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  • Misorientation Development in Continuous Dynamic Recrystallization of AZ31B Alloy Sheet and Polycrystal Plasticity Simulation
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-25
    Guowei Zhou, Zihan Li, Dayong Li, Yinghong Peng, Huamiao Wang, Peidong Wu

    A series of electron backscattered diffraction (EBSD) experiments is carried out to explore nucleation features in the continuous dynamic recrystallization (CDRX) of AZ31 Mg alloy sheets at 200°C. The CDRX mechanism that misorientation accumulated from the core area to grain boundary leads to nucleation of dynamic recrystallization grains around parent grains can be identified for the present fine-grained AZ31B Mg alloy rolling sheet. A crystal plasticity approach for DRX simulation is extended to simulate the hot deformation and CDRX of the AZ31B magnesium alloy sheets. The experimental results of uniaxial tension along rolling direction (RD) and compression tests along RD and normal direction of the AZ31B sheets at 200°C are numerically investigated by the current model in terms of mechanical behaviors, grains’ rotation, textures orientation and grain sizes evolution. The VPSC-DRX model that considers multiple slip systems and indirectly incorporates the misorientation can reproduce well the stress-strain curves, r-values, grain size change and texture evolution. The introduction of DRX will change the slip mode activities at 200°C. The VPSC-DRX model can better predict the texture evolution compared to the simulation results regardless of DRX effects.

    更新日期:2018-05-27
  • Aging characteristics and strengthening behavior of a low-carbon medium-Mn Cu-bearing steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-25
    Y. Zou, Y.B. Xu, D.T. Han, Z.P. Hu, H. Song, R.D.K. Misra, L.F. Cao, S.Q. Chen

    We describe here the aging characteristics and strengthening behavior of a low-carbon medium-Mn Cu precipitation-strengthened steel. Atom probe tomography (APT) was employed to characterize the evolution of Cu-rich precipitates in terms of mean radius, number density and volume fraction. Aging at 500 °C and 550 °C for 1 h resulted in substantial coherent body-centered cubic (bcc) Cu-rich precipitates with mean radius of 1.35 and 2.59 nm, respectively. The precipitation strengthening mechanism for these two aging conditions was shearing mechanism and the corresponding strengthening contribution was ~266 and ~312 MPa, respectively. Here, coherency strengthening and modulus strengthening played a major role, while the contribution of chemical strengthening was relatively small. With increased aging temperature to 600 °C, the precipitates grew and coarsened to elongated shape with incoherent face-centered cubic (fcc) structure, and the strengthening mechanism was Orowan mechanism with a contribution of ~232 MPa. Increasing the aging temperature also facilitated the formation of retained austenite, which was of great benefit to plasticity without pronounced deterioration on precipitation strengthening. Ultra-high yield strength of 1020 MPa with superior total elongation of 25.8% was obtained in the sample aged at 600 °C for 1 h. The excellent mechanical properties derived from the combination of precipitation strengthening by Cu-rich precipitates and plasticity effect of retained austenite can be considered as a design principle to simultaneously optimize strength and ductility.

    更新日期:2018-05-27
  • Nano-scale precipitate evolution and mechanical properties of 7085 aluminum alloy during thermal exposure
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-25
    Pan Dai, Xian Luo, Yanqing Yang, Zongde Kou, Bin Huang, Chen Wang, Jinxin Zang, Jigang Ru

    As a new generation of Al-Zn-Mg-Cu alloy, 7085 aluminum alloy is a promising structural material in the field of aerospace industry. However, research on its thermal stability is still lacking. In the present work, thermal exposure was carried out on the T7452-treated 7085 aluminum alloy under different temperatures (100 °C, 125 °C, 150 °C and 175 °C) for 500 h. Variations of tensile properties and hardness were exhibited. The microstructure, nano-scale precipitates and fracture characteristics of the alloy were investigated using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that with the increase of exposure temperature, the strength and hardness increase first and then decrease while the elongation and the reduction of area increase continuously as compared to those of the non-thermal exposed alloy. The transformation from η′ phase to η phase during thermal exposure occurs continuously during thermal exposure. In addition, as the exposure temperature increases, the average dimensions of precipitates and the average spacing of neighbor precipitates become larger. The influence of precipitates on mechanical properties of the alloy is discussed.

    更新日期:2018-05-27
  • Enhanced mechanical properties and high electrical conductivity in multiwalled carbon nanotubes reinforced copper matrix nanolaminated composites
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-25
    Jiapeng Liu, Ding-Bang Xiong, Zhanqiu Tan, Genlian Fan, Qiang Guo, Yishi Su, Zhiqiang Li, Di Zhang

    Multiwalled carbon nanotubes/copper (MWCNTs/Cu) composites with a nanolaminated structure have been successfully prepared via flake powder metallurgy. The key strategies are to achieve uniform dispersion of carbon nanotubes in copper matrix and laminated structure, leading to high strengthening efficiencies and architecture toughening. As a result, the composites show balanced failure strength and elongation and high electrical conductivity. The tensile strength of 1.0 vol% MWCNTs/Cu laminated composite is 395 MPa, 87% higher than that of coarse-grained Cu. At the same time, the enhancement on strength does not cause serious deterioration in failure elongation and electrical conductivity. A satisfied uniform elongation in excess of 20% and an electrical conductivity more than 90% International Annealed Copper Standard (IACS) are retained in the composite. Characterizations by in-situ digital image correlation and X-ray tomography indicate efficient stress transferring during loading as well as laminated structure guiding crack propagation, contributing to the good strength-elongation balance. Additionally, the electrical conductivity of the composites is anisotropic because of the laminated structure.

    更新日期:2018-05-27
  • Microstructure and thermomechanical behavior of Heusler phase Ni2AlHf-strengthened NiAl-Cr(Co) alloy produced by HIP of plasma-spheroidized powder
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-25
    Yu.Yu. Kaplanskii, A.V. Korotitskiy, E.A. Levashov, Zh.A. Sentyurina, P.A. Loginov, A.V. Samokhin, I.A. Logachev

    This study demonstrates that homogeneous high-purity micropowders with grain size of 10–45 μm from refractory NiAl-Cr(Co)-Hf alloy can be produced by combustion elemental synthesis and further treated in a thermal plasma flow. The optimal parameters of plasma spheroidization of the synthesized powders are shown at which the resulting spherical alloy particles are characterized by high sphericity (~98%) and the absence of satellites or internal porosity. The spherical powders were consolidated by HIP. The alloy structure was investigated by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The fine structure and features of segregation of hardening nanoparticles of Ni2AlHf (the Heusler phase) and hafnium solid solution (ssHf) in the NiAl matrix were thoroughly studied. Furthermore, in situ examination of structural phase transformations occurring in the as-HIP NiAl-based alloy was carried out upon heating in the temperature range of 25–850 °C. The α-Cr phase nanoparticles are formed inside the NiAl grains via two different mechanisms: (1) spinodal decomposition of solid solution at 250–450 °C resulting in segregation of the Guinier-Preston zone rich in Cr and its subsequent transformation into α-Cr precipitates 25–170 nm in size; (2) heterogeneous nucleation and growth of α-Cr nanocrystallites 7–40 nm long on prismatic dislocation loops at 750–850 °C. The thermomechanical properties of the alloy were studied in the temperature range of 596–1100 °C on a Gleeble System 3800 testing system. The correlation between the thermal load and the high-temperature creep rate, as well as the temperature dependences of the Young's modulus (E), the offset yield strength (σ0.2), and the rate sensitivity coefficient (m), were ascertained. The Young's modulus (E), the ultimate tensile strength, the offset yield strength σ0.2, and the proportional limit of the alloy being examined during uniaxial compression test of a cylindrical sample at 750 °C and at true strain rate of ~0.01 s−1 were as high as 137.8 GPa, 682, 455 and 358 MPa, respectively.

    更新日期:2018-05-27
  • In situ high-energy X-ray diffraction investigation of the micromechanical behavior of Fe-0.1C-10Mn-0/2Al steel at room and elevated temperatures
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-26
    Minghe Zhang, Qing Tan, Jie Ding, Haiyang Chen, Fangmin Guo, Yang Ren, Yan-Dong Wang

    The micromechanical behavior of medium-Mn transformation-induced-plasticity (TRIP) steels with nominal chemical compositions of Fe-0.1C-10Mn (mass%) (0Al steel) and Fe-0.1C-10Mn-2Al (mass%) (2Al steel) fabricated by intercritical annealing 600 °C for 1 h and 650 °C for 1 h, respectively, was investigated using in situ high-energy X-ray diffraction (HE-XRD) with uniaxial tensile tests at 25 °C and 100 °C. We found that Lüders band propagation promoted lower volume fraction of austenite transformation to martensite in 0Al steel than 2Al steel at 25 °C while higher volume fraction of austenite transformation to martensite in 0Al steel than 2Al steel at 100 °C. Portevin-Le Châtelier (PLC) band propagation promoted higher volume fraction of austenite transformation to martensite in 0Al steel than 2Al steel during deformation at 25 °C and 100 °C. Moreover, the addition of Al obviously suppresses the formation of intermediate ε martensite during tensile deformation, thus the zigzag change in lattice strain of austenite was completely depressed in 2Al steel. Due to the controlled stability of metastable austenite, the 2Al steel demonstrated the best combination of ultimate tensile strength and total elongation at 25 °C.

