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CN 21-1139/TG
Started in 1956

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    LU Fayun, YANG Ping, MENG Li, WANG Huizhen
    Acta Metall Sin, 2013, 49 (1): 1-9.  DOI: 10.3724/SP.J.1037.2012.00403
    Abstract   PDF (1287KB) ( 953 )

    High manganese steels demonstrate significant potential for industrial application due to their remarkable TRIP/TWIP effects at room temperature. Thus, they attract interest of many researchers. In this work, mechanical properties and microstructure evolutions of an Fe-22Mn-3Si-2Al TRIP/TWIP steel after tensile deformation were studied by mechanical tests and SEM analysis, and in particular, the crystallography of martensitic transformation was analyzed by EBSD technique. The results show that, plenty of harder thermalε-martensite existed before deformation and transformed to α’ -martensite during tensile deformation, namely the two kinds of martensite transformed asynchronously. During deformation, the TRIP effect by strain induced α’-martensite and the TWIP effect by deformation twinning coexisted, leading to good combination of strength and ductility in this steel, and furthermore, ε-martensite didn't cause the low temperature brittleness. The thermal ε-martensite showed tilting basal texture. When deformed,basal slip was an easy activity, whereas deformation twinning of 86°<1120> was also detected, which sometimes revealed 93°<7253> relationships by a combination of 30°<0001> misorientation due to mis-indexing of pseudosymmetry during EBSD measurement. During tensile deformation α’-martensite revealed a rotation around crystallographic <110> axis and scattered orientations, forming the phenomenon of comets trailing. Besides, austenitic orientations also rotated and increased low angle misorientations. And annealing twins in austenite was found to facilitate the martensitic transformation.

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    ZHANG Jikui, KANG Xinmeng, JIAO Qixiang, CHENG Xiaoquan, LI Zhengneng
    Acta Metall Sin, 2013, 49 (1): 10-16.  DOI: 10.3724/SP.J.1037.2012.00469
    Abstract   PDF (1001KB) ( 522 )

    SiCP/Al alloy is one of most competitive particle reinforced metal matrix composite materials. For the SiCP/Al alloy composites that volume fraction is lower than 25%, the mechanical properties were well known since many researcher worked hard in recent years. But when the volume fraction is higher than 35%, the mechanical properties of SiCP/Al alloy were not understood so well. In this paper, the tension properties were tested for the high volume fraction (55%) SiCP/Al alloy composites, the stress~strain curve showed obvious nonlinear properties. Macroscopic and microscopic fractography indicated that the fracture section of high volume fraction SiCP/Al alloy looks flat like traditional brittle materials, the shape of SiC particle was irregular and with sharp angle, some micro--defects such as void, crack and interface debonding exist inside the materials. A cell volume element model of polygonal prism with sharp angle at both ends was developed to simulate the mechanical behavior of high volume fraction. This model could simulate the irregular shape and uneven distribution of the SiC particle. Furthermore, the volume element cell and multi--particles combined cell with defects were built to consider the effect of the void and interface debonding. The analysis results showed that the interface debonding was the most important cause of the nonlinear mechanical behavior of high volume fraction SiCP/Al alloy composites.

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    DU Hongfei,ZENG Pan, ZHAO Jiaqing,LEI Liping, FANG Gang, QU Timing
    Acta Metall Sin, 2013, 49 (1): 17-25.  DOI: 10.3724/SP.J.1037.2012.00479
    Abstract   PDF (1570KB) ( 741 )

    When NiTi shape memory alloys thin strips under the uniaxial tensile deformation, the stress-induced martensitic transformation tends to exhibit strain localization and instability, and the sample shows reversible transformation bands evolution on macroscopic. The displacement, strain and temperature fields were investigated with in situ optical method, and the full-field strain and temperature information about martensitic localization were quantitatively obtained during uniaxial loading—unloading conditions. Strain field is calculated by digital image correlation (DIC) method, and the temperature field is captured by infrared thermograph, the thickness field and in-plane rotation angle are also calculated by DIC data.The strain, temperature variation, thickness, in-plane rotation both inside and outside of the transformation bands were studied when it nucleation, expansion, combination, reduction and disappear. The results show that transformation strain of the samples are mainly concentrated inside the transformation bands but small outside, and temperature variation mainly concentrated in the transformation fronts, the thickness field in transformation bands is 2% smaller than out of bands. In-plane rotation angle is not only concentrated in the transformation fronts, but also heterogeneous in the transformation bands. In addition, the maximum in-plane rotation angle during tension is 1.5 °. The whole loading-unloading progress is full thermal coupling, transformation localization, martensite and austenitic critical nucleation stress are greatly influenced by temperature variation.

