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ISSN 1005-0302
CN 21-1315/TG
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      20 April 2017, Volume 33 Issue 4 Previous Issue    Next Issue
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    Orginal Article
    Microstructural Evolution of Nb-V-Mo and V Containing TRIP-assisted Steels during Thermomechanical Processing
    Abbasi Erfan, Mark Rainforth William
    J. Mater. Sci. Technol., 2017, 33 (4): 311-320.  DOI: 10.1016/j.jmst.2016.08.019
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    The microstructural evolution and precipitation behaviour of Nb-V-Mo and single V containing transformation induced plasticity assisted steels were investigated during thermomechanical processing. A plane strain compression testing machine was used to simulate the thermomechanical processing. Microstructures were characterised by optical microscopy, scanning-transmission electron microscopy and microanalysis, and X-ray diffraction analysis, and Vickers hardness was obtained from the deformed specimens. The resulting microstructure of both Nb-V-Mo and V steels at room temperature primarily consisted of an acicular/bainitic ferrite, retained austenite and martensite surrounded by allotriomorphic ferrite. The TEM analysis showed that a significant number of Nb(V,Mo)(C,N) precipitates were formed in the microstructure down to the finishing stage in Nb-V-Mo steel (i.e. 830 °C). It was also found that the V(C,N) precipitation primarily occurred in both ferrite and deformed austenite below the finishing stage. The results suggested that Nb-Mo additions considerably increased the temperature stability of microalloy precipitates and controlled the microstructural evolution of austenite. However, the microalloy precipitation did not cause a significant precipitation strengthening in both Nb-V-Mo and V steels at room temperature.

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    Influence of Carbon Equivalent Value on the Weld Bead Bending Properties of High-Strength Low-Alloy Steel Plates
    Hyuk Kim Ki, Jun Moon In, Won Kim Ki, Bong Kang Ki, Gyu Park Byung, Seok Lee Kwang
    J. Mater. Sci. Technol., 2017, 33 (4): 321-329.  DOI: 10.1016/j.jmst.2016.07.009
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    The contribution of chemical compositions in terms of carbon equivalent value (CEV) on impending crack propagation during weld bead bend tests (WBBTs) was studied with five different high-strength low-alloy steel plates that underwent different rolling processes. The test showed that cracks developed at the weld joint, easily passed through the coarse-grain heat-affected zone (CGHAZ), and finally were arrested within the heat-affected zone (HAZ). An apparent decrease in the average crack length, which consequently indicated improvement in crack arrestability, was found with decreasing CEV. In addition, the relatively fine microstructure in the HAZ of low-CEV steel plates helped in preventing the crack from further propagation. Special emphasis was placed on the empirical expectation of critical CEV above which WBBT might fail.

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    Evolution Behaviors of Oxides in Severely Plastic Deformed Region of AISI 52100 Steel during Dry Sliding Wear
    Lao Yuanxia, Du Hao, Xiong Tianying, Wang Yuan
    J. Mater. Sci. Technol., 2017, 33 (4): 330-337.  DOI: 10.1016/j.jmst.2016.06.001
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    Under dry sliding wear, the evolution of oxides in severely plastic deformed (SPD) regions of metals has a great impact on the wear behaviors. To study the evolution behaviors of oxides in the SPD region, an SPD region was prefabricated on the surface of AISI 52100 steel by supersonic fine particle bombarding (SFPB) treatment. Dry sliding wear tests were carried out on both of the SFPB-treated and original samples. Wear volume loss of the SPBF-treated samples were compared with those of the original samples at different loads. Microstructure, element composition and oxides distribution in the SPD region were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and an electron probe microanalysis (EPMA). The results show that the evolution behaviors of the oxides in the SPD region change significantly with the load. Under low loads, oxides are usually formed on the contact surface. It inhibits adhesive wear on the steel. However, under high loads, oxides are apt to distribute along the cracks in the subsurface layer. The internal oxidation along the cracks can accelerate the cracks propagation, resulting in severe delamination wear on the steel.

