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  • Microstructural evolution and precipitation behavior of the warm-rolled medium Mn steels containing Nb or Nb-Mo during intercritical annealing
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-18
    Haijun Pan, Hua Ding, Minghui Cai

    The microstructural evolution and precipitation behavior of the warm-rolled (WRed) medium Mn steels with no microalloying addition, single Nb addition and multiple Nb-Mo addition treated at 650 °C for various durations were investigated. Different from the lamellar microstructure of hot rolled medium Mn steels and the equiaxed microstructure of cold rolled medium Mn steels after intercritical annealing, the WRed samples of medium Mn steels exhibited the equiaxed and lamellar mixed microstructure, and the equiaxed microstructure ratio became larger with increasing durations. The addition of Nb or Nb-Mo can respectively increase yield strength of ~50 MPa or ~180 MPa in average and ultimate tensile strength of ~70 MPa or ~100 MPa in average without sacrificing the tensile ductility. The stability of austenite can be improved for the smaller average austenite grain size due to the addition of Nb or Nb-Mo. The addition of Mo to the Nb-bearing medium Mn steel can increase the density of nano-precipitates by reducing the mismatch degree between the matrix and precipitates and inhibit the coarsening rate of precipitates.

    更新日期:2018-08-20
  • Preparation and mechanical properties of in-situ synthesized nano-MgAl2O4 particles and MgxAl(1-x)B2 whiskers co-reinforced Al matrix composites
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-17
    Fucheng Wang, Jiajun Li, Chunsheng Shi, Naiqin Zhao, Enzuo Liu, Chunnian He, Fang He

    Nano-MgAl2O4 particles and MgxAl(1-x)B2 whiskers co-reinforced Al matrix composites were prepared through powder metallurgy and hot working processes, involving ball-milling, cold pressing, sintering, repressing, and hot extrusion. The microstructure, tensile strength and hardness of the composites, the phase composition of the particles and the whiskers, and the interface between the whisker and the aluminum matrix were investigated systemically. The results show that during the hot extrusion, the reinforcements are uniformly distributed in the matrix, and the whiskers are arranged in the direction of extrusion. The whiskers are hexagonal prisms with close-packed hexagonal crystal structure and all the inner angles are 120°. Besides, the whisker elongates along the [0001] crystal orientation, and the six planes of the sidewalls belong to the {10 1 ( − ) 0} family of crystal planes. As the content of reinforcement increases, the strength and hardness of the composites increase, while the elongation decreases. The composite contained 25.2 wt.% reinforcements has a tensile strength of 400 MPa and a hardness of 124 HV.

    更新日期:2018-08-18
  • Effect of secondary phase particles on the tensile behavior of Mg-Zn-Ca alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-17
    Seung Mi Baek, Hyung Keun Park, Jae Ik Yoon, Jaimyun Jung, Ji Hyun Moon, Seok Gyu Lee, Jae H. Kim, Taek Soo Kim, Sunghak Lee, Nack J. Kim, Hyoung Seop Kim

    In the present study, the effect of secondary phase particles and deformation localization on tensile behaviors of Mg-Zn-Ca alloy were investigated using micro-digital image correlation method. To isolate the effect of the secondary phase particles on tensile behavior, the fractions of the secondary phase particles were adjusted by controlling the annealing temperature. Thermodynamic calculations were performed to identify the fractions of the secondary phase particles. The finite element method was conducted to confirm the localized deformation around the secondary phase particles considering 3-dimensional microstructure. The results suggest that the total fraction of the secondary phase particles affected the localized deformation around the particles and micro-crack propagation process during a tensile test.

    更新日期:2018-08-18
  • Experimental and numerical simulation analysis on creep crack growth behavior of CLAM steel
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-17
    Y. Zhao, S. Liu, J. Shi, X. Mao

    The experiment and numerical simulation were performed to analyze the creep crack growth behavior of China Low Activation Martensitic (CLAM) steel under temperatures ranging from 500 °C to 600 °C with a step of 50 °C. The creep crack growth micro-mechanism under multiaxial state of stress was investigated with the compact tension (CT) specimens. The creep crack growth rates were simulated based on the Nikbin-Smith-Webster (NSW) model. The difference of the crack propagation behavior is correlated with the evolution of two types of precipitates: Cr-rich M23C6 and W-rich M6C phases. The coarsening of M23C6 carbides and the precipitation of M6C phase particles lead to dissolving of the solid solution elements and deteriorating the creep properties during the testing processes.

    更新日期:2018-08-18
  • Strain hardening and tensile behaviors of gradient structure Mg alloys with different orientation relationships
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-17
    Jiangli Ning, Bo Xu, MingShuai Sun, Chongyang Zhao, Yunli Feng, Weiping Tong

    Surface mechanical attrition treatment (SMAT) was employed to produce gradient structures in AZ31B Mg alloy samples with two different initial textures. The structure of both samples can be regarded as an integration of two main layers: the severely deformed layer exhibited dramatic grain refinement to nano- and submicron-scale with weakened and randomized textures; the less deformed layer exhibited the inherited coarse grains with increased dislocation density, possessing the similar texture with the sample prior to SMAT. All the samples containing different layer constituents cut from the SMAT alloys showed remarkable increase of strength compared to the original Mg alloy. However, the two integral SMAT samples with different initial textures exhibited marked difference in uniform elongation (UE) during tension. That was attributed to the different strain hardening behaviors influenced by the deformation coordination and strain partitioning between layers with different orientation relationships. The 0° oriented SMAT sample showed no macroscopic strain transfer or slip transmission across layers, which was hard to cause dislocation accumulation throughout the whole thickness of the layered sample. That resulted in a limited strain hardening and UE. However, the favored orientation relationship between layers of the 45° oriented SMAT sample facilitated the strain transfer and slip continuity across the layers. That generated a dynamically migrating interface during tension, which allowed the dislocations accumulating over the whole sample volume. This caused a pronounced strain hardening and sustained UE to an appreciable value.

    更新日期:2018-08-18
  • Effect of grain size on stretch-flangeability of twinning-induced plasticity steels
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-18
    Jae Ik Yoon, Hak Hyeon Lee, Jaimyun Jung, Hyoung Seop Kim

    The effect of grain size on stretch-flangeability was investigated to determine its influence on stretch-flangeability of high strength steels. To avoid other effects of microstructure, single-phase twinning-induced plasticity (TWIP) steels were selected for the investigation. To control the grain size of two types of TWIP steels, 1) the initial specimen was annealed at 1100 ℃ to increase its grain size, or 2) subjected to high-pressure torsion then annealed at 650 ℃ to reduce the grain size. The microstructural features were analyzed using the electron backscatter diffraction. The stretch-flangeability of TWIP steels with various grain sizes was evaluated using a hole-expansion test. It was found that the hole-expansion ratio follows the Hall-Petch correlation as does fracture toughness. To improve the stretch-flangeability of high strength steels, microstructural features should be designed to increase their fracture toughness.

    更新日期:2018-08-18
  • Weak strengthening effect of the precipitated lamellar phase in the homogenized Mg-8Gd-4Y-1.6Zn-0.5Zr (wt.%) alloy followed by furnace cooling
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-18
    D.J. Chen, K. Zhang, T. Li, X.G. Li, Y.J. Li, M.L. Ma, G.L. Shi, J.W. Yuan

    The presence of small-scale precipitation is known to contribute to the high hardness and yield strength due to their hardening effect, which is widely applied in various engineering materials. The long period stacking ordered (LPSO) structure phases as well as stacking faults (SFs) are expected to play an important role in tuning the mechanical properties of Mg alloy. However, we found that the hardness and strength could not benefit from the lamellar LPSO structures and SFs precipitated in the matrix. Using transmission electron microscopy(TEM), high resolution transmission electron microscopy (HRTEM) and so on, we directly illustrate that the weak hardening effect is corresponding to the limited resistance of these basal precipitates in blocking the basal dislocation slip and the weaker solution strengthening of the matrix as the result of that much solute atoms have get into the precipitations. This finding demonstrates that the shape of the precipitations and their orientation relation to the glide planes of dislocation as well as the distribution of solute atoms are considerable factors to the strengthening effect.