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    WANG Bin, LIU Zhenyu, ZHOU Xiaoguang, WANG Guodong
    Acta Metall Sin, 2013, 49 (1): 26-34.  DOI: 10.3724/SP.J.1037.2012.00372
    Abstract   PDF (962KB) ( 661 )

    In order to meet both the development requirements for reduction in cost and strengthening, recently,the research of precipitation of cementites, as the most economical and common precipitates in steels, has drawn wide attention in the field of precipitation strengthening again, because if cementites could be effectively refined to the scale of a few nanometers, it could also generate very strong precipitation strengthening effects to replace the strengthening role of the precipitates of micro-alloying elements. However, the cementites in hypoeutectoid steels usually form lamellar pearlite structure in near-equilibrium conditions, unable to form the precipitation of nanoscale particles, and they tend to be coarsened significantly at high temperatures after hot rolling. Therefore,the non-equilibrium precipitation of cementites only could be realized by increasing cooling rate after hot rolling, and the thermodynamic feasibility for the formation of nanoscale cementite precipitates during cooling has to be determined. In this work, according to the austenitic transformation mechanism of KRC and LFG models in Fe—C alloys, the transformation driving force of undercooled austenite was calculated systematically in a thermodynamic view, and the effect of ultra fast cooling (UFC) after hot rolling on the precipitation behavior ofnano-scale cementite particles was investigated. Based on the calculation results, the driving force of degenerated pearlitic transformation is the most negative in the three transformation mechanisms, at the same undercooled temperature, which theoretically indicates that the degenerated pearlitic transformation of undercooled austenite can easily occur to form cementite and ferrite with the equilibrium concentrations. In practical manufacturing, the diffusion of carbon atoms could be restrained by decreasing temperature in short time in the application of UFC, as a result that cementites would most likely dispersed in the form of nano-scaled particles directly, rather than being fully grown up into lamellar pealites. Due to the UFC, a large number of dispersed nano-scaled cementite areas were found in the microstructure of hot-rolled hypoeutectoid experiment steels, where the size of the cementites was within the range of ten to tens nanometers. The precipitation of nano-scaled cementites was realized without the micro-alloying elements. Moreover, there were a lot of carbon-rich areas in the microstructure of undercooled austenite, based on the equilibrium concentration calculation, in which the local mole fraction of carbon could be from 0.04 to 0.08, and this part of austenite with the high carbon concentration was apt to decompose and form likely the precipitation of nano-scaled cementites.

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    JIA Xiaoshuai, ZUO Xunwei, CHEN Nailu, HUANG Jian, TANG Xinhua, RONG Yonghua
    Acta Metall Sin, 2013, 49 (1): 35-42.  DOI: 10.3724/SP.J.1037.2012.00351
    Abstract   PDF (1163KB) ( 1179 )

    This paper presents the mechanical properties and welding properties of Q235 steel with minimum yield strength of 235 MPa treated by a novel quenching-partitioning-tempering (Q-P-T) process.The experiments indicate that the strengths of Q-P-T treated Q235 steel (briefly called QPT235 steel) markedly raise compared with Q235 steel, and its yield strength and tensile strength are 435 and 615 MPa, respectively. In addition, the mechanical properties of the welding joint of QPT235 steel are markedly improved compared with Q235 steel when the same welding solder and process are performed for the steel with the two treatments, and the tensile strength and elongation of the former are about 532 MPa and 16.71%, respectively, while those of the latter are about 414 MPa and 12.4%. The microstructural characterization reveals two main factors resulting in the mechanical properties of QPT235 steel superior to those of Q235 steel: the grains of ferrite and interlamellar spacing of pearlite are both refined in the welding heat affected zone (HAZ), and a lot of widmanstatten structures in the welding joint of Q235 steel is avoided for QPT235 steel; there is a mixed microstructure of hard phases of martensite and bainite as well as remained austenite as soft phase in both base metal and HAZ, which replace parts of ferrite and pearlite in Q235 steel.

