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  • Determination of the DBTT of nanoscale ZrC doped W alloys through amplitude-dependent internal friction technique
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-17
    H.L. Ding, Z.M. Xie, Q.F. Fang, T. Zhang, Z.J. Cheng, Z. Zhuang, X.P. Wang, C.S. Liu

    In this paper, 1 mm thick W-0.5wt.%ZrC alloy plates were prepared by mechanical milling, hot pressing sintering and multistep hot and cold rolling. A newly developed testing technique based on the amplitude-dependent internal friction (ADIF) was used to determine the ductile-to-brittle transition temperature (DBTT) of this W-0.5wt%ZrC alloy. The DBTT obtained by ADIF technique is in the range of 50 to 80 °C, which is in consistence with the tensile tests. Qualitative relationship between the critical strain amplitude in ADIF and the yield stress in tensile test was also found. The ADIF technique was confirmed to be an available method to determine the DBTT of the materials. Microstructure analysis indicated that the relative low DBTT and the high strength of the W-0.5wt%ZrC alloy plates were resulted from the multistep hot and cold rolling and nanoscale particle pinning effects.

    更新日期:2018-01-17
  • Fatigue damage and fracture behavior of metallic glass under cyclic compression
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-17
    X.D. Wang, R.T. Qu, S.J. Wu, Z.Q. Liu, Z.F. Zhang

    A comprehensive understanding on the fatigue behavior of metallic glasses (MGs) is of vital importance for their structural applications. Here we present a systematic study on the fatigue damage and fracture behavior of a Zr52.5Cu17.9Ni14.6Al10Ti5 MG under cyclic compression of different stress levels, with the assistance of 3D X-ray tomography (XRT) and 2D scanning electron microscope to examine the origins of fatigue cracks. The results indicate that the fatigue fracture mode of the MG depends strongly on the stress level in such a manner similar to that of high-strength steels. The MG exhibits shear fracture along shear band under high cyclic stress level, just like the case for monotonic compression; under low cyclic stress level, in contrast, the fracture is dominated by splitting where the fatigue cracks originate from extrinsic defects. The increased propensity for cavitation and cracking under low stress level, as revealed by XRT, suggests a reduced resistance of the MG to cleavage failure which eventually leads to the transition of the fatigue fracture mode. The results support the idea that the fatigue performance of MGs can be improved by reducing the extrinsic defects and properly controlling the shear banding behavior.

    更新日期:2018-01-17
  • Effect of prior deformation on the subsequent creep and anelastic recovery behaviour of an advanced martensitic steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-17
    E. Hosseini, V. Kalyanasundaram, X. Li, S.R. Holdsworth

    The creep and anelastic recovery characteristics of a 10%Cr steel have been systematically investigated at 600 °C after subjecting the test material to various prior deformation histories. Constant-load forward creep tests on specimens, either with a tensile or compressive preloading history, indicated that over- and reverse-preloading respectively decreases and increases the early primary creep rate of the steel. The extent of decrease (or increase) in early primary creep rate is also found to be directly proportional to the magnitude of stress during prior loading while such a correlation is not clearly evident for material deformation in the secondary and tertiary stages. Specifically, the creep rate in the secondary and tertiary stages is lower for specimens with a compressive prior loading while the rupture time is notably shorter for tensile pre-loaded specimens. The observed effect of prior loading on the early primary creep behaviour can be explained by considering micro-backstress development (as a consequence of dislocation pile-up formation during the prior loading phase) that subsequently introduces a kinematic hardening effect to the material's viscoplastic response. The second set of experiments involve monitoring the anelastic recovery behaviour immediately after accumulation of a similar amount of time-dependent strain either under forward creep (load control mode) or stress relaxation (strain control mode) condition in completely unloaded 10%Cr steel specimens at 600 °C. Experimental observations indicate that the higher the stress magnitude during the prior loading phase the greater and faster the anelastic recovery at zero stress. Further findings show the mode of prior deformation (creep or relaxation) to also not noticeably influence the subsequent anelastic recovery behaviour. The observed anelastic recovery characteristic can be mechanistically interpreted by consideration of the time-dependent material back-flow due to the relaxation of dislocation bows and pile-ups generated during the prior deformation.

    更新日期:2018-01-17
  • Microstructure and tensile behavior of friction-stir welded TRIP steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-17
    S. Mironov, Y.S. Sato, S. Yoneyama, H. Kokawa, H.T. Fujii, S. Hirano

    In this work, electron backscatter diffraction and digital image correlation techniques were employed to study the microstructure and microstructure-property relationship in friction-stir welded TRIP steel. It was found that the thermal effect of the welding process led to material softening in the heat-affected zone and promoted martensite transformation in the stir zone. These microstructural changes provided rapid strain localization during subsequent transverse tensile tests and thus resulted in premature failure of the welds. Material softening in the heat-affected zone was deduced to be a combined result of dissolution and spheroidisation of retained austenite as well as recovery in bainitic ferrite. The stir zone martensite was characterized by significant orientation spread and therefore its orientation relationship with austenite essentially deviated from the ideal Kurdjumov-Sachs relation. Moreover, the martensite transformation was found to be influenced by variant selection.

    更新日期:2018-01-17
  • Microstructural evolution and mechanical property of a Ni-Fe-based weld metal during long-term exposure at 650? and 700?
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-17
    Dong Wu, Dianzhong Li, Shanping Lu

    A newly developed Ni-Fe-based weld metal has been researched for the microstructural and mechanical evolutions during prolonged thermal exposure using scanning electron microscopy and transmission electron microscopy. The M23C6-carbides precipitated at grain boundaries and around the primary (Nb, Ti)C phases in the Ni-Fe-based weld metal during prolonged exposure at 650°C and 700°C. The Cr, Mo-rich sigma(σ) and Cr-rich α-Cr formed as platelets and mainly located in the interdendritic areas during the exposure. σ phase had the orientation relationships of [001]γ//[ View the MathML source 11 2 ̅ ]σ, View the MathML source ( 2 ̅ 2 0 ) γ// View the MathML source ( 1 ̅ 1 0 ) σ and View the MathML source ( 2 ̅ 2 ̅ 0 ) γ//(111) ( 1 1 1 ) σ with the γ matrix. Higher (Al + Ti) content accelerated the precipitation of these Cr-rich phases and resulted in a significant deterioration of tensile ductility after 5000 h at 700°C. Precipitating and coarsening of spherical γ′ dominated the tensile strength evolution and the optimized γ′ radius with the best tensile strength was estimated to be 21~25 nm. (Al + Ti) content had no obvious effect on the γ′ coarsening rate but it affected the γ′ particle density Ns in two sides: On the one hand, higher (Al + Ti) content increased the Ns, which increased the 0.2% yield strength (Rp0.2)and tensile strength ( Rm). On the other hand, higher (Al + Ti) content increased the amount of σ phases which induced the γ′ envelopes to form and resulted in the decrease of Ns, and it would decreased the Rp0.2 and Rm.

    更新日期:2018-01-17
  • Mechanical and thermal properties of C/C composites modified with SiC nanofiller
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-16
    Danuta Mikociak, Arkadiusz Rudawski, Stanislaw Blazewicz

    The paper deals with carbon/carbon composites (C/C) modified with a ceramic nanofiller designed for high temperature applications. The C/C samples were manufactured from a coal tar pitch precursor (CTP) modified with a silicon carbide in the form of nanopowder (SiC). Carbon fibers were used as a reinforcement. The composite samples containing differing amount of the ceramic nanofiller were manufactured. The mechanical properties, thermal conductivity and oxidation resistance of C/C composites before and after modification with nanoSiC were determined. The structure and microstructure of the composites were examined using X-ray diffraction and SEM with EDS technique. The presence of the ceramic nanoparticles in carbon matrix precursor decreased the total porosity of carbon composites and redistributed pore sizes after annealing to 1000 °C. CTP-derived carbon matrix was composed from two carbon components that differed in crystallographic parameters. Modified C/C composites performed distinctly higher mechanical properties, thermal conductivity and oxidation resistance in comparison with the pure carbon/carbon samples.

    更新日期:2018-01-17
  • Heavy carbon alloyed FCC-structured high entropy alloy with excellent combination of strength and ductility
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-12
    L.B. Chen, R. Wei, K. Tang, J. Zhang, F. Jiang, L. He, J. Sun

    The effects of carbon content on the microstructure and room-temperature mechanical properties of Fe40Mn40Co10Cr10 high-entropy alloy (HEA) were systematically investigated. The results showed that heavy carbon alloyed HEA could possess supreme combination of high tensile strength (935 MPa) and high ductility (~74%).The excellent mechanical properties were ascribed to as follows: the high content interstitial carbon atoms strengthens the matrix greatly through suppressing dislocation motion and promoting the deformation-induced twinning at room temperature, which enhance the strength and ductility. Simultaneously, the ductility is further secured for single FCC structure maintained due to appropriate carbon alloying. Our findings provide a novel strategy for developing HEAs with excellent mechanical properties.