    更新日期:2018-08-18
  • Influence of {101¯2} twin characteristics on detwinning in Mg-3Al-1Zn alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-17
    Qi Sun, Ting Xia, Li Tan, Jian Tu, Min Zhang, Minhao Zhu, Xiyan Zhang

    In the present work, the influence of pre-induced {101¯2} twin characteristics on detwinning activation in a basal texture magnesium alloy is systematically investigated by a combination of electron backscatter diffraction technique and mechanical tests. Samples containing pre-induced {101¯2} twin variants with different morphological characteristics are prepared by pre-strain path change tests comprised two pre-strain paths: (i) compression along the rolling direction of magnesium alloy plate; (ii) tension along the normal direction of magnesium alloy plate. In the former case, a single {101¯2} twin variant or a {101¯2} twin pair is formed in one grain. While, multiple {101¯2} twin variants are usually activated simultaneously in one grain in the latter case. During subsequent compression along normal direction, detwinning process is effectively retarded for the samples containing multiple {101¯2} twin variants in one grain. Meanwhile, the yield stress of corresponding samples is enhanced as well, compared to that of the samples containing a single {101¯2} twin variant or a {101¯2} twin pair in one grain. According to these experimental features, the possible reasons behind such phenomena are proposed and discussed.

    更新日期:2018-08-17
  • Effects of hydrogen and microstructure on tensile properties and failure mechanism of 304L K-TIG welded joint
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-17
    Xiaogang Li, Baoming Gong, Xiuguo Liu, Caiyan Deng, Yizhe Li

    The effects of hydrogen on the mechanical properties of 304 L keyhole tungsten inert gas welded joint were investigated in the study. The presence of hydrogen resulted in a significant decrease in the macroscopic ductility of the weld metal, and fracture may be initiated at the sites of clustered dislocations due to hydrogen atoms. Moreover, it is found that hydrogen may result in more severe loss in ductility and noticeable decrease in the flow stress and strength of the base metal, because of the interplay between the hydrogen atoms and strain-induced α′ martensite. Strain-induced α′ martensitic transformation and phase boundaries in the base metal may provide more sites for the accumulation of hydrogen and dislocations, leading to an appreciable decrease in the ductility and strength of the base metal.

    更新日期:2018-08-17
  • Influence of post treatment on microstructure, porosity and mechanical properties of additive manufactured H13 tool steel
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-17
    M. Åsberg, G. Fredriksson, S. Hatami, W. Fredriksson, P. Krakhmalev

    Additive manufacturing (AM) is an attractive manufacturing technology in tooling applications. It provides unique opportunities to manufacture tools with complex shapes, containing inner channels for conformal cooling. In this investigation, H13, a widely used tool steel, was manufactured using a laser powder bed fusion method. Microstructure, tensile mechanical properties, hardness, and porosity of the AM H13 after stress relieve (SR), standard hardening and tempering (SR+HT), and hot isostatic pressing (SR+HIP+HT) were investigated. It was found that the microstructure of directly solidified colonies of prior austenite, which is typical for AM, disappeared after austenitizing at the hardening heat treatment. In specimens SR+HT and SR+HIP+HT, a microstructure similar to the conventional but finer was observed. Electron microscopy showed that SR and SR+HT specimens contained lack of fusion, and spherical gas porosity, which resulted in remarkable scatter in the observed elongation to break values. Application of HIP resulted in the highest strength values, higher than those observed for conventional H13 heat treated in the same way. The conclusion is that HIP promotes reduction of porosity and lack of fusion defects and can be efficiently used to improve the mechanical properties of AM H13 tool steel.

    更新日期:2018-08-17
  • Effect of banding on micro-mechanisms of damage initiation in bainitic/martensitic steels
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-17
    Behnam Shakerifard, Jesus Galan Lopez, Frank Hisker, Leo A.I. Kestens

    Multiphase bainitic steels, as a third generation of advanced high strength steels, show promising properties for automotive applications. Understanding the micro-mechanisms of damage initiation during plastic deformation is a key to further mechanical properties enhancement. The topological effect of martensite as a second phase constituents on local damage nucleation activity is significant. This effect has been studied in two different martensite banded microstructures produced by two various annealing cycles. The post mortem damage analysis by a scanning electron microscope on uniaxial loaded samples, revealed more damage nucleation along the dispersed and fragmented martensite phase within martensite banded regions. More pronounced strain partitioning was observed in coarse bainitic grains between adjacent martensite blocks. It is shown that the fracture strain is not controlled by local damage activities, implying that earlier damage initiation or an increased volume fraction of voids does not give rise to a reduced ductility.

    更新日期:2018-08-17
  • Influence of the Prior Athermal Martensite on the Mechanical Response of Advanced Bainitic Steel
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-17
    A. Navarro-López, J. Hidalgo, J. Sietsma, M.J. Santofimia

    The accelerated formation of bainite in presence of martensite is opening a new processing window for the steel industry. However, for a feasible industrial implementation, it is necessary to determine the mechanical behaviour of the steels developed under such conditions. This study focuses on analysing the effects of the formation of athermal martensite, followed by the formation of bainitic ferrite, on the mechanical response of a low-C high-Si steel. For this purpose, microhardness measurements and tensile tests have been performed on specimens that were thermally treated either above or below the martensite-start temperature (Ms). Specimens isothermally treated below Ms exhibit a good combination of mechanical properties, comparable with that of the specimens heat treated by conventional treatments above Ms, where there was no prior formation of martensite. Investigations show an increase of the yield stress and a decrease of the ultimate tensile strength as the isothermal holding temperature is decreased below Ms. The formation of prior athermal martensite and its tempering during the isothermal holding leads to the strengthening of the specimens isothermally heat treated below Ms at the expense of slightly decreasing their strain hardening capacity.

    更新日期:2018-08-17
  • Microstructural dependence of anisotropic fracture mechanisms in cold-drawn pearlitic steels
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-15
    Yonggang Zhao, Yuanbiao Tan, Xuanming Ji, Song Xiang

    The anisotropic fracture behavior of cold-drawn pearlitic steels was analyzed up to a true strain of 1.6. The structures were analyzed and quantified by scanning electron microscopy, electron backscatter diffraction and high-resolution electron microscopy. The aspect ratio Λ was defined to describe the fracture mechanism based on statistics pertaining to major axis and minor axis of pearlitic colonies, and the fitting formula was established as follows: Λ = 0.4 exp ( 2 x ) + 0.65 . At Λ ≤ 2.75, the fracture mechanism shifts from cleavage to quasi-cleavage fracture due to dislocation generation, as the measured parameters of interlamellar spacing (ILS) showed no obvious decrease. At values of Λ > 2.75, ductile fracture mechanism becomes dominant due to a drastic decrease in ILS. The <110> ferrite fiber microtexture formed during the cold drawing process and the component exhibiting a gradient distribution from the surface to the central regions increased gradually with drawing. The crack propagation path was then deflected and formed a ‘V’ shape at ε ≤ 1.5. However, as carbon migrated from cementite to the ferrite near the interphase, the fracture path deflected again. Furthermore, two reasonable models were formulated to explain fracture crack forming and anisotropic fracture behavior. In addition, this study illustrated that the relative crystallographic orientation of the ferrite and cementite components followed the Bagarytski relationship. With increasing strain, the cementite layers transformed from single crystals into nanostructured polycrystals and even evolved an amorphous structure at the interface at strains above 1.5.

    更新日期:2018-08-15
  • In-situ observations of the fracture and adhesion of Cu/Nb multilayers on polyimide substrates
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-14
    M.J. Cordill, A. Kleinbichler, B. Völker, P. Kraker, D.R. Economy, D. Többens, C. Kirchlechner, M.S. Kennedy

    Cu/Nb nanoscale metallic multilayers have been extensively investigated to understand how their mechanical behavior is influenced by the individual layer thickness. The general observed trend is that the yield stress of the multilayer increases with decreasing layer thickness. Important mechanical behaviors that have not been studied in-depth are the fracture of these multilayers and adhesion energy between the multilayer films and their substrate. Here, the influences of the layer thickness, layer order, and initial residual stresses of Cu/Nb multilayers on polyimide were examined using in-situ x-ray diffraction and confocal laser scanning microscopy under tensile loading. With these techniques, it was possible to calculate the stresses developing in the individual materials and measure buckles that could be used to evaluate the interfacial adhesion. Layer thickness, deposition order, and the initial residual stresses were not shown to influence the initial fracture strains of the Cu/Nb multilayer systems under tensile loading conditions. However, the adhesion energy between the multilayer and substrate was affected by the layer deposition order and by the initial residual stresses.

    更新日期:2018-08-15
  • Effect of plastic deformation of V nanowires on the transformation characteristics of NiTiV alloys
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-13
    Xiaobin Shi, Hongliang Yang, Hu Mao, Yongtao Li, Junsong Zhang, Xiaohui Yin

    The effect of plastic deformation of V nanowires on the transformation characteristics of two NiTiV alloys was investigated. A significant increase of B19’→B2 transformation temperatures in the deformed samples during the initial heating process was found. In the subsequent cooling process, the R→B19’ transformation temperatures were higher than the as-annealed samples. Upon second heating, the B19’→R transformation temperatures were higher than the as-annealed samples, but the change was much smaller than the R→B19’ transformation. All of these phenomena are closely related to the plastic deformation of V nanowires during deformation.

