF (3089KB)   In this study, the ultrafine grained (UFG) 6061 Al alloys fabricated by cold rolling were friction stir welded (FSW) with different rotation rates under both air cooling and rapid cooling in water. Low-heat-input parameters of 400 rpm rotation rate in water (400-Water) could effectively inhibit the coarsening of recrystallized grains, reduce the precipitation rate, and retain more dislocations of the UFG 6061 Al parent metal. 400-Water joint showed high lowest-hardness value, narrow low-hardness zone, and high tensile strength, attributing to the effect of dislocation, grain boundary, solid-solution, and precipitation hardening. This work provides an effective strategy to fabricate large-sized bulk UFG Al alloy by cold rolling with large deformation and low-heat-input FSW.
 Select Influence of a non-rotating shoulder on heat generation, microstructure and mechanical properties of dissimilar AA2024/AA7050 FSW joints Barbini Alessandro, Carstensen Jan, F. dos Santos Jorge J. Mater. Sci. Technol.    2018, 34 (1): 119-127.   DOI: 10.1016/j.jmst.2017.10.017 Abstract   HTML PDF (3631KB)   Friction stir welding (FSW) and stationary shoulder friction stir welding (SSFSW) were carried out for the butt joining of dissimilar AA2024-T3 and AA7050-T7651 aluminium alloys with thicknesses of 2 mm. A comparison between the two processes was performed by varying the welding speed while keeping the rotational speed constant. Through the analysis of the force and torque produced during welding and a simple analytical model, it was possible to show that in SSFSW there is more effective coupling with the tool and the heat produced is more efficiently distributed. This process decreases both the welding area and the diffusion at the interface of the two alloys compared with FSW. The minimum microhardness occurred at the advancing side (AS) at the interface between the thermo-mechanically affected zone (TMAZ) and the stir zone (SZ) in both processes, although the decrease was more gradual in SSFSW. This interface is also where all specimens failed for both welding technologies. An increase in tensile strength was measured in SSFSW compared with standard FSW. Furthermore, it was possible to establish the mechanical performance of the material in the fracture zone using digital image correlation.
 Select Computational fluid dynamics simulation of friction stir welding: A comparative study on different frictional boundary conditions Chen Gaoqiang, Ma Qingxian, Zhang Shuai, Wu Jianjun, Zhang Gong, Shi Qingyu J. Mater. Sci. Technol.    2018, 34 (1): 128-134.   DOI: 10.1016/j.jmst.2017.10.015 Abstract   HTML PDF (2565KB)   Numerical simulation based on computational fluid dynamics (CFD) is a useful approach for quantitatively investigating the underlying thermal-mechanical conditions during FSW, such as temperature field and material deformation field. One of the critical issues in CFD simulation of FSW is the use of the frictional boundary condition, which represents the friction between the welding tool and the workpiece in the numerical models. In this study, three-dimensional numerical simulation is conducted to analyze the heat transfer and plastic deformation behaviors during the FSW of AA2024. For comparison purposes, both the boundary velocity (BV) models and the boundary shear stress (BSS) models are employed in order to assess their performances in predicting the temperature and material deformation in FSW. It is interesting to note that different boundary conditions yield similar predictions on temperature, but quite different predictions on material deformation. The numerical predictions are compared with the experimental results. The predicted deformation zone geometry by the BSS model is consistent with the experimental results while there is large difference between the predictions by the BV models and the experimental measurements. The fact that the BSS model yields more reasonable predictions on the deformation zone geometry is attributed to its capacity to automatically adjust the contact state at the tool/workpiece interface. Based on the favorable predictions on both the temperature field and the material deformation field, the BSS model is suggested to have a better performance in numerical simulation of FSW than the BV model.
 Select Improving weld formability by a novel dual-rotation bobbin tool friction stir welding Wang F.F., Li W.Y., Shen J., Wen Q., dos Santos J.F. J. Mater. Sci. Technol.    2018, 34 (1): 135-139.   DOI: 10.1016/j.jmst.2017.11.001 Abstract   HTML PDF (2551KB)   A novel dual-rotation bobbin tool friction stir welding (DBT-FSW) was developed, in which the upper shoulder (US) and lower shoulder (LS) have different rotational speeds. This process was tried to weld 3.2 mm thick aluminum-lithium alloy sheets. The metallographic analysis and torque measurement were carried out to characterize the weld formability. Experimental results show that compared to conventional bobbin tool friction stir welding, the DBT-FSW has an excellent process stability, and can produce the defect-free joints in a wider range of welding parameters. These can be attributed to the significant improvement of material flow caused by the formation of a staggered layer structure and the unbalanced force between the US and LS during the DBT-FSW process.