    更新日期:2018-01-12
  • Study on the effect of ageing on laves phase evolution and their effect on mechanical properties of P92 steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-12
    Nitin Saini, Rahul S. Mulik, Manas Mohan Mahapatra

    P92 steel is candidate material for application in reactor pressure vessels in nuclear power plants. In present investigation, Laves phase evolution (at 650 ° C) with varying ageing time (upto 3000 hours) in P92 steel and their effect on mechanical properties have been investigated. During thermal ageing, the microstructure analysis showed the evolution of Laves phase that degrades the strength of P92 steel. The formation of Laves phase was observed after the thermal ageing of 720 h and it showed higher coarsening rate in ageing time range of 720 h −1440 h. The Laves phase formation was also confirmed by the XRD analysis, and line mapping. The strength and ductility decreased as a result of deprivation of solid solution strengthening and formation of Laves phase. The hardness of P92 steel was also affected by ageing time but less pronounced as compared to strength. Charpy toughness was also reduced continuously with increase in ageing time as a result of thermal straining of particles and Laves phase formation.

    更新日期:2018-01-12
  • Evolution of microstructure and tensile properties during the three-stage heat treatment of TA19 titanium alloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-12
    Dongrong Li, Ke Wang, Zhibing Yan, Yu Cao, R.D.K. Misra, Renlong Xin, Qing Liu

    This study reported the development of a three-stage heat treatment for TA19 titanium alloy to obtain a ternary microstructure consisting of equiaxed, lamellar and acicular α, which were successively acquired in the first stage (I-stage), second stage (II-stage) and third stage (III-stage) treatments. The content ratio among the equiaxed, lamellar and acicular α was tailored by controlling the I-stage and II-stage temperatures. Tensile test revealed that an increase of I-stage temperature improved both ultimate strength (UTS) and elongation (EL) due to the increased content of lamellar α. However, an excessively high I-stage temperature led to a very low content of equiaxed α, coarsening of β grain and precipitation of grain boundary α, which reduced the EL. An increase of II-stage temperature resulted in an increase of UTS and a decrease of EL, because the increased content of acicular α produced a number of α/β interfaces, which strengthened the alloy but was detrimental to plasticity. The fractography analysis indicated that majority of heat treated specimens exhibited a completely ductile fracture mode, while a mixed mode of brittle and ductile fracture was observed in specimens, which was subjected to an excessively I-stage temperature.

    更新日期:2018-01-12
  • Transient creep behavior of a novel tempered martensite ferritic steel G115
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-12
    Bo Xiao, Lianyong Xu, Lei Zhao, Hongyang Jing, Yongdian Han, Kai Song

    The transient creep deformation behavior and dislocation evolution in G115 steels were investigated in the temperature range 625–675 °C for applied stresses of 120–220 MPa. The transient creep curves showed that creep rate decreased with increases in time and creep strain. In this study, a novel method was proposed to determine the transient creep strain and the transient creep time. The strain decreased with increase in applied stress between 625 °C and 675 °C. A phenomenological constitutive equation was used to characterize the transient creep curves of G115 steels. The results showed that the constitutive equation for the G115 steel had a high precision. An internal stress was introduced to study the interactions of dislocations and precipitates, and a mechanism-based equation for transient creep in G115 steels was derived. The dependences of normalized strain and transient creep time on applied stress and temperature were analyzed in detail to better understand the transient creep deformation mechanism. It was concluded that dislocation annihilation at the grain boundaries was the dominant rate-controlling mechanism in the transient creep deformation of G115 steels. It was observed that the dislocation structures became more complex and no obvious textural features occurred after the transient creep deformation. The calculated dislocation density decreased at 650 °C relative to the initial value, which was mainly attributed to the annihilation processes occurring at the grain boundaries and the subsequent formation of subgrain boundaries.

    更新日期:2018-01-12
  • Influence of friction stir processing on the room temperature fatigue cracking mechanisms of A356 aluminum alloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-11
    Phalgun Nelaturu, Saumyadeep Jana, Rajiv S. Mishra, Glenn Grant, Blair E. Carlson

    Failure by fatigue is a common problem associated with cast aluminum alloys due to defects like shrinkage porosities, non-metallic inclusions, etc. Friction stir processing (FSP) has recently emerged as an effective technique for local modification of microstructure. This study investigates the fatigue crack initiation and growth mechanisms in cast and FSPed A356 aluminum alloy. Two sets of parameters were used to friction stir the cast alloy resulting in the complete modification the cast microstructure to a wrought microstructure. Both the FSPed microstructures exhibited severe abnormal grain growth (AGG) after heat treatment leading to a multimodal grain size distribution – the grain sizes ranging from a few microns to a few millimeters. One of the FSP conditions displayed an excellent improvement in fatigue life by an order of magnitude, while the other condition displayed an unexpectedly large scatter in fatigue lives. Detailed study of the fractured fatigue specimens by electron back scattered diffraction (EBSD) revealed that both, fatigue crack initiation and propagation, were intimately tied to the grain size as well as the grain misorientations in the microstructure.

    更新日期:2018-01-12
  • The role of fibrous morphology on the Charpy impact properties of low carbon ferrite-bainite dual phase steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-11
    Md. Basiruddin Sk, Irshad Alam, Debalay Chakrabarti

    Primarily ferrite-pearlite microstructure having coarse ferrite grain size (24 μm) and high pearlite fraction (42%) offered YS ∼ 575 MPa with poor impact properties such as, upper shelf energy (USE) of only 30 J and ductile brittle transition temperature (DBTT) as high as 27°C in an industrially hot-rolled plate of 0.25 wt.% C steel. In order to improve the strength along with the impact properties by developing ferrite-bainite microstructures, two different types of heat-treatments, namely step-cooling (SC) and intermediate cooling (IC) treatments, were carried out on the as-received material. Significant improvement in strength along with the impact toughness (with YS of 740 MPa, USE of 222 J and DBTT of -57°C) has been achieved by developing fibrous microstructure, with alternate thin-films (2-4 μm thick) of ferrite and bainite through intermediate cooling (IC) treatment. Fine film-like structure with large orientation difference across the ferrite-bainite interface boundaries not only increased the strength but also resulted in frequent deflection in cleavage crack propagation path which improved the low-temperature impact toughness and reduced the DBTT.

    更新日期:2018-01-12
  • Mechanism of microstructural refinement of deformed aluminum under synergistic effect of TiAl3 and TiB2 particles and impact on mechanical properties
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-11
    Renguo Guan, R.D.K. Misra, Yingqiu Shang, Yanan An, Yuxiang Wang, Yang Zhang, Di Tie

    To understand the synergistic effect of TiAl3and TiB2particles on grain refinement of α-Al, equal-channel angular pressing (ECAP) of Al-5Ti-1B (wt. %) alloy was carried out. The ECAPed Al-5Ti-1B (wt. %) alloy was characterized by X-ray Diffraction (XRD), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM).The results reveal that α-Al grains and TiAl3 phase of ECAPed Al-5Ti-1B (wt. %) alloy were refined, while the average size of TiB2was marginally decreased. The tensile strength was increased from 128 MPa to 218 MPa, and the elongation was reduced from 26% to 20.6%.The transformation from dislocations to low-angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs) resulted in continuous dynamic recrystallization (CDRX), which induced grain refinement during ECAP. From 3 to 6 passes of ECAP, CDRX occurred in the alloy, andas ECAP passes increased to 9, the rate of formation of LAGBs and the transformation rate from LAGBs to HAGBs reached a dynamic balance.The present study indicates that the second phase particles promote the formation of LAGBs but prolong complete-recrystallization time, such that complete-recrystallization leads to grain refinement.TiB2 particles are smaller than TiAl3 particles and this is caused by the composition of the alloy and its synthesis method, and compared with TiAl3, TiB2 with a smaller size had a more obvious influence on the formation of LAGBs and grain refinement.

    更新日期:2018-01-11
  • Effect of Pre-plastic-deformation on Mechanical Properties of TiZr-based Amorphous Alloy Composites
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-11
    Jianmin Yang, Ziyan Zhao, Juan Mu, Yandong Wang

    Phase transformation behaviors can modulate the stress distribution under loading and enhance the plasticity and work hardening ability of amorphous alloy composite. Effect of pre-plastic deformation on the phase transformation kinetics and the mechanical properties of the TiZr-based amorphous alloy composite were studied systematically in the current work. The plasticity of the composites was improved by lateral pre-compression deformation and pre-tensile deformation. However, the plasticity reduced after axial pre-compression deformation. These results were attributed to the distinguish of the directions between the pre-existing shear bands formed during pre-plastic deformation and new developed shear bands during the final compression, which can favor the severe interaction of shear bands.

    更新日期:2018-01-11
  • Over-aging influenced matrix precipitate characteristics improve fatigue crack propagation in a high Zn-containing Al-Zn-Mg-Cu alloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-11
    Kai Wen, Baiqing Xiong, Yongan Zhang, Zhihui Li, Xiwu Li, Shuhui Huang, Lizhen Yan, Hongwei Yan, Hongwei Liu

    A high Zn-containing Al-Zn-Mg-Cu alloy was researched due to excellent overall performances. The hardness, electrical conductivity and mechanical properties were investigated and detailed aging parameters subjected to T6, T79, T74 and T73 were proposed. The matrix precipitates of various aging tempers were investigated by transmission electron microscope (TEM) and high-resolution transmission electron microscope (HREM) techniques and quantitative information of matrix precipitates was extracted from the bright-field TEM images projected along 〈110〉Al orientation with the aid of an imaging analysis. The fatigue crack propagation (FCP) behaviors subjected to different aging tempers were investigated, the related fracture morphology of the stable expanding regions was analyzed and corresponding fatigue striations were measured to verify the FCP rates. The results showed that with the deepening of aging degree, the matrix precipitates coarsened with an expanding of precipitate size distribution and an enlargement of average precipitate size while the precipitates evolved from GP zones and η' phase to η' phase and η phase. The FCP resistance was improved with the aging degree deepens and the evolution of related tearing ridge, tearing dimple and fatigue striation also proved it. Due to a smaller cyclic plastic zone of the alloy with various tempers compared with the average grain size, the FCP rate was significantly influenced by matrix precipitate characteristics and a theoretical model which directly correlated FCP rate with matrix precipitate characteristics was proposed. From T6 state to T73 state, the enlargement of cuttable GP zones and η' phase, its evolution to η phase and the nucleation, growth and coarsening of η phase were in favor of enhancing the FCP resistance.