    更新日期:2018-08-14
  • Effects of Selective Laser Melting additive manufacturing parameters of Inconel 718 on porosity, microstructure and mechanical properties
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-13
    K. Moussaoui, W. Rubio, M. Mousseigne, T. Sultan, F. Rezai

    The effect of SLM parameters on porosity, microstructure and mechanical properties is studied. To this purpose, the Selective Laser Melting (SLM) technology is applied to manufacture Inconel 718 specimens. The material, the manufacturing process, the Hot Isostatic Pressure (HIP), heat treatment, observation procedures and characterisation of mechanical properties are presented. A columnar-dendritic microstructure was observed on all the SLM specimens and a Volumetric Energy Density (VED) effect on the latter was also noted. The rate of porosity varies in relation to the VED and is considerably reduced after HIP. The heat treatment erases the dendritic microstructure, significantly enhances microhardness and confers on the alloy tensile mechanical properties comparable to forged Inconel 718.

    更新日期:2018-08-14
  • Microstructure and mechanical properties of SiC joint with an in-situ formed SiC-TiB2 composite interlayer
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-13
    Zhiquan Wang, Huaxin Li, Zhihong Zhong, Ankang Yang, Yucheng Wu

    Diffusion bonding of SiC ceramic with in-situ formed SiC-TiB2 composite interlayer by reactive spark plasma sintering with B4C-Ti3SiC2-Si powder mixture was investigated. It was found that a dense composite interlayer between the SiC ceramic substrate could be obtained at 1600 °C for 10 min. Granular-shaped SiC with submicron-size and elongated TiB2 with micro-size were uniformly distributed in the bonding layer. The joint strength and hardness of the composite interlayer increased as the joining temperature increased up to 1600 °C due to the improved relative density and the higher fraction of elongated TiB2 phase. The grain growth and formation of micro-cracks deteriorated the mechanical strength of the 1700 °C joint. The maximum shear strength of 128.0 ± 7.3 MPa was achieved for the joint bonded at 1600 °C. The toughening mechanisms of crack deflection, crack bridging and crack branching, and hardening mechanism of dual phase strengthening were observed in the morphology of indentation cracks.

    更新日期:2018-08-14
  • Effects of Ti addition on the formation of LPSO phase and yield asymmetry of Mg-Zn-Y-Mn alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-14
    Jianqiang Hao, Jinshan Zhang, Chunxiang Xu, Yatong Zhang

    The microstructure and mechanical properties of Mg94Zn2.5Y2.5Mn1 (at.%) with different Ti contents were investigated. The addition of 0.3 at.% Ti yielded the best strength. However, Mg93.5Zn2.5Y2.5Mn1Ti0.5 exhibited the lowest yield asymmetry. These changes were attributed to the combined effects of grain refinement, increased LPSO phase, and reduced axial ration (c/a).

    更新日期:2018-08-14
  • Grain refinement of non-equiatomic Cr-rich CoCrFeMnNi high-entropy alloys through combination of cold rolling and precipitation of σ phase
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-13
    Ken Cho, Yumi Fujioka, Takeshi Nagase, Hiroyuki Y. Yasuda

    Grain refinement of non-equiatomic Cr-rich CoCrFeMnNi high-entropy alloys with an fcc structure was studied focusing on the precipitation of a Cr-rich σ phase along grain boundaries and shear bands. We found for the first time that the σ phase can be precipitated at the grain boundaries and the shear bands in the alloys at a significantly fast precipitation rate. The σ phase effectively suppressed the grain growth of the fcc matrix during recrystallization of the alloy after conventional cold rolling, resulting in a fine-grained microstructure with a mean diameter of approximately 1 μm. The alloys with this fine microstructure containing the σ phase exhibit high strength over a wide temperature range due to grain boundary strengthening and precipitation strengthening caused by the σ phase. Moreover, the alloys exhibit large ductility not only at room and high temperatures, but also at low temperatures. Deformation-induced twinning plays an important role in this ductility.

    更新日期:2018-08-13
  • Effect of Second Phase Particles and Stringers on Microstructures after Rolling and Recrystallization
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-11
    Guang Cheng, Xiaohua Hu, William E. Frazier, Curt A. Lavender, Vineet V. Joshi

    Second phase particles and particle stringers have strong influences on recrystallized grain structure and alloy performances. In this work, the effects of second phase particles on grain size and grain size distribution in the U-10 wt. % Mo (U-10Mo) after hot rolling and annealing were investigated and quantified. The particle volume fraction (PVF), distribution, morphology, size, and the formation of stringers during hot rolling were all considered. The criteria for stringers were determined using the edge-constrained post-necking model. Potts Model simulations of recrystallization were performed on six captured U-10Mo microstructures deformed using Finite Element Modeling (FEM). The stringer volume fraction (SVF) was found to be higher for captured microstructures with higher particle aspect ratios. It was determined from these simulations that the recrystallized grain size decreases with increasing PVF and the variation in grain size increases with increasing SVF. The current study will assist in the optimization of U-10Mo processing parameters to account for the effects of the second phase particles that exist in the alloy.

    更新日期:2018-08-13
  • Transformation of fracture mode of an Al-Mg-Si-Cu alloy subject to aging treatment
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-11
    Lin Gao, Lixin Zhang, Ji Gu, Xiaoqin Ou, Song Ni, Kai Li, Yong Du, Min Song

    In this work, the microstructure of Al-Mg-Si-Cu alloy was tailored by controlling the aging process and its effect on the fracture mechanism was systematically explored by in-situ tensile test and microstructural observations. It was found that the aging process influences the mechanical performance and leads to the alteration of the tensile fracture mode of the aged alloy. Microscopy observations revealed that the Al2Cu precipitates distribute heterogeneously in the peak-aged alloy. The density of the Al2Cu precipitates in the intergranular fracture regions is much higher than that in the transgranular regions. The effect of Al2Cu precipitates on the fracture mode was systematically discussed and a hybrid fracture mechanism was proposed.

    更新日期:2018-08-13
  • Cooperative deformation behavior between the shear band and boundary sliding of an Al-based nanostructure-dendrite composite
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-11
    J.T. Kim, S.H. Hong, Y.S. Kim, H.J. Park, T. Maity, N. Chawake, X. Bian, B. Sarac, J.M. Park, P.K. Gokuldoss, J.Y. Park, J. Eckert, K.B. Kim

    Investigation of the microstructural features and mechanical properties of the Al86Cu7Si7 nanostructure-dendrite composite revealed that the high yield strength of 615 MPa and its reasonable plasticity of ~20% at room temperature mainly originate from the evolution of dislocations in the micron-scale dendrites together with the cooperative deformation action of shear band and interfacial sliding throughout the whole volume of the material. Especially, shear band-induced rotation of dendrites was found to be an important deformation mechanism. Here, we sequentially elucidate the deformation behavior using atomic force microscopy, nanoindentation, and scanning electron microscopy to determine the surface topography of the deformed alloy.

    更新日期:2018-08-13
  • Thermal shock resistance of continuous carbon fiber reinforced ZrC based ultra-high temperature ceramic composites prepared via Zr-Si alloyed melt infiltration
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-11
    Yonggang Tong, Wentao Zhu, Shuxin Bai, Yongle Hu, Xinqi Xie, Yang Li

    Thermal shock resistance of continuous carbon fiber reinforced ZrC based ultra-high temperature ceramic matrix composite (C/C-ZrC) from ultra high temperatures (particularly >1500 °C) to the r°om temperature was evaluated using a novel self-developed equipment with high heating-cooling rates in controllable atmosphere. Residual strength and mass variation of the as-prepared composite under different thermal shock cycles and temperatures were tested to characterize the thermal shock resistance. Flexural strength of the composite slightly decreases initially without obvious weight loss below 1300 °C, while it decreases by relatively high values at 1300–1900 °C and then sharply decreases over 1900°C with obvi°us increase of weight loss. Microstructure of the thermally shocked specimens was examined to reveal the thermal shock damage. Matrix cracking, interfacial debonding and matrix pores were clearly observed, which were the main reasons for the strength degradation of the composite thermally shocked for different cycles and temperatures.