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    YANG Yan, LI Zili, WEN Chuang
    Acta Metall Sin, 2013, 49 (1): 43-50.  DOI: 10.3724/SP.J.1037.2012.00361
    Abstract   PDF (1329KB) ( 1004 )


    Alternating current (AC) corrosion is a significant problem to metal structures in close proximity to alternating current carrying conductors. While for direct current (DC) interference there is large agreement on protection criteria to be used for corrosion mitigation, but the mechanism and relationship between AC density and corrosion rate are still not clear, although some studies have been carried out for many years. In this work, electrochemical methods, including open curves, polarization curves and electrochemical impedance spectra (EIS) on X70 steel samples, were performed in soil-simulating conditions at various AC current densities from 0 to about 100 A/m2. The corrosion behavior and mechanism were discussed based on the electrochemical, XRD, stereo microscope and SEM results. The simulation experiment was set up to predict the effects of AC current density on corrosion potential and corrosion rates. The results indicate that the corrosion potential and corrosion rate increases with the rise of AC current density. When the alternating current goes across the X70 steel, its surface suffers from an obvious corrosion potential accompanied with the bubble crop up. It's because the hydrogen and oxygen reduction reactions happened on the surface of X70 steel. There is only one capacitive reactance arc on EIS at the corrosion potential under a low current density interference. Warburg impedance appears in a higher alternating current density of the low frequency area. It shows that the corrosion process of surface is controlled by diffusion. The amount of AC corrosion can be generally expressed as a percentage of the amount of DC corrosion with the equivalent corrosive effect. Alternating current causes less than 1% of the corrosion caused by the equivalent direct current. The X70 steel is corroded uniformly in the early AC interference, but corrosion pits appear gradually on the surface of X70 steel with the extension of time. Especially in the high current density interference, the pitting is more likely to occur. Furthermore, the higher current density increases pitting tendentiousness and depth. Alternating current has a major impact on metal polarization, and the asymmetry of positive and negative half cycle induces corrosion at alternating current interference. Alternating current could generate an “oscillating” effect in the metal/medium interface.
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    ZHU Li, YAO Meiyi, SUN Guocheng,CHEN Wenjue, ZHANG Jinlong,ZHOU Bangxin
    Acta Metall Sin, 2013, 49 (1): 51-57.  DOI: 10.3724/SP.J.1037.2012.00378
    Abstract   PDF (1023KB) ( 788 )

    The effect of Bi contents on the corrosion resistance of Zr-1Nb-xBi (x=0.05-0.3, mass fraction, %) was investigated in deionized water at 360 ℃ and 18.6 MPa by autoclave tests. The results show that the corrosion resistance of Zr-1Nb alloy can be improved by adding Bi,and the more the Bi content is, the better the corrosion resistance is.TEM and EDS analyses on the microstructures of the alloys show that there are two types of second phase particles (SPPs), including ZrNbFe and β-Nb. The Bi contents have little effect on the type, size and amount of SPPs, 0.3%Bi can be completely dissolved in α-Zr matrix and has no influence on the solution content of Nb inα-Zr matrix. From the fracture and inner surface morphology of oxide films observed by SEM, it can be seen that the Bi dissolved in theα-Zr could noticeably slow down the microstructural evolution of oxide film, including the propagation of micro--cracks and the transformation from columnar grains to equiaxed grains in the oxide film.