    更新日期:2018-05-27
  • Excellent combination of strength and ductility of Al-1.2Mn alloy with multi-scale lamellar structure
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-26
    X.P. Chen, S. Li, L. Mei, P. Ren, Q. Liu

    By the method of cold rolling, appropriate intermediate annealing and final annealing treatments, the multi-scale lamellar structure of ultrafine grains distributed in ribbons with coarse grains was produced in an Al-1.2Mn alloy, which displayed excellent combination of high strength and good ductility compared to coarse-grained and ultrafine-grained alloys. The high strength of the alloy with multi-scale lamellar structure is attributed to back stress strengthening associated with the formation of geometrically necessary dislocations and the good ductility results from the high strain hardening rate during plastic deformation. Loading-unloading-reloading tests were carried out to measure the back stresses of the alloys. The results show that the back stress of the specimen with multi-scale lamellar structure is much higher than that of the coarse-grained specimen.

    更新日期:2018-05-27
  • Two quite different primary Mn-rich phases in Al-Si-Cu-Mn heat-resistant alloy and its effect to mechanical properties
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-26
    Guangjing Li, Hengcheng Liao, Aiqun Xu

    Two quite different primary Mn-rich phases were observed by optical microscope and scanning electron microscope in the surface and central parts of Al-12wt.%Si-4wt.%Cu-2wt.%Mn heat-resistant alloy casting. Deep-etching observation reveals that one is as highly developed 3D-dendrites and the other is as simple 2D-laths. The results of SEM-EDS show the constituents of these two Mn-rich phases are quite different too. The results of TEM-EDS verify it further. By selected area electron diffraction pattern, the lath-shaped Mn-rich phase is identified as: Al13Mn4Si8, simple tetragonal, a=b=1.240 nm, c=0.489 nm. The tension test indicates the contribution to strength from dendritic Al15Mn3Si2 phase is larger than Al13Mn4Si8 phase whether at room temperature or at high temperature. The reason for it is thought to be tied with the much larger difference in elastic modulus of Al13Mn4Si8 phase with aluminum matrix than that of Al15Mn3Si2.

    更新日期:2018-05-27
  • Rate-dependent Shear Banding and Fracture Behavior in A Ductile Bulk Metallic Glass
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-26
    Ding Zhou, Bingjin Li, Shuangyin Zhang, Bing Hou, Yulong Li
    更新日期:2018-05-27
  • Effect of Solid-Solution Temperature on the Microstructure and Properties of Ultra-High-Strength Ferrium S53® Steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-26
    Yangpeng Zhang, Dongping Zhan, Xiwei Qi, Zhouhua Jiang

    Ferrium S53® is a new secondary hardening ultra-high-strength (UHS) stainless steel, with more Cr than other Co–Ni UHS steels (UHSSs). The solid-solution step is an important step in the heat treatment of Ferrium S53®. This study investigated the effect of solid-solution temperature on the microstructure and mechanical properties of Ferrium S53®. Steel samples were solid-solution-treated at six different temperatures between 935 °C and 1185 °C for one hour. The resulting microstructures were characterized by X-ray diffraction (XRD), optical microscopy, field emission scanning electron microscopy, and transmission electron microscopy. The tensile strength and impact toughness at different solid-solution temperatures were also evaluated, and their relationship with the microstructure was discussed. The secondary phase of Ferrium S53® steel precipitated during stress annealing was M23C6, mostly around 100 nm, having C, Cr, Mo, and W as main components. After one hour of solid-solution treatment at 1035 °C, the steel was entirely austenitized, but up to 1085 °C, the secondary phase was completely dissolved. The increase of the solid-solution temperature increased the retained austenite content and the toughness and led to the dissolution of the secondary phase. The last one is the main reason for the change of strength. The experimental results showed that the most appropriate solid-solution temperature for Ferrium S53® is 1085 °C.

    更新日期:2018-05-27
  • Microstructure, mechanical and thermo-physical properties of Al–50Si–xMg alloys
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-26
    Zhiyong Cai, Chun Zhang, Richu Wang, Chaoqun Peng, Xiang Wu, Haipu Li

    There is a growing demand to achieve better performance of electronic packaging materials owing to the development of modern electronics industry. In this work, the effects of elemental Mg addition on microstructural characteristics, mechanical properties, and thermo-physical properties of Al–50Si alloys were investigated. The tensile strength, bending strength, and hardness are improved significantly when the Mg content increases from 0% to 1%, but decreases as the content exceeds 1%. The enhanced strength is mainly attributed to the presence of small bar-like Mg2Si phase located at the interface. However, the extensive growth of Si phase in the high Mg contained alloys reduce the mechanical properties. The thermal conductivity and thermal expansion coefficient decrease gradually with the increase in the Mg content. Compared with the addition of Cu, minor amount of Mg addition (0.5%) can effectively increase the strength of Al–50Si alloys without substantially sacrificing the thermal conductivity.

    更新日期:2018-05-27
  • Tensile deformation characteristics of a Cu−Ni−Si alloy containing trace elements processed by high-pressure torsion with subsequent aging
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-25
    Hikaru Watanabe, Takahiro Kunimine, Chihiro Watanabe, Ryoichi Monzen, Yoshikazu Todaka

    Experiments were performed to investigate the tensile deformation characteristics of a Cu-2.5 wt%Ni-0.55 wt%Si alloy containing trace elements of Zn, Sn and Mg, processed by high-pressure torsion (HPT) under an applied pressure of 5 GPa for 10 revolutions, and then peak-aged or over-aged at 300 °C. The grain size of the HPT-processed (H) alloy was refined to 70 nm. The peak-aged (PA) alloy exhibited a higher tensile strength σu of over 1 GPa than the H alloy. The over-aged (OA) alloy exhibited a lower value of σu than not only the PA but the H alloy. The PA and the OA alloy revealed unique tensile deformation properties: an extremely small local elongation of 0.1% for the PA alloy, a very small uniform elongation of 0.2% for the OA alloy, and a sudden drop of tensile stress immediately after necking initiation for the OA alloy. The small local and uniform elongation for the PA and OA alloy were explained on the basis of intergranular fracture after and before necking initiation during tension. The stress drop was caused by the rapid propagation of intergranular fracture from the surface of necking part to the interior. Intergranular segregation of Sn promoted by aging at 300 °C resulted in the intergranular fracture of the PA and OA alloy.

    更新日期:2018-05-25
  • Nondestructive Measurements of Flow Properties of Nanocrystalline Al-Cu-Ti Alloy using Automated Ball Indentation (ABI) Technique
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-24
    H.R. Ammar, F.M. Haggag, A.S. Alaboody, F.A. Al-Mufadi

    The Stress-Strain Microprobe® (SSM) system and its Automated Ball Indentation® (ABI®) test technique were applied in the present study for the purpose of determining the flow properties of the alloy under study in a non-destructive manner. ABI tests were applied at 21 °C to the consolidated nanocrystalline Al-10wt.%Cu-5wt.%Ti alloy samples which were subjected to three and six hours of milling. The samples processed for six hours of milling were further tested at elevated temperatures of 200 °C and 400 °C. The results of the ABI tests of the alloy under study include the indentation load versus depth of penetration curves, true stress versus true-plastic-strain curves, determination of yield strength curves, ultimate tensile strength values, and ABI Brinell hardness number (BHN). X-Ray Diffraction (XRD) was applied to investigate the microstructure of the Al-10wt.%Cu-5wt.%Ti alloy subjected to different milling time before and after consolidation. XRD analysis includes identification of the main phases/elements present and calculating the crystallite size in the processed alloy for 3 and 6 hours of milling before and after consolidation. Apreo Field Emission Scanning Electron Microscope was applied to characterize the size and morphology of the indentation area after conducting the ABI tests. In addition, the same technique was applied to reveal the elemental distribution in the indentation area of the tested samples.

    更新日期:2018-05-25
  • A physically-based high temperature yield strength model for 9Cr steels
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-24
    Richard A. Barrett, Padraic E. O’Donoghue, Sean B. Leen

    The strength of 9Cr steels, which is controlled by chemical composition and microstructure, evolves significantly under high temperature loading. This paper presents a temperature-independent, physically-based model for evolving yield strength, including the interdependent effects of dislocations, solutes, precipitates and grain boundaries. The key roles of solute and precipitate strengthening in 9Cr steels are successfully predicted. The measured significant beneficial effect of up to 3 wt.% tungsten on solute strengthening, and hence, yield strength are successfully predicted. The new model demonstrates that the reported strength reduction in 9Cr-3W alloys under thermal aging can be primarily attributed to Laves phase formation and associated depletion of tungsten solutes, consistent with microstructural observations.

    更新日期:2018-05-25
  • Transformation induced plasticity effect under tensile and compression stresses in nanostructured bainite
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-25
    Behzad Avishan

    TRIP effect and mechanical stability of retained austenite are important factors which must be considered when talking about the ductility in nanostructured bainite. Meanwhile, different parameters affect austenite mechanical stability among those the deformation mode is of considerable importance. This article aims to compare the capability of high carbon retained austenite to transform to martensite when being subjected to external tensile and compressive mechanical loads in those materials. Scanning electron microscopy were used to investigate the microstructural features and interrupted tensile and compression tests were implemented to evaluate austenite stability. X-ray analyses were carried out to evaluate the austenite to martensite evolution during straining the samples and three different mathematical approaches were also implemented to express the austenite mechanical stability by mathematical constants. Results indicated that higher yield strength, enhanced true plastic strength and higher ductility could be obtained in compression deformation state. Moreover, tensile tests strongly simulated the martensite formation while compressive deformation mode opposed the volume expansion associated with austenite to martensite transformation. Accordingly, austenite was mechanically more stable during compression and it transformed to martensite more gradually comparing to that of tension.