    更新日期:2018-08-13
  • Influence of rhenium addition on microstructure, mechanical properties and oxidation resistance of NiAl obtained by powder metallurgy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-11
    Kamil Bochenek, Witold Węglewski, Jerzy Morgiel, Michał Basista

    The search for new materials capable of replacing nickel-based superalloys in aerospace applications has increased rapidly in the recent years. One of the candidates for this purpose is nickel aluminide NiAl provided that its main drawback, namely the inferior fracture toughness at room temperature is overcome. We propose rhenium as an addition to NiAl to improve its mechanical properties without compromising on the oxidation resistance. Two powder metallurgy techniques (HP and SPS) were used to obtain NiAl/Re sinters. Small amounts of rhenium (0.6 at.%; 1.25 at.%; 1.5 at.%) almost doubled the flexural strength of NiAl and improved its fracture toughness by 60%. Microscopic investigations revealed rhenium particles at the boundaries of NiAl grains resulting in an enhanced fracture toughness. Mass changes during oxidation at 900 °C, 1100 °C and 1300 °C were relatively low. Plausible mechanisms of the fracture toughness enhancement and the oxidation behavior are discussed.

    更新日期:2018-08-13
  • Multi-scale Mechanical Modeling of Al-steel Resistance Spot Welds
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-11
    Jian Chen, Zhili Feng, Hui-Ping Wang, Blair E. Carlson, Tyson Brown, David Sigler

    A multi-scale finite element modeling framework was developed to study the deformation and fracture behavior of Al-steel resistance spot welds. First, a micro-scale model was applied to simulate the mechanical responses of the intermetallic compound (IMC) layer having various morphologies and thicknesses under tensile and shear loading conditions. Second, the predicted tensile and shear strength of the IMC layer, that varied along the joint interface per the IMC layer morphology and thickness variation, was then introduced into 3D macro-scale models to predict the overall mechanical performance of weld coupons under coach peel, lap shear and cross-tension testing conditions. The numerical predictions agreed reasonably well with the experimental data.

    更新日期:2018-08-13
  • Carbon Nanotube Conductive Additives for Improved Electrical and Mechanical Properties of Flexible Battery Electrodes
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-11
    Sarah Jessl, David Beesley, Simon Engelke, Christopher J. Valentine, Joe C. Stallard, Norman Fleck, Shahab Ahmad, Matthew T. Cole, Michael De Volder

    Flexible electronics are being pursued as replacements for rigid consumer electronic products such as smartphones and tablets, as well as for wearable electronics, implantable medical devices, and RFIDs. Such devices require flexible batteries with electrodes that maintain their electro-chemical performance during multiple bending cycles. These electrodes typically consist of an active battery material blend with a conductive additive and a binder. Whilst the choice of active battery material is typically dictated by the desired battery power and energy requirements, there is more freedom in changing the conductive additives to cope with strain induced during the bending of flexible batteries. Here we compare the mechanical and electrical properties of free standing cathodes using lithium cobalt oxide (LiCoO2) as the active material and 10 to 20 wt% of amorphous carbon powder (CP) or carbon nanotubes (CNTs) as conductive additives. We found that the CNT based electrodes showed less crack formation during bending and have a Young's modulus up to 30 times higher than CP electrodes (10 wt% loading). Further, the electrical resistance of pristine CNT electrodes is 10 times lower than CP electrodes (20 wt% loading). This difference further increases to a 28 times lower resistance for CNT films after 2000 bending cycles. These superior properties of CNT films are reflected in the electrochemical tests, which show that after bending, only the electrodes with 20 wt% of CNTs remain operational. This study therefore highlights the importance of the conductive additives for developing reliable flexible batteries.

    更新日期:2018-08-13
  • A micro-alloyed Mg-Sn-Y alloy with high ductility at room temperature
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-11
    Qinghang Wang, Yaqun Shen, Bin Jiang, Aitao Tang, Jiangfeng Song, Zhongtao Jiang, Tianhao Yang, Guangsheng Huang, Fusheng Pan

    Here, Mg-0.4wt.%Sn based alloys containing different Y contents, Mg-0.4Sn-xY (x=0, 0.7 and 2.0 wt.%) alloys, were extruded into the sheets to systemically investigate the role of Y element on the microstructure, texture and mechanical properties of extruded Mg-0.4Sn alloy. We found that, with the Y addition, the average grain size gradually was reduced and the typical basal texture was transformed into the splitting one tilted to the extrusion direction. Moreover, the evidence of low number density and rough grain boundary cracks after tension indicated the strengthening in grain boundary cohesion with the Y addition. Besides more basal slips, the activation of prismatic slip and high intergranular strain propagation capacity were observed, which efficiently accommodated the sheet strain at room temperature. Those mentioned key factors contributed to the high room-temperature ductility of Mg-0.4Sn-0.7Y sheet. However, the formation and coarsening of Sn3Y5 and MgSnY phases were increasingly severe with increasing the Y addition to 2.0%. Those coarse secondary phases served as crack sources during tension deteriorating the ductility of Mg-0.4Sn-2.0Y sheet at room temperature. Therefore, we concluded that Y micro-alloying provided a new insight to achieve a superior room-temperature ductility of Mg-0.4Sn sheet.

  • Microstructural evolution and mechanical properties of 27Cr-4Mo-2Ni ferritic stainless steel during isothermal aging
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-10
    Hui-Hu Lu, Yi Luo, Hong-Kui Guo, Wen-Qi Li, Jian-Chun Li, Wei Liang

    The microstructural evolution and its effect on mechanical properties of 27Cr-4Mo-2Ni super ferritic stainless steels during intermediate temperature aging treatment were investigated. Experimental results demonstrated that Laves phase started to nucleate at dislocations and sub-grain boundaries before chi and sigma phase, while the chi phase and sigma phase precipitated at grain boundaries. Laves phase was partially dissolved into the matrix to provide additional Cr and Mo atoms for the rapid coarsening of sigma phase with increasing aging time during 800 °C aging treatment. Meanwhile, the average grain size increased from ~68 to ~111 μm. Mechanical properties such as RT impact toughness, tensile properties and micro hardness were significantly influenced by the brittle intermetallic and grain coarsening. When the alloy was aged at 800 °C, the rate of microhardness increase was accelerated by the formation and coarsening of sigma phase, and the value of Vickers hardness was positively correlated with the volume fraction of sigma phase. Impact toughness was much more sensitive to brittle precipitates than both the tensile properties and hardness. Precipitation of sigma phase induced brittle fracture during impact testing, especially in the situation where the grain boundaries were completely covered by sigma phase.

    更新日期:2018-08-11
  • Deformation behavior of Nickel-based superalloy Su-263: Experimental characterization and crystal plasticity finite element modeling
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-09
    S. Chandra, M.K. Samal, Rajeev Kapoor, N.Naveen Kumar, V.M. Chavan, S. Raghunathan

    The deformation behavior of a Ni-based superalloy, Su-263, was investigated using a blend of compression experiments, microstructural characterization and crystal plasticity finite element modeling. Uniaxial compression tests were performed at different strain rates ( 800 s − 1 - 2500 s − 1 ) and temperatures (300 K- 1073 K) using Split-Hopkinson pressure bar. The microstructure was examined using scanning electron microscopy and electron back-scatter diffraction technique was employed to provide necessary inputs required for crystal plasticity modeling. The simulations used a phenomenological crystal plasticity model based on thermally activated theory of plastic flow. The model was found capable of reproducing the stress-strain curves and texture evolution for all mechanical tests using a single set of hardening parameters. In addition, an attempt was made to determine an optimized set of hardening parameters at the level of crystal plasticity. This was achieved by simulating different initial crystallographic textures of the material, each of which led to a different set of hardening parameters. Using these, a single optimized set of parameters was extracted from the achieved band, which could adequately predict the mechanical response of the material for all the initial textures.

    更新日期:2018-08-10
  • Reinforcing effects of carbon nanotube on carbon/carbon composites before and after heat treatment
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-10
    Liyuan Han, Kezhi Li, Jiajia Sun, Qiang Song, Yawen Wang

    Different contents of carbon nanotubes (CNTs) were in situ grown on the surface of carbon fiber bundles by injection chemical vapor deposition. The fibers were then stacked into unidirectional preforms and densified by pyrocarbon (PyC) via chemical vapor infiltration. The effects of CNT on the strength and toughness of carbon/carbon composites (C/Cs) before and after heat treatment at 2100℃ were investigated. Results show that both the tensile strength and work of fracture achieves the optimum performance when the CNT content is 1.5 wt%. After heat treatment, the tensile strength increases by 25.68% for CNT reinforced C/Cs (CNTs-C/Cs), while only 4.36% for pure C/Cs. The refinement effect of CNT promotes the resistance of PyC matrix against destruction and makes it maintain the structural integrity and continuity even after heat treatment. Before heat treatment, the presence of CNT results in a decreased in-plane lattice size (La) compared with pure C/Cs. while interestingly, after heat treatment, La becomes larger due to the stress graphitization of CNT. The stress graphitization induced by CNT gives the carbon matrix a stronger ability to resist crack propagation, thereby enhancing the strength of the C/Cs. In addition, the existence of CNT changes the fracture mode of the C/Cs and increases the way of energy consumption during the tensile test. Thus, both fibers and the interface of the composites are fully utilized.