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    YU Honglei,LIN Xin, LI Junjie, WANG Lilin, HUANG Weidong
    Acta Metall Sin, 2013, 49 (1): 58-62.  DOI: 10.3724/SP.J.1037.2012.00569
    Abstract   PDF (651KB) ( 717 )

    Grain selection in directional solidification is a fundamental process in single-crystal super alloy production. In fact, many factors can influence this process. This work only focuses on the effect of pulling rates on competitive divergence in directional solidification. The competitive growth of bi-crystal atdifferent pulling rates was observed during 2D directional solidification using the transparent model alloy SCN-1.7%Eth (mass fraction). The results show that as the pulling rate increases, it becomes harder for preferred grain to eliminate slant grain. This is because the increase of pulling rate decreases primary dendrite arm spacing and slightly reduces the frequency of new primary dendrite arms. When the growth direction of preferred grain is not well aligned with temperature gradient, the preferred grain can also be eliminated by the slant grain at specific pulling rates.

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    ZENG Qiang, YAN Ping, SHAO Chong, ZHAO Jingchen, HAN Fengkui,ZHANG Longfei
    Acta Metall Sin, 2013, 49 (1): 63-70.  DOI: 10.3724/SP.J.1037.2012.00428
    Abstract   PDF (883KB) ( 888 )

    Serrated grain boundaries in conventional cast nickel base superalloys can enhance high temperature mechanical properties of the alloy due to a large number of precipitates arranged in grain boundaries, which effectively inhibited the movement of dislocations. During service at elevated temperatures, the ripening, coalescence or degeneration process may occur in these precipitates, resulting in the coarsening of grain boundaries with time. This paper aims to investigate the coarsening kinetics and microstructure evolution of serrated grain boundaries in a conventional nickel base superalloy during long term aging. After solution and aging heat treatment, a long term aging treatment at 900 ℃ for 3000 h was carried out on a cast K480 nickel base superalloy. The microstructure of grain boundaries (GBs) was observed using OM and SEM respectively, and phase composition was measured using EDS. The results showed that the GBs of K480 alloy were the irregular serrated GBs composed of carbide GBs andγ’ GBs respectively, both of which coarsened with aging time. During the course of aging, the MC carbides in the carbide GB were decomposed with the formation of new phases of M6C carbide andη phase; the discontinuous large-sizedγ’ particles in the γ’ GB were coalesced with each other along the GB direction with aging time andγ’ bands formed after aging treatment.The coarsening ratio of the carbide GB was higher than that ofγ’ GB at the beginning of aging treatment and the difference in these two values became larger with increase in aging time. A Johnson-Mehl-Avrami-Kolmogorov (JMAK) type of function was employed to quantify the evolution of coarsening behaviors of the two kinds of GBs with aging time. The good agreement between calculated results and experimental data indicated that the coarsening behaviors of carbide GB andγ’ GB in the serrated GB were evolved as a JMAK type of function of aging time in K480 nickel base superalloy long term aged at 900 ℃.

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    YANG Jian, DONG Jianxin, ZHANG Maicang
    Acta Metall Sin, 2013, 49 (1): 71-80.  DOI: 10.3724/SP.J.1037.2012.00445
    Abstract   PDF (1208KB) ( 775 )

    Powder metallurgy superalloys are important materials for manufacturing aero engine turbine disks which are subjected to loading in the forms of fatigue and creep-fatigue in service. In order to meet the increasing demands for advanced aero engines with high thrust-weight ratios, a novel generation of Ni-based powder metallurgy superalloy FGH98 was developed, which was expected to have high strength and good damage tolerance property. For the sake of examining the fatigue crack growth resistance of FGH98, the fatigue crack growth rate of this novel superalloy was investigated at 650 ℃ in air and then compared with those of the first two generations of powder metallurgy superalloys FGH95 and FGH96. The effects of microstructures and hold--time on the fatigue crack growth behavior of FGH98 were studied. It was found that the fatigue crack growth resistance of FGH98 was significantly improved in comparison with those of FGH95 and FGH96. Conducting proper cooling methods after solution could make the secondary and tertiaryγ’ phase precipitate in a uniform order, causing that the alloy could have good fatigue crack propagation resistance. It was also found that FGH98 with coarser grains showed a lower fatigue crack growth rate, especially in the near--threshold regime, and its fatigue crack growth rate increased with increasing hold-time, and correspondingly, its fracture mode changed from a mixture of transgranular-intergranular into pure intergranular.