    更新日期:2018-05-25
  • Fracture properties of zinc coating layers in a galvannealed steel and an electrolytically galvanized steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-25
    Jinshan He, Junhe Lian, Anke Aretz, Napat Vajragupta, Ude Hangen, Frank Goodwin, Sebastian Münstermann

    The zinc coating layer fracture properties of a galvannealed steel and an electrolytically galvanized steel were analyzed by conducting the in-situ bending test with newly designed samples. It is found that the fracture develops much earlier in the coating layer of galvannealed steel than that of electrolytically galvanized steel. Using transmission electron microscope and energy dispersive X-ray spectroscopy, the intermetallic phases are characterized and it is found that the early crack initiation in a galvannealed steel is mainly triggered in the gamma phase. Combining with nano-indentation tests and corresponding simulation, the deformability of intermetallic phases were analyzed to explain the failure behavior of coating layers in the two steels.

    更新日期:2018-05-25
  • Fine-grained W-NiTi heavy alloys with enhanced performance
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-25
    Yang Shao, Huan Ma, Lishan Cui

    In this study, the effect of infiltration temperature and holding time on the microstructure, transformation behaviors and mechanical properties of W-NiTi heavy alloys prepared by infiltration and hot pressing were investigated by SEM, DSC and compression tests. It was found that the average W grain sizes of all the W-NiTi heavy alloys prepared at temperatures varied from 1350 to 1450 ℃ with different holding times were less than 5 μm. The average W grain size was found to increase with increasing sintering temperature and holding time. The morphology of W grain was still polygonal with less growth. The NiTi matrix in these alloys was observed to dissolve up to 8.75 wt.% W, significantly less than that of conventional tungsten heavy alloys, which could suppress the growth of W grains and make W grains more stable in NiTi matrix than in NiFe or NiCu matrix. All the W-NiTi alloys exhibited reversible martensite transformation. The stable microstructure and the increasing bonding strength resulted in the increasing of the strength and ductility of the alloys with infiltration temperature and holding time in compression tests.

    更新日期:2018-05-25
  • Microstructure and mechanical properties of a Cu-Zr based bulk metallic glass containing atomic scale chemical heterogeneities
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-24
    Reza Rashidi, Mehdi Malekan, Reza Gholamipour

    In this research, Tin was alloyed to the Cu47Zr47Al6 bulk metallic glass to induce chemical heterogeneities formation and study its effect on the mechanical and thermal properties. The large difference in enthalpy of mixing of Sn-Zr (−43 kJ/mol) and Sn-Cu (+7 kJ/mol) causes chemical heterogeneities. Microstructural examination showed composition fluctuations including Cu-rich and Zr-rich zones in (Cu47Zr47Al6)99 Sn1 alloy, improves ductility and fracture strength by 72.15% and 26.37%, respectively. Correlation between microstructure and mechanical properties was investigated through work hardening behavior, serration density and fractography of compression samples. Chemical heterogeneities lead to increase in shear bands, consequently enhance ductility and work hardening rate.

    更新日期:2018-05-24
  • Compressive deformation and fracture behaviors of AZ31 magnesium alloys with equiaxed grains and bimodal grains
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-23
    Xinfeng Li, Jin Zhang, Dewen Hou, Qizhen Li

    The compressive mechanical properties, microstructural evolution and fracture behaviors of AZ31 alloys with an equiaxed grain (E-grain) and bimodal grain (B-grain) were investigated using optical microscopy, scanning electron microscopy and electron backscattered diffraction. The results indicate that the ratio of twinned grains to all grains increases for both E-grain and B-grain samples with increasing strain levels during compression deformation along the extruded direction. The average Schmid factor for {10–12} twins of B-grain samples is higher than that of E-grain samples, which lowers required activating stress for {10–12} twins in B-grain samples. This causes a higher quantity of twins in B-grain samples than that of E-grain samples at the same pre-compression level. The E-grain samples mainly exhibit parallel twins regardless of pre-compression levels, whereas the twins in coarse grains of B-grain samples are parallel to each other at low compressive strain level and then transform to crossed twins at high compressive strain level with parallel twins in fine grains. Additionally, the peak strength, strain hardening rate and fracture strain of E-grain samples are lower than those of B-grain samples, and they possess similar yield strength. The coarse grains in the B-grain samples effectively resist crack growth by deflecting and branching crack tips, bridging of the cracks and forming the secondary cracks. Meanwhile, the fine grains in B-grain samples accommodate plastic strain and mediate multiple grains’ deformation. The concurrent effect of coarse grains and fine grains results in the improvement of plasticity.

    更新日期:2018-05-24
  • Effect of Quenching Temperature on Stretch Flangeability of a Medium Mn Steel Processed by Quenching and Partitioning
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-23
    Ji Hoon Kim, Eun Jung Seo, Min-Hyeok Kwon, Singon Kang, Bruno C. De Cooman

    The stretch flangeability of a medium Mn steel prepared by quenching and partitioning was examined using a hole-expansion test. Samples were heat-treated at quenching temperatures 90 °C ≤ TQ ≤ 170 °C, at which no secondary martensite is formed upon final quenching. The hole-expansion test used a 60° conical punch. The hole at the center of the specimens was prepared by punching or wire cutting. In both sample preparation conditions, hole expansion ratio (HER) increased as TQ decreased. The dependence of HER on TQ was more pronounced in the samples prepared by punching than in samples prepared by wire cutting. While there is no clear correlation between HER and tensile properties, the HER decreased as retained austenite increased. By punching, the retained austenite transformed to strain-induced martensite in the shear-affected zone near the hole edge. This martensite has a negative impact on the HER.

    更新日期:2018-05-24
  • Effect of Ta and Nb additions in arc-melted Co-Ni-based superalloys: microstructural and mechanical properties
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-22
    A.M.S. Costa, J.P. Oliveira, M.V. Salgado, C.A. Nunes, E.S.N. Lopes, N.V.V. Mogili, A.J. Ramirez, A.P. Tschiptschin

    The current work characterizes the microstructural and mechanical properties of two arc melted Co-Ni-based superalloys containing: 1) Co-40Ni-10Al-7.5W-10Cr-3Ta-0.06B-0.6 C (at. %) and 2) Co-40Ni-10Al-7.5W-10Cr-3Nb-0.06B-0.6 C (at. %). These two materials named 3Ta and 3 Nb, respectively, were characterized by X-Ray diffraction analysis (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and compression tests were performed from the room temperature up to 1000 °C. The 3Ta and 3 Nb alloys showed different as-cast microstructures that were confirmed by Scheil calculations and microstructural characterization. The phase transformation temperatures of minor phases were measured and calculated using a thermodynamic approach and some discrepancies observed on the calculated γ′-solvus temperatures reflect that the thermodynamic description of γ′ phase does not take into account the strong partition effect of Ta, Nb and Ti into γ′. As shown by DSC results, the γ′ phase was stable for both alloys at 1000 °C, being a main reason for the 3Ta and 3 Nb alloys to keep the yield stress above 500 MPa.

    更新日期:2018-05-23
  • Simultaneous enhancement of strength and plasticity by nano B2 clusters and nano-γ phase in a low carbon low alloy steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-22
    G. Han, Z.J. Xie, B. Lei, W.Q. Liu, H.H. Zhu, Y. Yan, R.D.K. Misra, C.J. Shang

    Nano B2 FeCu ordered clusters and multiphase microstructure consisting of intercritical ferrite, tempered martensite and nano-γ phase (reverted austenite) were obtained by two-step heat treatment involving intercritical annealing and intercritical tempering. The experimental steel with nano Cu precipitates and nano-γ phase exhibited high strength and high ductility combination. The yield strength and total elongation of the experimental steel increased from 758 MPa and 16.8% to 984 MPa and 29.5% after the second step intercritical tempering for 5 min. High resolution transmission electron microscopy (HRTEM) and three-dimensional atom probe (3DAP) studies provided evidence to support that high density of nano B2 FeCu ordered clusters contributed to high strength. First principle calculations suggested that the feasibility of B2 FeCu nano-ordered clusters, and the stability of B2 structure is related to the coherent stress field at the interface between clusters and the BCC-Fe matrix. The average size of B2 FeCu nano-ordered clusters was 4 nm with a lattice constant of 0.2893 nm and an orientation relationship of (1 1 0)B2∥(1 1 0)α and [0 0 1]B2∥[0 0 1]α. 9 R Cu without twinned structure was discovered at different tempering times. The proportion of Cu in Cu precipitates varied from 24.4 at.% to 61.2 at.% with change in crystal structure and increase in the size of precipitates. The significant increase in yield strength is discussed in terms of ordered domain strengthening, modulus strengthening and lattice misfit strengthening mechanisms. Bright field TEM image and selected area electron diffraction (SAED) pattern discovered and proved that spindle nano-γ phase was present at phase boundary between tempered martensite and intercritical ferrite. The size of them were among 242 −375 nm in length and 52 − 80 nm in width. The two needle tip ends of nano-γ phase were just at phase boundaries, indicating the preferred growth direction. (1 −1 1)γ plane of nano-γ phase was parallel to (1 −1 0)α plane of the adjacent tempered martensite. The effect of nano-γ phase on enhancing the plasticity was revealed by instantaneous work hardening index curve.