    更新日期:2018-08-10
  • Effects of processing temperatures on FGH4097 superalloy fabricated by hot isostatic pressing: microstructure evolution, mechanical properties and fracture mechanism
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-10
    Qing Teng, Qingsong Wei, Pengju Xue, Chao Cai, Hui Chen, Hongxia Chen, Yusheng Shi

    A newly-developed γ′-hardenable powder metallurgy (P/M) nickel based surperalloy FGH4097 developed by China was fabricated by hot isostatic pressing (HIP) with different processing temperatures. The microstructure evolution, tensile properties and fracture mechanism of the FGH4097 superalloy were systematically studied. Grains size, strengthening γ′ phase, prior particle boundaries and carbide precipitations, were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), meanwhile the mechanical properties were assessed by hardness measurements and tensile tests at room temperature (RT) and 650 °C. A dendrite structure inherited from the pre-alloy powder was partially reserved in the as-sintered alloy processed at 1100 °C. It transformed into a cellular structure when the HIP temperature was increased to 1200 °C. With increasing the HIP temperature, the contents of γ′ phase forming element decreased, accompanied by the decrease of γ′ lattice parameters and volume fraction. Due to a promoted atomic diffusion at higher temperatures, MC carbide and two types of HfO2 precipitations formed in the γ matrix that were not present in the pre-alloy powder. Besides, the grain size increased with increasing HIP temperatures, and the average grain sizes of the sintered samples were 5.8 μm (1100 °C), 17.8 μm (1200 °C) and 74.7 μm (1300 °C), respectively. Compared with two other processing temperatures, samples fabricated with at 1200 °C had the highest ultimate tensile strengths (1410 MPa at room temperature and 1236 MPa at 650 °C), along with a superior elongation (33.0 at room temperature and 31.4 at 650 °C), which was attributed to an appropriate microstructure and mediate volume fraction of γ′ phase. Owing to the incomplete recrystallization and prior particle boundaries, the yield strength and ultimate tensile strength at 1100 °C exhibited a bit less than that of at 1200 °C, but showed obvious difference in elongation. However, the tensile strengths decreased significantly at 1300 °C as a result of coarse grains. With increasing the HIP temperature, the fracture appearances indicated that the fracture changed from dimple ductile mode to brittle mode, because of the formation of coarsening MC carbide at grain boundaries.

    更新日期:2018-08-10
  • Superior mechanical properties induced by the interaction between dislocations and precipitates in the electro-pulsing treated Al-Mg-Si alloys
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-10
    Yi-tong Wang, Yu-guang Zhao, Xiao-feng Xu, Dong Pan, Wen-qiang Jiang, Xue-hui Yang, Zhe Wang

    The effect of electric pulsing treatment (EPT) on the solid-solutioned Al-Mg-Si alloy was investigated in this paper. It was interestingly found that EPT promoted the precipitation of the stable Al5FeSi phase. Besides, the precipitates pinned the dislocations effectively, thus the dislocations would be remained after the aging process. Moreover, the most interesting phenomenon was that the deformation bands induced by the remained tangled dislocation made the precipitates Mg2Si from rod-like to globular. Consequently, the superior strength (47.2% higher than T6 temper) alloy nearly without loss of ductility was obtained.

    更新日期:2018-08-10
  • Anti-aging treatment of nuclear power plant steel
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-10
    Yan Zhao, Binyan He, Sébastien Saillet, Christophe Domain, Patrick Le Delliou, Michel Perez, Rongshan Qin

    Duplex stainless steel loses impact toughness quickly during its service at nuclear power plant station as pipe and boiler. Aging induced spinodal decomposition in ferrite phase is the mechanism behind this degradation. This work uses electropulsing to treat the aged steel at the service temperature. The charpy impact toughness and Vickers micro-hardness were recovered significantly. Thermoelectric power is recommended to measure the degree of spinodal decomposition in the aging processing, which was recovered by >83% by the electropulsing treatment. It was proved that the anti-aging treatment has nothing to do with the Ohm heating. Instead, the electropulsing-induced extra free energy change of −891 J/mol provided thermodynamic driving force for the regeneration processing. Electropulsing-enhanced diffusivity enables the anti-aging processing to be completed quickly.

    更新日期:2018-08-10
  • Influence of Vanadium on the Hydrogen Embrittlement of Aluminized Ultra-High Strength Press Hardening Steel
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-10
    Lawrence Cho, Eun Jung Seo, Dimas H. Sulistiyo, Kyoung Rae Jo, Seong Woo Kim, Jin Keun Oh, Yeol Rae Cho, Bruno C. De Cooman

    Improved safety standards and reduced automotive body-in-white weight have led to a strong interest in martensitic press hardening steel (PHS). As the sensitivity to hydrogen embrittlement of martensite increases at higher strength level, the very small uptake of diffusible hydrogen by aluminized PHS during the austenitization stage of the hot press forming process is of concern. The hydrogen uptake was found to reduce the plasticity of the PHS considerably. The PHS with a higher strength was more susceptible to hydrogen-induce brittle fracture. The present work reports that vanadium additions, which serve to trap the hydrogen, considerably reduce the negative impact of the hydrogen uptake in aluminized 1800MPa and 2000MPa PHSs. The contribution also proposes a mechanism for the uptake of hydrogen during the processing of aluminized PHS.

    更新日期:2018-08-10
  • Microstructure and mechanical properties of a low C steel subjected to bainitic or quenching and partitioning heat treatments
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-10
    Sandra Ebner, Clemens Suppan, Ronald Schnitzer, Christina Hofer

    The third generation of advanced high strength steels represents a promising approach to meet the requirements of the automotive industry. A profound knowledge of the mechanical performance and the possible limitations of the different steel grades is necessary to achieve the best properties for a given application. In the present work, the main differences between transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) and quenching and partitioning (Q&P) heat treatments was studied on the same low C steel. The mechanical properties obtained from tensile testing were correlated to the microstructure, which was investigated by scanning electron microscopy and dilatometry. In a first step, the influence of varying process parameters was evaluated. The results revealed that both heat treatment concepts offer an interesting combination of strength and ductility. While TBF heat treated samples achieved higher uniform elongation, superior reduction of area was gained by Q&P cycles, which is assumed to be beneficial for local formability.

    更新日期:2018-08-10
  • Fracture toughness investigation of Al1050/Cu/MgAZ31ZB multi-layered composite produced by accumulative roll bonding process
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-07
    Davood Rahmatabadi, Moslem Tayyebi, Ahmad Sheikhi, Ramin Hashemi

    In this article, fracture behavior of multi-layered composite processed via accumulative roll bonding (ARB) method has been investigated. At first, Al1050/Cu/MgAZ31B multi-layered composite has been prepared by ARB through seven passes. The microstructure and mechanical properties have been evaluated using uniaxial tensile test, microhardness test and optical microscope, respectively. Then, the plane stress fracture toughness of Al1050/Cu/MgAZ31B have been studied via R-curves. Also, tensile fracture surfaces have been demonstrated by scanning electron microscope (SEM). The results of microstructure investigations have indicated that plastic instability occurred for both pure Cu and Mg AZ31B reinforcing at the primary sandwich and uniform distribution has been processed. By increasing the applied strain, the values of microhardness for the three layers Al1050, pure Cu, and Mg AZ31B as well as ultimate tensile strength (UTS) have been significantly increased, continully, and UTS has reached to the maximum value of 355.5 MPa. SEM images of the tensile rupture surfaces in the different ARB passes have demonstrated that with increasing the applied strain, the fracture mode converted to shear ductile at the last ARB pass. Results of fracture test have shown that by increasing the applied strain, the value of fracture toughness have been raised, continually and at the third pass reached to the maximum value of 40.4 MPam1/2. Also, the trends of fracture toughness for Al1050/Cu/MgAZ31B were in great matching with the conclusions of the fracture behavior investigation of Al1050 produced by ARB.

    更新日期:2018-08-08
  • Hydrogen Absorption and Embrittlement of Ultra-High Strength Aluminized Press Hardening Steel
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-07
    Lawrence Cho, Dimas H. Sulistiyo, Eun Jung Seo, Kyoung Rae Jo, Seong Woo Kim, Jin Keun Oh, Yeol Rae Cho, Bruno C. De Cooman

    The use of press hardening steel (PHS) in structural safety-related parts has experienced a rapid growth in the automotive industry, due to increased passenger safety standards, which require an improved vehicle intrusion resistance. Very small concentrations of diffusible hydrogen in ultra-high strength PHS deteriorate their mechanical properties. The focus of the present study was the analysis of the diffusible hydrogen uptake during the hot press forming process of the aluminized PHS and its impact on the mechanical properties of PHS. The effect of the paint baking on the properties of aluminized PHS was also evaluated by mechanical testing and hydrogen thermal desorption analysis. A mechanism for the hydrogen absorption of aluminized PHS was proposed.

