ISSN 0412-1961
CN 21-1139/TG
Started in 1956

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    EFFECT OF STRAIN RATE ON DYNAMIC DEFORMATION BEHAVIOR OF LASER WELDED DP780 STEEL JOINTS
    DONG Danyang, LIU Yang, WANG Lei, YANG Yuling, LI Jinfeng, JIN Mengmeng
    Acta Metall Sin, 2013, 49 (12): 1493-1500.  DOI: 10.3724/SP.J.1037.2013.00341
    Abstract   PDF (4618KB) ( 1076 )

    Dual phase (DP) steels have good combinations of strength and ductility, and are being increasingly used in vehicle body structures to meet enhanced government regulations and safety standards. The use of DP steels in automotive industries involves laser welding, which would lead to changes in local material properties and create potential safety and reliability issues under dynamic loads. The present work aimed to study the effects of strain rate on tensile properties and deformation behavior of laser welded DP780 steel joints. The results showed that the deformation behavior of laser welded joints was more sensitive to strain rate as compared to base metal of DP780 steel. The strength of DP780 steel joint increased with increasing strain rate, while the ductility decreased first with increasing strain rate from 10-3 to 101 s-1, and then increased up to a strain rate of 102 s-1. The strain rate sensitivity of the deformation behavior of DP780 steel joints was mainly dependent on the change of deformation behavior and its mechanisms of base metal at various strain  rates. The distance of the tensile failure location from the weld centerline decreased obviously with the increase of strain rate. And the failure location changed from the base metal to the softened heat—affected zone (HAZ) as strain rate increased. The mechanism for changing failure location can be related to the strain rate dependence of the plastic deformation behaviors of microstructures in various regions across a joint.

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    EFFECT OF MICROSTRUCTURE ON FRACTURE TOUGHNESS OF NEW TYPE HOT—ROLLED NANO—SCALE PRECIPITATION STRENGTHENING STEEL
    SUN Qian, WANG Xiaonan, ZHANG Shunhu, DU Linxiu, DI Hongshuang
    Acta Metall Sin, 2013, 49 (12): 1501-1507.  DOI: 10.3724/SP.J.1037.2013.00349
    Abstract   PDF (3155KB) ( 832 )

    The fracture toughness of new type hot—rolled nano—scale precipitation strengthening steels (tensile strength of 700 MPa grade and 780 MPa grade) were evaluated by crack tip opening displacement (CTOD) experiments, and the influence mechanisms of microstructure, high angle grain boundaries, dislocation density and nano—scale precipitation on fracture toughness were discussed. The results indicated, when experimental temperature were room temperature, -10 and -30℃, the δQ0.2BL value of 700 MPa grade carriage strip were 0.468, 0.333 and 0.248 mm, and the δ0.2 value of 700 MPa grade carriage strip were 0.298, 0.234 and 0.215 mm, respectively. However, the δQ0.2BL value of 780 MPa grade crossbeam strip were 0.311, 0.290 and 0.247 mm, and the δ0.2 value of 780 MPa grade crossbeam strip were 0.212, 0.212 and 0.198 mm, respectively. Therefore, the fracture toughness of 700 MPa grade steel was better than 780 MPa grade steel. The differences of microstructure between 700 MPa grade steel and 780 MPa grade steel mainly included four aspects: (1) the microstructure of 700 MPa grade steel was mainly ferrite, while the microstructure of 780 MPa grade steel was mainly bainitic ferrite; (2) the carbide shape of 700 MPa grade steel was granular or short rod, and 780 MPa grade steel was strip carbide; (3) the dislocation density of 780 MPa grade steel was significantly higher than 700 MPa grade steel; (4) the proportion of large—angle grain boundaries of 700 MPa grade steel and 780 MPa grade steel were 85.6 % and 76.8%, respectively. Therefore, improving the volume fraction of ferrite and the proportion of high angle grain boundaries, refining carbide size and reducing dislocation density could effectively improve the fracture toughness of steels. Coarse precipitation (Nb, Ti)CN and grain boundary precipitation in microstructure deteriorated fracture toughness of steel, and semi—coherent precipitates nano—scale (Nb, Ti)C on ferrite or bainite matrix have less damaging effect on fracture toughness.

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    SIMULATION AND EXPERIMENTAL STUDIES ON GRAIN SELECTION BEHAVIOR OF SINGLE CRYSTAL SUPERALLOY :I. Starter Block
    ZHANG Hang, XU Qingyan, SUN Changbo, QI Xiang,TANG Ning, LIU Baicheng
    Acta Metall Sin, 2013, 49 (12): 1508-1520.  DOI: 10.1016/j.jmst.2013.07.001
    Abstract   PDF (7938KB) ( 1074 )

    The rapid development of advanced aero—engine and industry gas turbine requires high performance of single crystal (SX) blade. Spiral selector is very important to produce SX blade, which includes starter block and spiral part. In this research, grain density changing and orientation deviating as the height of grain growth (the distance between section studied and the undersurface of the sample) increasing were studied by the experiment and simulation, and the designing rules for the starter block were given out. EBSD orientation mapping technology was used to get grains' morphology and orientation. Mathematical and physical models were built for the directional solidification process. Adopting CA—FD method, the 3D macro temperature field of solidification process was calculated as well as grain growth. The properties of grains competitive growth and evolution process during directional solidification in starter block were analyzed based on macro and micro modeling results, and rules for grains competitive growth was explained, which provided theoretical supports for designing starter block.

