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  • Strengthening in Al0.25CoCrFeNi high-entropy alloys by cold rolling
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-21
    Jinxiong Hou, Min Zhang, Shengguo Ma, Peter. K Liaw, Yong Zhang, Junwei Qiao

    The effects of cold rolling on the evolution of microstructure and the mechanical behavior of Al0.25CoCrFeNi high-entropy alloys were investigated. Cold rolling resulted in extensive grain elongation, formation of deformation bands within the grains, and development of crystallographic textures that depended on the rolling reduction. The textures were investigated by electron backscattered diffraction after cold rolling. The present cold-rolled alloy indicates a strong brass-type texture. Cold rolling results in a substantial strengthening of the alloy; its ultimate tensile strength approaches 1,479 MPa, which was 2.8 times of that in the as-cast condition, but at the expense of low ductility (ε~ 2.3%).

    更新日期:2017-09-21
  • The deformation behavior of AZ31 Mg alloy with surface mechanical attrition treatment
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-21
    Xiangchen Meng, Meng Duan, Lan Luo, Duochan Zhan, Bin Jin, Yuhai Jin, Xi-xin Rao, Yong Liu, Jian Lu

    In present work, the deformation behavior of AZ31 Mg alloy with surface mechanical attrition treatment (SMAT) had been studied. The microstructure and mechanical properties of AZ31 Mg alloy with SMAT were investigated. The results indicated that a gradient nanostructure could be formed in sample by SMAT, in which the grain size increased gradually from surface to matrix. A depth-dependent gradient microhardness was also formed due to the corresponding gradient microstructure. Yield strength and ultimate tensile strength of AZ31 Mg alloy with SMAT were significantly improved combining with decrease of fracture elongation. The effect of SMAT on anisotropy of mechanical properties of AZ31 alloy had been discussed and analyzed. The plastic anisotropy of the sample increased significantly after SMAT, which was related to the texture variation of rolled sheet and special deformation behavior of gradient nanostructure. Finally, in order to illuminate the difference in deformation behavior between fine and coarse grained microstructure, the in-situ tensile deformation behavior of AZ31 Mg alloy with one-side gradient structure had been studied by SEM. The deformation mechanism of AZ31 Mg alloy with gradient structure had been put forward.

    更新日期:2017-09-21
  • Microstructure evolution and fracture mechanism of a novel 9Cr tempered martensite ferritic steel during short-term creep
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-21
    Bo Xiao, Lianyong Xu, Lei Zhao, Hongyang Jing, Yongdian Han, Zhengxin Tang

    In this work, the microstructure evolution and fracture mechanism of a novel 9% chromium tempered martensite ferritic steel G115 were investigated over the temperature range of 625 to 675 °C using uniaxial creep tests. The creep curves consist of a primary transient stage followed by an apparent secondary stage, and an accelerated tertiary creep regime. The relationship between the minimum creep rate and the applied stress followed Norton's power law. Based on the EBSD analysis, there were no obvious textural features formed after creep deformation, and with the increase in creep time, the number of subgrains slightly increased, and then sharply increased, indicating dynamic recrystallization (DRX) occurs after creep deformation. In addition, three types of precipitates can be observed after creep deformation: W-rich Laves phase, Nb-rich MX, and Cu-rich precipitates. The Nb-rich MX with a square shape and Cu-rich precipitates with an ellipsoidal shape remain very stable. However, the W-rich Laves phases distributed mainly on the grain boundaries have rod-like, chain-like, and bulky shape, which are coarsened significantly. Representative fractographs of the G115 steel after creep deformation exhibit significant necking with an elliptical shape. A dense array of deep and equiaxed dimples appear in the central region under the tested creep conditions. Ductile fracturing is the dominant fracture mechanism during short-term creep deformation.

    更新日期:2017-09-21
  • Origin of the Bauschinger effect in a polycrystalline material
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-21
    A.A. Mamun, R.J. Moat, J. Kelleher, P.J. Bouchard

    There is a long and lively debate in the literature about the origin of the Bauschinger effect in polycrystalline materials, the most widely accepted explanation being the easier movement of dislocations during reverse loading causing the reduction of the yield stress. Other explanations include incompatible deformation at the grain scale and change of dislocation cell structures during forward and reverse loading, but recent publications show these phenomenological explanations of the Bauschinger effect are not holistic. In the experimental work presented here, we have investigated the role of micro residual lattice strain on the origin of the Bauschinger effect in type 316 H austenitic stainless steel using in-situ neutron diffraction. Standard cylindrical specimens were tension-compression load cycled at room temperature with the loading interrupted at incrementally larger compressive and tensile strains followed by reloading to the tensile loop peak strain. Mirror symmetric compression-tension cyclic tests were also performed with tensile and compressive load interruptions followed by compressive reloading to the compressive loop peak strain. A strong correlation is demonstrated between the evolution of residual lattice strain in the grain families and the change in magnitude in macroscopic yield stress, peak stress and the shape of the yielding part of the stress-strain curve for both the cyclic tension yield and compression yield tests. This implies that the residual lattice strain generated by grain scale elastic and plastic deformation anisotropy is the primary source of the Bauschinger kinematic hardening effect observed in type 316 H austenitic stainless steel.

    更新日期:2017-09-21
  • Integral method of preparation and fabrication of metal matrix composite: selective laser melting of in-situ nano/submicro-sized carbides reinforced iron matrix composites
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-21
    Bo Song, Zhiwei Wang, Qian Yan, Yuanjie Zhang, Jinliang Zhang, Chao Cai, Qingsong Wei, Yusheng Shi

    This paper presents a study on the formation, microstructure and mechanical property of Cr3C2/Fe nanocomposites using selective laser melting (SLM). Scanning electron microscope (SEM), transmission electron microscope (TEM) and tensile tests were conducted to investigate the influence of Cr3C2 particulates on the microstructure and mechanical properties. It was found that the porosity decreased when volumetric energy density increased from 27 to 37 J/mm3, then rose to 55 J/mm3. The results showed that the Cr3C2 particulates decomposed completely during SLM process and interacted with Fe matrix to form (α-Fe, Cr) solid solution and M(Fe, Cr)23C6 carbides. In addition, due to the characteristic of rapid solidification, very fine grains were obtained throughout the composites. The tensile tests showed that the ultimate tensile strength could reach up to 1158 MPa.

    更新日期:2017-09-21
  • Microstructure and mechanical properties of medium Mn steel containing 3%Al processed by warm rolling
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-21
    Weijun Hui, Chengwei Shao, Yongjian Zhang, Xiaoli Zhao, Yuqing Weng

    The present study was attempted to evaluate the microstructural evolution and mechanical properties of a 3 wt.%Al containing medium Mn steel which was warm rolled with different thickness reductions directly after intercritical annealing at 750 ℃ by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction and tensile tests. The results show that the laths of ferrite and retained austenite (RA) not only gradually rotated their longitudinal axis to the rolling direction but also were refined with increasing rolling thickness reduction, and thus a well developed lamellar structure of bimodal size distributed lathy ferrite and RA was obtained after heavy rolling reduction. Though the strength increases while the ductility decreases with increasing rolling thickness reduction, an excellent combination of strength and ductility expressed by the product of ultimate tensile strength (UTS) to total elongation (TEL) of ~50 GPa% was obtained through a wide process window of warm rolling. It is thus proposed that warm rolling is a promising way to simplify the traditional multi-stage rolling and annealing processes of Al-containing medium Mn steels, especially to overcome the cold rolling difficulty of medium Mn steels with high carbon content.

    更新日期:2017-09-21
  • Influence of Microstructure on Fatigue Crack Nucleation and Microstructurally Short Crack Growth of an Austenitic Stainless Steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-21
    Jonathan W. Pegues, Michael D. Roach, Nima Shamsaei

    In this study, the effect of microstructure on crack nucleation and microstructurally short fatigue crack growth is investigated for a metastable austenitic stainless steel. Fatigue tests were conducted at an intermediate fatigue life regime between 104 and 106 cycles such that martensitic phase transformation occurs given sufficient localized deformation. Through the use of scanning electron microscopy, along with electron backscatter diffraction, several micro-cracks were analyzed and compared. The influence of microstructural features such as twin boundaries, slip band intrusions/extrusions, grain boundaries, inclusions, and martensitic transformed areas on the crack initiation life is discussed. The initiation stages of crack nucleation and the subsequent microstructurally short crack growth for each microstructural feature are compared revealing that twin boundaries and slip bands are the most dominant initiation features. However, the initiation mechanism governing crack nucleation for each feature was different. Additionally, the phase transformation behavior showed only minor effects on the microstructurally short crack growth leading up to an engineering crack. It was found that while the cracks that propagated more quickly had larger transformed martensitic zones around the crack tip, this was due mostly to the size of the crack. Interestingly, the initiation life in the transitional fatigue regime was observed to be more sensitive to crack initiation feature than the martensitic transformation.

    更新日期:2017-09-21
  • Improving joint performance of friction stir welded wrought Mg alloy by controlling non-uniform deformation behavior
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-20
    Q. Shang, D.R. Ni, P. Xue, B.L. Xiao, Z.Y. Ma
    更新日期:2017-09-21
  • Bimodal laminated Ti3Al matrix composite achieved by in situ formed Ti5Si3 reinforcements
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-20
    Tongtong Zhang, Guohua Fan, Kesong Miao, Kai Chen, Zhihao Pan, Shuai Chen, Xiping Cui, Meng Huang, Lin Geng

    The low density and high-temperature mechanical properties of intermetallic Ti3Al based alloys are highly desirable for a wide range of critical applications. In this paper, we designed and prepared a novel laminated Ti5Si3/Ti3Al composite (LTTC) consisting of alternating Ti5Si3-lean/coarse-grained Ti3Al layers (TCLs) and Ti5Si3-rich/fine-grained Ti3Al layers (TFLs). The processing route involves hot pressing of a stack of alternating Ti foils and Al-6Si foils, followed by a two-step reaction annealing. The tensile properties of the LTTC have also been evaluated, and the LTTC exhibited a good strength-ductility combination when compared with monolithic Ti3Al. By in-depth mechanism discussion, we propose the possible approaches to further enhance the mechanical properties of such LTTC.