    更新日期:2018-08-08
  • Microstructure, mechanical properties and strengthening mechanism of titanium particle reinforced aluminum matrix composites produced by submerged friction stir processing
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-07
    Guoqiang Huang, Jie Wu, Wentao Hou, Yifu Shen

    In spite of the improved strength, aluminum matrix composites (AMCs) reinforced with ceramic particle generally suffer a great loss in ductility. A solution to this problem is to use rigid metallic particles as a substitute for ceramic particles. In the present study, multi-pass submerged friction stir processing (SFSP) was employed to efficiently scatter titanium (Ti) particles into 5083Al matrix to form bulk AMCs. The multi-pass processing accompanied by water cooling could ensure no only the rapid acquisition of well-distributed particle dispersion, but also the absence of Al/Ti interface reaction products as well as the formation of ultrafine grains. A continuous type of dynamic recrystallization process was responsible for grain refining. The additional water cooling had a strong suppression effect on the growth of recrystal grains, and meanwhile the addition of Ti particles could boost the recrystallization due to the generation of extra dislocations at Ti/Al interfaces. As a result, ultrafine grains with the average size of about 1 μm were created in the resultant AMCs. The SFSPed AMCs exhibited an improvement of about 78 MPa in the YS and 153 MPa in the UTS respectively as compared with as-received Al and simultaneously kept a considerable amount of ductility (23.2%). The fracture surfaces of the SFSPed AMCs indicated well-developed small and uniform dimples corroborating appreciable ductility. Strength contribution from various strengthening mechanisms for the YS improvement of SFSPed AMCs was analyzed in detail. Quantitative analysis indicated that grain boundary strengthening contributed most to the YS of SFSPed AMCs.

    更新日期:2018-08-08
  • Strain rate sensitivity in commercial pure titanium: the competition between slip and deformation twinning
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-07
    Qinmeng Luan, T. Benjamin Britton, Tea-Sung Jun

    Titanium alloys are widely used in light weight applications such as jet engine fans, where their mechanical performance under a range of loading regimes is important. Titanium alloys are mechanically anisotropic with respect to crystallographic orientation, and remarkably titanium creeps at room temperature. This means that the strain rate sensitivity (SRS) and stress relaxation performance are critical in predicting component life. In this work, we focus on systematically exploring the macroscopic SRS of Grade 1 commercially pure titanium (CP Ti) with varying grain sizes and texture using uniaxial compression. Briefly, we find that Ti samples had positive SRS and samples compressed along the sheet rolling direction (RD) (i.e. soft grains dominant) were less rate sensitive than bars compressed along the sheet normal direction (ND) (i.e. hard grains dominant). We attribute this rate sensitivity to the relative activity of slip and twinning. Within the grain size range of ~   317   ± 7   μ m , we observe an increase in the rate sensitivity, where volume fraction of { 10 1 ̅ 2 } < 10 1 ̅ 1 >   ̅ ̅ T1 tensile twins was low, and the twin width at different strain rates were similar. These observations imply that the macroscopic rate sensitivity is controlled by the ensemble behaviour of local deformation processes: the amount of slips accumulated at grain boundaries affects the SRS, which is grain size and texture dependent. We hope that this experimental study motivates mechanistic modelling studies using crystal plasticity, including strain rate sensitivity and twinning, to predict the performance of titanium alloys.

    更新日期:2018-08-08
  • Shape memory characteristics of a nanocrystalline TiNi alloy processed by HPT followed by post-deformation annealing
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-07
    Hamed Shahmir, Mahmoud Nili-Ahmadabadi, Yi Huang, Jai Myun Jung, Hyoung Seop Kim, Terence G. Langdon

    A martensitic TiNi shape memory alloy was processed by high-pressure torsion (HPT) for 1.5, 10 and 20 turns followed by post-deformation annealing (PDA) at 673 and 773 K for various times in order to study the microstructural evolution during annealing and the shape memory effect (SME). Processing by HPT followed by the optimum PDA leads to an appropriate microstructure for the occurrence of a superior SME which is attributed to the strengthening of the martensitic matrix and grain refinement. A fully martensitic structure (B19' phase) with a very small grain size is ideal for the optimum SME. The results indicate that the nanocrystalline microstructures after PDA contain a martensitic B19' phase together with an R-phase and this latter phase diminishes the SME. Applying a higher annealing temperature or longer annealing time may remove the R-phase but also reduce the SME due to grain growth and the consequent decrease in the strength of the material. The results show the optimum procedure is a short-term anneal for 10 min at 673 K or only 1.5 min at 773 K after 1.5 turns of HPT processing to produce a maximum recovered strain of ~8.4% which shows more than 50% improvement compared with the solution-annealed condition.

    更新日期:2018-08-08
  • The effect of microstructure on the mechanical properties of friction stir welded 5A06 Al Alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-08
    Shujun Chen, Xiaoxu Li, Xiaoqing Jiang, Tao Yuan, Yazhou Hu

    The effect of grain size, grain orientation and precipitates on mechanical properties of friction stir welded 5A06 aluminum alloy was investigated with varying rotation speeds. The precipitation behavior and grain coarsening were discussed. Recrystallization occurred in the weld zones and recrystallization texture components were replaced by the shear texture components with increased rotation speeds. The development of shear texture, precipitates dissolution, and dislocations pinned by precipitates may result in the coarsening of the grains in the weld zones at higher rotation speeds. The effect of grain size on hardness is consistent with Hall-Petch relationship. All the welds fractured in the boundary between TMAZ and the HAZ at the AS due to the dislocations pinned by the precipitates and the sharp change of microstructure at the AS.

    更新日期:2018-08-08
  • Microstructure and compression strength of Co-based superalloys hardened by γ´ and carbide precipitation
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-08
    Marta Cartón-Cordero, Mónica Campos, Lisa P. Freund, Markus Kolb, Steffen Neumeier, Mathias Göken, José M. Torralba

    A Co-based superalloy, Co-9Al-9W (at%), was processed by mechanical alloying by high-energy milling of elemental powders and consolidated by field assisted hot pressing (FAHP). The milled powder particles mainly consist of undissolved bcc-W as well as WC and an Al and W rich fcc-γ Co solid solution. After consolidation and heat treatment a fine grained microstructure with a high fraction of carbides and a γ/γ′ microstructure was obtained. The compressive yield strength at room temperature was found to be 45% higher than that of previously reported results for Co-based superalloys. A similar level of strength was found at 700 ºC. These extraordinary properties can be explained due to the multitude of hardening mechanisms that sintered Co-based superalloys possess: γ′ precipitation, carbide formation and the ultra-fine γ-grain size promoted by the fast consolidation technique.

    更新日期:2018-08-08
  • Compression behavior of individual thin-walled metallic hollow spheres with patterned distributions of microporosity
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-08
    Jinliang Song, Quansheng Sun, Shengmin Luo, Sanjay R. Arwade, Simos Gerasimidis, Yi Guo, Guoping Zhang
    更新日期:2018-08-08
  • High-temperature rotary-bending fatigue characteristics of a high Nb-containing beta-gamma TiAl alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-08
    Yulun Wu, Rui Hu, Jieren Yang, Xiangyi Xue

    The rotary-bending fatigue tests of a high Nb-containing beta-gamma TiAl alloy based on microstructure control have been conducted at 800 °C. A fine nearly lamellar microstructure with the lamellar colony of 80 μm and slight content of B2 phase of 2% is obtained by an improved two-step heat treatment. The rotary-bending fatigue results perform the scattered S-N data, flat S-N fitting curve and an excellent fatigue limit reached at 355 MPa. SEM results exhibit an obvious transition between stable crack growth and fast fracture surface. Massive cracks initiate at lamellar interface, colony boundary and B2 phase on the specimen surface, which coalesce and propagate from the margin to the center of specimen. TEM analysis indicate that dislocation glide and mechanical twinning are the mainly deformation mechanism. The interlamellar and translamellar fracture result from dislocation pile-up and the inclined slip band or mechanical twinning, respectively.