    更新日期:2018-05-23
  • Finite element analysis on flexural strength of Al2O3-ZrO2 composite ceramics with different proportions
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-22
    Hui Yu, Zhenhao Hou, Xiaodong Guo, Yongjun Chen, Jianlin Li, Lijie Luo, Jianbao Li, Tao Yang

    Al2O3-ZrO2 composite ceramics with different proportions were fabricated by isostatic compaction and sintered by the normal pressure sintering method. The effect of proportions on the microstructures, mechanical properties and sintering properties of Al2O3-ZrO2 ceramics were investigated. The results show that as the volume fraction of ZrO2 increases from 0 to 50%, the relative bulk density gradually decreases from 90 to 88.5% and the porosity increases from 0.182 to 0.197%, but the flexural strength increases from 291 to 423 MPa. In order to explain this “abnormal phenomenon”, a finite element method was applied to calculate the maximum tensile stress on the Al2O3-ZrO2 grain interface under tensile condition for the first time. The results indicate that the maximum grain interface tensile stress (MTS) of the Al2O3-ZrO2 composite gradually decreases from 148.18 to 96.89 MPa as the volume fraction of ZrO2 increases from 25 to 100%, which may be the main reason for their progressive flexural strength of Al2O3-ZrO2 composite from 320 to 604 MPa. In addition, the pure Al2O3 model has low MTS, and its ceramic material also has good sintering performance, but its flexural strength is worst compared to other Al2O3-ZrO2 composite ceramics. This is due to the large sizes of defects generated by abnormal growth of Al2O3 particle size.

    更新日期:2018-05-23
  • In-situ SEM study of crack initiation and propagation behavior in a dissimilar metal welded joint
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-22
    W. Wang, T.G. Liu, X.Y. Cao, Y.H. Lu, T. Shoji

    In this paper, the in-situ fracture tests were carried out to study the fracture mechanism of different regions (base region, weld region and interface region) in a dissimilar metal welded joint of nuclear power plant. The local microstructural effect on short crack initiation and propagation behavior was investigated. The results indicated that the mechanism of crack initiation and propagation was strongly depended on the microstructures of the difference zone in the dissimilar metal welded joint. There were large numbers of twins in the base region, and the cracks tended to initiate along the twin boundary. The base region exhibited the characteristics of ductile fracture. There was no twin existed in the weld metal. The crack tended to initiate at the slip band and propagate along the columnar grain boundary due to existence of the second phase particles along the grain boundary. The fracture mode of the weld metal was intergranular. In the interface region, the crack tended to propagate from the weld zone to the epitaxial zone, and then along the grain boundary of the epitaxial zone. The fracture mode was the combination of the brittle and ductile fracture. Meanwhile, the weld region has a lower resistance of crack initiation and propagation compared with the base region and interface region.

    更新日期:2018-05-23
  • Long-time stability of metals after severe plastic deformation: Softening and hardening by self-annealing versus thermal stability
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-22
    Kaveh Edalati, Yuki Hashiguchi, Hideaki Iwaoka, Hirotaka Matsunaga, Ruslan Z. Valiev, Zenji Horita

    Despite superior properties of ultrafine-grained (UFG) materials processed by severe plastic deformation (SPD), their thermal stability is a concern because of the supersaturated fractions of lattice defects. In this study, the microstructural stability of various UFG materials (2 alloys and 15 pure metals) after SPD processing through the high-pressure torsion (HPT) were investigated at room temperature for up to 10 years. While most of the metals with high melting temperatures remained stable, a softening by self-annealing occurred in pure silver, gold and copper with moderate melting temperatures, and an unusual hardening occurred in pure magnesium, Al-Zn alloy and Mg-Li alloy with low melting temperatures. These softening/hardening behaviors by grain coarsening were attributed to the contribution of grain boundaries to dislocation activity or grain-boundary sliding, respectively. It was shown that the self-annealing was accelerated by increasing the processing pressure and strain and by decreasing the processing temperature and stacking fault energy, due to the enhancement of stored energy and/or atomic mobility.

    更新日期:2018-05-23
  • Negligible effect of twin-slip interaction on hardening in deformation of a Mg-3Al-1Zn alloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-22
    Peng Chen, Bin Li, Duke Culbertson, Yanyao Jiang

    Extruded magnesium alloys with a strong basal texture present a strong tension-compression asymmetry in deformation due to the fact that dislocation slips dominate plastic deformation in tension along the extrusion direction (ED) whereas twinning dominates plastic deformation in compression along the ED. The deformation characteristics allows for a study of the twin-slip interaction by introducing dislocations in the material before activating twinning. In the current work, dislocations of different densities are first introduced by prestraining in tension an extruded Mg-3Al-1Zn alloy along the ED to 5% and 10% of total strain, respectively. Then the loading direction is reversed to compression such that deformation twinning is activated. The results show that the hardening rate after different prestrains remains nearly unchanged, indicating that twin-slip interaction produces negligible effect on strain hardening. The yield stress in the reversed compression increases only slightly with increasing prestrain: ~20 MPa at every 5% increase in prestrain. Electron backscatter diffraction results show that prestraining can retard twin nucleation at the very early stage of plastic deformation, but it has little effect on twin growth.

    更新日期:2018-05-22
  • Tensile properties of spark plasma sintered AISI 316L stainless steel with unimodal and bimodal grain size distributions
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-22
    B. Flipon, C. Keller, L. Garcia de la Cruz, E. Hug, F. Barbe

    Powder metallurgy associated to spark plasma sintering was used to elaborate near full-dense samples of 316L austenitic stainless steel with unimodal or bimodal grain size distributions. To this aim, two different precursor powders were employed: a ball-milled one giving rise to ultrafine grains and a coarse one, as-received, for grains with conventional size. Sintered specimens were characterized in mechanical tension and their microstructure was revealed using transmission and scanning electron microscopy. Unimodal ultrafine grained samples show a large yield stress and a low ductility with a breakdown in the Hall-Petch relationship. For bimodal samples, a compromise between yield stress and ductility can be found. These features are then discussed in terms of strain mechanisms, grain size distribution and backstress. It is shown in particular that coarse grains contribute to enhance the ductility of the ultrafine grains matrix by modifying both the strain hardening mechanisms and the stress concentration areas.

    更新日期:2018-05-22
  • Dynamical Mechanical Analysis of metallic glass with and without miscibility gap
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-22
    Y.Y. Wang, C.X. Peng, X.L. Li, Y. Cheng, L.J. Jia, L. Wang

    The dynamic mechanical properties of the miscible Cu50Zr50 and immiscible Cu50Ag50 amorphous materials are investigated to explore the relationship between deformation mechanism and relaxation of glass through molecular dynamics simulation with modified embedded atom method (MEAM). It is found that the mechanical hysteresis of Cu50Ag50 glass is more pronounced than that of Cu50Zr50 glass. The storage modulus decreases with increasing loading period or amplitude; while the loss modulus increases till the maximum, corresponding to the beginning of α−α− relaxation. The β− β− relaxation in both Cu50Zr50 and Cu50Ag50 glass shows excess tails in the loss modulus curves. However, the peak height on the left part in the curve of loss modulus as a function of temperature for Cu50Ag50 glass is higher than that for Cu50Zr50 system, which indicates that β− β− relaxation in Cu50Ag50 glass is more likely to be activated than that in Cu50Zr50 system due to lower number of icosahedra-like clusters. The primary α−α− relaxation always takes place when the most probable atomic displacement reaches a critical fraction (~23%) for Cu50Zr50 and (~21%) for Cu50Ag50 of the average interatomic distance, irrespective of whether the relaxation is induced by temperature (linear response) or by mechanical strain (nonlinear).The fast atom is defined by the atom motion displacement to explore the dynamical heterogeneity of the glass. We find that the internal fraction shows linear with the number of fast atom.

    更新日期:2018-05-22
  • The role of twinning and nano-crystalline ω phase on the fatigue behavior of the metastable β Ti-15Mo alloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-21
    Leonardo Contri Campanelli, Francisco Gil Coury, Yaofeng Guo, Paulo Sergio Carvalho Pereira da Silva, Michael Joseph Kaufman, Claudemiro Bolfarini

    This work evaluated in depth the fatigue and fracture behavior of the metastable β Ti-15Mo alloy considering the presence of deformation twins and athermal nano-crystalline ω phase in the microstructure. Regardless of the microstructural condition studied, the as-received and the solution treated and quenched materials, ω phase remained unchanged, guaranteeing the static mechanical properties at acceptable levels. The mechanism of fatigue fracture was related to the existence of twins and maximum shear stress planes near the direction of screw dislocation motion in the BCC structure. The significant amount of deformation twins in the initial microstructure did not alter the fatigue limit, because the solution treated and quenched material unexpectedly developed twins during the fatigue test.

    更新日期:2018-05-21
  • Investigation on the dislocation evolution in nanoindentation with 2.5D discrete dislocation dynamics simulation and experiment
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-21
    Qingshun Bai, Jinxuan Bai, Chao Hu, Hui Guo

    This study investigates the influence of materials’ dislocation source density and initial dislocation density on dislocation evolution mechanism during nanoindentation process. On the basic of discrete dislocation dynamics (DDD) algorithm, a novel 2.5D nanoindentation numerical model was by proposed by coupling with finite element (FE) contact analysis. In particular, new reduced integration method and stress transmission mode were performed in this simulation scheme. Results showed that the dislocation multiplication and slip behavior were both significantly impacted by the initial dislocation source density. Meanwhile, the influence of initial dislocation density focused on the spatial arrangement of discrete dislocations. In addition, transmission electron microscope (TEM) analysis was used to reveal the dislocation configurations during nanoindentation process. Typical dislocation patterns including Shockley partial dislocation and Frank partial dislocation were found in extrusion region, which verifies the reasonable slipping conditions of DDD simulation. The results of this study can be useful in developing DDD model and predicting dislocation evolution rules in nanoindentation.