    更新日期:2017-09-20
  • Initiation and propagation of microcracks in Cu thin films on flexible substrates through the thickness direction during a cyclic bending test
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-20
    Atanu Bag, Shi-Hoon Choi

    The initiation and propagation of microcracks in the copper thin film on flexible polyimide substrates was examined through the thickness direction following cyclic bending test using a focused ion beam (FIB) and electron backscattering diffraction (EBSD) technique. The EBSD observations of the cross-sectional plane clearly indicated that intergranular fracture was predominant during the initiation and propagation of microcracks. During the cyclic bending testing, through the thickness direction microcracks were propagated mostly along the high-angle grain boundaries (HAGBs) that separated the neighboring grains with a high Schmid factor (SF), instead of at the twin boundaries (TBs).

    更新日期:2017-09-20
  • Effects of annealing treatment on the microstructure evolution and the strength degradation behavior of the commercially pure Al conductor
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-20
    J.P. Hou, Q.Y. Chen, Q. Wang, H.Y. Yu, Z.J. Zhang, R. Li, X.W. Li, Z.F. Zhang

    Strength degradation during the electron transmission process is always a hidden danger to the overhead transmission lines. In this study, the microstructure evolutions and the strength degradation behaviors of a cold-drawn commercially pure Al conductor (CPAC) were investigated systematically by a series of annealing experiments. The results show that the texture evolution, dislocation recovery and subgrain growth during the recrystallization should be responsible for the strength degradation of CPAC. Besides, the microstructure evolution depends on the annealing temperature. For instance, some of the <111> texture was changed into the <001> one in the CPACs annealed at a temperature of 90 °C; while, there is an obvious increase in the subgrain width when the CPACs were annealed in the high temperature range from 150 °C to 300 °C. Finally, the strength degradation due to the texture evolution and the subgrain coarsening was quantitatively calculated based on the crystallographic analysis.

    更新日期:2017-09-20
  • Effects of multiple trace alloying elements on the microstructure and properties of Cu-4wt.% Ti alloys
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-20
    Huan Wei, Yanchao Cui, Huiqi Cui, Yinghui Wei, Lifeng Hou

    Effects of multiple trace alloying element Fe and Cr addition on the solid solution-treated Cu-4wt.% Ti alloys were studied with respect to microstructure and mechanical properties. It was elucidated that the addition of trace alloying elements results in the formation of intermetallic phase CuTi, lowering the titanium content of Cu solid solution phase, and refining the microstructure. The contributions of different strengthening mechanisms were calculated, which are in good agreement on experimental results. It turns out that the solute solution strengthening in Cu- Ti alloys plays a crucial role, even though the fine grain size and precipitation strengthening mechanism are contributed to the strength of Cu-Ti-Cr -Fe alloys. Therefore, the Cu-Ti-Cr-Fe alloys exhibits lower strength and higher elongation compared with Cu-4wt.% Ti alloys. However, Cu-Ti-Cr-Fe alloys show a great potential to balance the mechanical properties and electrical conductivity during aging treatment as the titanium content decreases in the Cu matrix, and the strength is higher than that of others in previous study.

    更新日期:2017-09-20
  • Mechanical Behavior and Deformation Mechanism of Commercial Pure Titanium Foils
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-20
    L. Niu, S. Wang, C. Chen, S.F. Qian, R. Liu, H. Li, B. Liao, Z.H. Zhong, P. Lu, M.P. Wang, P. Li, Y.C. Wu, L.F. Cao

    The work presents a detailed investigation of mechanical properties and deformation mechanism of ultrathin commercial pure Ti foils subjected to uniaxial tensile. The initial microstructure of the annealed Ti foils is fine equiaxed grains, with an average grain size of 3.3 μm. The main types of texture in pure Ti foils are {10 View the MathML source 1 ̅ 3} and {11 View the MathML source 2 ̅ 4}. Dimples, tear ridges and slip traces are found on the fracture surface, and the elongation reaches 16.7%. The large tensile elongation of pure Ti foils can be attributed to the slip of dislocations, twinning and phase transformation of HCP-Ti to FCC-Ti, which is confirmed by both X-ray diffraction and transmission electron microscope. The orientation relationship of the two phases is determined to be: <0001>HCP // <001>FCC, <11 View the MathML source 2 ̅ 0 >HCP // <1 View the MathML source 1 ̅ 0 >FCC and {01 View the MathML source 1 ̅ 0}HCP // {110}FCC. According to this relationship, the HCP to FCC phase transformation expands the volume of the unit cell by about 10.5%. Furthermore, the volume fraction of FCC-Ti is determined to be about 3.44%. Therefore, the transformation is expected to contribute about 3.6% to the applied strain.

    更新日期:2017-09-20
  • Plastic flow behavior based on thermal activation and dynamic constitutive equation of 25CrMo4 steel during impact compression
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-20
    Dingyuan Li, Zhiwu Zhu, Shoune Xiao, Guanghan Zhang, Yesen Lu

    To evaluate the dynamic mechanical properties of 25CrMo4 steel, the stress-strain curve of 25CrMo4 steel have been obtained by performing quasi-static and impact compression experiments at strain rates ranging from 0.001 s−1 s − 1 to 4163 s−1 s − 1 . The 25CrMo4 steel displays an obvious strain rate effect and its hardening rate changes with the strain and strain rate. It even shows softening effect at high strain rates. Further, the grains of 25CrMo4 steel show features of both body-centered cubic (BCC) and face-centered cubic (FCC) structures and are severely stretched and fragmented during impact loading, as evidenced by microscopic observations. Based on these two points, the Zerilli-Armstrong (ZA) constitutive equations have been improved reasonably. These can provide the mechanical properties of 25CrMo4 steel during impact compression loading, which can in turn be used as reference values for practical engineering analysis.

    更新日期:2017-09-20
  • Selective laser melting of pure tantalum: Densification, microstructure and mechanical behaviors
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-20
    Libo Zhou, Tiechui Yuan, Ruidi Li, Jianzhong Tang, Guohua Wang, Kaixuan Guo

    In this study, selective laser melting (SLM) of pure tantalum (Ta) was systematically investigated, with emphasis on densification, microstructure and mechanical properties of Ta specimen. The high scanning speed of laser resulted in micropores and discontinuous scan tracks, owing to the elevated instability of the liquid induced by Marangoni convection and the balling effect. However, the interlayer thermal microcracks were produced at a low scanning speed, due to the thermal stress and balling effect. The microhardness and tensile strengths of the optimally SLM-processed Ta parts were improved to 445 HV and 739 MPa, respectively, which were considerably higher than those of the specimens processed by cast (110 HV and 205 MPa) or powder metallurgy (120 HV and 310 MPa) method, due to the fine-grain strengthening. The fracture morphology of the tensile-failed SLM-processed specimens showed that the porosities and incompletely melted particles are responsible for the fracture of porous sample. While for dense sample, cleavage fracture and minor ductile fracture both account for the fracture. And the failure mechanisms were discussed. The reduced coefficient of friction of 0.3 and lowest wear rate of 7.1×10−3 mm3·N−1·m−1 in dry sliding wear tests were obtained for the optimally prepared Ta parts due to the formed adhesion of hardened tribolayers.

    更新日期:2017-09-20
  • Investigation of pop-in events and indentation size effect on the (001) and (100) faces of KDP crystals by nanoindentation deformation
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-19
    Jarosław Borc, Keshra Sangwal, Igor Pritula, Elena Dolzhenkova

    Results of an investigation of the nature of load-displacement curves, pop-in events and indentation size effect of the (001) and (100) faces of KDP crystals by nanoindentation deformation under Berkovich indenter at different loads are presented and discussed. It was found that: (1) load-displacement curves reveal numerous pop-in events but these pop-in events occur less frequently on the (100) face than on the (001) face, (2) different segments of individual load-displacement curves are explained by plastic deformation under the indenter, (3) the value of indentation displacement excursion d of pop-in events lies between 3 and 20 nm, (4) nanohardness H lies between 2.2 and 3.3 GPa and between 1.6 and 3.2 GPa for the (001) and (100) faces, respectively, and is comparable with that reported before, whereas penetration independent hardness H0 obtained from analysis of the H(h) data is 2.04 and 1.41 GPa for the (001) and (100) faces, respectively.

    更新日期:2017-09-20
  • Corrosion fatigue crack initiation and initial propagation mechanism of E690 steel in simulated seawater
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-19
    Tianliang Zhao, Zhiyong Liu, Cuiwei Du, Chunduo Dai, Xiaogang Li

    In the present paper, the corrosion fatigue crack initiation and initial propagation mechanism of E690 steel in simulated seawater were studied by stress-controlled fatigue tests and a series of subsequent characterizations on the fracture surface, microstructure and secondary cracks. Results show that the corrosion fatigue crack initiation and initial propagation mechanism evolves with elevated peak stress level in simulated seawater. When peak stress is far below the proof stress, cracks preferentially initiate at the parent austenite grain boundaries (PAGBs) with 68.4% probability and at the ferrite lath boundaries (FLBs) with 31.6% probability. Meanwhile, the cracks also preferentially propagate along the PAGBs and FLBs. Upon the peak stress close to or above the proof stress, cracks turn to initiate from the emerging corrosion pits and propagate without zigzag detour but by splitting the ferrite laths which transversely block its propagation way.