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    SIMULATION AND EXPERIMENTAL STUDIES ON GRAIN SELECTION BEHAVIOR OF SINGLE  CRYSTAL SUPERALLOY :II. Spiral Part
    ZHANG Hang, XU Qingyan, SUN Changbo, QI Xiang,TANG Ning, LIU Baicheng
    Acta Metall Sin, 2013, 49 (12): 1521-1531.  DOI: 10.3724/SP.J.1037.2013.00214
    Abstract   PDF (4316KB) ( 657 )

    The spiral selector is the key part for producing single crystal (SX) blades and ensures the integrity of crystal, which mainly includes starter block and spiral part. In this work, the influence of spiral part on the grain selection process was studied. Both of the metallography results and EBSD results proved that the prior location and the special orientation of the second dendrite arms were important for thegrains competitive growth during the directional solidification process. Based on the experimental results, two geometrical restrict mechanisms of grain selection were proposed. They were the competitive stimulating effect on the second dendrite arms in horizontal direction, which was resulted from the spiral arc shape, and the growing blocking effect on the primary dendrites in vertical direction, which was resulted from the take—off angle of the spiral part. These models could successfully explain the grain selecting effects of the spiral part. The modified cellular automaton  (MCA) technology was used to simulate the grains' competitive growth in spiral part. The changes of grains structure and orientation as the grain growing on were studied. The simulated and experimental results were compared and agreed well. Based on the simulated and experimental results, Influences of structural parameters on the grain selection behavior were proposed. The criteria for designing spiral part were also presented.

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    THREE—DIMENSIONAL MATHEMATICAL SHAPE MODEL AND PROCESS RESEARCH OF SPRAY—FORMED BILLET
    YUAN Hao, FAN Junfei, REN Sanbing, WANG Haowei, ZHANG Yijie
    Acta Metall Sin, 2013, 49 (12): 1532-1542.  DOI: 10.3724/SP.J.1037.2013.00320
    Abstract   PDF (3878KB) ( 758 )

    Spray forming is an advanced technology that is used to produce a variety of high—performance materials with the characteristics of rapid solidification. By properly controlling of spray forming parameters, it is possible to fabricate near—net—shaped preforms, such as rod—billets, strips, and tubes. During the billet spray forming process, the shape of deposit strongly influences the solidification and consequently end—product quality. Therefore, it is necessary to reveal the shape forming mechanism in spray forming process. In the present work, a three—dimensional (3D) model, tracing the coordinates of the moving surface of a growing spray—formed billet, has been formulated to predict the shape evolution of the general deposit. This geometric model takes into account geometrical process parameters in the whole spray forming process: mass flux and mass distribution, position of the atomizer, distance between atomizer and the preform, substrate withdrawal velocity and rotation speed. This makes it possible to model not only the growth of a Gaussian shaped deposit in which case the spray axis and the rotation axis coincide, but also the profile evolution as there is a spray angle between these two axes. For this purpose, “shadowing effect” must be taken into account as a core part of the surface evolution algorithm. On the basis of this 3D model, a timesaving and accurate methodology is established to determine the shadowing effect coefficient, using the “triangular element checking” algorithm coupled with back face culling (BFC). The transient shape modeling has been validated by numerical algorithms and experimental investigation, and has proved that the simulated billet profiles are in good agreement with the experimental data. The effect of spray forming parameters, such as spray distribution parameters, withdrawal velocity, initial eccentric distance, spray angle and angular velocity of rotation, are analyzed. According to the obtained simulation results, the most dominant parameters affecting the shape evolution of deposit are the spray distribution parameters, withdrawal velocity, and initial eccentric distance. It is also found that the spray angle mainly affects the profile  of the top transition region of the rod. The effect of the angular velocities of substrate on the shape evolution of the deposit is not significant. Finally, the maximum withdrawal velocity and maximum initial eccentric distance are deduced based on the analysis of shape form mechanism, which can be used to guide the process optimization during spray forming.