    更新日期:2018-05-21
  • Strain hardening and nanocrystallization behaviors in Hadfield steel subjected to surface severe plastic deformation
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-19
    Chen Chen, Bo Lv, Xiaoyong Feng, Fucheng Zhang, Hossein Beladi

    A gradient nanocrystalline layer with a thickness in a range of millimeter magnitude was successfully produced on the surface of Hadfield steel by a novel severe plastic deformation technology, high speed pounding. The surface hardness was measured, and the microstructure evolution during nanocrystallization process was characterized by X-ray diffraction and transmission electron microscopy. Results showed that the hardness increment and nanocrystallization in Hadfield steel were obtained at different stages under high speed pounding. The first stage was strain hardening, where surface hardness of the Hadfield steel increased gradually during high speed pounding until a steady-state value was obtained. The hardening degree and rate of the Hadfield steel were determined by deformation stress and strain rate, respectively. The second stage was microstructure nanocrystallization, at which twin boundaries interacted with dislocations to form general high angle grain boundaries. In this stage, the surface hardness of Hadfield steel remained basically the same. Moreover, a physical model was established to explain the strain hardening and surface nanocrystallization behaviors in accordance with the microstructure evolution at different stages in Hadfield steel.

    更新日期:2018-05-19
  • High-temperature Tensile Properties and Deformation Mechanism of Polycrystalline Magnesium Alloys with Specifically Oriented Columnar Grain Structures
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-19
    Sheng-shi Zhao, Xiao-ping Lin, Yun Dong, Yi Niu, Dan Xu, Heng Sun

    A Mg–4.78Zn–0.45Y–0.10Zr (wt.%) alloy with specifically oriented columnar grain structures (crystal growth direction <01—10>) was prepared using directional solidification. The columnar grain structures possessed parallel-growing primary arms and straight longitudinal grain boundaries. The results from an electron backscatter diffraction analysis demonstrated that the angle between the c-axis and the growth direction was 74–87°. As the tensile tests were performed along the <01—10> growth direction, the Schmid factors for the cylindrical and conical plane slips were as high as 0.45. When the tensile test temperature was 300 ℃, dynamic recovery was observed and the cylindrical, conical, and conical slip systems were all activated. This phenomenon, together with the high orientation consistency between the adjoining grains, resulted in an elongation increase up to 42%. Fractography of the Mg alloy revealed extended or parabola-shaped dimples with specifically oriented columnar grain structures, suggesting good ductility for this material.

  • Evolution of the microstructure in aged G115 steels with the different concentration of tungsten
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-19
    Zhen Liu, Zhengdong Liu, Xitao Wang, Zhengzong Chen, Longteng Ma

    The microstructural evolution and certain mechanical properties of three G115 steels, 9 Cr-(2.3, 2.6, 3.0)W-3 Co (wt.%), were investigated after thermal aging at 675 °C for different times using optical microscopy, field emission transmission electron microscopy, X-ray diffraction, post-aged tensile tests and impact tests, focusing on martensitic lath, dislocations and precipitates. The results show that the coarsening rates of the lath and Laves phase particles increased with the increase in W concentration during thermal aging. The rates of the decreases in the dislocation and particle densities of the Laves phase particles increased with the increase in W concentration during thermal aging. Both the coarsening rates of the lath and precipitates (i.e., M23C6 and Laves phase particles) and reduction rate of the dislocation density are higher in the first 1000 h compared to those in the following 4000 h during the thermal aging process. As predicted using the Orowan stress formula, the 2.3 W G115 steel revealed the best creep strength during service, among the three steels.

    更新日期:2018-05-19
  • Enhanced ductility in dynamic strain aging regime in a Fe-25Ni-20Cr austenitic stainless steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-19
    Abdullah S. Alomari, N. Kumar, K.L. Murty

    Contrary to the commonly observed embrittlement during dynamic strain aging, we report in this note distinct enhancement in ductility in a Nb-containing and nitrogen stabilized Fe-25(wt.%)Ni-20Cr austenitic stainless steel (Alloy 709) at temperatures from 623 K to 873 K at 10−4 s−1 where serrated flow is noted. This observation is rationalized in terms of the influence of strain hardening parameters and strain rate sensitivity on uniform elongation and ductility respectively.

    更新日期:2018-05-19
  • Influence of small amount and different morphology of secondary phases on impact toughness of UNS S32205 Duplex Stainless Steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-19
    Claudio Gennari, Luca Pezzato, Enrico Piva, Renato Gobbo, Irene Calliari

    Duplex Stainless Steels (DSSs) are biphasic austenite-ferrite stainless steels, with higher mechanical properties and corrosion resistance than the other stainless steels grades. Impact toughness is a very important mechanical property and even though DSSs have a lower impact toughness as compared to the austenitic grades, the transition to ductile to brittle behavior is more gradual and it occurs at lower temperature than ferritic grades. On the other hand, DSSs suffer from embrittlement due to secondary phase precipitation, which affects all mechanical and corrosion properties, in particular impact toughness, even in low amount. In this research paper, the influence of a small amount (<2%) and different morphologies of secondary phases (coarse and finely dispersed) on the ductile to brittle transition of standard duplex stainless steel UNS S32205 was studied. Two isothermal heat treatments were conducted on the solubilized DSS in order to precipitate the same amount but different morphologies of secondary phases. Charpy tests were conducted on a temperature range between 20 °C and −196 °C. The wrought material retained a very good impact toughness even at −90 °C (100 J), but a small volume fraction affected the impact toughness even at room temperature. Coarser secondary phases affect the impact toughness largely as compared to small and finely dispersed particles.

    更新日期:2018-05-19
  • Multi-scale investigation of highly anisotropic Zinc alloys using crystal plasticity and inverse analysis
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-18
    Ludovic Cauvin, Balaji Raghavan, Salima Bouvier, Xiaodong Wang, Fodil Meraghni

    Zinc and its alloys are important industrial materials due to their high corrosion resistance, low cost and good ductility. However, the characterization of these materials remains a difficult task due to their highly anisotropic behavior, the latter being due to the influence of microstructural effects, i.e. loading orientation-dependent activation of different families of slip systems and subsequent texture evolution, rendering the development of a reliable material model considerably difficult. A micro-mechanical approach based on polycrystal plasticity would better describe the physical mechanisms underlying the macroscopic behavior. This improved model should ostensibly improve the comprehension of the mechanical behavior, compared to the macroscopic approach using solely phenomenological anisotropy models along with a prohibitively large number of experiments required to identify the material parameters. In this paper, a multi-scale Visco-Plastic Self-Consistent (VPSC) approach is used. It is based on a micro-scale model calibrated by microstructural and deformation mechanism information based on Electron Back-Scattered Diffraction (EBSD) to describe the macroscopic anisotropic mechanical response during sheet metal deformation. The critical resolved shear stress (CRSS) as well as the micro-scale crystal parameters are obtained by an inverse analysis comparing the simulated and experimental results in terms of obtained tensile curves along three different directions. In order to obtain a global solution for the identification, we then use the Covariance Matrix Adaptation-Evolution Strategy (CMA-ES) genetic algorithm to the inverse problem. We validate our approach by comparing the simulated and experimental textures and activated slip systems. Finally, the identified mechanical parameters are used to investigate the anisotropy of the alloy and predict its formability by determining the corresponding R-values and Hill yield coefficients.

    更新日期:2018-05-18
  • Solid Solution Softening and Enhanced Ductility in Concentrated FCC Silver Solid Solution Alloys
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-18
    Yongjun Huo, Jiaqi Wu, Chin C. Lee

    The major adoptions of silver-based bonding wires and silver-sintering methods in the electronic packaging industry have incited the fundamental material properties research on the silver-based alloys. Recently, an abnormal phenomenon, namely, solid solution softening, was observed in stress vs. strain characterization of Ag-In solid solution. In this paper, the mechanical properties of additional concentrated silver solid solution phases with other solute elements, Al, Ga and Sn, have been experimentally determined, with their work hardening behaviors and the corresponding fractography further analyzed. Particularly, the concentrated Ag-Ga solid solution has been discovered to possess the best combination of mechanical properties, namely, lowest yield strength, highest ductility and highest strength, among the concentrated solid solutions of the current study. Microscopically, a clear evidence of narrow twin lamella feature has been observed in the Ag-Ga solid solution, and further identified as the deformation twinning, using high resolution transmission electron microscopy (HRTEM). To explain solid solution softening mechanism in low Peierls potential FCC metals, the authors have proposed a self-contained theoretical interpretation associated with fractional kink-pairs formation, and originally introduced the concept of the short range order (SRO) induced localized homologous temperature (LHT). Furthermore, the mechanism of twinning-induced plasticity (TWIP) can be referred as the underlying reason of the enhanced ductility in the Ag-Ga solid solution alloy. With such excellent mechanical properties, the Ag-Ga solid solution alloy is expected to have a great potential in the development of advanced joining materials for the various applications in electronics industries.