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    HUANG Mingliang, ZHOU Shaoming, CHEN Leida, ZHANG Zhijie
    Acta Metall Sin, 2013, 49 (1): 81-86.  DOI: 10.3724/SP.J.1037.2012.00401
    Abstract   PDF (983KB) ( 452 )

    The consumption of electroless Ni-P and its effect on the failure mechanism of solder joints duringelectromigration under a current density of 1.0×104 A/cm2 at both 150 and 200℃ wereinvestigated using line-type Cu/Sn/Ni-P solder joints. Before the electroless Ni-P was completely consumed,the microstructural evolution of the Sn/Ni-P interface (cathode) was the formation of Ni2SnP and Ni3P accompanied by the consumption of the electroless Ni-P. Ni atoms diffused from the electroless Ni--P into the Sn solder under electron current stressing. Most Ni atoms precipitated as (Cu, Ni)6Sn5 or (Ni, Cu)3Sn4 in the Sn solder, and few Ni atoms could arrive at the opposite Cu/Sn interface (anode). After the electroless Ni-P was completely consumed, the microstructural evolutions of the Sn/Ni-P interface (cathode) were the formation of voids and the transformation from Ni3P to Ni2SnP. Furthermore, cracks that resulted from the propagation of voids significantly increased the current density through solder joints, and thus greatly enhanced the Joule heating of solder joints, resulting in the failure of solder joints by the fusing of Sn solder.

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    YAO Zhanquan, ZHAO Zengqi, JIANG Liping,HAO Hongbo, WU Shuangxia,ZHANG Guangrui, YANG Jiandong
    Acta Metall Sin, 2013, 49 (1): 87-91.  DOI: 10.3724/SP.J.1037.2012.00498
    Abstract   PDF (648KB) ( 566 )

    Fe-Ga alloy, which was a new type of magnetostrictive material with high magnetostriction, low hysteresis, high tensile strength and good machinability, has been widely studied. However, the magnetostrictive properties of the practically prepared Fe-Ga alloys were quite small at present. In order to improve magnetostrictive properties of the Fe-Ga alloy, the Fe83Ga17Cex (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) alloys were prepared by non--consumable vacuum arc melting furnace using high purity elements under a protective argon atmosphere. The crystal structures and surface morphologies of the alloys were intensively studied by X--ray diffraction (XRD) and scanning electron microscopy (SEM) combined with energy—dispersive spectroscopy (EDS), respectively. The magnetostriction coefficients of the alloys were measured by means of the resistance strain method. The results showed that Fe83Ga17 alloy is composed only of a single phase of A2 with bcc structure. However, the Fe83Ga17Cex alloys are composed of the A2 phase and a small amount of CeFe2 secondary phase with MgCu2 structure except the alloy with x=0.2, which is composed only of a single phase of A2. Furthermore, the microstructure of the Fe83Ga17 alloy presents the equiaxial morphology with coarse grains. However, the microstructure of the Fe83Ga17Ce0.8 alloy is a columnar structure with fine grains. Compared with the Fe83Ga17 alloy, the magnetostriction coefficients of the Fe83Ga17Cex alloys are increased significantly except the alloy with x=0.2. The magnetostriction coefficient of the Fe83Ga17Ce0.2 alloy (81×10-6) is slightly smaller than that of the Fe83Ga17 alloy (84×10-6). With the increase of the rare earth Ce content, the magnetostriction coefficients of the Fe83Ga17Cex alloys increase firstly and then decrease. When x=0.8, the maximum magnetostriction coefficient of 356×10-6 is obtained at the magnetic field of 557 kA/m.For the Fe83Ga17Cex alloys except the alloy with x=0.2, the noticeable increase of the magnetostriction coefficients derives from the following reasons: (1) the secondary phase of CeFe2 with MgCu2 structure appears and increases with the increase of the rare earth Ce content in the alloys. (2) the preferred orientation along <100>of A2 phase of the Fe83Ga17Cex alloy is more favorable to the increase of magnetostriction coefficient of Fe-Ga alloy. As for the Fe83Ga17Ce0.2 alloy, the decrease of magnetostriction coefficient is attributed to the fact that the rare earth Ce dissolves in the Fe-Ga alloy and forms the solid solution alloy.