    更新日期:2018-01-11
  • 更新日期:2018-01-11
  • Effects of nano-graphite content on the characteristics of spark plasma sintered ZiB2–SiC composites
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-10
    Mehdi Shahedi Asl, Mehran Jaberi Zamharir, Zohre Ahmadi, Soroush Parvizi

    In this study, ZrB2–25 vol% SiC composite containing 0, 2.5, 5, 7.5 and 10 wt% graphite nano-flakes were prepared by spark plasma sintering (SPS) process at 1900 °C for 7 min under 40 MPa. The fabricated composite samples were compared to examine the influences of nano-graphite content on the densification, microstructure and mechanical properties of ZrB2–SiC-based ultrahigh temperature ceramics. Fully dense composites were obtained by adding 0–5 wt% nano-graphite, but higher amounts of additive led to a small drop in the sintered density. The growth of ZrB2 grains was moderately hindered by adding nano-graphite but independent of its content. The hardness linearly decreased from 19.5 for the graphite-free ceramic to 12.1 GPa for the sample doped with 10 wt% nano-graphite. Addition of graphite nano-flakes increased the fracture toughness of composites as a value of 8.2 MPa m½ was achieved by adding 7.5 wt% nano-graphite, twice higher than that measured for the graphite-free sample (4.3 MPa m½). The in-situ formation of ZrC and B4C nano-particles as well as the presence of unreacted graphite nano-flakes led to a remarkable enhancement in fracture toughness through activating several toughening mechanisms such as crack deflection, crack bridging, crack branching and graphite pullout.

    更新日期:2018-01-10
  • Indentation-Based Rate-Dependent Plastic Deformation of Polycrystalline Pure Magnesium
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-10
    M. Haghshenas, Y. Wang, Y.-T Cheng, M. Gupta

    Using a Berkovich pyramidal indenter and an instrumented nanoindentation platform, dual stage nanoindentation creep tests, including loading to a pre-set load and holding at the constant load then unloading, were performed on polycrystalline pure magnesium at 300 K. Indentation tests were performed at four indentation loading rates of 0.05, 0.5, 5, and 50 mN/sec at constant load of 45 mN and holding time of 400 s. These were performed to assess indentation force-displacement response along with average indentation stress, indentation strain rate sensitivity and ambient temperature rate dependent plastic deformation response of the material. To this end, activation volume, a kinetic characteristic of plastic deformation, and density of mobile dislocations in plastically deforming material in the plastic zone around the indenter are discussed in detail. Uncertainties and sources of error, i.e. indentation size effect, surface roughness, and thermal drift, and pile-up/sink-in in measuring creep response through nanoindentation are provided in the current paper as well. The microstructure of the material was also studied through optical and scanning electron microscopy to further investigate the microstructure/property correlations in the tested polycrystalline pure magnesium. The results show the dependency of indentation stress, strain rate sensitivity, and activation volume upon depth and loading rate. According to the creep stress exponent measurements, the dominant mechanism of rate dependent plastic deformation for polycrystalline pure Mg at ambient temperature is attributed to obstacle limited dislocation glide. Finally, the contribution of mechanical twins in the plastic zone around the indenter in the studied polycrystalline pure magnesium is briefly discussed.

    更新日期:2018-01-10
  • The mode I fatigue behaviours of notched freestanding Al-3Mg foils
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-10
    Yi Zhao, Jiaqing Zheng, Jinzhang Liu, John W. Holmes, Liu Liu, Jiangbo Sha

    The effect of the thickness on fatigue crack propagation behaviour of rolled and annealed Al-3Mg alloy foils was investigated using notched rectangular specimens with thicknesses of 30 μm and 100 μm and elastic finite element method analysis. A specialized apparatus in combination with a force sensor and a video graphics array digital camera to record the crack growth was employed to investigate the mode I fatigue behaviour of freestanding Al-3Mg alloy foils. Tests were conducted at ambient temperature, with a loading frequency of 2 Hz and a loading ratio R of 0.1. The foil thickness was found to play a distinct role in the fatigue properties of micron-foil metals. Compared to the 100-μm-thick foil, the 30-μm-thick foil exhibited higher average yield stress (σ0.2), but lower elongation, fracture toughness and fatigue life. The crack propagation rate da/dN of both foils as a function of the stress intensity factor ΔK followed the Paris relationship. The Paris constant m, increased slightly, from 4.6 to 5.6, as the foil thickness decreased from 100 μm to 30 μm. A smaller plastic zone size at the crack tip and a higher value for the self-similarity parameter z were found in the thinner foils, resulting in the higher Paris constant m value. With typical fatigue striations, the 30-μm-thick foils demonstrated failure in a brittle mode during the stable fatigue crack growth stage. However, the 100-μm-thick foils had abnormal striations and dimples in the stable fatigue crack growth stage, they failed in a mixed ductile and brittle mode.

    更新日期:2018-01-10
  • Characterization of 3ʺ through-thickness friction stir welded 7050-T7451 Al alloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-09
    Mageshwari Komarasamy, Karthik Alagarsamy, Laura Ely, Rajiv S. Mishra
    更新日期:2018-01-10
  • Comparison of long-term radii evolution of the S-phase in aluminum alloy 2618A during ageing and creep
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-09
    Christian Rockenhäuser, Sina Schriever, Philipp von Hartrott, Benjamin Piesker, Birgit Skrotzki

    A study was made on the effect of creep loading on the precipitate radii evolution of the aluminum alloy 2618 A. The overageing process of the alloy was investigated under load at a temperature of 190 °C with stresses between 79 and 181 MPa and compared to stress free isothermal aging. The precipitates responsible for strength were characterized using dark-field transmission electron microscopy (DFTEM). This allows the experimental determination of radii distributions of the rod-shaped Al2CuMg precipitates and the evaluation regarding their mean precipitate radius. It was found that the mean precipitate radius enables the comparison of the different microstructural conditions of crept and uncrept samples. The mean precipitate radii of the samples experiencing creep are significantly higher than those of undeformed samples. It was shown that the acquired radii distributions are viable to determine averaged particle radii for comparison of the aged samples. A ripening process including pipe diffusion along dislocations describes the data on coarsening very well for the creep samples.

    更新日期:2018-01-10
  • A hierarchical nanostructured Fe34Cr34Ni14Al14Co4 high-entropy alloy with good compressive mechanical properties
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-09
    Yang Zhou, Xi Jin, Lu Zhang, Xingyu Du, Bangsheng Li

    A non-equiatomic Fe34Cr34Ni14Al14Co4 high-entropy alloy was designed and prepared. The alloy displayed novel nanostructures consisting of hierarchical Fe-Cr-rich and Ni-Al-rich phases. As a result, the alloy exhibited good compressive mechanical properties both at room and elevated temperatures, superior to many high-entropy alloys and conventional alloys.

    更新日期:2018-01-10
  • A study on the susceptibility of high strength tempered martensite steels to hydrogen embrittlement (HE) based on incremental step load (ISL) testing methodology
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-09
    Tuhin Das, Sriraman K. Rajagopalan, Salim V. Brahimi, Xiang Wang, Stephen Yue

    Incremental step load (ISL) technique was used to determine the material susceptibility of three different grades of tempered martensite steels 35CrMo4, 41Cr4 and 36NiCrMo4 to hydrogen embrittlement (HE). In addition to testing the steel grades in air, the environmental hydrogen embrittlement susceptibilities (EHE) of each material were determined by in-situ charging of hydrogen at three different cathodic potentials −0.85 VSCE, −1VSCE and −1.2VSCE to obtain entire threshold curves for these materials. Overall investigation implies that 35CrMo4 is more susceptible than 41Cr4 and 36NiCrMo4. The fracture surface morphology in case of 35CrMo4 is entirely intergranular at −1VSCE and −1.2VSCE showing the severity of embrittlement. The higher matrix-cementite interfaces alongwith high cementite and low manganese sulphide (MnS) inclusion content of 36NiCrMo4 influencing the hydrogen transport kinetics within the material are responsible for the lower material susceptibility. The thermal desorption analyses (TDA) further corroborate the observations.

    更新日期:2018-01-10
  • Microstructure and mechanical properties of CP-Ti fabricated via powder metallurgy with non-uniformly dispersed impurity solutes
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-09
    J. Shen, B. Chen, J. Umeda, K. Kondoh

    Oxygen and nitrogen have been both known to have a strong hardening effect on Ti and its alloys, while also imposing a serious embrittlement effect. In the present work, Ti samples with non-uniformly dispersed oxygen and nitrogen solid solutions were produced using high purity Ti via powder metallurgy (PM) methods. The experimental results suggested that, when the solutes are non-uniformly distributed, the high solute content region can serve as strengthening particles as that in metal matrix composites. In light of this, we propose a design of novel structure in high oxygen/nitrogen Ti materials to achieve both improved strengths and ductility. The highlight of the structure is a design of strain gradient at the matrix-particle interface, which can mitigate the strain compatibility that commonly reported in general MMCs as the crack initiator. Instead of using ceramics as reinforcements, here we propose to use the matrix phase itself that hardened by high oxygen/nitrogen solutes as reinforcing particles.