    更新日期:2017-09-20
  • Microstructure-tensile property correlation in 304 stainless steel after cold deformation and austenite reversion
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-19
    P. Mallick, N.K. Tewary, S.K. Ghosh, P.P. Chattopadhyay

    The present study aims to investigate the evolution of microstructure of 304 austenitic stainless steel (SS) due to reversion annealing of cold deformed samples under different schedules. Evolution of phases in selected samples was identified and quantified by X-ray diffraction analysis along with corresponding microstructural characterisation through optical, scanning and transmission electron microscopy. Mechanical properties of the samples were determined by carrying out tensile test of the samples. Electron microscopy of selected samples has revealed that, while strain induced martensite, deformation twins, ε-martensite and high dislocated structures are the dominant microstructural features of the cold deformed samples, reversion annealing of the same results into ultrafine-grained reverted austenite, cell type dislocation sub-structures with some amount of untransformed martensite. The maximum tensile strength of 1589 MPa with an elongation of 9% has been obtained after 20% deformation at −196°C (20LND), whereas 40% deformation at 0°C (40ZCD) results into a tensile strength of 1225 MPa with 13% elongation. Annealing of 40ZCD and 20LND specimens at 300°C results in significant improvement in tensile strength, while annealing at 725°C increases the elongation with a decrease in tensile strength. During reversion annealing, strain induced martensite transforms to austenite through shear and diffusional processes. However, the reversion is essentially diffusional in nature at a higher temperature.

    更新日期:2017-09-20
  • The deformation and fracture behaviors of 316 L stainless steels fabricated by spark plasma sintering technique under uniaxial tension
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-19
    Amol B. Kale, Atanu Bag, Ji-Hyun Hwang, Elinor G. Castle, Mike J. Reece, Shi-Hoon Choi

    In this study, 316 L stainless steel (SS) specimens with different relative densities were fabricated using the spark plasma sintering (SPS) technique. These SPS specimens were used to capture the effect of microstructure heterogeneity on deformation and fracture behaviors during uniaxial tension. Microstructure analysis indicated that the SPS specimens consisted of fully sintered and partially sintered regions and contained initial pores which are located at the grain boundaries. Mini-tension tests combined with the digital image correlation (DIC) technique were carried out at room temperature to measure the mechanical properties of the SPS specimens and the evolution of strain heterogeneity on tensile specimens during uniaxial tension. In order to reveal the fracture mechanisms of the SPS specimens, the surfaces of the fractured specimens were analyzed via field emission scanning electron microscope (FE-SEM). The fracture mechanism in the fully sintered region was identified as a ductile fracture by the formation of cup-like dimples, while the fracture mechanism in the partially sintered region was identified as a decohesion of the interface between the powder and the matrix.

    更新日期:2017-09-19
  • Stress relaxation behavior and mechanisms in Ti-6Al-4V determined via in situ neutron diffraction: Application to additive manufacturing
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-19
    Zhuqing Wang, Alexandru D. Stoica, Dong Ma, Allison M. Beese

    The complex thermal histories present during additive manufacturing (AM) of metals result in the generation of residual stresses, which may result in distortion and early failure of the fabricated component. The amount of residual stress built up or relieved during deposition depends on the stress relaxation behavior of the deposited material as well as the substrate onto which the component is built, over the typical timescale for depositing a few layers in AM (seconds to minutes), corresponding to the timescale over which the material is subjected to both stress and elevated temperature. This work presents a method for investigating stress relaxation behavior and mechanisms in conventionally processed and additively manufactured Ti-6Al-4V (CP Ti-6Al-4V and AM Ti-6Al-4V) through compression tests at 600 °C and 700 °C with in situ neutron diffraction. The results show that with an applied plastic deformation, 60–80% of the initial stress in Ti-6Al-4V was relieved in ten minutes and the stress stabilized at a negligibly low level. With the same applied strain, the stress relaxation rate at 700 °C was 2–4 times higher than that at 600 °C, and the peak stress at 600 °C was twice as high as that at 700 °C. It was determined that neither stress partitioning nor phase transformation was active in Ti-6Al-4V at the temperatures studied. Thus, it was hypothesized that the stress relaxation was primarily due to dislocation glide and climb. The presently reported relaxation behavior can be used in the development and validation of thermomechanical models used to predict and mitigate residual stresses and distortion in AM, or to predict distortion in Ti-6Al-4V used in structural applications at elevated temperatures.

    更新日期:2017-09-19
  • Microstructure, mechanical and tribological properties of nickel-aluminium bronze alloys developed via gas-atomization and spark plasma sintering
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-18
    Wenzheng Zhai, Wenlong Lu, Po Zhang, Mingzhuo Zhou, Xiaojun Liu, Liping Zhou

    This work investigated the effect of sintering temperatures (600–750 °C) on mechanical and tribological behaviors of nickel-aluminium bronze (NAB) alloys developed by gas atomization and spark plasma sintering. Results indicated an increase of the volume fraction of B2-type NiAl precipitates with increasing sintering temperature, leading to an improvement of the yield strength and the wear resistance. Specifically, detailed microstructural analyses of sintered NAB alloys at 750 °C showed the presence of ultrafine grains with an average size of 367 nm, nanoscale twins with a volume fraction of 11.7% and dislocations with a density of 1.3 ± 0.1 × 1014 m−2. A much higher yield strength of 620 MPa was obtained in NAB, if compared to that of conventional cast counterparts (280–440 MPa). Estimations of strengthening mechanisms suggested the predominant mechanism of grain boundary strengthening (335 MPa) for NAB alloys with contributions from precipitate strengthening (54 MPa), dislocation strengthening (75 MPa), twin boundary strengthening (89 MPa) and solid solution strengthening (50 MPa). Moreover, dislocations was blocked at twin boundaries to form complex dislocation barriers and networks, further contributing to the high strength. The high wear resistance of NAB could be ascribed to the reduction of the local stress around crack tips due to the high elastic modulus mismatch (ENiAl/ECu), and to the crack extension toughening near the interface of the matrix and the precipitate caused by the high plastic mismatch (σNiAl/σCu).

    更新日期:2017-09-19
  • An improved viscoplastic constitutive model and its application to creep behavior of turbine blade
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-18
    Chan Wang, Duoqi Shi, Xiaoguang Yang, Shaolin Li, Chengli Dong

    This paper improved Chaboche viscoplastic constitutive model in order to simulate the mechanical behavior of engineering components during service. The anisotropic tensor and Kachanov damage evolution equation were added to the constitutive equation to simulate the creep behavior of anisotropic material. Based on that, the Levenberg-Marquardt nonlinear optimization algorithm was used to obtain material parameters and the constitutive model was compiled as the subprogram of finite element software ABAQUS for its simulation application. Then the improved constitutive model was used to simulate and analyse the uniaxial tensile and creep behavior of specimens made of anisotropic material DZ125 at different loads, and the accuracy and feasibility of material parameters and constitutive model were verified by corresponding test results. Finally, the improved constitutive model was used to simulate the creep behavior of a hollow turbine blade, achieving its engineering application. And the calculated results have great significance for structure design and life prediction of turbine blade.

    更新日期:2017-09-19
  • The creep deformation and fracture behaviors of nickel-base superalloy M951G at 900 °C
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-18
    Luqing Cui, Huhu Su, Jinjiang Yu, Jinlai Liu, Tao Jin, Xiaofeng Sun

    The creep behaviors of M951G alloy were carried out under stress ranging from 240 MPa to 400 MPa at 900 ℃, and the corresponding deformation mechanisms and fracture behaviors after rupture had been investigated by various techniques. Results showed that both the deformation mechanisms and fracture behaviors were dependent on the applied stress. According to the transmission electron microscope (TEM) observations, the dominant deformation mechanism changed from a combined process of slip and climb of dislocations in matrix channel to shearing of dislocations in γ′ precipitates and cross-slip of dislocations in matrix channel with the applied stress increasing. Fracture behaviors of M951G alloy were characterized using optical microscope (OM) and scanning electron microscope (SEM), which changed from intergranular to transgranular with the increase of applied stress. At low applied stress, M951G alloy was failure in the form of intergranular owing to coalescence of micropores along the grain boundaries. However, at higher applied stress the microcracks initiated at broken carbides in the grain interior, and finally resulted in transgranular fracture. Additionally, creep strain rate also played a key role in determining the transition of creep fracture modes by effect the corresponding temperature of equal strength for grain boundary and grain interior. The values of apparent stress exponent at low and high stress regions were calculated to be 5.14 and 11.13 respectively, which was due to the change of deformation mechanisms and fracture modes with the increase of applied stress.

    更新日期:2017-09-19
  • Crystallographic features of α variants and β phase for Ti-6Al-4V alloy fabricated by selective laser melting
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-18
    Y. Yang, Y.J. liu, J. Chen, H.L. Wang, Z.Q. Zhang, Y.J. Lu, S.Q. Wu, J.X. Lin

    The present study investigated the crystallographic features of α variants and β phase for Ti-6Al-4V alloy fabricated by selective laser melting. The <100>β fiber texture parallel to the building direction was ascertained on the basis of a reconstruction method realized by the manipulation of stereographic projection. The SLMed alloy has no α/α′ variants selection but contains a special crystallographic area exhibiting random orientation which cannot be reconstructed as a parental columnar β grain with the present introduced method due to its nature as the overlapped area between adjacent melt pools resulting from the heterogeneous nucleation in front of the liquid-solid interface. With increasing the heat treated temperature, α variants selection occurs. Especially at higher temperature of 905 °C, the intergranular β phase following a reversed crystallographic path as “parental β phase”→“α variants”→“intergranular β phase” would be precipitated, therefore the intergranular β phase keeps the same orientation with the parental β phase. Once the alloy was heat treated at 975 °C close to Tβ, the microstructure is characterized by primary α variants selection and a large amount of secondary α widmanstatten structure with a homogeneous orientation which accounts for the lowest tensile strength. The decomposition of twelve α variants proved the BOR <110>β//(0001)α, <111>β//<2-1-10>α. The misorientation between two variants sharing a common parental <100>β pole with a similar color consisting of all Euler angles was identified to be [0001] α/10.53°.