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    PAN Zhiping, LI Shuangming, XU Lei, FU Hengzhi
    Acta Metall Sin, 2013, 49 (1): 92-100.  DOI: 10.3724/SP.J.1037.2012.00483
    Abstract   PDF (1134KB) ( 629 )

    Dendrite, which plays a key role in determining the mechanical properties of the casting parts, is the most important growth form in cast ingot and its pattern has received a wide range of investigations. In this work, the three-dimensional (3D) dendritic morphology of primary phase Cu2Mg in Cu-10.25%Mg hypereutectic alloy has been studied using the serial sectioning technique during directional solidification, due to the fact that the Cu2Mg phase is a typical Laves phase existing in most intermetallic compounds. The experimental results showed that the secondary dendrite profile of primary phase Cu2Mg had different morphologies at the pulling rate of 20 μm/s, which included plate-like, octahedral and faceted patterns. The occurrence of these patterns was related to alloy macro--segregation and the thermo--solutal convection during directional solidification. The growth dendrites and their side-branching morphologies in the three-dimensional section were clearly interpreted differently from those in the two-dimensional section. The two-dimensionally discontinuous dendrites in growth direction were the secondary dendrites of primary phase Cu2Mg and the branching dendrites in two-dimensional section were part of the primary dendrites. Furthermore, the primary and secondary dendrite arm spacing were calculated as 112.53 and 40.78 μm, close to those of the plate-like dendrites measured at the solidified fraction of 0.6 with the values of 123.30 and 44.82 μm, less than those of the octahedral dendrites measured at the solidified fraction of 0.8, which were 198.00 and 47.66 μm. In addition, the growth rates of secondary dendrites with octahedral, faceted and plate-like patterns were calculated as 1.40, 1.94 and 2.66 μm/s, respectively, an order of magnitude less than the pulling rate of 20 μm/s.

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    ZHANG Jianfeng, LE Qichi, CUI Jianzhong
    Acta Metall Sin, 2013, 49 (1): 101-106.  DOI: 10.3724/SP.J.1037.2012.00521
    Abstract   PDF (969KB) ( 778 )

    The electric resistance of Sn-20%Pb alloy was measured at different temperatures when the AC magnetic field was applied. The results showed that the AC magnetic field had no effect on the electric resistance of solid Sn-20%Pb alloy, but when it was applied on the liquid Sn-20%Pb alloy, the electric resistance of alloy increased gradually. After the AC magnetic field was stopped, the electric resistance remained at a constant value for a long time, and then slowly decreased to the value before applying the AC magnetic field. The results of metallographic microscope and SEM showed that the AC magnetic field not only refined the primary β-Sn grain, but also increased the Pb content in primary β-Sn grain, so that the distribution of Pb was more uniform than that in the sample without magnetic field. The Anderson's theory of disorder has been used to explain the experimental results. It was considered that the electromagnetic stirring caused by AC magnetic field destroyed the short range order of the Sn-20%Pb melt, improved the disorder degree of the system, transformed some extended states electrons into localized states electrons,consequently increased the electric resistance of the melts.

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    GUO Weiwei, QI Chengjun, LI Xiaowu
    Acta Metall Sin, 2013, 49 (1): 107-114.  DOI: 10.3724/SP.J.1037.2012.00411
    Abstract   PDF (1141KB) ( 468 )