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    Effect of Substrate Type on Deposition Behavior and Wear Performance of Ni-Coated Graphite/Al Composite Coatings Deposited by Cold Spraying
    Huang Chunjie, Li Wenya, Xie Yingchun, Planche Marie-Pierre, Liao Hanlin, Montavon Ghislain
    J. Mater. Sci. Technol., 2017, 33 (4): 338-346.  DOI: 10.1016/j.jmst.2016.11.016
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    This work focused on the deposition characteristics and wear behavior of Ni-coated graphite mixed with 40 vol.% Al (Ni-Gr/Al) composite coatings sprayed on an Al alloy and a steel substrate by cold spraying (CS). The morphology of the flattened Ni-Gr particles was examined by single-impact tests. Cross-sectional microstructure and wear performance of the Ni-Gr/Al composite coatings were studied. Results showed that a larger number of Ni-Gr particles were finally bonded with the steel substrate, whereas many craters existed on the Al alloy substrate after the single-impact tests. The coating on the steel substrate had a high thickness, high graphite content and low coefficient of friction (COF) compared to those on the Al alloy substrate. In addition, the CS coatings presented a homogeneous distribution and uniform morphology of graphite, and a comparative COF to that of conventional thermal sprayed coatings. It was shown that CS could avoid the decomposition and transformation of graphite phase.

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    Formation of the Surface Eutectic of a Ni-based Single Crystal Superalloy
    Cao Liang, Yao Li, Zhou Yizhou, Jin Tao, Sun Xiaofeng
    J. Mater. Sci. Technol., 2017, 33 (4): 347-351.  DOI: 10.1016/j.jmst.2016.08.014
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    The microstructure, size, elemental composition and hardness of the surface eutectic layer formed during directional solidification of a Ni-based single crystal superalloy were studied. The formation mechanisms of the surface eutectic on the outer surface of the casting were also discussed. The metal/mould interactions did not play any role in the formation of the surface eutectic. The formation cause of surface eutectic layer was attributed to the interdendritic residual liquid for excretion caused by solidification shrinkage.

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    Phase Selection in Solidification of Undercooled Co-B Alloys
    Wei X.X., Xu W., Kang J.L., Ferry M., Li J.F.
    J. Mater. Sci. Technol., 2017, 33 (4): 352-358.  DOI: 10.1016/j.jmst.2016.09.012
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    A series Co-(18.5-20.7) at.% B melts encompassing the eutectic composition (Co81.5B18.5) were solidified at different degrees of undercooling. It is found that the metastable Co23B6 phase solidifies as a substitute for the stable Co3B phase in the alloy melts undercooled above a critical undercooling value of ~60 K. The Co23B6 and α-Co phases make up a metastable eutectic. The corresponding eutectic composition and temperature are Co80.4B19.6 and 1343 K, respectively. On exposure of the metastable Co23B6 phase at a given temperature above 1208 K, it does not decompose even after several hours. But it transforms by a eutectoid reaction to α-Co + Co3B at lower temperature.

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    Intermetallic Compounds in the Banded Structure and Their Effect on Mechanical Properties of Al/Mg Dissimilar Friction Stir Welding Joints
    Shi Hui, Chen Ke, Liang Zhiyuan, Dong Fengbo, Yu Taiwu, Dong Xianping, Zhang Lanting, Shan Aidang
    J. Mater. Sci. Technol., 2017, 33 (4): 359-366.  DOI: 10.1016/j.jmst.2016.05.006
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    Dissimilar friction stir welding (FSW) between aluminum and magnesium alloy was performed, using various tool rotational speed (TRS) at a fixed travel speed, with tool offset to aluminum to investigate the formation of intermetallic compounds (IMCs) in the banded structure (BS) zone and their effect on mechanical properties. Large quantities of IMCs, in the form of alternating bands of particles or lamellae, were found in the BS zone, where drastic material intermixing occurred during FSW. The BS microstructural characters in terms of the morphology of the bands and the quantity and distribution of IMC particles varied with TRS. All welds exhibited brittle fracture mode with their fracture paths propagating mainly in/along the IMCs in the BS. It is shown that these BS microstructural characters have significant effect on the mechanical properties of the joints. Suggestions on tailoring the BS microstructure were proposed for improving the strength of the BS zone and the final mechanical properties of the Al/Mg FSW joints.