    更新日期:2017-09-19
  • Low cycle fatigue behaviors of pure Mo and Mo-La2O3 alloys
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-18
    P.M. Cheng, Z.J. Zhang, G.J. Zhang, J.Y. Zhang, K. Wu, G. Liu, W. Fu, J. Sun

    Pure molybdenum (PM) and La2O3 dispersion strengthened Mo alloy (ODS-Mo) were prepared by powder metallurgy through two different mixing method (solid-solid or SS mixing and solid-liquid or SL mixing). After annealed at 1050 °C for 1 h, the PM, SS-Mo and SL-Mo were compared in microstructure and mechanical properties. The microstructural examinations showed that the PM was mostly recrystallized with rather coarse grains, but the ODS-Mo alloys remained fine elongated grains. This discrepancy is due to a higher recrystallization temperature held in the ODS-Mo alloys. The uniaxial testing results showed that the ODS-Mo had higher tensile mechanical properties compared to the PM, which are attributed to the remarkable strengthening and ductilizing effect induced by the La2O3 particles. The low cycle fatigue (LCF) testing results revealed that the PM experienced cyclic hardening behaviors, while the ODS-Mo alloys exhibited cyclic softening behaviors. The Basquin-Manson-Coffin analyses results demonstrated that the ODS-Mo possessed a higher fatigue ductility and longer fatigue life than the PM. Two kinds of cracks, i.e., cleavage cracks and intergranular cracks, were experimentally observed to coexist on the fracture surface. In particular in the SL-Mo alloy, the two crack interacted and propagated forward until final fracture, leading to a step-like crack growth path and concomitantly enhanced fatigue ductility. In addition, different cycling dislocation structures were revealed between the PM and ODS-Mo alloys. Cycle fatigue mechanisms responsible for the fatigue behaviors and microstructure evolution during LCF testing were discussed. The best LCF resistance combined with superior uniaxial tensile mechanical properties found in the SL-Mo alloy were rationalized.

    更新日期:2017-09-19
  • Effect of Surface Etching on the Tensile Behavior of Coarse- and Ultrafine-Grained Pure Titanium
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-18
    Seung Mi Baek, Alexander V. Polyakov, Ji Hyun Moon, Irina P. Semenova, Ruslan Z. Valiev, Hyoung Seop Kim

    For metallic biomaterials, surface treatment is essential for improving osseointegration. Acid etching is the basic method used for removing undesirable contaminants and increasing surface roughness. Although the obvious effect of the etching process is a change in surface characteristics, interestingly, surface etching can also affect the tensile properties of the material. In this paper, the influence of etching on the tensile behavior of pure titanium was investigated. The surface morphology and tensile properties of coarse- and ultrafine-grained pure titanium specimens were examined after hydrofluoric acid etching, in terms of the etching time. The ultrafine-grained titanium was prepared using conform equal-channel angular pressing to enhance the mechanical properties of the pure titanium. Micro-scale pores were observed, and tensile strength was decreased in the 1 min etched coarse-grained titanium. The 20 min etched coarse-grained specimen and the ultrafine-grained specimen with relatively homogeneous pore structure maintained or slightly increased their tensile strengths. These results provide a new foundation for the production of reliable metallic biomaterials.

    更新日期:2017-09-19
  • High strength-toughness combination of a low-carbon medium-manganese steel plate with laminated microstructure and retained austenite
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-18
    Y. Zou, Y.B. Xu, Z.P. Hu, S.Q. Chen, D.T. Han, R.D.K. Misra, G.Z. Wang

    Three different grain structures of low-carbon medium-manganese steel were prepared through appropriate controlled rolling process. The laminated microstructure with a strong <110>//rolling direction (RD) fiber texture was characterized by ultra-fine elongated ferrite, retained austenite and martensite phase arranged alternately along the RD. The steel with equiaxed grain structure exhibited a relatively low tensile strength of 960 MPa and an extremely poor low-temperature toughness of ~8 J at −196 °C. An enhanced upper shelf energy (> 450 J) and low-temperature toughness (~105 J at −196 °C), as well as an improved tensile strength (1145 MPa) was obtained in the steel with laminated microstructure. The laminated microstructure enabled the steel to be significantly stronger and tougher along the RD, which contributed to the high tensile strength to some extent. It is concluded that the combined effect of the ultra-fine elongated laminated microstructure, the possible interface decohesion and the existence of numerous {001} cleavage planes resulted in the occurrence of delamination. The delamination fracture enhanced the upper shelf energy mainly by promoting crack branching along the RD and thus suppressing crack propagation along the v-notch direction, which finally resulted in greater plastic deformation and significant increase in absorbed energy. Besides delamination toughening, transformation-induced plasticity (TRIP) effect of metastable retained austenite is believed to be responsible for the high cryogenic toughness, which can release stress concentration of crack tips and thus blunting cracks propagation.

    更新日期:2017-09-19
  • Effects of Cryogenic Treatment on Mechanical Properties and Micro-structures of IN718 Super-alloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-18
    Jing Li, Jianzhong Zhou, Suqiang Xu, Jie Sheng, Shu Huang, Yunhui Sun, Qi Sun, Emmanuel Agyenim Boateng

    The main purpose of this study is to investigate the mechanical properties and micro-structures of IN718 super-alloy subjected to Cryogenic Treatment (CT). Tensile tests were performed and their engineering stress-strain curves were examined. Fracture morphologies of samples were observed with the help of Scanning Electron Microscopy (SEM). The micro-structural evolution of IN718 super-alloy before and after CT was investigated by both Optic Microscope (OM) and SEM. The composition of the precipitated phase was detected by Energy Dispersive Spectrometer (EDS). In order to better analyze the evolution of micro-structures, Transmission Electron Microscopy (TEM) observation was also carried out. Additionally, XRD technique was used to analyze the surface residual stress and microstructural phase. The results show that after two cycles of CT (CT-2), the tensile properties of IN718 super-alloy were found to be substantially improved at room temperature. Also, in comparison to the ductile fracture mode with very small dimples and some quasi-cleavage planes of untreated specimen,CT-2 specimen exhibited a more ductile fracture mode. The grains of CT-2 sample were significantly refined and the size became much more uniform. Compared to untreated specimen, the quantity of precipitated phases at grain boundaries was increased while the size of the precipitates was smaller and the distribution was more uniform after CT-2. High density dislocations were found to pile up at the grain boundaries. The occurrence of this phenomenon is expected to be explained as a result of micro plastic deformation and the increase of internal stress induced by volume shrinkage during the CT process.

    更新日期:2017-09-19
  • Implications of dynamic strain aging under LCF-HCF interactions in a type 316LN stainless steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-18
    Aritra Sarkar, A. Nagesha, P. Parameswaran, R. Sandhya, M. Okazaki

    Influence of dynamic strain aging (DSA) under sequential low cycle fatigue (LCF) and high cycle fatigue (HCF) loading was investigated by conducting HCF tests on specimens subjected to prior LCF over a wide range of temperature from 573–923 K. DSA was found to be pronounced at 823 K depending on the magnitude of the stress employed under HCF cycling. DSA was seen to have contrasting implications under LCF and HCF deformation resulting in an anomalous fatigue behavior in terms of remnant HCF life under LCF-HCF interaction. LCF-HCF interaction was found to be pronounced at intermediate levels of prior LCF exposure, where the remnant HCF life is dictated by competitive damage mechanism resulting from the influence of DSA under LCF as well as HCF. Detailed fracture surface examination revealed that extensive hardening associated with DSA leads to an extended zone of faceted appearance with river markings (Stage-I crack) under HCF cycling (with or without LCF exposure). This reduces the crack growth rate, delaying the transition of crack from Stage-I to Stage-II, thereby leading to an extension of life in such cases. On the other hand, a highly striated fracture surface indicating a quick transition in crack from Stage-I to Stage-II, was observed for loading conditions with minimal or no influence of DSA, thus leading to lower life compared to the previous case.

    更新日期:2017-09-19
  • Influence of squeeze casting pressure and heat treatment on microstructure and mechanical properties of Mg94Ni2Y4 alloy with LPSO structure
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-17
    Liangyan Hao, Xiong Yang, Shulin Lü, Xiaogang Fang, Shusen Wu

    Ternary Mg94Ni2Y4 (at.%) alloy was prepared by conventional gravity casting or direct squeezing casting and then subjected to T6 heat treatment. The influence of squeeze casting pressure and heat treatment on microstructure evolution and mechanical properties of Mg94Ni2Y4 alloy containing long period stacking ordered (LPSO) structure was studied. The results show that gravity cast Mg94Ni2Y4 alloy consisted of primary α-Mg, 18 R LPSO and eutectic structure containing α-Mg, LPSO and Mg2Ni phases. The application of pressure cannot change phase constituents and the type of LPSO phase. With the increase of squeeze casting pressure, microstructure was refined, the volume fraction of LPSO was decreased while that of eutectic structure was increased. Under the highest pressure of 320 MPa, kinking of LPSO phase was induced and the best mechanical properties were obtained, with the ultimate tensile strength of 250 MPa, the yield strength of 140 MPa and the elongation of 8%. They were improved by 20.2%, 22.8% and 45.5%, respectively, than those of gravity cast Mg94Ni2Y4. After heat treatment, bulk-shaped LPSO phase transformed into rod-shaped one, but no lamellar 14 H LPSO was precipitated in the matrix. Besides, the application of squeeze casting pressure prevented LPSO from delaminating during heat treatment.

    更新日期:2017-09-19
  • High strain rate deformation of ARMOX 500T and effects on texture development using neutron diffraction techniques and SHPB testing
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-17
    Michael Saleh, Muhammad M. Kariem, Vladimir Luzin, Karl Toppler, Huijun Li, Dong Ruan

    The authors evaluated the crystallographic texture, defined as the distribution of orientation of crystals (or grains), to gauge the deformation and microstructural evolution of ARMOX 500 T armour plates at elevated strain rates. Using neutron diffraction, the authors examined a number of specimens deformed at room temperature and high strain rates and contrasted these with equivalent samples deformed quasi-statically. Since crystallographic texture can play a part in the armour's ballistic response the authors were able to observe a rate dependent textural development, with the strengthening of the rolling α-fibre. The hot rolling process used in the manufacture of these steels leads to a through thickness texture variation that leads to an asymmetric transitional texture in the strain regime (1–2%) but with increased strain a symmetric texture develops irrespective of the strain rate, albeit with different intensities. By extending the testing program the authors were also able to deduce the strength parameters for the Johnson-Cook model through split Hopkinson pressure bar testing at high strain rates (1000–3000 s−1) and elevated temperatures (20–600 °C). The results, when compared with existing literature, show deviations in the strain rate sensitives of the tested specimens and, subsequently, variations in the computed flow stress parameters.