    更新日期:2018-01-10
  • Deteriorated tensile creep resistance of a high-pressure die-cast Mg–4Al–4RE–0.3Mn alloy induced by substituting part RE with Ca
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-08
    Qiang Yang, Xin Qiu, Shuhui Lv, Fanzhi Meng, Kai Guan, Baishun Li, Deping Zhang, Yaqin Zhang, Xiaojuan Liu, Jian Meng

    Tensile creep resistance of a high-pressure die-cast Mg–4Al–4RE–0.3Mn (AE44) alloy was significantly deteriorated after substituting part RE with Ca. According to traditional power-law creep theories, the stress exponent and the activation energy were revealed as 6 and 217 kJ/mol, which indicate inconsistent mechanisms of dislocation climb and dislocation cross-slip, respectively. Then, transmission electron microscopy (TEM) observations illustrate that dislocation substructures developed during creep are variational with precipitate characters in α-Mg grains, creep stress levels and creep temperatures. Therefore, both stress exponent and activation energy obtained from traditional power-law creep theories are meaningless for the AE44 alloy with part RE substituted by Ca. Finally, the shrink of C36 phase lattice, the precipitation of Al2Ca precipitates and the denuded zones were observed in the crept samples, and all of them are responsible for the deterioration in creep resistance of the AEX422 alloy. Also, this paper provides insight into alloy design principles for further development of creep-resistance Mg–Al–RE based alloys.

    更新日期:2018-01-09
  • Effect of strain path on the evolution of microstructure, texture and tensile properties of WE43 alloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-08
    R.K. Sabat, P.K. Samal, Muzzamil Ahamed S

    Hot rolling of extruded and solutionized WE43 alloy was carried out at 400 °C through different strain paths such as unidirectional rolling (UDR) and multistep cross rolling (MSCR). The effect of strain path on the evolution of microstructure and texture was investigated. The grain size was ∼ 7 μm and 18 μm after hot rolling of UDR and MSCR samples, respectively. A strong basal texture was observed during rolling which is independent of the strain path of the samples and the recrystallization mechanism was identified to be continuous dynamic recovery and recrystallization type. Further, the texture of the deformed and recrystallized grains was observed to be same. Hence, the texture was simulated successfully using visco-plastic self-consistent (VPSC) simulation. The VPSC simulation of UDR sample showed the dominant activity of basal and prismatic slip system up to a true strain of ∼ 0.5 and after that pyramidal <c+a> activity dominants, whereas MSCR sample showed the combined activity of basal and pyramidal <c+a> up to a true strain of ∼ 0.5 and later on basal activity dominants on further deformation. The room temperature tensile testing showed that the yield strength of MSCR sample was greater than UDR sample except along RD of UDR sample and the tensile strength was more for MSCR compared to UDR sample which suggests that higher volume fraction of metastable precipitates in MSCR than UDR sample during high temperature rolling. The higher volume fraction of metastable precipitates was attributed to lower dislocation activity on prismatic planes of MSCR compared to UDR sample during hot rolling. The current VPSC simulation showed that the prismatic slip activity was four times higher in UDR than MSCR sample. A decrease in mean free path of dislocations by the presence of metastable precipitates led to higher tensile strength of MSCR compared to UDR sample. The mean free path of dislocations increased from RD to TD and hence, results in a decrease in tensile strength from RD to TD of both the samples.

    更新日期:2018-01-09
  • Effects of martensite-austenite constituent on crack initiation and propagation in inter-critical heat-affected zone of high-strength low-alloy (HSLA) steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-08
    Seok Gyu Lee, Seok Su Sohn, Bohee Kim, Woo Gyeom Kim, Kyung-Keun Um, Sunghak Lee

    Crack susceptibility of martensite-austenite constituent (MA) in inter-critical heat-affected zone (ICHAZ) is varied with properties of adjacent matrix, hardness of MA, and MA/matrix interfacial characteristics, but reasons why MAs are voided at MA/matrix interfaces or cracked at themselves still remain to be addressed. Effects of microstructural features including MA and matrix composed of granular bainite (GB) and quasi-polygonal ferrite (QPF) on crack initiation and propagation behavior, which affected critical crack tip opening displacement (CTOD), were investigated in this study. In the ICHAZ mainly composed of GB (75.2 vol.%) and QPF (19.5 vol.%), together with 5.3 vol.% of MA, QPF areas were more deformed than GB areas during the interrupted three-point bending tests, while some MAs were voided or cracked as MA areas were also highly deformed. The Nano-indentation test data indicated that the hardness of MA depended on boundary characteristics of nearby matrix, e.g., QPF having high-angle boundaries vs. GB having low-angle boundaries. According to detailed analyses on MA/QPF interface by using an atom probe, C, Mn, Mo, Ni, and Cr atoms were segregated at the MA/QPF interface, whereas Si, Cu, Al, and P were hardly segregated. These results indicated that major alloying elements (C, Mn, Ni, and Cr) were segregated at the MA/QPF interface, which influenced the deterioration of critical CTOD by reducing the mean free path for the crack initiation and propagation.

    更新日期:2018-01-09
  • Crystallographic characterization of the middle shelf in a two-step ductile to brittle transition for a ferrite + pearlite structure steel sheet
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-08
    Hiroyuki Kawata, Osamu Umezawa

    The middle shelf (MS) exhibits medium absorbed energy during a two-step ductile to brittle transition and appears clearly in a ferrite + pearlite steel sheet. The analysis involved using the electron backscattered diffraction technique and indicated that the traces of fracture surface and secondary cracks at MS and lower shelf (LS) correspond to low index crystallographic planes in bcc iron. Although the {001} cleavage plane in the ferrite grain is dominant for the fracture propagation path at LS, the traces at MS correspond to {001}, {112}, and {011} planes. The potential of fracture in the {001} plane at MS (i.e., the number of traces normalized by the number of equivalent planes) exceeds that in the {112} plane. However, the difference between the same is significantly lower than that at LS. Furthermore, the{112} and {011} planes are typical slip planes in bcc iron. This suggests that the fracture at MS propagates on the {001} plane with cleavage fracture as well as on the {112} and {011} planes with a slipping-off mechanism.

    更新日期:2018-01-09
  • Effects of Cobalt on the structure and mechanical behavior of non-equal molar CoxFe50-xCr25Ni25 high entropy alloys
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-08
    Wei Fang, Ruobin Chang, Xin Zhang, Puguang Ji, Xinghua Wang, Baoxi Liu, Jia Li, Xinbo He, Xuanhui Qu, Fuxing Yin

    Non-equal molar high entropy alloys (HEAs) are promising to obtain better mechanical properties owing to abundant composition designs compared with single equal molar HEAs point. CoxFe50-xCr25Ni25 HEAs were designed to investigate the effect of Co compositional variation on the structure and mechanical properties of single phase solid solutions. Microstructure evolution at different strain levels was studied by electron backscatter diffraction. The results indicate that lattice parameter tends to decrease and yield strength are enhanced, with the increase of Co content. The ultimate tensile strength and ductility are improved at Co 30~35 at. %, especially the ductility, compared to the equal molar CoCrFeNi HEAs. Meanwhile, grain rotation is slower and more difficult in non-equal molar Co35Fe15, and the greater low angle boundary fraction indicates the higher dislocation density. In different deformation stages, the dislocation forest strengthening and twinning-induced plasticity effect result in better strength-ductility combination in Co35Fe15.

    更新日期:2018-01-09
  • The High Cycle Fatigue, Deformation and Fracture ofCompacted Graphite Iron:Influence of Temperature
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-07
    C.L. Zou, J.C. Pang, M.X. Zhang, Y. Qiu, S.X. Li, L.J. Chen, J.P. Li, Z. Yang, Z.F. Zhang

    The microstructure, tensile strength, high-cycle fatiguepropertyand corresponding damage mechanisms of compacted graphite iron at room temperature (25 °C), 400 °C and 500 °C, were investigated. It is found that the fatigue strength increases at first and then decreases with the increase of the testingtemperature.At 25 °C,the fatigue crackmainlyinitiatesfrom graphite debonding and propagates along the graphiteclusters.At 400 °C,the fatigue crack initiationis influenced by oxidation; the fatigue strengthmay be improved by dynamic strain aging.At 500 °C, theoxidation becomes more serious andthe oxide layer accelerates the crack to propagate along the matrix. At the same time, the phenomenon of grain boundarysoftening, one of the reasons resulting in the reduction of fatigue strength,is found.Then, the model ofdamage mechanismwas proposedaccording to the propagation behavior of fatigue crack at high temperatures, and the quantitative relationship betweenthefatigue strength and the ratio of the interphase corrosion depth to the critical crack lengthwas established.This investigationmay enrich the fundamental understandingon the damage mechanism of compacted graphite iron.