    更新日期:2018-08-08
  • Microstructure characterization and mechanical behavior analysis in a high strength steel with different proportions of constituent phases
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-08
    Fei Peng, Yunbo Xu, Xingli Gu, Yuan Wang, Jianping Li, Hua Zhan

    A high strength steel with Nb-V-Ti addition was heat treated by a series of isothermal bainite holding (IBH) processes and quenching and partitioning (Q&P) processes with different intercritical annealing temperatures. The microstructure observation showed that some martensite was formed during quenching in IBH condition with annealing temperature above 850 °C, mainly ascribed to heterogeneous carbon content distribution in parent austenite with relatively large size. Meanwhile, film-like RA in martensite, relatively coarser lath-like RA in bainite and blocky RA located in grain boundaries or phase boundaries were observed in both IBH and Q&P treatments. Moreover, Q&P process was proved to be more beneficial to retain austenite than IBH process, while the ability of austenite retention was similar in IBH conditions. An excellent combination of strength and ductility was obtained in Q&P process annealed at 880 °C with tensile strength of 1126 MPa and total elongation of ~18%, attributing to TRIP effect mainly occurred in the latter part of strain and fine microstructure with homogeneous distribution.

    更新日期:2018-08-08
  • Properties of a high-strength ultrafine-grained CoCrFeNiMn high-entropy alloy prepared by short-term mechanical alloying and spark plasma sintering
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-07
    Filip Průša, Alexandra Šenková, Vojtěch Kučera, Jaroslav Čapek, Dalibor Vojtěch

    An equiatomic CoCrFeNiMn high-entropy alloy was prepared by induction melting and a progressive combination of mechanical alloying and compaction via spark plasma sintering done at temperatures of 800 °C and 1000 °C. The chosen methods of preparation had a significant impact on the microstructure and mechanical properties of the alloy. In comparison, the as-cast alloy had a much coarser microstructure while simultaneously obtaining inferior mechanical properties compared to those of the 8-h mechanically alloyed and spark plasma sintered alloy compacted at 1000 °C, which achieved a hardness of 424 ± 7 HV, and the alloy compacted at 800 °C showed a lower but still highly comparable hardness of 352 ± 12 HV. Both alloys showed good thermal stability, as expressed by almost negligible hardness changes during 100 h of annealing at temperatures of 400 °C and 600 °C. The investigated alloys also showed their superiority during compressive stress-strain tests at ambient and elevated temperatures of 400 °C and 600 °C. At ambient temperature, the highest compressive yield strength of 1534 MPa was observed for the sample compacted at 800 °C. As the temperature of the compressive test increased, the investigated alloys reduced their compressive yield strengths.

    更新日期:2018-08-07
  • Quantitative study on yield point phenomenon of low carbon steels processed by compact endless casting and rolling
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-06
    Jung Gi Kim, Ho Yong Um, Ji Yun Kang, Hyeok Jae Jeong, Kang Hyun Choi, Sang-Hyeon Lee, Seong-Yeon Kim, Jae-Sook Chung, Hyoung Seop Kim

    Compact endless cast and rolling mill (CEM) is an innovative process to manufacture a hot-rolled steel strip by combining casting and hot-rolling processes. However, the yield point phenomenon (YPP) of low carbon steel strip-processed by CEM induces a negative effect in the steel product and an additional post-process is required to remove the YPP. In this study, the influence of the dislocation density and the grain interior solute atoms on the yield point elongation (YPE) of low carbon steels were quantitatively investigated using 3-dimensional atom probe tomography analysis and X-ray convolutional multiple whole profile fitting. The YPE of low carbon steel is suppressed as the dislocation density increases and carbon atom content decreases. Because the dislocation density of low carbon steel by the CEM process is increased by lowering the processing temperature, the yielding behavior of the CEM products can be eliminated without any additional post-processing. The quantitative study on the carbon/dislocation density and YPP of low carbon steel not only represents the theoretical basis of the role of carbon-dislocation interaction on YPP but also provides an effective solution to optimize the CEM process for superior products.

    更新日期:2018-08-06
  • Effects of Sn addition on dynamic recrystallization of Mg-5Zn-1Mn alloy during high strain rate deformation
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-06
    Jiankun Zou, Jihua Chen, Hongge Yan, Weijun Xia, Bin Su, Yi Lei, Qin Wu

    Effects of Sn addition (0–1.5 mass%) on dynamic recrystallization (DRX) of Mg-5Zn-1Mn alloy during high strain rate ( ε ̇ =9.1 s−1) deformation are investigated by hot compression testing. With a higher Sn addition, the strain corresponding to the maximum softening rate and the critical strain corresponding to the onset of DRX decrease at first and then increase, while the DRX fraction is just the reverse. The Mg-5Zn-1Mn-0.9Sn alloy shows the biggest DRX extent. Continuous DRX with twin nucleation is the dominant DRX mechanism for all the alloys. The high density dislocations inside twins gradually evolve into the cellular structures and sub-grains by dynamic recovery, and sub-grains turn into the DRX nuclei. The Sn addition can adjust the nucleation rate and the nucleus growth rate of DRX by regulating the inclination for twinning, adjusting stacking fault energy (SFE) and changing the amount of precipitates. Solute Sn atoms not only promote DRX nucleation by increasing twin density but also stimulate the DRX nucleus growth by reducing SFE. With the higher amount of MgZn2 and Mg2Sn dislocation-induced precipitates, the negative effect on DRX derived from the precipitates becomes effective, including hindering the formation of cellular structures and preventing the rotation and the growth of sub-grains by the pinning effect.

    更新日期:2018-08-06
  • Carbon Fiber/SiC Composites Modified SiC Nanowires with Improved Strength and Toughness
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-06
    Fang He, Yongsheng Liu, Zhuo Tian, Chengyu Zhang, Fang Ye, Laifei Cheng, Litong Zhang

    In this study, Cf/SiC composite was modified with SiC nanowires (SiCnws) grown in-situ in SiC matrix. Effects of SiCnw on bending strength and fracture toughness were investigated by comparison with conventional Cf/SiC. Results showed that SiCnw significantly improved both strength and fracture toughness of Cf/SiC. Compared to Cf/SiC, flexural strengths of Cf/SiC-SiCnw/PyC and Cf/SiC-SiCnw composites increased from 363.1 MPa to 375 MPa and 466.9 MPa, respectively. Bending strength and fracture toughness were estimated to 565 MPa and 20.30 MPa·m1/2 for Cf/SiC containing SiCnw at high temperature (1000 °C), respectively. These values were 13.2% and 16.1% higher than that of conventional Cf/SiC. The latter was attributed to deflection, bridging, and pull-out of SiC nanowires to cracks. Improvements in mechanical properties of modified C/SiC composites were mainly attributed to deflection and bridging of SiC nanowires on cracks and pulling out of SiC nanowires from the matrix.

    更新日期:2018-08-06
  • The influence of temperature on twinning behavior of a Mg-Gd-Y alloy during hot compression
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-06
    S.H. Lu, D. Wu, R.S. Chen, E.H. Han

    The microstructure evolution and plastic deformation mechanism of Mg-6.58Gd-5.7Y-0.55Zr alloy were investigated by uniaxial compression at temperatures ranging from 300 °C to 450 °C and with an initial strain rate of 0.1 s-1, and characterized by optical microscope (OM), scanning electron microscope (SEM) and electron back-scattering diffraction (EBSD). The effect of temperature on twinning behavior was focused in detail. At relatively low temperature, such as 300 °C, multiple twin variant pairs can be activated in one grain, and the twining process was dominated by nucleation. However, at high temperature, most of the twins activated in one grain belong to the single twin variant pair with the highest Schmid Factor (SF), and twin growth seems to represent the twinning behavior at 450 °C. The twinning behavior of Mg-6.58Gd-5.7Y-0.55Zr alloy at temperatures ranging from 300 °C to 450 °C basically follows Schmid law, and can be well understood in terms of variation of stress state and activated deformation modes with compression temperature.

    更新日期:2018-08-06
  • Synchronous improvement of the strength and plasticity of Ni-Co based superalloys
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-04
    C.L. Yang, Z.J. Zhang, P. Zhang, C.Y. Cui, Z.F. Zhang

    Synchronous improvement of strength and plasticity (SISP) is realized for the first time in a precipitated-phase strengthened Ni-Co based superalloy through reducing stacking fault energy (SFE). Microstructure characterizations indicate that, with lowering SFE, the dislocation slip mode transforms from wavy to planar slip, meanwhile, the twinnability is also improved. Both the planar slip mode and the increased twinnability enhance the strain-hardening capability of the superalloy, which postpones the initiation of necking. Thus, the SISP in the double-phase superalloy is achieved.