    更新日期:2017-09-19
  • Microstructural characterization, strengthening and toughening mechanisms of a quenched and tempered steel: Effect of heat treatment parameters
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-17
    Bo Jiang, Meng Wu, Mai Zhang, Fan Zhao, Zhigang Zhao, Yazheng Liu

    A quenched and tempered steel for a large bearing ring was investigated. The heat treatment experiments were designed by using the L9 (34) type orthogonal form. Based on these conditions, a better combination of mechanical properties was obtained. The results showed that the quenching and the tempering temperatures were the most influential factors on the strength and toughness. The dislocation strengthening and the solid solution strengthening of the dissolved alloying carbides are the main mechanisms of increasing the strength by decreasing the tempering temperature and increasing the quenching temperature, respectively. The stripped carbides and long chain carbides strongly influence the differences in the tensile strength of the steels under different processes. The toughness AKv at −20 °C was increased by 42.2 J when the quenching temperature increased from 800 to 900 °C. The stripped undissolved carbides at lower quenching temperature promoted crack propagation and cleavage fracture and thus decreased the toughness of the steel. The AKv was increased by 47.4 J when the tempering temperature increased from 550 to 650 °C. The long chain carbides distributed along the grain boundary and the martensitic laths with a high density of dislocations at the lower tempering temperature decreased the toughness. Oil quenching can improve both the strength and toughness by refining the martensitic microstructure.

    更新日期:2017-09-19
  • Fatigue behavior of a harmonic structure designed austenitic stainless steel under uniaxial stress loading
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-17
    Zhe Zhang, Hantuo Ma, Ruixiao Zheng, Qian Hu, Masashi Nakatani, Mie Ota, Gang Chen, Xu Chen, Chaoli Ma, Kei Ameyama

    Harmonic structured materials present a good balance of high strength and high ductility due to their peculiar network structure topology. Since long-term durability is critical for their practical applications, so the present work investigated the fatigue properties of a harmonic structured austenitic stainless steel at room temperature under uniaxial stress loading. The harmonic structure designed SUS316L steels were prepared by mechanical milling and subsequent hot isostatic pressing. The enhanced tensile strength in the harmonic structured SUS316L steels was attributed to the ultrafine grains (shell region), which also resulted in the improved resistance to fatigue crack initiation during cyclic loading. Compared to the conventional SUS316L bulk, increased fatigue limit can be achieved in the harmonic structured SUS316L steels. However, the fatigue ratio tends to be a constant value in SUS316L steels having homogeneous grain structure or bimodal grain structure. Moreover, the change of grain size was not significant after fatigue, which demonstrated the harmonic structured SUS316L steels showed good cyclic stability. In addition, the fatigue cracks tended to initiate at core/shell surface due to strain localization. The harmonic structure designed stainless steel demonstrates great attraction for commercial applications due to its good combination of high yield strength, large uniform elongation, good fatigue resistance and cyclic stability.

    更新日期:2017-09-19
  • Tensile Property and Microstructure of Fe-22Mn-0.5C TWIP Steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-17
    Peng Lan, Jiaquan Zhang

    The tensile property and microstructure evolution of Fe-22Mn-0.5 C TWIP steel under different annealing conditions was experimentally investigated. The matrix phase of this steel after annealing and tensile test is single austenite. The (111) peak is the most pronounced under as annealed condition, however, its intensity decreases significantly after stretched to fracture where (220) peak becomes the most dominant. The yield strength and tensile strength of Fe-22Mn-0.5 C TWIP steel mainly decrease as annealing temperature increases, while the total elongation largely increases. The variation in the product of tensile strength and total elongation against temperature is related to the annealing time. The most obvious change in strength and elongation is located between 750–850 °C, due to the grain coarsening in the recrystallized matrix. The grain size and density of annealing twins in Fe-22Mn-0.5 C TWIP steel increases with increasing anneal temperature. The recrystallized grains grow obviously as annealing temperature and time increase. The influence of annealing time changing by 15 min on grain growth is not as obvious as temperature changing by 100 °C under the current condition. The tensile strength of Fe-22Mn-0.5 C TWIP steel decreases as grain size increases while the total elongation increases. The variation in strength and elongation against the square root of grain size follows Hall-Petch's law well. In order to get a good combination of strength and ductility, the grain size between 10–30 μm for TWIP steel is preferentially suggested, within which the product of strength and ductility can be easily adjusted to higher than 65 GPa%. High strength and ductility can be obtained in full-recrystallized Fe-22Mn-0.5 C TWIP steel with fine-grained matrix. The high strength TWIP steel with tensile strength above 1030 MPa and total elongation about 60% can be prepared by annealing at 650–750 °C for 15–20 min. The high ductility TWIP steel with tensile strength around 900 MPa and total elongation beyond 70% can be manufactured by annealing at 850–950 °C for 5–10 min.

    更新日期:2017-09-19
  • Effect of artificial aging on the Cu-Mg co-clustering and mechanical behavior in a pre-strained Al-Cu-Mg alloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-15
    Puyou Ying, Zhiyi Liu, Song Bai, Jian Wang, Junlin Li, Meng Liu, Linyan Xia

    In the present work, the effect of artificial aging on the Cu-Mg co-clustering and mechanical behavior in a pre-strained Al-Cu-Mg alloy are investigated by tensile and fatigue testing, X-ray diffraction (XRD), transmission electron microscope (TEM) and atom probe tomography (APT). Results show that in the pre-strained samples, sample with artificial aging (170 °C/30 min) possesses unchanged strength and higher elongation in comparison with naturally aged sample. This is due to the greater strengthening effect caused by cluster hardening but the weaker strengthening effect caused by strain hardening in 170 °C/30 min sample. The fatigue crack propagation (FCP) resistance of 170 °C/30 min sample is higher than that of naturally aged sample. APT analysis indicates that artificial aging remarkably increases Cu-Mg co-cluster size, which leads to a greater critical shear stress for dislocation motion. This undoubtedly enhances fatigue crack closure effect and FCP resistance. Meanwhile, artificial aging reduces the dislocation density, which enhances FCP resistance as well. Compared to the sample without pre-strain, the pre-strained samples exhibit a higher FCP rate as a high density dislocation favors a detrimental effect on FCP resistance.

    更新日期:2017-09-15
  • Strain-rate Effect on Work-hardening Behavior in β-type Ti-10Mo-1Fe Alloy with TWIP Effect
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-15
    Xin Ji, Satoshi Emura, Xiaohua Min, Koichi Tsuchiya

    The strain rate effect (2.8×10−5 ~ 2.8×10−1 s−1) on the tensile properties and microstructure evolution of a β-type Ti-10Mo-1Fe (wt.%) alloy has been investigated. With increasing strain rate, the yield strength increased, while the ultimate tensile strength, total elongation and uniform elongation decreased. It was found that deformation at a lower strain rate led to an enhanced work hardening rate (θ). This is reflected in the decreasing strain rate sensitivity of flow stress, m, with increasing strain. Strain rate sensitivity was positive at a smaller strain level (< 0.13), and decreased to a negative at a larger strain. The strain rate dependence of work-hardening behavior has been investigated and discussed in terms of the microstructure evolution, such as {332}<113> twins and dislocations. Electron Backscattered Diffraction (EBSD) and X-ray diffraction (XRD) analyses revealed lower increasing rates of {332}<113> twins and dislocation density at higher strain rates, which may be caused by adiabatic heating. This may lead to the reduced work-hardening rate on deformation at the higher strain rates.

    更新日期:2017-09-15
  • Effect of SiC particles and the particulate size on the hot deformation and processing map of AZ91 magnesium matrix composites
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-15
    Li Zhang, Qudong Wang, Guoping Liu, Wei Guo, Haiyan Jiang, Wenjiang Ding

    Isothermal hot compression at the temperature range of 573~698 K and strain rates of 0.005~1.0 s−1 was used to investigate the flow behavior and processing characteristics of the nano-SiCp/AZ91 composites. Effects of the incorporated particles and their particulate size on the workability of the base alloy were then compared and discussed. Results show that compared with the monolithic AZ91 alloy, the incorporated nano-SiC particles effectively increase the flow stress of the composites by blocking the strain-induced dislocations, while effect of the micro-SiC particles varies due to the competition between pinning effect and particle stimulating nucleation (PSN) mechanism. Three domains of peak energy dissipation efficiency are identified in the processing map and the corresponding microstructures examined by EBSD indicate that continuous dynamic recrystallization (DRX) occurs during the compression. The instability characteristics at low temperature are severe mechanical twinning and micro-cracks, while that at high temperature is intergranular cracking. The incorporation of SiC particles enhances the high temperature (>655 K) workability of AZ91 by increasing the upper limit of the processing strain rate and enables low temperature processing by decreasing the lower limit of the temperature. However, the added particles impose a side effect by enlarging the instability domain of the base alloy to a lower strain rate and even higher temperature.