    更新日期:2018-01-07
  • Toward an understanding of post-necking behavior in ultrafine-scale Cu/Ni laminated composites
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-07
    H.F. Tan, B. Zhang, G.P. Zhang

    Cu/Ni laminated composites with the same total thickness of 128 μm, but different modulation wavelength λ (the sum of monolayer thickness of constituent layers) of 32 μm and 3 μm, respectively were prepared using a dual-bath electrodeposition technique. The tensile testing results and the quantitative characterization of strain gradient in the post-necking region of the composites reveal that the λ=3 μm composites have the higher tensile strength and the better strain hardening ability, as well as the stronger resistance to post-necking compared with the λ=32 μm ones. A high density of layer interfaces in the ultrathin laminated composite may play a key role in delaying the development of unstable plastic deformation along the layer thickness direction through periodical strain gradient and a number of geometrically necessary dislocations near the interfaces. Thus, the fatal localized plastic deformation along the through thickness direction has to be spread over along the loading direction, leading to the large neck breadth in the ultrathin laminated composites.

    更新日期:2018-01-07
  • Local mechanical properties in the vicinity of (11̄0)Σ3/[111] symmetric tilt grain boundary in aluminum bicrystal
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-06
    Yoshiyuki Tokuda, Sadahiro Tsurekawa, Dmitri A. Molodov

    The local mechanical properties near the View the MathML source ( 1 1 ̅ 0 ) Σ3/[111] symmetric tilt grain boundary in a high purity aluminum bicrystal were studied by the nanoindentation technique. The first pop-in load and its width significantly decreased near / at the grain boundary compared to those in the grain interior. The critical stress required to nucleate dislocations at the grain boundary was estimated to be notably lower than that in the grain interior. Moreover, it was found that the slope of the load-penetration curve prior to the first pop-in was smaller near the grain boundary than in the grain interior. This result was discussed in terms of possible micro plasticity effects and decreasing the elastic modulus in the close vicinity of the grain boundary.

    更新日期:2018-01-06
  • Microstructure evolution and micro-mechanical behavior of secondary carbides at grain boundary in a Fe–Cr–W–Mo–V–C alloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-06
    Jing Guo, Liqun Ai, Tingting Wang, Yunli Feng, Decheng Wan, Qingxiang Yang

    The microstructure evolution and micro-mechanical behavior of secondary carbides at grain boundary in a Fe–Cr–W–Mo–V–C alloy for cold work roll were systematically investigated in this study. The typical microstructures at the characteristic temperature of 1240 °C, 1200 °C and 1150 °C were observed by Optical Microscope and Field Emission Scanning Electron Microscope. The hardness values of secondary carbides were predicted and measured by first-principles calculation, Vickers hardness tester and nanoindentation technique. The fracture toughness (KC) values were calculated by a method known as Indentation Microfracture. Single-pass scratch tests were carried out to investigate the micro-scale wear behavior of secondary carbides. The macroscopic pin-on-disk test was also performed. The results show that the secondary carbide at grain boundary contains secondary MC, M2C and M7C3. The formation of secondary MC and M7C3 belongs to the precipitation and growth process, while secondary M2C is the result from the growth of eutectic M2C. In the studied alloy, M7C3 is a dominant carbide in quantity, and has higher hardness than secondary M2C and the matrix, and processes the better toughness than secondary MC, whose hardness almost reaches 30 GPa, and can also effectively resist the crack initiation and propagation, which therefore makes a significant contribution to the wear resistance of the alloy.

    更新日期:2018-01-06
  • Effect of prior austenitic grain size and tempering temperature on the energy absorption characteristics of low alloy quenched and tempered steels
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-06
    Elakkiya Mani, Thendralarasu Udhayakumar

    The aim of the present work was to investigate the combined effect of prior austenitic grain size and the tempering temperature on the energy absorption characteristics of low alloy quenched and tempered steel. High frequency (HF) electric resistance welded tubes made of Boron added low carbon steels were used for the study. Induction hardening at two different temperatures (above upper critical temperature) led to two different prior austenitic grain sizes 20 µm and 100 µm.Both the set of samples were subjected to conventional tempering at different temperatures and it resulted in variation in the distribution and morphology of the carbides. Microstructural evolution at each tempering temperature, carbide morphology and distribution of carbides was investigated using SEM. Low temperature tempering leads to precipitation of rod like carbides and with increase in tempering temperature the carbide morphology turns spherical followed by carbide coarsening at higher temperatures. Three point bend test of the tempered samples was carried out using Schimadzu Universal testing machine to determine the energy absorption characteristics. Test results indicate that optimum combination of high energy absorption and better mechanical properties was delivered by tempered martensite with fine spherical carbides and fine prior austenitic grain size. Thus the work established the correlation between energy absorption; prior austenitic grain size and tempering temperature in low alloy quenched and tempered steel.

    更新日期:2018-01-06
  • Simultaneously improving the strength and ductility of Fe–22Mn–0.6C twinning-induced plasticity steel via nitrogen addition
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-06
    H.K. Yang, Y.Z. Tian, Z.J. Zhang, Z.F. Zhang

    The effect of nitrogen addition on the mechanical properties of Fe–22Mn–0.6 C (wt.%) twinning-induced plasticity steel was studied. It was found that the stacking fault energies of the two steels were comparable, and the twinned grain fractions of FeMnC and FeMnC-N steels were similar before the true strain of 0.5. With increasing the strain to 0.7, the fraction of secondary twinned grain rose to support the further strong work-hardening rate of FeMnC-N steel. Moreover, the nitrogen addition suppressed the dynamic strain aging, which can trigger early shear fracture in FeMnC steel. Therefore, the work-hardening rate was kept increasing and the frequency of plastic instability was suppressed by nitrogen addition, and the ultimate tensile strength and uniform elongation simultaneously increased in FeMnC-N steel.

    更新日期:2018-01-06
  • Effects of Nb addition on the microstructures and mechanical properties of a precipitation hardening Cu-9Ni-6Sn alloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-06
    Minqiang Gao, Zongning Chen, Huijun Kang, Rengeng Li, Wei Wang, Cunlei Zou, Tongmin Wang

    The effects of Nb addition on the microstructures and mechanical properties of the casting Cu-9Ni-6Sn alloy were investigated in the present work. Microstructures of the as-cast and heat treated samples were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results demonstrate that, with the addition of Nb from 0.0 wt.% to 0.35 wt.%, the average grain size of Cu-9Ni-6Sn alloys in the as-cast condition is reduced from 570 μm to 156 μm due to the formation of Nb-bearing intermetallic compounds. The occurrence of NbNi3 particles after solution treatment is considered as an important factor that leads to the postponed hardening response to age treatment in the early stage. For the peak-aged samples with 0.35 wt.% Nb addition, the tensile strength is increased from 768.9 MPa to 913.3 MPa when compared to that of the Nb-free samples. In addition, for the peak-aged samples, the hardness increment is mainly attributed to the coherent stress generated by the ordered γ' phases, which results in practically identical hardness (~350 HV) regardless of the Nb additions. Furthermore, an optimized combination of UTS (719.5 MPa), elongation (13.76%) and electrical conductivity (11.37% IACS) of the Cu-9Ni-6Sn alloy can be achieved by the 0.35 wt.% Nb addition accompanied with aging treatment at 375 ℃ for 1 h.

    更新日期:2018-01-06
  • Effect of heat-treatment on microstructural evolution and mechanical behaviour of severely deformed Inconel 718
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-06
    Prabhat Chand Yadav, Sandeep Sahu, Anandh Subramaniam, Shashank Shekhar

    Severe plastic deformation (SPD) techniques impose very high level of strains and it can enhance the strength of a material several folds. In the current work, Inconel 718 alloy was severely deformed by machining process resulting in inherently “bi-modal” grain size distribution consisting of sheared zone with nano-structured grains and moderately refined grain zone. Hardness of machined chips were found to be much higher than that of bulk and increased further upon giving heat-treatment because of precipitation of γ’’ and γ’ nano-precipitates. However, as with most severely deformed materials, ductility of the machined chip is known to be very low, primarily because of the presence of large fractions of dislocation-saturated nano-structured grains which hinder any more dislocation generation or movement. In this work, we gave short heat-treatment to these deformed samples at elevated temperature to ensue controlled recrystallization in the sheared zone. However, heat-treatment is also expected to result in coarsening of precipitates as well as the grains of the matrix. This phenomenon may, not only reduce the strength, but may also reduce the pinning ability of the precipitates which endow the microstructure with thermal stability. Hence, the specific objective of this work is to understand the interplay of grain boundary pinning and recrystallization, both of which occur at elevated temperatures. Short heat-treatment of the severely deformed samples was performed for 15 minute between 700 °C to 900 °C. It was found that temperatures up to 800 °C do not lead to appreciable recrystallization, while 900 °C heat-treatment can cause appreciable recrystallization, albeit, limited to the shear zone. Size of precipitates was also found to grow with increasing temperature, nonetheless, samples heat-treated at 900 °C were found to be thermally stable with a good fraction of coincidence site lattice boundaries, low grain size and improved hardness.

    更新日期:2018-01-06
  • Compressive behavior of a rolled open-cell aluminum foam
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-06
    Vasanth Chakravarthy Shunmugasamy, Bilal Mansoor

    In this paper, we investigate compressive behavior of an open-cell 6101 aluminum foam in as-cast and as-rolled conditions. The as-cast foam with a relative density of ρ⁎ = 7% was rolled to create two distinct as-rolled conditions with relative densities of ρ⁎ = 29% and ρ⁎ = 42%, respectively. The quasi-static and high strain rate compressive behavior was studied over a range of strain rates 10−4 /s ≤ View the MathML source ε ̇ ≤ 5×103 /s. It was found that the internal structure of as-rolled foams significantly differs from as-cast foams. Rolling causes plastic bending, stretching, and buckling of individual struts and ligaments, resulting in reduced pore size and an increase in relative density. In as-rolled condition, compressive plateau strength increased with increase in ρ⁎; it was up to 26 times higher as compared to the as-cast condition. In the range of strain rates considered here, strain rate sensitive compressive behavior was not observed; instead, the intrinsic strain rate sensitivity of parent material governed the compressive behavior in all conditions. Our results demonstrate the feasibility of using conventional rolling as a simple way to modify internal structure and relative density of as-cast foams, thereby enabling mechanical properties more attractive for various structural applications.