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    Tensile Properties and Deformation Behaviors of a New Aluminum Alloy for High Pressure Die Casting
    Zhang Peng, Li Zhenming, Liu Baoliang, Ding Wenjiang
    J. Mater. Sci. Technol., 2017, 33 (4): 367-378.  DOI: 10.1016/j.jmst.2016.02.013
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    Effects of natural aging and test temperature on the tensile behaviors have been studied for a high-performance cast aluminum alloy Al-10Si-1.2Cu-0.7Mn. Based on self-strengthening mechanism and spheroidization microstructures, the alloy tested at room temperature (RT) exhibits higher 0.2% proof stress (YS) of 206 MPa, ultimate tensile strength (UTS) of 331 MPa and elongation of 10%. Increasing aging time improves the YS and UTS and reduces the ductility of the alloy. Further increasing aging time beyond 72 h does not significantly increase the tensile strengths. Increasing test temperature significantly decreases the tensile strengths and increases the ductility of the alloy. The UTS of the alloy can be estimated by using the hardness. The Portevin-Le Chatelier effect occurs at RT due to the interactions between solid solution atoms and dislocations. Similar behaviors occurring at 250 °C are attributed to dynamic strain aging mechanism. Increasing aging time leads to decrease in the strain-hardening exponent (n) value and increase in the strain-hardening coefficient (k) value. Increasing test temperature apparently decreases the n andk values. Eutectic phase particles cracking and debonding determine the fracture mechanism of the alloy. Final failure of the alloy mainly depends on the global instability (high temperature, necking) and local instability (RT, shearing). Different tensile behaviors of the alloy are mainly attributed to different matrix strengths, phase particle strengths and damage rate.

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    Microstructure and Mechanical Properties of 7005 Aluminum Alloy Components Formed by Thixoforming
    Jiang Jufu, Atkinson H.V., Wang Ying
    J. Mater. Sci. Technol., 2017, 33 (4): 379-388.  DOI: 10.1016/j.jmst.2016.07.014
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    In the present research, semisolid billet of 7005 aluminum alloy was fabricated by using recrystallization and partial remelting (RAP), then thixoformed at different isothermal temperatures, preheating temperatures and load routes. Mechanical properties and microstructure of the thixoformed product were investigated. The results showed that microstructure achieved by three-step induction heating warm extruded 7005 aluminum alloy consists of a uniform and spheroidal microstructure suitable for thixoforming. Preheating temperature of the die affected significantly the filling status of semisolid billet of 7005 aluminum alloy. Complete filling status with good surface quality was obtained at a preheating temperature of 365 °C. Thixoformed microstructures consisting of relatively spheroidal grains illustrate the dependence of filling process on the sliding and rotating of solid grains rather than plastic deformation of solid grains. A non-uniform distribution of liquid phase was found in the different regions of the thixoformed product due to the slower adjustable velocity of solid grains as compared with liquid phase. Increase of isothermal temperatures led to a slight decrease of mechanical properties of the thixoformed product due to coarsening of solid grains. The highest yield strength, ultimate tensile strength and elongation of thixoformed components with T6 heat treatment are 237 MPa, 361 MPa and 16.8%, respectively, which were achieved at the isothermal temperature of 605 °C. Load route has a significant effect on mechanical properties and microstructure of the thixoformed product. Defects, such as crack and microporosity occurred in the microstructure of the thixoformed product obtained under load route 2. It led to an obvious reduction of mechanical properties as compared with route 1. A better compatibility of deformation caused by more liquid fraction at the isothermal temperature of 612 °C is beneficial to reducing non-uniformity of liquid phase in the different regions of the thixoformed product.