    更新日期:2017-09-15
  • High-temperature low-cycle fatigue behavior of a 9Cr-ODS steel: Part 1 - pure fatigue, microstructure evolution and damage characteristics
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-15
    Ankur Chauhan, Jan Hoffmann, Dimitri Litvinov, Jarir Aktaa

    Low cycle fatigue (LCF) behavior of a tempered martensitic Fe-9%Cr-based oxide dispersion strengthened (ODS) steel at elevated temperatures is correlated to the microstructural evolution after cyclic straining. The fully reversed strain-controlled tests were conducted in air at 550 °C and 650 °C for different strain amplitude values ranging from ± 0.4% to ± 0.9%. Apart from the higher cyclic stress levels, the steel manifests complex cyclic softening which is significantly lower in comparison to that observed for similar non-ODS steels. This is due to the fact that highly stable nano-oxide particles act as persistent barriers for dislocation motion which as a consequence slows down the typical microstructural evolution realized in the conventional non-ODS steels. Upon cycling at 550 °C, microstructure exhibits only minor changes. The main modifications are in respect to the dislocations rearrangement and/or annihilation which finally result in their reduced density. At 650 °C, microstructural evolution hastens and becomes prominent mainly in nano-oxides/carbides deficient regions. Here, in addition to the reduced dislocation density, partially eliminated original sub-grain structures, grain growth, M23C6 carbides coarsening and Cr-W enriched Laves phase precipitation were evident. The microstructural modifications, concerning dislocation density and sub-grain structures, intensify even further with increase in applied strain amplitude. Nevertheless, annealing at 650 °C for similar duration has no major influence on microstructure. Damage studies revealed expeditious as well as pronounced damage with increase in applied strain amplitude. Cracks initiation, eventuates at the early stage of the test, and their propagation were further assisted by oxidation. The stable crack growth region manifests secondary cracks, and at higher magnification the classical fine-scale transgranular ductile fatigue fracture features called striations. In addition, at 650 °C, crack path also acquires an intergranular tendency under higher strain amplitude.

    更新日期:2017-09-15
  • Effects of overaging on microstructure and tensile properties of the 2055 Al-Cu-Li-Ag alloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-14
    E. Balducci, L. Ceschini, S. Messieri, S. Wenner, R. Holmestad

    The lightweight, unconventional 2055 Al-Cu-Li-Ag alloy exhibits an excellent specific strength in the T83 state, but no literature reports the effects of overaging on this alloy. In the present work, the suitability of the alloy for lightweight components operating at high temperature is evaluated. Thermal exposure in the range 215 – 305 °C was investigated, highlighting its consequences on both microstructure and mechanical properties. In the most severe overaging state (24 h at 305 °C), the typical T1 precipitates (Al2CuLi) are dissolved, leading to the formation and coarsening of ϑ’ and Ω phases. In all overaging conditions, the alloy performance was superior or at least comparable to that of another third generation Al-Li alloy, AA2099, which is characterised by a slightly lower density and encouraging mechanical properties for high temperature applications. Compared to AA2099, the AA2055 alloy provides a higher specific strength (the basic requirement for mass savings) both in the T83 and in the most severe overaging state (24 h at 305 °C). This work highlights that AA2055 is a promising candidate for lightweight components operating up to 305 °C, and it lays the basis for high temperature tests of the alloy.

    更新日期:2017-09-15
  • Fracture behavior of nanostructured heavily cold drawn pearlitic steel wires before and after annealing
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-14
    B.N. Jaya, S. Goto, G. Richter, C. Kirchlechner, G. Dehm

    In situ micro-cantilever fracture testing is used to demonstrate changes in fracture behavior of nanostructured, heavily cold drawn pearlitic steel wires as a function of drawing strain and annealing conditions. It is shown that these steels exhibit a sharp transition in fracture behavior between a drawing strain of 320 and 520% with a drop in fracture toughness from 7.5 to 4 MPam1/2. This is confirmed from the nature of fracture which is stable with some degree of plasticity at drawing strains below 320% and changes to catastrophic cleavage fracture at drawing strains of 420% and above. This transition and associated brittleness is attributed to structural (cementite decomposition and strain induced increase in tetragonality) and microstructural (increasing nanocrystallinity and dislocation density) evolution that these steels undergo at higher drawing strains. On heat treating the 420% strained sample, brittle cleavage fracture continues for low temperature (200 °C) annealing with no visible changes in microstructure, while crack growth is suppressed and large-scale plasticity is recovered for high temperature (500 °C) annealing with accompanying grain coarsening, and re-precipitation of spherodized cementite at grain boundaries.

    更新日期:2017-09-15
  • Effect of long term aging on the fatigue crack propagation in the β titanium alloy Ti 17
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-14
    Sasaki Layla, Henaff Gilbert, Arzaghi Mandana, Villechaise Patrick, Delfosse Jérôme

    This work aims to evaluate the impact of a prolonged thermal aging on the fatigue crack growth properties of a commercial Ti 17 (Ti-5AI-2Sn-2Zr-4Mo-4Cr) titanium alloy. Aging was operated at 450 °C for extended times, of 1000 h and 10 000 h respectively. In terms of room-temperature tensile properties, aging is mainly responsible for a strain hardening of the material and a decrease in ductility, the total elongation dropping from 5.1% in the as-received state to 1.5% after 10 000 h. A significant increase in crack propagation rates at room temperature for R=0.1 is observed, emphasizing an embrittling effect of aging. This last effect is however only noticed beyond a critical value of ΔK (ΔKcr), which decreases with increasing aging times. It is observed that the effect of long-term aging on the mechanical properties is more pronounced during the early stage of the aging process, and tends to saturate for longer times. Tests with different load ratio of R=0.4 and 0.7 were performed to gain a better understanding of the occurrence of these ΔKcr values. The results indicate that the increase in propagation rates is actually governed by the static Kmax component of the fatigue loading. Fractographical analysis of the failed specimens reveal that aging affects the material resistance at a very fine scale, namely the α lamellae/β matrix interfaces. The detrimental role played by both grain boundary α phase and precipitate-free zones on the fatigue crack growth resistance of the material were finally discussed.

    更新日期:2017-09-15
  • Competitive twinning behavior in magnesium and its impact on recrystallization and texture formation
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-14
    I. Basu, T. Al-Samman

    Rolled pure Mg and Mg-1wt.% Gd alloy were subjected to room temperature in-plane compression along the rolling direction, followed by isochronal annealing treatments for 1 hour. The results of deformation texture and microstructure showed substantial differences due to rare earth alloying. In spite of imposed c-axis extension during deformation, the Mg-1Gd alloy retained the initial texture with the majority of basal poles concentrated near the longitudinal direction of the used channel-die tool. Electron back scatter diffraction analysis of the deformation microstructure revealed a predominance of { View the MathML source 10 1 ̅ 1 } compression and { View the MathML source 10 1 ̅ 1 }-{ View the MathML source 10 1 ̅ 2 } double twins relative to coexisting { View the MathML source 10 1 ̅ 2 } tension twins. This behavior was significantly contrasting in comparison with that of pure Mg, wherein first and second generation { View the MathML source 10 1 ̅ 2 } tension twins were observed in profuse quantities. Continuous dynamic recrystallization took place inside compression and double twins by means of slip assisted subgrain rotation about the [0001] axis giving rise to a sharp prismatic fiber of recrystallized orientations. This fiber was transformed into a randomized texture pattern during subsequent static recrystallization and grain growth due to a different discontinuous recrystallization mechanism. This resulted in a significant annealing texture weakening and an increase of the overall Schmid factor for basal slip.

    更新日期:2017-09-15
  • One-step quenching and partitioning treatment of a commercial low silicon boron steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-14
    H. Kong, Q. Chao, M.H. Cai, E.J. Pavlina, B. Rolfe, P.D. Hodgson, H. Beladi

    In the current study, the microstructure evolution and mechanical behaviour in a commercial low silicon boron steel were investigated after one-step quenching and partitioning (Q&P) treatment under different isothermal temperatures in the range 260–320 °C. This processing approach resulted in a complex microstructure consisting of a tempered martensite matrix with bainitic ferrite, fresh martensite, and retained austenite, offering a superior combination of mechanical properties compared to the directly water-quenched condition, where a fully martensitic structure was exhibited. The Q&P processed steel isothermally held at a high temperature of 320 °C showed lower work-hardening, which was mainly associated with the lower fraction of initially formed martensite and the corresponding tempering responses, compared to the steels held at lower temperatures. The phase transformation behaviour throughout the heat treatment was studied by dilatometry, regarding the martensitic transformation upon initial cooling, carbon partitioning combined with the martensite tempering and austenite decomposition during isothermal holding, as well as the fresh martensite formation on the final cooling. Additionally, the microstructure characteristics were examined using X-ray diffraction, scanning electron microscope, and electron backscatter diffraction techniques.

    更新日期:2017-09-15
  • Tension-Compression Asymmetry in Yielding and Strain Hardening Behavior of CP-Ti at Room Temperature
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-14
    Peng Lin, Yonggang Hao, Baoyou Zhang, Shuzhi Zhang, Chengzhong Chi, Jun Shen

    The tension-compression asymmetry including the yielding and strain hardening asymmetry of a commercially pure titanium (CP-Ti) at room temperature was studied using uniaxial tensile and compressive tests. The deformation twinning modes responsible for the tension-compression asymmetry were analyzed using the electron back scattered diffraction technology. The CP-Ti exhibits a strong tension-compression asymmetry in yielding and strain hardening. The prismatic slip is the most easiest to activate either for tension or compression. The deformation twinning mode can be characterized by the secondary View the MathML source { 10 1 ¯ 2 } twin variants in primary View the MathML source { 11 2 ¯ 2 } twins for tension and the secondary View the MathML source { 11 2 ¯ 2 } twin variants in primary View the MathML source { 10 1 ¯ 2 } twins for compression. The secondary twin variants are the dominated twinning modes whether for tension or compression, determining the final yielding behavior of the titanium. The stress required to activate the secondary extension twins in tension is larger than that to activate the secondary contraction twins in compression, leading to the tension-compression asymmetry in yielding. The strain hardening enhancement of the CP-Ti is a result of the combination of the Hall-Petch hardening by grain refinement and texture hardening by lattice re-orientation both resulting from the deformation twinning. The stronger effect of these two hardening mechanisms on compression than on tension leads to the tension-compression asymmetry in strain hardening.