    更新日期:2018-05-18
  • Crack propagation mechanisms of AISI 4340 steels with different strength and toughness
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-18
    H.F. Li, S.G. Wang, P. Zhang, R.T. Qu, Z.F. Zhang

    In this paper, the crack propagation characteristics in AISI 4340 steel with different strength and toughness have been investigated in quasi-situ via 3D X-ray tomography during fracture toughness testing. Three kinds of crack propagation characteristics were observed by the 3D image analysis, which were determined by the competition between cleavage fracture and voids growth/coalescence. It is found that the main crack would connect the micro-voids near the crack tip by cleavage fracture if the local stress achieves the cleavage fracture strength, resulting in rapid crack propagation. However, the growth and coalescence of micro-voids ahead of the crack tip would be linked with the main crack by strain-controlled, leading to the ductile fracture. The above two mechanisms would exist simultaneously in materials, which might be explained by the competition balance between strain and stress. Studying on transition of crack propagation mechanisms in materials would further understand the essence of fracture toughness. It will provide new insight into the fracture behavior of engineering materials and some new materials such as nanostructured materials and metallic glasses. Additionally, based on the energy principle, we propose a quantitative relationship between the microscope fracture surface morphology and fracture toughness of materials.

    更新日期:2018-05-18
  • Effect of multipass deformation at elevated temperatures on the flow behavior and microstructural evolution in Ti-6Al-4V
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-18
    Baoqi Guo, Clodualdo Aranas, Anes Foul, Xiankun Ji, Ameth Fall, Mohammad Jahazi, John J. Jonas

    Simulated multipass deformation experiments were carried out via torsion testing on a Ti-6Al-4V alloy in the two-phase region under both isothermal and continuous cooling conditions. Flow softening was observed during the isothermal multipass deformation tests as indicated by the evolution of the mean flow stress (MFS). The MFS values increased with interpass time from 2 s to 32 s. Using BSE-SEM characterization techniques, dynamic phase transformation (alpha to beta) and coarsening of the alpha phase took place. The quantitative results indicate that the alpha phase transforms into beta during straining, but it retransforms statically into alpha by amounts that increase with interpass time. The flow softening observed is the net result of softening by dynamic transformation and hardening by reverse transformation.

    更新日期:2018-05-18
  • New role of screw dislocation in twin lamella during deformation: An in situ TEM study at the atomic scale
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-17
    Zongde Kou, Yanqing Yang, Lixia Yang, Bin Huang, Yanxia Chen, Xian Luo

    Two dynamic processes were revealed in Cu by in situ tensile tests at the atomic scale: a lattice screw dislocation forming by the combination of two twinning dislocations; and two 60° full dislocations evolving from an extended screw dislocation. The results indicate that screw dislocation can trigger the transition of dislocation slip mode by its formation and dissociation, and also nucleate non-screw dislocations.

    更新日期:2018-05-18
  • Microstructure and properties of bulk Al0.5CoCrFeNi high-entropy alloy by cold rolling and subsequent annealing
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-17
    Tong Guo, Jinshan Li, Jun Wang, William Yi Wang, Yi Liu, Ximing Luo, Hongchao Kou, Eric Beaugnon

    Cold rolling combining subsequent annealing is implemented to tune the microstructure and mechanical properties of Al0.5CoCrFeNi high-entropy alloy. Results show that the microstructure of Al0.5CoCrFeNi HEA after 80% thickness reduction is severely deformed along the rolling direction. Prominent work hardening effect on Al0.5CoCrFeNi HEAs is evidenced by the remarkable enhancement of the hardness and strength at the sacrifice of ductility. After annealing, detailed description of microstructure evolution, texture analyses as well as tensile properties are given. Strongly temperature dependent recrystallized microstructure manifests the rather high recrystallization temperature (0.81 Tm) that can be attributed to original coarse as-cast grains, severe lattice distortion effect and sluggish diffusion effect. Progressively smaller intensity of deformed texture proves the function to eliminate texture effectively of recrystallization in spite of the detection of weak recrystallization texture. Tensile tests of annealed Al0.5CoCrFeNi HEAs show the strength decreases dramatically while ductility increases remarkably accompanying increased annealing temperature. To sum up, cold rolling combining subsequent annealing is a valid method to gain grain refined Al0.5CoCrFeNi HEAs with adjustable properties that are prospective for many kinds of application.

    更新日期:2018-05-18
  • Effects of Sn and Y on the microstructure, texture, and mechanical properties of as-extruded Mg-5Li-3Al-2Zn alloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-16
    Yuehua Sun, Richu Wang, Chaoqun Peng, Yan Feng

    The effects of combined addition of Sn and Y on the microstructure, texture, and mechanical properties of as-extruded Mg-5Li-3Al-2Zn alloy were investigated. The results show that the microstructure of as-extruded Mg-5Li-3Al-2Zn alloy mainly consists of α-Mg and AlLi phases. With the addition of Sn and Y, Mg2Sn and Al2Y compounds form, while AlLi phase gradually disappears with increasing Y content. These second phases are crushed into smaller particles during hot extrusion process, and Al2Zn particles are found in Mg2Sn-rich regions and distribute along the grain boundaries. Meanwhile, the microstructure is significantly refined, and the combined addition of 0.8 wt.% Sn and 1.2 wt.% Y produces the smallest grain size of 4.78 μm and the uniform microstructure is entirely composed of equiaxed grains. Additionally, the addition of Sn and Y weakens the basal texture of as-extruded alloy and introduces recrystallization texture due to solute drag effect and dynamic recrystallization. As-extruded Mg-5Li-3Al-2Zn-0.8Sn-1.2Y alloy exhibits a superior combination of tensile properties with ultimate tensile strength and elongation of 328.0 MPa and 25.1%, which are increased by approximately 32.2% and 41.0% compared with those of as-extruded Mg-5Li-3Al-2Zn alloy (248.2 MPa and 17.8%), respectively. The improvement of mechanical properties is mainly attributed to solution, dispersion, and grain refinement strengthening.

    更新日期:2018-05-17
  • Assessment of the stress-oriented precipitation hardening designed by interior residual stress during ageing in IN718 superalloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-16
    Hailong Qin, Zhongnan Bi, Hongyao Yu, Guang Feng, Ruiyao Zhang, Xia Guo, Hai Chi, Jinhui Du, Ji Zhang

    The following study presents a novel approach that utilizes residual stresses inside a component to produce stress-oriented precipitation and achieves texture strengthening during the ageing treatment. Refined investigations for the stress-oriented precipitation of γ″ were achieved via transmission electron microscopes (TEM) and neutron diffraction (ND). The results showed that all three variants of γ″ nucleated at short ageing times. The orienting effect of stress was appreciable only during the Ostwald ripening, due to the change of the γ″/γ lattice mismatch. The effect that the stress-oriented precipitation had on the mechanical properties was evaluated and analyzed via tensile tests.

    更新日期:2018-05-17
  • Back-stresses and geometrical hardening as competing mechanisms enhancing ductility in HCP metals
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-16
    D. Esqué-de los Ojos, C.-T. Nguyen, A. Orozco-Caballero, G. Timár, J. Quinta da Fonseca

    By recourse of computational mechanics, back-stresses are unveiled as a major source for the increase in work hardening during forming of hexagonal close-packaged (hcp) metals. Polycrystalline visco-plastic self-consistent (VPSC) and crystal plasticity finite element modelling (CPFEM) simulations of tensile uniaxial experiments were used along with experimental texture information. Simulations took into account the analogous variation in the critical resolved shear stress (CRSS) values of each slip family that could result from an increase in the test temperature. As the CRSS ratio between secondary and primary slip families increased, two different contributions to the variation of the work hardening rate were observed depending on the simulation framework: (i) a decrease in the work hardening rate in VPSC simulations attributed to texture evolution or geometrical hardening and (ii) an increase in the work hardening rate in CPFEM simulations due to back-stresses. While geometrical hardening is present in both simulation frameworks, only CPFEM is able to capture the influence of back-stresses on the increase of the work hardening rate with temperature. The results provided here contribute to a better understanding of the deformation mechanisms present in warm forming of hcp metals, showing also that CPFEM is a better simulation framework to study warm forming of hcp metals.

    更新日期:2018-05-16
  • Microstructure refining of Co-29Cr-6Mo-0.16N alloy in rapid hot-forging process
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-16
    Xianjue Ye, Bohua Yu, Xiaojuan Gong, Biaobiao Yang, Yan Nie, Yunping Li, Akihiko Chiba

    Explosive dynamic recrystallization (DRX) of Co-29Cr-6Mo-0.16 N (CCMN) alloy with extremely low stacking fault energy (SFE) was observed in rapid hot-forging processes; the DRX was considered to be closely related to the prevailing planar-defects formation in parent grains such as mechanical twin and stacking fault bands as well as their interactions, that finally break the parent grains into fine pieces accompanied by instantaneous reorientation and recovery in the refined grains.

    更新日期:2018-05-16
  • Strain-rate dependent hot deformation behavior and mechanism of interphase- precipitated Ti-Mo-xNb steels: physical modeling and characterization
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-15
    P.R. Yang, M.H Cai, C.F. Wu, J.H. Su, X.P. Guo

    The hot flow behaviors of interphase precipitated Ti-Mo-xNb (x=0, 0.02) steels were investigated by conducting compression tests at higher strain rates of 0.1~10 s−1 and temperatures of 800~1150 °C using a Gleeble-2000 thermo-mechanical simulator. The hot deformation behavior was modeled through combining the dislocation density based Bergstrom and the diffusional transformation based KJMA models. The results show that even small amount of Nb addition can effectively increase the activation energy for hot deformation, thus delaying the occurrence of dynamic recrystallization. However, the ascend rates in flow stress σ, peak stress σp and dislocation density ρp with Nb appeared to be more significant at higher strain rates, implying that the effectiveness of Nb in Ti-Mo-xNb steels was sensitive to strain rate. The microstructual features demonstrated that the hardening effect caused by Nb addition was offset by the softening effect of DRX at 0.1 s−1, causing the weak strain-rate dependence; whereas, at the strain rate of 10 s−1, the softening mechanism was considered as DRV, giving rise to a relatively large discrepancy in flow behaviors with Nb addition.