    更新日期:2018-01-06
  • An effective method to obtain Cu-35Zn alloy with a good combination of strength and ductility through cryogenic rolling
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-05
    Pengfei Wang, Jinchuan Jie, Chenbin Liu, Lijuan Guo, Tingju Li

    In the present study, commercial Cu-35Zn α-brass sheets were subjected to cryogenic rolling (CR) to obtain samples with different amounts of deformation in the thickness direction. A self-designed liquid nitrogen cooling system that can simultaneously cool the work rollers and samples was used to ensure an ultra-low temperature condition during the rolling process. The grains, deformation twins, and dislocation density of the samples were studied by optical microscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). Uniaxial tensile tests and Vickers hardness measurements were conducted to measure the mechanical properties of the samples. The microstructures and mechanical properties of CR samples were apparently improved compared to the room temperature rolling (RTR) samples with increasing deformation amount. As a typical example, when the deformation amount is 90%, the CR sample possesses ultrafine microstructures and demonstrates extraordinary mechanical properties. The average tensile and yield strengths of a 90% deformation CR sample can be improved to 835.3 MPa and 711.5 MPa, while those of a 90% deformation RTR sample are 718.6 MPa and 481.2 MPa. The average elongation of the CR sample is 2.9%, which is acceptable compared with the RTR sample whose average elongation is 3.1%. The ultrafine microstructures containing ultrafine grains, high density dislocation, and nanometer scale deformation twins in the 90% deformation CR sample may be the main reason for its extraordinary mechanical properties. Therefore, samples with a good combination of strength and ductility were obtained in the present study. This may be a valuable exploration to fabricate Cu-35Zn alloy sheets with excellent microstructures and mechanical properties.

    更新日期:2018-01-06
  • Influence of stress state on microstructure evolution of AZ31 Mg alloy rolled sheet during deformation at room temperature
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-05
    Dabiao Xia, Guangsheng Huang, Qianyuan Deng, Bin Jiang, Shuaishuai Liu, Fusheng Pan

    In this paper, two kinds of micro-tensile test systems for in-situ Electron Back-Scattered Diffraction (EBSD) are developed to explore the difference in microstructure evolution of AZ31 alloy under biaxial tensile stress state (BTSS) and uniaxial tensile stress state (UTSS). The in-situ EBSD results show that there is an apparent difference of texture evolutions during deformation under BTSS and UTSS and the different responses of tensile twins (TTs) in various stress states are proved to be the main reason. A modified calculation of Schmid factor (SF) for varied stress states further reveals that the distribution of TT SF on {0001} pole figure (PF) determined by BTSS significantly differs from that under UTSS. In addition, the change of stress states also affects the deformation compatibility of TT to slip systems. The poor formability of Mg alloy rolled sheets partly results from the incompatibility between TT and slip systems or tensile twinning under BTSS.

    更新日期:2018-01-06
  • Dislocation binding as an origin for the improvement of room temperature ductility in Mg alloys
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-05
    Ki-Hyun Kim, Ji Hyun Hwang, Hyo-Sun Jang, Jong Bae Jeon, Nack Joon Kim, Byeong-Joo Lee

    Improving room temperature ductility and formability is a bottleneck for a wide industrial application of Mg alloys, but even the mechanism for the effect of alloying elements on the deformation behavior of Mg is not clearly known. Here, using a molecular dynamics simulation, we clarify the role of alloying elements in improving the room temperature ductility of Mg alloys: Solute atoms have stronger dislocation binding tendency and solid solution strengthening effect on basal <a> slip planes than on non-basal <c+a> slip planes, reduce the anisotropy in the critical resolved shear stress between slip systems, and eventually improves the room temperature ductility. We predict that any solute elements with a size difference from Mg can improve the room temperature ductility, once the alloying amount is carefully controlled. By proving the validity of the prediction experimentally, we provide a new guide for designing Mg alloys with improved room temperature ductility and formability.

    更新日期:2018-01-05
  • Fatigue properties of Ti-6.5Al-3.5Mo-l.5Zr-0.3Si alloy produced by direct laser deposition
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-05
    Zheng Liu, Pengfei Liu, Lu Wang, Yunzuo Lu, Xing Lu, Zuo-Xiang Qin, Hua-Ming Wang

    The fatigue properties of the Ti-6.5Al-3.5Mo-l.5Zr-0.3Si alloy produced by direct laser deposition (DLD) have been investigated. The fatigue limit of the alloy perpendicular to and parallel to the deposition direction at specified life of 107 and stress ration of 0.1 is 337 MPa and 365 MPa, respectively. The porosities of specimens are among 0.014%~0.028%, averaged porosity is 0.02%. The fatigue cracks initiate at the pores near the surface, which are located mostly at the column grain boundary during solidification. The size of pore and its distance from the surface are two important factors to control the fatigue life of specimen, i.e., bigger the pore or nearer to the surface, shorter the fatigue life. The fatigue fracture consists of fatigue source, radial slow crack growth and rough fast fracture region. The columnar grains boundaries of Ti-6.5Al-3.5Mo-l.5Zr-0.3Si alloy by epitaxial growth from the underlying template grains layer-by-layer during direct laser deposition are the weak joints under the cyclic stress and crack formed propagates preferentially along the grain boundaries, which should be a key factor leading to the difference of fracture morphology in the region of fatigue source and final fast fracture, and also the lower fatigue limit of specimen perpendicular to the deposition direction than that parallel to the deposition direction.

    更新日期:2018-01-05
  • The Deformation Induced Martensitic Transformation and Mechanical Behavior of Quenching and Partitioning steels under Complex Loading Process
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-05
    DQ. Zou, SH. Li, Ji He, Bin Gu, YF. Li

    The quenching and partitioning (QP) steels have received much attention due to its high strength and good ductility, which results from deformation induced martensitic transformation (DIMT). However, due to the stress state, temperature and strain rate dependent DIMT behavior, the DIMT related flow behavior of QP steels might be more complex than other advanced high strength steels (AHSS) without martensitic transformation. During forming process of such materials (non-linear loading process), effects of DIMT behavior and DIMT related flow behavior on springback prediction are still unknown. In this paper, we focused on the martensitic transformation behavior of QP steels under complex loading (linear loading and cyclic loading). The X-ray diffraction and digital image correlation techniques were performed to monitor the retained austenite evolution and strain history of the specimen, respectively. Then, flow behaviors of QP sheets steels subjected to complex loading are analyzed and compared with DP980 sheets steels (without DIMT). Finally, modified Yoshida and Uemori model calibrated by two different cyclic loading tests, i.e., tension-compression test and cyclic shear test, are adopted in this study to investigate the stress state dependent DIMT effect on springback prediction. The results show, for QP steels, the DIMT is stress state dependent. Accordingly, compared with DP steels, its flow behavior exhibits remarkable stress state sensitivity. Due to these special DIMT behavior and flow behavior of QP steels, its springback prediction should be treated differently compared with DP steels.

    更新日期:2018-01-05
  • Micro-crack Initiation and Propagation in a High Strength Aluminum Alloy during Very High Cycle Fatigue
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-05
    Luopeng Xu, Qingyuan Wang, Min Zhou

    The behavior of the new AA2198-T8 high strength aluminum alloy during tensile and very high cycle fatigue (VHCF) tests is studied. The results of the tensile tests show that the alloy in this heat treatment state has excellent mechanical properties compared with the same alloy with other heat treatment conditions and the 2024 and 7075 conventional aluminum alloys. The VHCF tests show a fatigue life plateau with the fatigue life reaching 109 cycles under a stress amplitude of 200 MPa, suggesting the existence of an endurance limit. Two different crack initiation modes are observed under scanning electron microscopy (SEM). Cracks tend to initiate from the interior of the specimen when fatigue life is in the VHCF regime. The interior crack initiation mode has a characteristic of a fish-eye, at the center of which is a white rough area (WRA) where the original micro-crack initiates. The thermo-mechanical process during ultrasonic fatigue is also studied. Quantitative analyses on micro-crack initiation and propagation show that the threshold stress intensity factor for stable micro-crack propagation is 1.66 MPa·m1/2, and the threshold stress intensity factor for unstable macro-crack propagation is 3.17 MPa·m1/2. Finally, analyses based on the Paris-Hertzberg law suggest that the micro-crack initiation stage likely consumes most of the fatigue life. The analyses do not apply to fatigue lives less than 106 cycles. The prediction of fatigue life is confirmed by the analyses of the thermo-mechanical process.