  • Effect of Contact Pressure and Stress Ratio on the Fretting Fatigue Behaviour of Ti-6Al-4V
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-14
    Virendra Kumar Verma, Hamza Naseem, S. Ganesh Sundara Raman, H. Murthy, Anuradha Nayak Majila, D. Chandru Fernando

    Fretting fatigue behaviour of Ti-6Al-4V was studied at two different contact pressures (150 MPa and 300 MPa) and two different stress ratios (0.1 and 0.7). The variation of frictional force and tangential force coefficient (TFC) with the number of fretting cycles was studied to understand the fretting fatigue behaviour of the alloy. As evidenced by the variation of frictional force with time as well as frictional force versus cyclic load hysteresis-type plots, gross sliding was present at lower contact pressures and lower stress ratios, leading to higher TFC values and surface roughness. Oxide particles were detected in the slip region and in the initiated fretting cracks indicating fretting debris. While the effect of increasing contact pressure in increasing the fretting fatigue life was clearly seen at higher stress ratio, it was not observed at lower stress ratio. At both contact pressures, a significant increase in life with an increase in stress ratio was noticed. The contact problem was analysed using the existing numerical tools to obtain the contact stresses. From these stresses, fretting fatigue lives were estimated as a sum of initiation lives estimated from multi-axial fatigue parameters and propagation lives estimated from fracture mechanics approach. A good agreement while using Findley parameter indicates that the adopted procedure can effectively account for the effects of contact pressure as well as stress ratio.

    更新日期:2017-09-14
  • Structural modifications in sub-Tg annealed CuZr-based metallic glass
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-14
    B. Sarac, A. Bernasconi, J. Wright, M. Stoica, F. Spieckermann, M. Mühlbacher, J. Keckes, X. Bian, G. Wang, J. Eckert

    Short term sub-glass transition annealing controls the structural reordering and nanocrystallization of CuZr− CuZr − based bulk metallic glasses (BMGs). Using this method, 5 times higher resistance against strain softening as confirmed by three-point bending tests is attained. Homogenously dispersed nanoparticles in sizes of 20 − 50 nm accounts for the remarkable shifts and peak formations on reciprocal space together with the diffraction spots observed on 2D diffraction pattern. Real-space pair distribution function analysis reveals noticeable changes in peak shapes and positions correlated with the changes in short- to medium-range ordering. The differences in the partial coordination numbers upon annealing mark the dominant Zr−Zr Zr − Zr pair to diminish whereas the Cu−Cu Cu − Cu pair becomes prominent, hinting that Cu Cu diffusion is mainly responsible for structural reordering and formation of new phases. The pursued study using hard X-ray synchrotron radiation reveals important aspects of structural changes preceding nanocrystallization, which in turn enhances the mechanical performance in BMGs.

    更新日期:2017-09-14
  • Effect of MgO on the Physical, Mechanical and Microstructural Properties of ZTA-TiO2 Composites
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-14
    Abdullah Al Mahmood, M.A. Gafur, M.E. Hoque

    Ceramic composites of alumina (Al2O3), 3 mol% yittria stabilized zirconia (ZrO2), Magnesia (MgO) and Titania (TiO2) were produced and different physical and mechanical properties were characterized. This multicomponent toughened Al2O3 was also characterized by observing XRD and SEM. The weight percentages of ZrO2 (15 wt%) and TiO2 (0.5 wt%) were constant where the amount of MgO varied from 0–1.5 wt%. The batch compositions were properly weighed, dried, mixed with polyvinyle alcohol binder, again dried and pressed into 10 mm diameter shaped samples by uniaxial pressure of 160 MPa. The prepared green samples were sintered at 1450 °C, 1520 °C and 1590 °C for 5 hours under pressureless condition. The effect of MgO addition and variation of sintering temperature was observed on the density, porosity, hardness, fracture toughness and diametral tensile strength of the sintered pellets. Hardness and fracture toughness were calculated by indentation technique. SEM and XRD were used to characterize the microstructure and phases present in the sintered ceramic composites. Results show that, the addition of MgO upto 1% influences all the physical and mechanical properties to improve, where further addition of MgO shows no fruitful effect. From the phase analysis and rietveld analysis one extra phase of spinel (MgAl2O4) founded when MgO added more than 0.5%. The perfect sintering temperature was found 1520 °C. Further increasing the sintering temperature causes structural imperfection of the composites and the physical and mechanical properties diminished at the sintering temperature at about 1600°C.

    更新日期:2017-09-14
  • Synchrotron and neural network analysis of the influence of composition and heat treatment on the rolling contact fatigue of hypereutectoid pearlitic steels
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-14
    W. Solano-Alvarez, M.J. Peet, E.J. Pickering, J. Jaiswal, A. Bevan, H.K.D.H. Bhadeshia

    A series of experimental hypereutectoid pearlitic steels were tested under rolling contact sliding conditions using a lubricated twin-disc setup to study the influence of different chemical compositions and heat treatments on rolling contact fatigue life. Tested samples were then characterised using microscopy and synchrotron measurements as a function of depth from the contact surface. Results, analysed through neural networks, indicate that the most influential factor in lengthening the number of cycles to crack initiation of hypereutectoid steels is hardness, attained by increasing the cooling rate from the hot rolling temperature, but adequate alloying additions can enhance it further. The harder, fast-cooled samples displayed less plastic flow at the surface than the softer slow-cooled ones. With regard to chemical composition, silicon was found to strengthen the ferrite thus reducing strain incompatibilities with the cementite, preventing in this way the fragmentation and eventual dissolution of the lamellae. This is beneficial since larger depths of cementite dissolution were found in samples with lower cycles to crack initiation for a given cooling rate (hardness). Samples containing vanadium lasted longer and displayed less plastic deformation at the surface than those without, at a similar hardness.

    更新日期:2017-09-14
  • Dynamic nano precipitation behavior of as-cast Mg-4Li-4Zn-Y alloy during high temperature deformation
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-13
    Yucheng Zhou, Zhaoyun Chen, Jinghan Ji, Zhijie Sun

    Dynamic nano precipitation behavior of as-cast Mg-4Li-4Zn-Y alloy during high temperature compression at 450 ℃ was investigated in this paper. The grains were coarse, interdendritic W-Mg3Y2Zn3 mainly existed at grain boundaries and MgZn2 mainly existed in grains in the as-cast alloy. After hot deformation at 400 ℃/0.1 s−1 and 450 ℃/0.1 s−1, complete dynamic recrystallization occurred, MgZn2 was dissolved and Mg24Y5 appeared. Compared with 400 ℃, the number of second phases was more and grains were finer at 450 ℃. Bulk W-Mg3Y2Zn3 particles were broken and partly dissolved during hot deformation, and Zn and Y atoms diffused from grain boundaries into inner grains. Diffused atoms were aggregated at defects caused by deformation, then nano precipitates with a size of tens of nanometers formed at these places without growth. High-density ultrafine nano precipitates with a size of several nanometers were also found. Grains were refined due to PSN progress promoted by relatively large particles and growth restraint by nano precipitates at grain boundaries. The combined strengthening effect of particle dispersion and grain refinement can significantly improve the mechanical properties. Texture in the alloy was randomized during high temperature deformation as well, which was beneficial to reduce mechanical property anisotropy.

    更新日期:2017-09-13
  • On microstructure and mechanical properties of additively manufactured AlSi10Mg_200C using recycled powder
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-13
    Hamed Asgari, Carter Baxter, Keyvan Hosseinkhani, Mohsen Mohammadi

    Additive manufacturing of reactive metals has opened door to consider using metal 3D printing to manufacture aluminum alloys for different applications from automotive and aerospace to defense. However, one of the major milestones of adopting the technology is still the high price of metal powder in comparison to casting methods. Using recycled powder to additively manufacture parts can be considered one way to decrease the final price in this technology. In the present paper, the microstructural evolution and mechanical properties of the DMLS-AlSi10Mg_200 C alloy manufactured by recycled powder were investigated. As the first step of this research, the powder characteristics, i.e. morphology, average particle size, microstructure and composition, of virgin, condensate and recycled AlSi10Mg powder were studied. It was inferred that virgin and recycled powders had comparable powder characteristics, which are very different in comparison to the condensate powder. In the second step of this research, microstructural and mechanical characterization of the as-built DMLS-AlSi10Mg_200 C alloy using recycled powder revealed that elongation of horizontally built DMLS-AlSi10Mg_200 C was higher compared to that of vertically built. In addition, the fracture surfaces of vertically and horizontally built samples were investigated and possible fracture modes discussed. It is confirmed that, the microstructure and mechanical behavior of the as-built as-built DMLS-AlSi10Mg_200 C manufactured by recycled powder were similar to that of virgin powder.

    更新日期:2017-09-13
  • Thermal cycle-dependent metallurgical variations and their effects on the through-thickness mechanical properties in thick section narrow-gap welds
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-12
    Dinesh W. Rathod, John A. Francis, Matthew J. Roy, Gideon Obasi, Neil M. Irvine

    The primary components in pressurised water reactors are manufactured by welding thick sections of either SA508 or SA533 pressure vessel steel using processes such as submerged arc welding (SAW) or gas-tungsten arc welding (GTAW). Narrow-groove (NG) variants of these processes have reduced welding times, but thick-section welds still require a large number of passes. In this work, the effects of a large number of welding thermal cycles on the through-thickness variability in microstructure and mechanical properties have been analysed for NG-GTAW and NG-SAW joints made in 78 mm thick SA533 steel. Microstructures were characterised using optical and scanning electron microscopy, while mechanical properties were captured in cross-weld tensile tests using digital image correlation and through tests on coupons extracted exclusively from the weld metal and from the heat-affected zone. Charpy impact testing was used to assess toughness. While the toughness was relatively consistent throughout the SAW joint, significant through-thickness variations in toughness were observed in the NG-GTAW joint, which can be attributed to the varying degree to which the weldment was tempered by subsequent welding thermal cycles.