    更新日期:2018-05-15
  • Microstructure and mechanical properties of 7075-Al alloy joint ultrasonically soldered with Ni-foam/Sn composite solder
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-15
    Yong Xiao, Shan Li, Ziqi Wang, Yong Xiao, Zhipeng Song, Yongning Mao, Mingyu Li

    Ultrasound-assisted soldering of 7075 Al alloy was performed using Ni-foam reinforced Sn composite solder. The phase composition, interfacial microstructure and mechanical properties of Al/Ni-Sn/Al joints soldered for different times were investigated. Results showed that, the bonding ratio of joint was increased with increasing ultrasonic soldering time and was stabled at approximately 98% when the soldering time was longer than 20 s. The Ni-foam in joint was compressed into a strip type and a Ni3Sn4 intermetallic compound (IMC) layer was formed on the Ni skeleton surface, whilst an Al3Ni IMC layer was formed on the Al substrate surface. The Al3Ni phase was dot-distributed in joint soldered for 5 s then formed continuously in joint soldered for 15 s. However, further increasing the soldering time to 30 s resulted in the drastic growth of Al3Ni IMC layer, accompanied with the depletion of Ni3Sn4 phase. The formation and microstructure evolution mechanisms of the Al3Ni IMC layer was discussed. The measured shear strength of joint was first increased then decreased with increasing ultrasonic soldering time, and a highest shear strength of 58.0 MPa was obtained for joint soldered for 20 s.

    更新日期:2018-05-15
  • On the origin and contribution of extended kinks and jogs and stacking fault ribbons to deformation behavior in an ultrahigh strength cobalt-free maraging steel with high density of low lattice misfit precipitates
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-12
    K. Li, B. Yu, R.D.K. Misra, G. Han, S. Liu, Y.F. Shen, C.J. Shang

    We elucidate here the deformation mechanisms and underlying reasons that contributed to high ductility (10.2%) and high static toughness (112.5 MJ·m−3) in an ultrahigh strength (1860 MPa) cobalt-free 19Ni3Mo1.5Ti maraging steel characterized by high density (2.3×1024 m−3) of η-Ni3(Ti,Mo) and B2-Ni(Mo,Fe) nanoscale precipitates with low lattice misfit of <1% with the martensite matrix. Multiple deformation processes occurred during plastic deformation. Lath-morphology of martensite was dramatically segmented with angles of 30°, 60° or 120° with large pile-up of dislocations at the segmented boundaries. This occurred because of the interactive ability of edge and screw dislocations along the martensite habit planes, which led to kinks and jogs. The low lattice misfit (0.6% ~ 0.9%) precipitates interacted with dislocations leaving stacking fault ribbons within precipitates that build a large long range of back stress producing a high strain-hardening response. Additionally, nanoscale twinning occurred. The above contributions to ductility are envisaged to be in addition to the significantly reduced elastic interaction between the low lattice misfit nanoscale precipitates and dislocations that reduces the ability for crack initiation at the particle-matrix interface.

    更新日期:2018-05-12
  • Improvement of room and high temperature tensile properties of NiAl-strengthened ferritic heat-resistant steels through Mo addition
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-12
    Ken Cho, Kenshi Ikeda, Hiroyuki Y. Yasuda

    In order to improve the room and high temperature strength–ductility balance of ferritic Fe-Al-Ni based alloys strengthened by the B2-type NiAl precipitates, Fe-Al-Ni-Cr-Mo alloys was developed. The microstructures and tensile properties of the alloys were examined focusing on Mo content. We found that the 2 at.% or 4 at.% Mo addition is effective in the improvement of the elongation of the Fe-Al-Ni based alloys at room temperature (RT) while maintaining high strength, which reflects the suppression of intergranular fracture by the strengthening of the bcc matrix by Mo addition. We also found that the RT tensile properties and deformation behavior of the alloys depend strongly on the volume fraction and the size of the NiAl precipitates. For instance, the yield stress decreases by the change in the primary slip direction from <111> to <001> and the formation of the Orowan loops with increasing the volume fraction and the mean diameter of the NiAl precipitates. In addition, the alloy with the appropriate Mo content exhibits a high yield stress above 660 MPa up to 923 K, owing to the solid-solution hardening by Mo addition and the low growth rate of the NiAl precipitates even at high temperatures. These findings indicate that the alloys have a great potential for steam turbines of advanced ultra‐supercritical thermal power plants.

    更新日期:2018-05-12
  • Effect of building direction on porosity and fatigue life of selective laser melted AlSi12Mg alloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-12
    Junwen Zhao, Mark Easton, Ma Qian, Martin Leary, Milan Brandt

    Gas porosity is one of the most common defects in aluminum alloy parts manufactured by solidification processing, and can have a strong influence on fatigue properties. This study shows that gas pores with a fraction of 0.2–1.6% and an average size of 20–55μm are present in the Al-Si alloy parts manufactured by Selective Laser Melting (SLM). Failure after fatigue testing was found to initiate from surface or subsurface gas pores and fatigue life prediction equations were developed considering the influence of pores. The building direction did not have a statistically verifiable effect on the average gas porosity fraction, size and distribution, although the scatter in porosity fraction was greater in the vertically built specimens. At the same applied stress, the fatigue life of SLM manufactured specimens decreased with an increase in pore size, and specimens built horizontally exhibited a greater fatigue life than those built vertically. The cause is attributed to greater propensity of cracks to propagate along lower strength melt pool boundary layers in vertically built specimens.

    更新日期:2018-05-12
  • An isothermal forming process with multi-stage variable speed for magnesium component assisted by sensitivity analysis
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-12
    Y. Cai, C.Y. Sun, W.R. Wang, Y.L. Li, L. Wan, L.Y. Qian

    We have developed a multi-stage variable speed (MSV) isothermal forming process based on strain rate sensitivity (SRS) analysis, and successfully produced a large size of rib-web component of magnesium alloy from a cylindrical billet. The obtained rib-web component has been processed into parts for mechanical properties examination and the examination results met the standard of being safely used in aircrafts. The process parameters of the forming technology and microstructure change were explicitly linked by the SRS coefficient. The finite element (FE) simulation and SRS distribution were implemented to identify the relationship among process parameters, shaping characteristics, and microstructure development. The MSV speed process was employed for reducing the forming load, refining grain sizes and increasing the strength of the component. With the selected process parameters the maximum forming load was reduced to be less than 20 MN, which is 55.3% lower than that used in the same process with the initial constant speed unchanged. Fine dynamic recrystallization (DRX) were observed from all parts of the forged cover plate including the convex, ribs, and web region. It was also found that higher strain resulted in more uniform distribution of fine β-Mg17Al12 precipitates and larger DRX volume fraction. Importantly the developed isothermal forming process with severe plastic deformation can be applied for obtaining grain refinement so improving the mechanical properties of Mg products.

    更新日期:2018-05-12
  • Micro-plastic deformation behavior of Al-Zn-Mg-Cu alloy subjected to cryo-cycling treatment
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-11
    Kaixuan Gu, Kaikai Wang, Liubiao Chen, Jia Guo, Chen Cui, Junjie Wang

    The micro-plastic deformation behavior of Al-Zn-Mg-Cu alloy subjected to cryo-cycling treatment was investigated experimentally in the present work. Micro-plastic deformation was produced by continuous loading-unloading method with applied stress far below the yield strength of this alloy, and the deformation characteristics of grains were detected by Optical Microscope (OM) and Scanning Electron Microscope (SEM). The results showed that cryo-cycling treatment reduced the residual strain (εp) after each cycle of loading-unloading, and the degree of grain deformation was decreased apparently, which indicated that cryo-cycling treatment enhanced the capacity of resistance to micro-plastic deformation. Transmission Electron Microscope (TEM) was employed to reveal the mechanisms of cryo-cycling treatment on Al-Zn-Mg-Cu alloy. The results showed that higher content of homogenous η phases precipitated in the matrix and grain boundaries of Al-Zn-Mg-Cu alloy after cryo-cycling treatment. Meanwhile, no clear precipitate-free zones (PFZs) can be observed in the microstructure of cryo-treated specimen. The changes of microstructure had great contribution to improve the pinning effect of dislocations during micro-plastic deformation. Internal stress was released by the cryo-cycling treatment which was demonstrated by the in-suit cryogenic X-Ray (XRD) measurement, as a result, the stability of dislocation was improved. Cryo-cycling treatment had no obvious influence on the tensile strength and yield strength of Al-Zn-Mg-Cu alloy, while elongation and fracture toughness were improved by the extent of 7% and 10.3% respectively, compared to those of the untreated specimen.