    更新日期:2018-01-05
  • A Mechanism for Energy Absorption: Sequential Micro-kinking in Ceramic Reinforced Aluminum Alloy Lattices during Out-of-plane Compression
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-05
    B. Yu, K.H. Chien, K. Abu Samk, G.D. Hibbard

    The current study examines the out-of-plane compressive response of ceramic/aluminum hybrid lattice materials with an anodic alumina outer shell and an aluminum alloy core. The combination of increasing truss angle and oxide coating thickness results in a six-fold increase in compressive strength, two-fold increase in densification strain, twelve-fold increase in energy absorption per volume, and twenty-fold increase in energy absorption per mass. Such improvements are caused by the change in failure mode from mid-strut buckling to a hinge kinking mode as the oxide coating thickness increases. Microscopy and analytical modelling reveal that kink formation is most likely initiated by aluminum shear band formation followed by oxide rotation and fracture (analogous to micro-kinking in fiber composites). In terms of energy absorption, the best performing lattice materials in the current study were on par with the best available cellular materials in existing literature.

    更新日期:2018-01-05
  • Second-phase hardening and rule of mixture, microbands and dislocation hardening in Fe67.4-xCr15.5Ni14.1Si3.0Bx (x=0, 2) alloy systems
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-04
    Mohsen Askari-Paykani, Hamid Reza Shahverdi, Reza Miresmaeili, Hossein Beladi

    The work-hardening mechanisms of two novel advanced high-strength steels (Fe67.4-xCr15.5Ni14.1Si3.0Bx [x = 0 (0B), 2 (2B)] wt.%) were investigated by means of field emission gun scanning electron microscopy coupled with angle-selective backscattered detection, transmission electron microscopy, and electron backscattered diffraction. The 0B and 2B specimens combined low yield stresses and high ultimate tensile strengths with good total elongation percentages, with results of 219 MPa, 568 MPa, and 83% and 357 MPa, 703 MPa, and 42%, respectively. The 0B and 2B alloys were characterized by a decreasing work hardening rate, followed by a constant and finally a steep decreasing change tendency. Detailed angle-selective backscattered and electron backscattered diffraction microscopy observations on interrupted tensile test specimens revealed that the work hardening rate in these alloys was facilitated by planar (extended stacking faults) and wavy (dislocation cell and wavy microbands) characteristics and mechanical nano-twins. The total flow stresses of the 0B and 2B specimens were calculated from the dislocation density and twin spacing. This indicated that the work hardening contribution of the microband mechanism can be estimated via a dislocation hardening formula. The rule of mixture was also used to evaluate the effect of a boron addition on the total flow stress of the 2B specimen; this illustrated that, in addition to the strengthening contribution of the second hard phase to the yield stress, the rule of mixture must also be considered. The calculated values of the contribution of the mechanical nano-twins and dislocations on the work-hardening for 0B and 2B specimens were about 62% and 18.6% and 52% and 31.8%, respectively.

    更新日期:2018-01-05
  • The influence of grain boundaries and grain orientations on the stochastic responses to low load nanoindentation in Cu
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-04
    B.J. Schuessler, P.C. Wo, H.M. Zbib

    Mechanical properties that are considered to be deterministic in the macro-scale have been shown to be stochastic in the sub-micron length scale. The origin of such stochastic responses is not well understood. This work examines the potential influence of grain boundaries and grain orientations on the stochastic nature of pop-in and hardness measurement in annealed high purity polycrystalline Cu samples during low load nanoindentation. Statistical analysis on pop-in load and hardness showed that variations of these measurements depend on crystal orientations and is influenced by the indenter probe size. Analysis on the pop-in load statistics showed that pop-ins are likely initiate from an atomic sized precursor that leads to dislocation generation or expansion. Variation in hardness measurements near an arbitrary chosen grain boundary and the apparent grain boundary hardening effect observed may be related to the higher density of dislocations at and near the grain boundary.

    更新日期:2018-01-05
  • Anisotropy of slip behaviour in single-colony lamellar structures of Ti‒6Al‒4 V
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-04
    Yin Htwe, Kwangsik Kwak, Daichi Kishi, Yoji Mine, Rengen Ding, Paul Bowen, Kazuki Takashima

    Micro-tensile tests on single-colony specimens of Ti‒6Al‒4 V alloy with a fine lamellar microstructure revealed that the critical resolved shear stress of the basal slip was lower than that of the prismatic slip. This has been attributed to easier slip transmission within the habit plane at the α/β boundaries.

    更新日期:2018-01-05
  • Enhancement of the Strength and Ductility of Martensitic Steels by Carbon
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-04
    Ke Zhang, Ping Liu, Wei Li, Fengcang Ma, Zhenghong Guo, Yonghua Rong

    For most engineering materials, the properties of strength and ductility are mutually exclusive, and no process has been reported to enhance the strength and ductility of steels by carbon. Nevertheless, the quenching and partitioning (Q&P) process, which was originally proposed in 2003, is a promising method for the fabrication of carbon-alloyed steels with high strength and ductility. In this work, we show that the development of a composite-like microstructure in steels during Q&P process can enhance the strength and ductility of Fe–Mn–Si–Nb–C steels containing 0.2–0.4 wt% C. In contrast to the traditional quenching and tempering process, the novel Q&P (or Quenching-Partitioning-Tempering) process can link strength enhancement, which is conferred by the hard martensite phase, with ductility enhancement, which is conferred by the retained soft austenite fraction. The origin of such different effects of carbon is revealed by the microstructural characterization.

    更新日期:2018-01-04
  • The role of the microstructure on the influence of hydrogen of some advanced high-strength steels
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-04
    Qinglong Liu, Qingjun Zhou, Jeffrey Venezuela, Mingxing Zhang, Andrej Atrens

    The role of microstructure was studied for dual-phase (DP), quenched and partitioned (Q&P), and twinning induced plasticity (TWIP) steels. The hydrogen influence changed the fracture mode at the ultimate tensile strength, there being no subcritical crack growth at a lower stress. The fractures initiated (i) in the hard martensite and/or at the interfaces of ferrite and martensite for DP steels, (ii) in the martensite and/or at the interfaces of retained austenite and martensite for Q&P steels, and (iii) at mechanical twins for TWIP steels. Tempering may improve the resistance to hydrogen of DP and Q&P steels.

    更新日期:2018-01-04
  • Different precipitation hardening behaviors of extruded Mg–6Gd–1Ca alloy during artificial aging and creep processes
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-03
    Rongguang Li, Jinghuai Zhang, Guangyan Fu, Lin Zong, Beitao Guo, Yongmei Yu, Yong Su, Yongsheng Hao

    How to effectively improve the precipitation hardness response of Mg alloys with relatively low RE content is an interesting and important topic. In the present study, an extra stress is added instead of just exposing the sample (extruded Mg–6Gd–1Ca alloy) in high temperature (175 °C) to modify the precipitation behavior and improve the hardness of alloy. Microstructural examinations of the aged alloy by transmission electron microscopy (TEM) show that the weak age-hardening response is mainly attributed to the low number intensity and relatively large size of precipitates in grains. The results suggest that the creep process can effectively promote precipitation response compared with the aging process. TEM analyses indicate a large amount of nano-scale precipitates (espically β' phase) can be formed in grains of the crept alloy, which is mainly responsible for the obvious hardness increase of the alloy after creep. Electron backscattered diffraction (EBSD) data suggest that the dislocation increase caused by strain during creep is responsible for the precipitate increase through providing more nucleation sites.

    更新日期:2018-01-04
  • Improvement of interfacial interaction and mechanical properties in copper matrix composites reinforced with copper coated carbon nanotubes
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-03
    Hu Wang, Zhao-Hui Zhang, Zheng-Yang Hu, Qi Song, Shi-Pan Yin, Zhe Kang, Sheng-Lin Li

    The incorporation of low-dimensional nanofillers into 3D metal matrix is promising to translate their excellent properties from nanoscale to the macroscopic world. However, the design of firm nanocarbons and efficacious fabrication of such advanced composites remain challenging. In this paper, we report an optimized strategy of carbon nanotubes (CNTs) for reinforcing copper by combining the use of electroless deposition (ED), spark plasma sintering (SPS) and hot-rolling. We finished a perfect match of enhanced yield strength, high plasticity, and good electrical conductivity (e.g. 264 MPa, 29%, 96.6% IACS for 1 vol.% CNTs/Cu) in 3D bulks, which is attributed to inherent properties and strong interfacial bonding. In addition, the particularly aligned arrays of CNTs contributed to the transfer stress from the matrix without sacrificing ductility and conductivity. We revealed the strengthening and toughening mechanisms of CNTs in the CNTs/Cu composite through the dislocation theory. This study provides a new approach for the designing and fabricating of novel low-dimensional nanomaterials into 3D metal matrix.

    更新日期:2018-01-04
  • The effect of nano β-TCP on hot compression deformation behavior and microstructure evolution of the biomedical Mg-Zn-Zr alloy
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-03
    Haoran Zheng, Zhen Li, Minfang Chen, Chen You, Debao Liu

    Aiming to investigate the effects of nano β-tricalcium phosphate (β-TCP) on the hot deformation mechanism of Mg-Zn-Zr alloy (MZZ) and confirm the optimum hot working conditions of 2β-TCP/Mg-3Zn-0.8Zr composites (MZZT), thermal simulation test at deformation temperature and strain rate in the range of 523–673 K and 0.001–1 s−1 was performed on MZZ and MZZT by using Gleeble-1500 simulator. The results of strain-stress behavior indicated that the flow stress of MZZT is higher than that of MZZ under the same hot deformation condition in the deformation temperature range of 523–623 K, whereas this trend opposed when specimens deformed at 673 K. Based on the establishment of constitutive equation, the average activation energy of MZZT for hot deformation is calculated to be 184.1 kJ/mol, and almost 60 kJ/mol higher than that of MZZ which can be attributed to the increasing of deformation resistance after the addition of β-TCP particles. Furthermore, according to the processing map constructed at strains of 0.8 and microstructure observation, the optimum hot working condition for MZZT with higher value of power dissipation (37%) is determined to be 600–645 K and 0.01-0.001 s−1.