    更新日期:2018-08-05
  • Effect of loading direction on the deformation and annealing behavior of a zirconium alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-04
    Weijun He, Adrien Chapuis, Xin Chen, Qing Liu

    Zr alloys are used as cladding materials in nuclear reactor, and new alloys and process are continuously developing. This work focuses on the influence of loading direction on deformation and subsequent annealing behavior of hexagonal Zr-4 alloy. As-received Zr-4 sheets with recrystallized microstructure and strong crystallographic texture were compressed along the normal direction (ND) and rolling direction (RD) at room temperature. Different strain levels were applied and subsequent annealing at various temperatures was carried out. Microstructures and textures were characterized by electron backscatter diffraction (EBSD) method. For a same strain level, the fraction of low angle misorientation is higher after ND compression than RD compression, indicating higher stored dislocation density. During annealing, obvious recrystallization causes grain growth in ND compressed specimens when the annealing temperature is higher than 650 oC, while no significant recrystallization happens in RD compressed specimens, even annealed at 700 oC. Visco-plastic self-consistent (VPSC) simulations are used to evaluate the type of stored dislocations, their density and the stored energy, and explain the different annealing behaviors after ND compression and RD compression.

    更新日期:2018-08-05
  • The temperature dependence of high-temperature strength and deformation mechanism in a single crystal CoNi-base superalloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-04
    Zhongding Fan, Chenchong Wang, Chi Zhang, Yunhe Yu, Hao Chen, Zhigang Yang

    The thermal stability and high-temperature strength of single crystal CoNi-base superalloys, Co-30Ni-10Al-7W (at.%) and Co-30Ni-10Al-7W-2Ta-3Ti (at.%), containing γ’(L12) phase have been investigated. The investigated CoNi-base superalloy with alloying additions of Ti and Ta exhibits excellent γ’ solvus temperature, 1236℃, and high-temperature strength, 622 MPa at 900℃, compared to reported γ’-strengthening Co-base superalloys. Detailed analyses of the deformation mechanisms for the investigated alloys were carried out by using the transmission electron microscopy (TEM) characterization. Two different transitions of deformation mechanisms for the γ’ phase, from superpartial dislocation shearing to dislocation bypassing and to stacking fault shear, occurred in the investigated alloys as the deformation temperature increased above the peak temperatures. Meanwhile, the high-temperature strength of the investigated alloys largely decreased as the transitions of deformation mechanism took place. The influence of alloying effects on energies of faults and the relationship between high-temperature strength and deformation mechanisms were discussed.

    更新日期:2018-08-05
  • Annealing-induced hardening in high-pressure torsion processed CoCrNi medium entropy alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-04
    S. Praveen, Jae Wung Bae, Peyman Asghari-Rad, Jeong Min Park, Hyoung Seop Kim

    Nanocrystalline CoCrNi equiatomic alloy processed by high-pressure torsion shows annealing induced hardening at 500 °C and 600 °C. The microstructural characterization indicates the hardening phenomenon is not because of precipitation hardening. The annealing-induced hardening is explained based on the reduced dislocation density and grain boundaries relaxation.

    更新日期:2018-08-05
  • Ultrafine Grained Dual-phase Martensite/Ferrite Steel Strengthened and Toughened by Lamella Structure
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-04
    Junjie Sun, Tao Jiang, Yingjun Wang, Shengwu Guo, Yongning Liu
    更新日期:2018-08-05
  • Behaviour of Al6061-T6 alloy at different temperatures and strain-rates: experimental characterization and material modelling
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-05
    M. Scapin, A. Manes

    The simulation of impact scenario against a structure requires the use of material models able to reproduce all aspects of the mechanical behaviour of the involved materials; plastic flow is one of the main aspects to be reproduced. In more detail, attention has to be paid to the investigation of strain-rate and temperature sensitivities, as well as their interaction, which necessitates the use of a reverse engineering approach. The present paper mainly focuses on the tensile behaviour and an ad-hoc testing campaign was performed on cylindrical dog-bone specimens made in Al6061T6 at different temperatures and strain-rates extending the range up to a level where, at present, there is a lack in the scientific literature. The thermal softening effect was investigated in quasi-static as well as in dynamic loading conditions from room temperature up to 400 °C; while the material strength dependence on the strain-rate was studied up to 104 s−1 on miniaturized samples. Microstructure analyses were performed to better investigate the mechanical response at different loading conditions. The parameters of the Johnson-Cook model were identified starting from experimental data via a numerical inverse approach based on FEM simulations. These parameters can be used for simulations of extreme loading scenario like ballistic impact events.

    更新日期:2018-08-05
  • Hydrogen behavior in high strength steels during various stress applications corresponding to different hydrogen embrittlement testing methods
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-05
    Yu Matsumoto, Tomonori Miyashita, Kenichi Takai

    Atomic scale phenomena during various types of hydrogen embrittlement testing were examined by investigating hydrogen desorption during the tests using tempered martensitic steel and cold-drawn pearlitic steel specimens. Hydrogen desorption increased in the elastic stage of constant stress/strain and cyclic stress testing, implying hydrogen transportation by dislocations. In contrast, hydrogen desorption increased in the elastic stage but turned downward near proof stress and finally deceased to less than that before stress application. This implies that hydrogen-enhanced strain-induced lattice defects such as dislocations and vacancies formed in addition to hydrogen accumulation by mobile dislocations. These results suggest that one of the reasons for the high hydrogen embrittlement susceptibility of high strength steels is hydrogen accumulation on the grain boundaries and cementite interfaces due to hydrogen transportation by mobile dislocations under elastic deformation.

    更新日期:2018-08-05
  • Microstructural and mechanical properties of low-carbon ultra-fine bainitic steel produced by multi-step austempering process
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-05
    Hamid Mousalou, Sasan Yazdani, Behzad Avishan, Naghi Parvini Ahmadi, Ali Chabok, Yutao Pei

    The multi-step isothermal austempering heat treatment to achieve an ultra-fine bainitic microstructure and maximum volume fraction of bainite was conducted on a steel containing 0.26 wt.% carbon. The microstructural and crystallographic characteristics, as well as the mechanical properties and fracture behavior were studied. The results showed that the subsequent austempering heat treatment at a lower temperature, immediately after partial bainite formation at a higher temperature, would replace the coarse austenite/martensite areas with much refined bainite consisting nanoscale plates of bainitic ferrite and filmy austenite which ultimately leads to the refinement of the bainitic microstructure. This microstructural modification, in addition to the increased yield strength, causes a significant increase in the impact fracture toughness of the multi-step austempered steels. The EBSD analysis also showed that the subsequent austempering heat treatment at a lower temperature results in a finer structure of Bain groups and increase in the fraction of high angle grain boundaries leading to higher resistance against crack propagation and subsequently higher impact energy absorption.

    更新日期:2018-08-05
  • Enhancement in mechanical properties of selectively laser-melted AlSi10Mg aluminum alloys by T6-like heat treatment
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-08-01
    L.F. Wang, J. Sun, X.L. Yu, Y. Shi, X.G. Zhu, L.Y. Cheng, H.H. Liang, B. Yan, L.J. Guo

    This work proposes a T6-like heat treatment, including solid-solution treatment at 535 ºC and an artificial aging treatment for 10 h at 158 ºC, to control the mechanical behavior of selective-laser- melting (SLM)-produced AlSi10Mg alloys. The mechanical properties of the AlSi10Mg alloys, such as densification, hardness, and tensile/bending strength, were investigated, and the microstructure of the alloys was analyzed. The results reveal that the tensile strength of the heat-treated samples slightly decreased by 19.97% (from 334 MPa to 267.3 MPa of the as-fabricated samples), while the elongation showed a remarkabe increase by up to 155% (from 3.64% to 9.28%). Likewise, the bending strength slightly decreased by 6.1%, while the fracture deflection dramatically increased by up to 122.9 % after T6 heat treatment. Thus, the T6 heat treatment can critically enhance plasticity/ductility without any significant loss in the tensile/bending strength of the alloy. The corresponding mechanism is also elucidated based on the spheroidization and diffusion of silicon precipitation during the T6 heat treatment. The results of this study offer an intriguing insight to tailor the mechanical properties of SLM-fabricated AlSi10Mg alloys using suitable solid solutions and artificial aging treatment.

    更新日期:2018-08-01
  • Enhancement of mechanical properties of low carbon dual phase steel via natural aging
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-07-31
    Mehran Zamani, Hamed Mirzadeh, Mehdi Maleki

    The natural aging behavior of a low carbon dual phase (DP) steel was studied and the effect of martensite content was taken into account. The enhancement of strength and hardness by aging at room temperature was related to quench aging via the formation of fine precipitates in the ferrite grains. It was revealed that increasing the intercritical annealing temperature diminishes this precipitation hardening effect, which was related to the decreased degree of supersaturation of carbon in ferrite. The maximum quench aging effect was observed by annealing the ferritic-pearlitic steel at a subcritical temperature close to the eutectoid temperature, where the solute carbon content of ferrite reaches the maximum value. However, in contrast to aged DP steels, the tensile stress-strain curves showed yield-point phenomenon. The aging effect was rationalized by the Ashby-Orowan relationship and the effect of the silicon in the chemical composition of the studied steel was discussed.

    更新日期:2018-07-31
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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