    更新日期:2018-05-11
  • Studies on creep-fatigue interaction behaviour of Alloy 617M
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-11
    Sunil Goyal, K. Mariappan, Vani Shankar, R. Sandhya, K. Laha, A.K. Bhaduri

    In this investigation, response of Alloy 617 M to creep-fatigue interaction loading has been studied. The creep-fatigue interaction tests were conducted in air environment at a constant strain rate of 3 × 10−3 s−1 and strain amplitude of ± 0.4% with hold periods of 1 min, 10 min and 30 min at peak tension or compression at 973 K. The material showed initial hardening to a saturation followed by reduction in stress initiated by crack nucleation, propagation and final failure. The fatigue life decreased in presence of hold period. The decrease in fatigue life was more in tensile hold than in compressive hold. The fatigue life showed saturating trend for higher hold periods. Fractographic studies revealed significant intergranular creep cavitation under tensile hold whereas compressive hold exhibited transgranular fatigue striations in association with intergranular creep cavitation. Significant stress relaxation was found to take place during hold and was more in compressive hold than under tensile hold. The creep and fatigue damage have been assessed based on linear damage summation rule. Creep damage fraction was assessed from the stress relaxation considering the average stress, minimum stress and integrated stress over the relaxation curve. The creep-fatigue damage assessment considering integration of creep damage during stress relaxation showed most conservative compared to the other two approaches.

    更新日期:2018-05-11
  • The superior elevated-temperature mechanical properties of Al-Cu-Mg-Si composites reinforced with in situ hybrid-sized TiCx-TiB2 particles
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-09
    Yu-Yang Gao, Bai-Xin Dong, Feng Qiu, Run Geng, Lei Wang, Qing-Long Zhao, Qi-Chuan Jiang
    更新日期:2018-05-09
  • Effect of Ausforming Temperature on Creep strength of G91 investigated by means of Small Punch Creep Tests
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-09
    J. Vivas, C. Capdevila, E. Altstadt, M. Houska, M. Serrano, D. De-Castro, D. San-Martín

    The stability of the martensitic microstructure in ferritic/martensitic G91 steel at operating temperatures up to 700 °C might be improved by means of ausforming thermomechanical treatments. The goal sought is to promote the formation of a high number density of MX nanoprecipitates in the martensitic microstructure obtained after a subsequent tempering. This work is focused on the effect of the ausforming temperature. The results show that the lower the ausforming temperature is the higher is the dislocation density obtained in ausformed fresh martensite and the larger the number density of MX carbonitrides after tempering is. Creep strength, evaluated by Small Punch Creep Tests has allowed us to conclude that the best creep resistance was obtained for the steel ausformed at the lower temperature due to the higher pinning force for dislocation motion triggered by the distribution of MX. The creep results obtained on the ausformed samples were compared with those after the conventional heat treatment, showing that the high density of MX carbonitrides after an ausforming thermomechanical treatment is a promising processing to raise the operation temperature of this steel.

    更新日期:2018-05-09
  • Microstructural Evolution and Precipitation Strengthening In a New 20Cr Ferritic Trial Steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-09
    Mujin Yang, Jiahua Zhu, Chao Wu, Shuiyuan Yang, Zhan Shi, Cuiping Wang, Xingjun Liu

    Microstructural evolution and precipitation strengthening of the newly-developed 20Cr steel were investigated in present work. Three types of precipitates, including G-phase (Ni16Ti6Si7, intermetallic silicide), Laves phase (Fe2Ti, intermetallic compound) and carbide (TiC), were observed and their crystal structures were resolved with combined electrochemical phase extraction, X-ray diffraction and transmission electron microscopy analysis. G-phase particles were observed in the grains and Laves phase occurred at grain boundaries while carbides were present at both of the two sites. A temperature-dependency sketch map for these precipitates was also plotted to illustrate the characteristics of G-particles below 750 °C, Laves phase at 850 °C~1050 °C and carbide below 1150 °C. In particular, G-phase precipitated and had the cubic-cubic orientation with the ferritic matrix, thus showing a strong aging hardening characteristic. Nanodispersion of these G-particles greatly enhanced the yield strength, which was estimated to be up to ~1700 MPa. Finally, theoretical calculations for phase equilibria, critical radius and precipitation strengthening helped to understand the precipitation thermodynamics and strengthening mechanisms for developing high-performance steel by making use of G-phase.

    更新日期:2018-05-09
  • Effect of Heat Treatment on Microstructure and Properties of 1045 Steel Modified with (NbTi)C Nanoparticles
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-09
    Hong-wei Zhu, Kai Xu, Sen Qin, Fu-ren Xiao, Bo Liao

    Heat treatment is an important mean to obtain excellent properties of all kinds of metal materials. The microstructure of 1045 steel modified with (NbTi)C nanoparticles was studied by means of optical, scanning electron, and transmission electron microscopy techniques. The effects of (NbTi)C nanoparticles in as-cast, normalized, as well as quenched and tempered (Q&T) steel were determined. The metallographic results showed that the normalized steel had a finer grain size than the as-cast steel. Tensile tests results showed that the strength and elongation of as-cast steel were significantly improved after heat treatment. Normalized steel showed a higher elongation and Q&T steel exhibited a higher strength. Under the same heat treatment condition, (NbTi)C nanoparticles modified 1045 steel showed a similar elongation with the base metal, whereas, the strength and hardness were evidently improved. The improved properties of the modified steel can be attributed to the refinement of the microstructure and uniform dispersion of (NbTi)C nanoparticles. Theoretical calculations of the strengthening contributions to different strengthening mechanisms indicated that particle strengthening plays a key role in strength improvement.

    更新日期:2018-05-09
  • Understanding the deformation mechanism of individual phases of a dual-phase beta type titanium alloy using in situ diffraction method
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-08
    Hongjia Li, Wupeng Cai

    An in situ tensile X-ray diffraction (XRD) set-up with an area detector was employed to characterize deformation behaviors of individual phases in a commercial dual-phase β-Ti alloy. A developed analytical approach based on the modified Williamson-Hall plot accounting for dislocation evolution was proposed to elucidate the deformation mechanism of each phase from the in situ XRD data. A commercial purity titanium (alpha type, α-Ti) alloy was examined as a comparison to assist identifying the influence of inter-phase interaction on the deformation behavior of the β-Ti alloy. We found that the small amount of the fine lamella shaped hcp (α) precipitates, which is only 5% in volume, has great effects on the deformation mechanism of the bcc (β) matrix in the β-Ti alloy due to the Burger's orientation relationship (BOR) between the two phases. Due to the reason that slip on the coherent plane of (211)β of the β phase (i.e. (10 View the MathML source 1 ̅ 0 )α plane of the α phase) is more predominant than that of (110)β plane (i.e. (0002)α plane) in a hcp ploycrystal, slip on (211)β plane is easier than that on (110)β plane in the β-Ti alloy (cf. {110}<111> is the typical slip system in single phase bcc polycrystals). Although slip on (211)β plane is the most facile, lattice strain on (211)β plane is not the smallest, manifesting that stress/strain redistribution between α precipitates and β matrix occurred to accommodate the inter-phase interaction.

    更新日期:2018-05-09
  • A new type of high entropy alloy composite Fe18Ni23Co25Cr21Mo8WNb3C2 prepared by mechanical alloying and hot pressing sintering
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-08
    Chongfeng Sun, Panpan Li, Shengqi Xi, Yun Zhou, Shengwen Li, Xigang Yang

    In this work, a new type of high entropy alloy (HEA) composite Fe18Ni23Co25Cr21Mo8WNb3C2 was prepared by mechanical alloying (MA) and hot pressing sintering (HPS). Phase transitions, microstructure characteristics, and tensile properties of samples were analyzed. During the MA process, a powder mixture of Cr-Mo was milled to synthesize the amorphous alloy powder Cr(Mo)a. Then, the mixture of amorphous alloy powder Cr(Mo)a and other elemental powders (Fe, Ni, Co, W, Nb, and C) was milled to obtain the HEA powder.. This HEA powder is composed of single-phase austenite (γ γ ). Finally, the HEA powder was consolidated using HPS. The sintered HEA composite contains the matrix phase austenite A and second-phase M6C carbide. The precipitation reaction that occurs during the sintering process can be summarized as: γ γ →A+M6C. Tensile properties of the sample sintered at 1050 °C were tested. At room temperature, ultimate tensile strength and elongation are ~1452 MPa and ~3.9%, respectively; at 650 °C, ultimate tensile strength and elongation are ~795 MPa and ~13.3%, respectively.

    更新日期:2018-05-09
  • High-temperature mechanical properties and thermal cycling stability of Al-50Si alloy for electronic packaging
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-05-08
    Zhiyong Cai, Chun Zhang, Richu Wang, Chaoqun Peng, Xiang Wu, Haipu Li

    With the rapid development of electronics industry, there is an increasing demand for high performance electronic packaging materials. Furthermore, the high-temperature performance and thermal cycling stability are required owing to the harsher service environment. In this work, Al-50Si alloy was prepared via rapid solidification/powder metallurgy technique, and the tensile properties at elevated temperature and the thermo-physical properties after thermal cycling were investigated. The results show that the tensile strength decreases gradually with the increase in the environmental temperature. Whereas, pullout of the Si particle is seldom observed in the specimens at elevated temperatures owing to the strong interfacial bonding. With the progress of thermal cycling, the coefficient of thermal expansion (CTE) is stable, but the plastic strain increases rapidly at the initial stage. However, the thermal conductivity decreases significantly with the increase in the temperature or the number of thermal cycles. This phenomenon is mainly ascribed to the CTE mismatch between the Al matrix and Si phase. The variation of CTE and thermal conductivity is discussed in terms of thermally induced stress.

    更新日期:2018-05-09
Some contents have been Reproduced with permission of the American Chemical Society.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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