    更新日期:2018-01-04
  • Effect of compound addition of Nb-B on hot ductility of Cr-Mo alloy steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-03
    Yaxu Zheng, Fuming Wang, Changrong Li, Jin Cheng, Yongliang Li

    The hot tensile tests were conducted in this study to investigate the combined effect of Nb and B on hot ductility of 25CrMo alloy steel in temperature range of 700–1100 °C with a strain rate of 0.5 s−1. Besides, the influences of DRX grains growth behavior, ferrite transformation, precipitates and B non-equilibrium segregation on hot ductility were also investigated by use of OM, FE-SEM, TEM, AES and SIMS. The results indicated that the amount of Nb(C,N) particles decreased with increasing deformation temperatures. Moreover, the precipitation of BN on Nb(C,N) particles, which precipitated in matrix abundantly rather than at grain boundaries, reduced stress acting on grain boundaries, and the coarsening BN particles precipitating on Nb(C,N) reduced the pining effect of fine Nb(C,N) particles on dislocations. Besides Nb(C,N) and BN particles, there were M2B, M23C6 and M7C3 precipitates in γ+α two-phase region. The mechanism for hot ductility improvement by compound addition of Nb-B in single phase austenite region was that although the DRX was retarded, the DRX grains were refined obviously by adding Nb and B. Meanwhile, the B atoms segregated rapidly to grain boundaries occupying the voids and enhancing grain boundary cohesion. Besides, the BN particles, precipitating on Nb(C,N) particles in matrix rather than at grain boundaries, reduced the adverse effects on hot ductility. The improvement of hot ductility in γ+α two-phase region was attributed to the restraint of ferrite transformation by the addition of B and the deformation induced precipitation of carbides, which consumed strengthening alloy elements in austenite.

    更新日期:2018-01-03
  • Mechanical properties and thermal stability of pure W and W-0.5 wt %ZrC alloy manufactured with the same technology
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-02
    H.W. Deng, Z.M. Xie, Y.K. Wang, R. Liu, T. Zhang, T. Hao, X.P. Wang, Q.F. Fang, C.S. Liu

    The mechanical properties, microstructure and thermal stability of hot rolled pure W and W-0.5 wt%ZrC alloy manufactured using the same technology are systematically studied. The results of Vickers hardness and tensile tests show that the W-0.5 wt%ZrC alloy has a higher hardness, ultimate tensile strength, ductility and thermal stability compared to pure W. The recrystallization temperature of W-0.5 wt%ZrC alloy is about 1300 °C which is 100 °C higher than that of hot rolled pure W (1200 °C). The ductile to brittle transition temperature (DBTT) of W-0.5 wt%ZrC alloy is 50 °C lower than that of the rolled pure tungsten. The above results indicate that at the fully same fabrication technology, the trace ZrC does improve the mechanical properties and thermal stability of W. The possible micro-mechanisms were investigated and proposed.

    更新日期:2018-01-03
  • Is there an optimal grain size for creep resistance in Ni-based disk superalloys?
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-02
    Louis Thébaud, Patrick Villechaise, Coraline Crozet, Alexandre Devaux, Denis Béchet, Jean-Michel Franchet, Anne-Laure Rouffié, Michael Mills, Jonathan Cormier

    The high temperature creep properties of next generation cast and wrought AD730 superalloy have been investigated taking into consideration three microstructural parameters: the grain size, the presence of grain boundaries and the γ′ precipitates size and distribution. Definitive analysis of the influence of the grain boundaries and γ′ precipitates size distribution has been enabled by the study of single crystalline versions of the polycrystalline alloys studied. At high temperature (equal to or in excess of 850 °C), the grain size controls creep properties. Comparisons between polycrystalline and single crystalline specimens indicate that the grain boundaries provide a strengthening effect, especially in the small strain regime. At intermediate temperature (700 °C), the γ′ precipitates size is the main creep-rate controlling parameter. In this temperature domain, creep strength seems to be mainly controlled by dislocation motion. A very striking grain boundary strengthening mechanism is observed at small creep strain and intermediate temperature.

    更新日期:2018-01-03
  • Strengthening of cobalt-free 19Ni3Mo1.5Ti maraging steel through high-density and low lattice misfit nanoscale precipitates
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-02
    K. Li, B. Yu, R.D.K. Misra, G. Han, S. Liu, C.J. Shang

    The concept of low lattice misfit and high-density of nanoscale precipitates obtained through solution treatment was adopted to obtain ultrahigh strength maraging steel without compromising elongation. An “ultrahigh strength-high toughness” combination was successfully obtained in 19Ni3Mo1.5Ti maraging steel with ultimate strength of ~1858 MPa and static toughness of ~110 MJ·m−3. Maraging steel had extremely high density (2.3×1024 m−3) of nanoscale precipitates with minimum lattice misfit of less than 1% at the solutionization temperature of 820 °C. Two kinds of nanoscale precipitates, namely, η-Ni3(Ti,Mo) and B2-Ni(Mo,Fe) contributed to ultrahigh strength. The size of nanoscale precipitates governed the movement of dislocations, cutting versus by-passing. Theoretical estimate of ordering and modulus contribution to strengthening suggested that ordering had a dominant influence on strength. The toughness was closely related to the characteristic evolution of nanoscale precipitates such that the high density of nanoscale precipitates contributed to increase of elastic deformation and low lattice misfit contributed to increase of uniform deformation. The nanoscale size and low lattice misfit of precipitates were the underlying reasons for the high-performance of maraging steel. Moreover, the combination of high-density of nanoscale precipitates and low lattice misfit is envisaged to facilitate the futuristic design and development of next generation of structural alloys.

    更新日期:2018-01-03
  • Influence of Original Austenite Grain Size on Tensile Properties of a High-manganese Transformation-induced Plasticity (TRIP) Steel
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-01
    Xing Li, Liqing Chen, Yang Zhao, Xiaoyun Yuan, Raja Devesh Kumar Misra

    Original austenitic grain size and stacking fault energy (SFE) of a high-Mn transformation-induced plasticity (TRIP) steel was varied by annealing at different temperatures. The microstructural evolution during cooling and tensile deformation were investigated to explain the work hardening behavior. The study suggested that the amount of ε-martensite was increased with annealing temperature. When SFE was greater than 13.3 mJ/m2, twin may form in austenite on cooling; and ε-martensite cannot be thermally induced in austenite once its SFE was higher than 20.6 mJ/m2. Work hardening behavior of the steel annealed at different temperatures could be divided into two stages using Hollomon analysis. The grain refinement strengthening and deformation induced γ→ε transformation rendered 800 °C annealed sample to have the largest work hardening exponent in Stage-I, in this case ultimate tensile strength was also highest. In Stage-II, significant deformation-induced ε→α’ transformation in samples annealed at high temperatures resulted in greater improvement of work hardening exponent. The elongation of the steel was also improved with annealing temperature because local stress concentration was effectively released by ε→α’ transformation.

    更新日期:2018-01-01
  • Influence of Aging on Twin boundary Strengthening in Magnesium Alloys
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-01
    Jianwei Teng, Xiaojuan Gong, Yunping Li, Yan Nie

    In order to investigate the effect of aging on twin boundary (TB) strengthening in magnesium alloys, the cyclic compressions along two orthogonal directions of samples with and without intermediate aging are carried out. The results show that the grains in all Mg alloys AZ31, AZ61 and AZ91 are significantly refined by twin boundaries (TBs). The TB strengthening is strongly dependent on both alloying element content and aging. Although grain refinement by TB formation occurs in AZ31, it exhibits slight strengthening effect even after aging. In contrast, significant TB strengthening after aging is observed in Mg alloys AZ61 and AZ91 with high contents of alloying elements.

    更新日期:2018-01-01
  • High-strength Mg-6Zn-1Y-1Ca (wt. %) alloy containing quasicrystalline I-phase processed by a powder metallurgy route
    Mater. Sci. Eng. A (IF 3.094) Pub Date : 2018-01-01
    J. Medina, P. Pérez, G. Garcés, A. Stark, N. Schell, P. Adeva

    A high-strength Mg-6Zn-1Y-1Ca (wt.%) alloy has been processed by a powder metallurgy route. Rapidly solidified powders with a particle size below 100 μm were used as a way for preventing formation of ternary MgZnCa compounds during subsequent extrusion at 250 °C. The microstructure of the extruded alloy consists of an ultrafine-grain magnesium matrix, with an average grain size of 444 nm, embedding a high volume fraction of fine I-phase particles aligned along the extrusion direction. The alloy combines an excellent ductility (14% of elongation to failure) with a high strength (ultimate strength of 469 MPa and yield stress of 461 MPa) at room temperature, mainly due to grain size refinement (around 70% of the yield stress). The strength is kept high up to 150 °C (yield stress of 279 MPa). Above this temperature, the mechanical strength falls to very low values but the ability to deform plastically is considerably enhanced, exhibiting superplastic behaviour from 200 to 350 °C, with a maximum elongation of 477% at 350 °C.

    更新日期:2018-01-01
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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