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    Selective Laser Melting of an Al-Fe-V-Si Alloy: Microstructural Evolution and Thermal Stability
    Sun Shao-Bo, Zheng Li-Jing, Liu Jin-Hui, Zhang Hu
    J. Mater. Sci. Technol., 2017, 33 (4): 389-396.  DOI: 10.1016/j.jmst.2016.09.015
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    Selective laser melting was used to produce an aluminum alloy Al-8.5Fe-1.3V-1.7Si (wt%). The effects of heat treatment on microstructure evolution and phase stability during long-term thermal exposure of the deposits were investigated. Results show that the microquasi-crystalline phase, Al12(Fe,V)3Si and AlmFe metastable phases coexisted with α-Al in the as-produced alloy. Annealing at 400 °C resulted in decomposition of microquasi-crystalline phase and supersaturated α-Al into Al12(Fe, V)3Si phase in the fusion zone, accompanied by the decrease in alloy hardness. The activation energy of this decomposition process was 115 kJ/mol. A more homogenous microstructure was obtained after annealing at 400 °C for 60 min, which was resistant to coarsening exposed at 425 °C up to 500 h. The Al12(Fe,V)3Si and AlmFe phases were coarsened at 475 and 525 °C with increasing the exposure time. Coarsening of Al12(Fe,V)3Si phase was attributed to a combination of volume diffusion and grain boundary diffusion mechanism of Fe. Heat treatment at 600 °C resulted in accelerated microstructure coarsening and formation of large-sized equilibrium phases, which significantly degraded the room temperature microhardness.

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    Reactivity of Al-rich Alloys with Water Promoted by Liquid Al Grain Boundary Phases
    He Tiantian, Wang Wei, Chen Wei, Chen Demin, Yang Ke
    J. Mater. Sci. Technol., 2017, 33 (4): 397-403.  DOI: 10.1016/j.jmst.2016.11.013
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    Al-Ga-Sn, Al-Ga-In and Al-Ga-In-Sn alloys were prepared using arc melting technique. Their microstructures were investigated by X-ray diffraction and scanning electron microscopy with energy dispersed X-ray. Based on microstructure analysis, the phase constituents of alloys at Al grain boundaries were identified. The melting points of Al grain boundary phases were measured using differential scanning calorimeter. The reactivities of Al-water at different water temperatures indicate that liquid Al grain boundary phases promote Al-water reactions of alloys. The melting points of Al grain boundary phases affect the reaction temperatures of Al-water, leading to different reaction temperatures of alloys. The measured H2 generation rate and yields of alloys are related to the compositions of alloys. The theory of micro-galvanic cell is used to explain the observed different H2generation rates of alloys.

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    Effect of Squeeze Casting on Microstructure and Mechanical Properties of Hypereutectic Al-xSi Alloys
    Li Runxia, Liu Lanji, Zhang Lijun, Sun Jihong, Shi Yuanji, Yu Baoyi
    J. Mater. Sci. Technol., 2017, 33 (4): 404-410.  DOI: 10.1016/j.jmst.2017.02.004
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    The effect of squeeze casting on microstructure and mechanical properties of hypereutectic Al-xSi alloys (x = 15, 17.5, 22 wt%) was investigated in this study. Results show that microstructure of the hypereutectic Al-xSi alloys was obviously improved by squeeze casting. The amount of coarse primary Si phase decreased, while that of fine primary α-Al dendrites increased with the increase of squeeze casting pressure. Due to the decrease of coarse primary Si particles, cracking of the matrix was reduced, whilst the fine microstructure, and mechanical properties of the squeeze casting alloys were improved. Compared with gravity casting alloys, mechanical properties of the hypereutectic Al-xSi alloys solidified at 600 MPa were improved significantly. Hardness of the squeeze casting hypereutectic Al-(15, 17.5, 22 wt%) Si alloys was improved by 15.91%, 12.23%, 17.48%, ultimate tensile strength was improved by 37.85%, 32.27%, 22.74%, and elongation was improved by 55.83%, 167.86%, 126.76%, respectively. Due to the uniform distribution of Si phases in squeeze casting Al-xSi alloys, their wear resistance was markedly enhanced.

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ISSN: 1005-0302
CN: 21-1315/TG
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