 Select Solid state crack repair by friction stir processing in 304L stainless steel Gunter C., Miles M.P., Liu F.C., Nelson T.W J. Mater. Sci. Technol.    2018, 34 (1): 140-147.   DOI: 10.1016/j.jmst.2017.10.023 Abstract   HTML PDF (4208KB)   Friction stir processing (FSP) was investigated as a method of repairing cracks in 12 mm thick 304L stainless steel plate. Healing feasibility was demonstrated by processing a tapered crack using a PCBN/W-Re tool with a 25 mm diameter shoulder and a pin length of 6.4 mm. The experiment showed that it was possible to heal a crack that begins narrow and then progressively grows up to a width of 2 mm. Bead on plate experiments were used to find the best parameters for creating a consolidated stir zone with the least amount of hardness difference compared to the base metal. Grain refinement in some specimens resulted in much higher stir zone hardness, compared to base metal. A plot of grain size versus microhardness showed a very strong inverse correlation between grain size and hardness, as expected from the Hall-Petch relationship. Corrosion testing was carried out in order to evaluate the effect of FSP on potential sensitization of the stir zone. After 1000 h of intermittent immersion in 3.5% saline solution at room temperature it was found that no corrosion products formed on the base material controls or on any of the friction stir processed specimens.
 Select Corrosion fatigue behavior of friction stir processed interstitial free steel Wang Wen, Xu Ruiqi, Hao Yaxin, Wang Qiang, Yu Liangliang, Che Qianying, Cai Jun, Wang Kuaishe, Ma Zongyi J. Mater. Sci. Technol.    2018, 34 (1): 148-156.   DOI: 10.1016/j.jmst.2017.11.013 Abstract   HTML PDF (5707KB)   In this study, interstitial free (IF) steel plates were subjected to double-sided friction stir processing (FSP). The fine-grained structure with an average grain size of about 12 μm was obtained in the processed zone (PZ) with a thickness of about 2.5 mm. The yield strength (325 MPa) and ultimate tensile strength (451 MPa) of FSP IF steel were significantly higher than those of base material (BM) (192 and 314 MPa), while the elongation (67.5%) almost remained unchanged compared with the BM (66.2%). The average microhardness value of the PZ was about 130 HV, 1.3 times higher than that of the BM. In addition, the FSP IF steel showed a more positive corrosion potential and lower corrosion current density than the BM, exhibiting lower corrosion tendency and corrosion rates in a 3.5 wt% NaCl solution. Furthermore, FSP IF steel exhibited higher fatigue life than the BM both in air and NaCl solution. Corrosion fatigue fracture surfaces of FSP IF steel mainly exhibited a typical transgranular fracture with fatigue striations, while the BM predominantly presented an intergranular fracture. Enhanced corrosion fatigue performance was mainly attributed to the increased resistance of nucleation and growth of fatigue cracks. The corrosion fatigue mechanism was primarily controlled by anodic dissolution under the combined effect of cyclic stress and corrosive solution.
 Select Tensile properties and fracture behavior of friction stir welded joints of Fe-32Mn-7Cr-1Mo-0.3N steel at cryogenic temperature Li Yi-jun, Fu Rui-dong, Li Yan, Peng Yan, Liu Hui-jie J. Mater. Sci. Technol.    2018, 34 (1): 157-162.   DOI: 10.1016/j.jmst.2017.11.034 Abstract   HTML PDF (2648KB)   This study investigates the cryogenic tensile properties and fracture behavior of fiction stir welded and post-weld heat-treated joints of 32Mn-7Cr-1Mo-0.3N steel. Cryogenic brittle fracture, which occurred in the as-welded joint, is related to the residual particles that contain tungsten in the joint band structure. Post-weld water toughening resulted in the cryogenic intergranular brittleness of the joint, which is related to the non-equilibrium segregation of solute atoms during the post-weld water toughening. Annealing at 550 °C for 30 min can effectively inhibit the cryogenic intergranular brittleness of the post-weld water-toughened joint. The yield strength, ultimate tensile strength, and uniform elongation of the annealed joint are approximately 95%, 87%, and 94% of the corresponding data of the base metal.
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