    Although comprehensive research findings of the cyclic deformation and dislocation structures of Cu single crystals with various orientations have been well established over the four decades, studies on the thermal stability of dislocation structures in fatigued Cu single crystals are still rarely reported. In the present work, [233] Cu single crystals oriented for coplanar double slip were firstly cyclically deformed at different plastic strain amplitudes γpl up to saturation, and then annealed at different temperatures for 30 min.The dislocation structures induced by cyclic deformation as well as the microstructural changes resulting from subsequent annealing treatments were detected by using the electron channeling contrast (ECC) technique in SEM and TEM. It was found that the dislocation structures have undergone an obvious process of recovery at 300 ℃,and the recrystallization even partially takes place in the sample fatigued at highγpl. However,at 500 and 800 ℃, the violent recrystallization takes place in all crystals and a large number of annealing twins have appeared. As the plastic strain amplitude and accumulated plastic strain increase, the degree of strain concentration would be significantly aggravated, providing a higher local strain energy for the occurrence of recrystallization and the initiation of twins, so that the recrystallization takes place more noticeably, and annealing twins become coarser and the number of them increases notably. The formation of annealing twins is closely related to the appearance of stacking faults. The DSC measurements demonstrated that the recrystallization process and the formation process of twins should be a gradually-developing process, instead of a suddenly-forming process.

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    KE Changbo, CAO Shanshan, MA Xiao, HUANG Ping, ZHANG Xinping
    Acta Metall Sin, 2013, 49 (1): 115-122.  DOI: 10.3724/SP.J.1037.2012.00264
    Abstract   PDF (896KB) ( 556 )

    The precipitation behavior and distribution of Ni4Ti3 particles in NiTi alloys have a significant influence on the subsequent martensitic transformation, which may consequently affect shape memory effect and superelasticity of NiTi alloys. The latest experimental studies confirmed that in single crystal NiTi alloy, the Ni4Ti3 particles nucleate and grow as an autocatalytic way leading to a step--like configuration. The autocatalytic nucleation known as collective manner has been widely studied in martensitic transformation while causing relatively less attention in diffusion transformation such as precipitation. Previous studies have been launched to investigate the nucleation and orientation issues of Ni4Ti3 precipitate mainly by analyzing the stress or  concentration field around the Ni4Ti3 particle. However, the coupling between stress and concentration field is necessary in Ni4Ti3 precipitation due to its diffusion nature, also the coupling and cross influence of each energyin the simulation system should be taken into account. In thispaper, the mechanism of autocatalytic effect of Ni4Ti3 precipitate has been studied by means of phase field method,the favorable Ni4Ti3 precipitates array has been investigated through thevariation of each energy in the simulation system. The simulation results show that in the initial short time of Ni4Ti3 precipitation, due to the morphological accommodation of Ni4Ti3 and NiTi phases, the system chemical free energy gradually increases, while the elastic and interfacial energy decrease; after the self-accommodation stage, each energy shows the inverse trend compared with the initial stage; in the whole phase transformation process, the system total energy develops toward the lower energy state. In all of the feasible precipitate arrays, it is found that the step-like array formed by same orientation Ni4Ti3 variants has the supreme priority, secondly the horizontal array followed by the edge-face array formed by different orientation variants, the vertical array constituted by same orientation variants was confirmed as the least possible way, these simulation results have deepened and expanded the understanding obtained in previous experiment studies.

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    ZHU Yiguo, ZHANG Yang, ZHAO Dan
    Acta Metall Sin, 2013, 49 (1): 123-128.  DOI: 10.3724/SP.J.1037.2012.00319
    Abstract   PDF (663KB) ( 656 )

    Shape memory alloys (SMAs) are new functional material featured by the excellent properties including shape memory effect and superelasticity. NiTi SMAs have some important implications in avitation, medical device,etc. The main objective of this work is to derive a simple Taylor model for NiTi polycrystal.By the assumption of laminated microstructure and perfect interfacial relationship for NiTi single crystal transformation, microscopic strain for each phase can be transformed to overall strain of respective volume element, and expression of phase transformation driving force is derived, then control equation of phase transformation is constructed. Based on the single crystal model, the NiTi polycrystal constitutive model is constructed by Taylor assumption. The model is used to simulate the mechanical response of NiTi polycrystal alloys with strong {111} texture, the results are in agreement with those observed experimentally. The predicted result of NiTi alloy with strong texture shows asymmetry of tension and compression. Simulation results behave softening as the free energy function is non-convex during phase transformation. Transformation stress level rises as temperature is raised. 

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