    更新日期:2017-09-12
  • Effects of artificial defect on the material residual strength of SiC ceramics after thermal-shock
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-12
    Xiao Yang, Xuejian Liu, Lujie Wang, Hui Zhang, Xiumin Yao, Zhengren Huang

    Thermal shock behaviors of SiC ceramics with artificial defects were studied. Artificial surface cracks were simulated by Vickers and Knoop indentations, and artificial inner cracks were obtained using novel method of introducing plastic fibers. The residual flexural strength and microstructure were analyzed. The flexural strength of artificial defect free SiC ceramic maintains minimum 87% of the original strength after thermal shock ΔT no more than 400 ℃, and the residual strength of those with artificial crack, no matter surface type or inner type, can still be improved by the joint effect of thermal treatment, thermal shock and the stress relaxation by artificial crack if ΔT no more than 400 ℃. It is believed that inner and surface artificial defects ease the crack propagation during the thermal shock process.

    更新日期:2017-09-12
  • Microstructure and Mechanical Properties of an AA6060 Aluminum Alloy after Cold and Warm Extrusion
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-12
    Nadja Berndt, Philipp Frint, Marcus Böhme, Martin F.-X. Wagner

    Severe plastic deformation techniques can produce ultrafine-grained aluminum alloys with high strength and ductility, but further processing often requires costly and/or time-consuming machining. In this study, we investigate the potential of an alternative processing route that operates below recrystallization temperature and that can produce fine-grained materials. Cast billets of the age hardening aluminum alloy AA6060 were solution annealed and then extruded at room temperature or at 170 °C (which corresponds to the aging temperature of the alloy, and allows for simultaneous forming and aging). The materials were then subjected to an aging treatment. Electron microscopy and mechanical testing were performed to characterize the resulting microstructural features and mechanical properties. Both extruded profiles exhibit similar, strongly graded microstructures with submicron-sized grains in the highly deformed surface layers, and a mixture of coarse and fine grains in the center regions. Despite different processing routes (i.e., cold vs. warm extrusion), both materials are characterized by similar mechanical properties in terms of maximum hardness, strength and ductility; these results are discussed in the light of the relevant microstructural deformation and precipitation mechanisms.

    更新日期:2017-09-12
  • Artificial neural network application to microstructure design of Nb-Si alloy to improve ultimate tensile strength
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-12
    Guangxu Liu, Lina Jia, Bin Kong, Shangbin Feng, Huarui Zhang, Hu Zhang

    In this paper, a feed forward neural network with back propagation artificial neural network (BP ANN) was developed to predict ultimate tensile strength (UTS) and optimize microstructure. The alloys were produced by directional solidification and heat treatment. The UTS was measured for ANN output. Five characteristic factors used for ANN input were abstracted and measured. As the result of this study, the ANN model with high accuracy and good generalization ability to predict UTS within the range of 343.5~1063.3 MPa was established and mutual verified with sensitivity analysis. Based on the optimized ANN model, a new way to design microstructure of Nb-Si alloy to obtain required UTS was proposed. With silicide design maps made by ANN model, the microstructure of the sample of 343.5 MPa was optimized and the UTS reached the target UTS (600 MPa) successfully.

    更新日期:2017-09-12
  • Influence of Ni-P Content on Microstructure and Mechanical Properties of Fe-x(Ni+P)−1Cu Alloys
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-11
    Runjian Jiang, Ai Li, Guodong Cui, Chengsong Zhang, Hongtao Chen, Yixue Wang

    The present paper is devoted to investigating the effects of various Ni-P contents on microstructure and related mechanical behavior of Fe-x(Ni+P)−1Cu alloys fabricated by semi-liquid phase sintering (SLPS) at 950 °C, with a special emphasis on strengthening effects of Cu nanoscale precipitates. Uniaxial compression testing and scanning electron microscope (SEM) were performed to correlate the microstructure with compression properties. Transmission electron microscope (TEM) studies were conducted to characterizing the ultrastructure of Fe-Ni-P-Cu alloy and accurately elucidating the strengthening effect of Cu nanoprecipitates. Experimental results show that the optimal Vickers hardness (465.4HV0.1), ultimate compressive strength (1920MPa) and yield strength (1223 MPa) would be available provided that alloys contain appropriate amount of Ni-P (around 25.3 wt.%). But excess Ni-P (>28.5 wt.%) and resulting coarse phosphide phase at grain boundaries had a detrimental effect on the ductility (21.1%) of Fe-Ni-P-Cu alloy. A quantitative expression had been proposed to elaborate the relationship between yield strength, porosity and component content of as-prepared alloys. It was confirmed that, due to a high level of Ni content, the ultra-fine and dense Cu precipitates obtained could strongly strengthen Fe-Ni-P-Cu alloys even they were air-cooled rapidly. The observed yield strength increment contributed by Cu precipitates coincided with theoretical arithmetic.

    更新日期:2017-09-11
  • Strain energy based low cycle fatigue damage analysis in a plain C-Mn rail steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-11
    P.P. Sarkar, P.S. De, S.K. Dhua, P.C. Chakraborti

    Strain-controlled low cycle fatigue tests were performed on 60 kg/m 90-UTS plain C-Mn rail steel with fully pearlitic microstructure. Analysis of hysteresis loop shape and properties revealed that the steel deviated from the ideal Masing type material behavior to a little extent. To evaluate the fatigue damage in rail steel, energy-based analytical approach proposed by Morrow based on Masing hypothesis was employed. The Morrow energy model resulted in reasonably good approximation of both average plastic strain energy and fatigue life in comparison to experimentally observed values. In addition to that the plastic strain energy density (ΔWp) was correlated to fatigue life (2Nf) through a simple power law and very well represented by Coffin-Mansion type relationship similar to conventional strain-life (Δεp − 2Nf) relationship. Finally, the concept of fatigue toughness, an energy parameter, was introduced to life prediction in rail steel which was proved to be a suitable and reliable alternative to strain-life approach in fatigue damage evaluation with high degree of accuracy.

    更新日期:2017-09-11
  • Direct Observation of a Coincident Dislocation- and Grain Boundary-Mediated Deformation in Nanocrystalline Iron
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-11
    G. Vetterick, A.C. Leff, M. Marshall, J.K. Baldwin, A. Misra, K. Hattar, M.L. Taheri

    The vast majority of our understanding about the deformation mechanisms in nanocrystalline materials is limited to information gained from experimental and theoretical characterization of FCC materials. Related behavior in nanocrystalline BCC materials is not as frequently studied, and thus outstanding questions remain regarding deformation regimes and Hall-Petch trends. Using in situ TEM, we investigate the deformation mechanisms of nanocrystalline BCC iron films with an average grain size of 35 nm produced by physical vapor deposition. The tensile experiments showed that fracture resulted after strains of about 5%. Crack propagation occurred primarily by separation of grain boundaries at the crack tip, which was accompanied by localized intragranular ductile (often superplastic) fracture. Deformation at the crack tip was accommodated by dislocation motion, grain rotation, and grain growth. No evidence was observed of twinning in nanocrystalline BCC iron. The concurrent nature of the grain rotation and dislocation motion indicates that grain rotation occurs at fairly large grain sizes and there is no sharp transition from dislocation-mediated to grain boundary sliding mechanisms as grain size is decreased in BCC iron.

    更新日期:2017-09-11
  • Room temperature tensile fracture characteristics of the oriented TiB whisker reinforced TA15 matrix composites fabricated by pre-sintering and canned extrusion
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-10
    Yangju Feng, Wencong Zhang, Guorong Cui, Wenzhen Chen, Yang Yu

    In situ oriented TiB whisker reinforced TA15 matrix composites (TiBw/TA15) exhibited an improvement of tensile properties. The strain hardening after yield point was almost counteracted by the reduced effective cross-sectional area caused by the whisker fracture, thus resulting in a large yield strength ratio.

    更新日期:2017-09-10
  • Micro/nano ductile-phases reinforced Fe-based bulk metallic glass matrix composite with large plasticity
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-10
    Shengfeng Guo, Chen Su

    Fe-based bulk metallic glasses (BMGs) often show an extremely poor plastic deformation ability at room temperature, which seriously restricted their wide application. In this work, a series of in-situ (Fe77Mo5P9C7.5B1.5)100-xCux (x=0, 0.1, 0.3, 0.5, 0.7, 1 at.%) BMG matrix composites were successfully developed by low purity industrial raw materials using copper mold casting. An (Fe77Mo5P9C7.5B1.5)99.9Cu0.1 BMG matrix composite reinforced by the dual heterogeneous structure with micro/nano α-Fe exhibits the highest ever reported compressive plasticity of ~6.6% during the testing rod with a diameter of 1.5 mm. The origin of a large plasticity of this Fe-based BMG composite has been discussed according to the initiation of the shear bands and the instability of the shear banding.

    更新日期:2017-09-10
  • Surface effected fracture behavior of nano-spherical electrodes during lithiation reaction
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2017-09-09
    Xiaopeng Hu, Yanjie Zhao, Rui Cai, Jianqiu Zhou

    Degradation mechanisms that caused by repetitive insertion and extraction of the lithium ion on the electrodes can lead to a grand challenge for optimizing the design of silicon (Si) based anode with high capacity and rate capacity. However, with decreasing the electrode size into nanometer scale, the surface-to-volume ratio will become very high, meaning that surface effect will have a significant role in determining the mechanical behavior of the electrode. And these effects suppress crack nucleation and propagation, which may become a resistance to brittle fracture. In our work, we establish a theoretical model to study the diffusion induced stress (DIS) evolution and firstly discuss the crack growth by using stress intensity factor (SIF) coupled with surface effects. The results show that DIS, especially the tensile stress, would decrease noticeably due to the surface mechanism. Surface cracks will propagate when SIF is larger than the fracture toughness of materials. It can also be revealed that smaller particles exhibit higher structural integrity. Significantly, the critical nanoparticle electrode size is arrived, below which the anode will not be broken and this value is in good agreement with experimental observations. Overall, the present work maybe provides physical underpinnings for optimized structural design to mitigate the mechanical degradation in high-performance anodes for Li-ion batteries.

    更新日期:2017-09-10
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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