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    THEORETICAL ANALYSIS OF AN ABRUPT INCREASE IN GROWTH RATE ON THE DIRECTIONALLY SOLIDIFIED MICROSTRUCTURES OF OFF—EUTECTIC ALLOYS
    LI Shuangming, FU Hengzhi
    Acta Metall Sin, 2013, 49 (12): 1543-1548.  DOI: 10.3724/SP.J.1037.2013.00428
    Abstract   PDF (893KB) ( 522 )

    Coupled growth eutectic microstructures at an abrupt increase in growth rate in directional solidification of off—eutectic alloys have been analyzed based on the maximum interface growth temperature criteria. The abrupt growth rate possibly required for the growth of coupled eutectics during the abrupt increase in growth rate is obtained by considering the competitive growth between the eutectic and primary dendrites. The theoretical predictions show that the eutectics can grow out of coupled zone, differing from the conventional eutectic solidification concept that indicates the entirely coupled eutectics only form within the coupled zone. In addition, the theoretical analysis are employed in two kinds of regular eutectics such as lamellar eutectic in the Al—40% Cu hypereutectic alloy and rod eutectic in the Al—12 %Ni hypereutectic alloy; a good agreement between the theoretical calculation results and reported experimental data has been achieved.

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    EFFECT OF TEMPERING TEMPERATURE ON MICRO-STRUCTURE AND PROPERTY OF 690 MPa GRADE OCEAN ENGINEERING STEEL UNDER FAST HEATING RATE
    ZHANG Jie, CAI Qingwu, WU Huibin, FAN Yanqiu
    Acta Metall Sin, 2013, 49 (12): 1549-1557.  DOI: 10.3724/SP.J.1037.2013.00290
    Abstract   PDF (5541KB) ( 1116 )

    Ocean engineering steel has been rapidly developed with the exploration of ocean resourses. A 690 MPa grade low carbon bainite steel was designed for ocean engineering, to upgrade performance by microstructure control and the refinement and dispersion control of precipitates. This steel was tempered on--line with rapid heating rate after control rolling and accelerated cooling process. The results show that the mechanical properties, especially the strength--toughness balance, are strongly influenced by the transformation of untransformed austenite and the condition of precipitates. When fast tempering at 550℃, microstructure recovered and few precipitates appeared, bringing strength down seriously, untransformed austenite turned into martensite/austenite (M/A) islands on cooling. When the tempering temperature reached 600℃, the size of the M/A islands transformed from untransformed austenite decreased slightly, Cu and Nb/Ti precipitates increased greatly, bringing an apparent improvement in strength. When tempering temperature reached 660℃, precipitates got coarsened and made the strength decreases, the untransformed austenite formed retained austenite film between the laths, improving toughness and making the best strength—toughness balance. When the tempering temperature reached the top at 700℃, the precipitates got further coarsened, the untransformed austenite on cooling turned into large M/A islands, bringing toughness and the strength—toughness balance down again. In general consideration, fast heating tempering at 600—660℃ could make the steel has the best strength—toughness balance.

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    EFFECT OF PRECURSOR MICROSTRUCTURE ON MORPHOLOGY FEATURE AND MECHANICAL PROPERTY OF C—Mn—Si STEEL
    REN Yongqiang, XIE Zhenjia, ZHANG Hongwei, YUAN Shengfu, SONG Tingting, SHANG Chengjia
    Acta Metall Sin, 2013, 49 (12): 1558-1566.  DOI: 10.3724/SP.J.1037.2013.00301
    Abstract   PDF (2302KB) ( 866 )

    The effects of different precursor microstructure on the morphology and mechanical properties of the 0.22C—1.9Mn—1.32Si multiphase steel which was obtained by the treatment of intercritical reheating—quenching and partitioning (IQ&P) heat treatment were examined. Under the same IQ&P heat treatment parameters, multiphase microstructure which contains lath—like ferrite matrix and film or short needle—like retained austenite can be obtained by the martensite (M) precursor steel; while multiphase steel which has a bainite—ferrite (B—F) precursor can obtain a microstructure of equiaxed—like ferrite matrix and particale like retained austenite. After the IQ&P process, tensile strength of the multiphase steel which has a B—F precursor is up to 976 MPa, but elongation of this kind of steel is only 26.7%, and thus the product of strength and elongation of this kind of steel is only 26 GPa•%; while multiphase steel which has a M precursor has realized the combined properties of high strength and excellent ductility, product of strength and elongation of this kind of steel reaches 31 GPa•%. As for the work hardening behavior of the uniform elongation stage, although B—F precursor multiphase steel has a higher work hardening index n than the M precursor multiphase steel, stability of the retained austenite in this kind of steel is relatively poor, variation behavior curve of the instantaneous n value with true strain for this kind of steel shows a notched—like shape; as for the multiphase steel which has a M precursor, retained austenite in this kind of steel is relatively stable, variation behavior curve of the instantaneous n value with true strain for this kind of steel is much more steady, which shows a trend of gradual increasing. The reason for the different tensile testing and work hardening results above is related to the morphology, proportion and distribution state of the retained austenite and matrix microstructures, which is due to the effect of different morphology and microstructure characteristics of the precursor phases by the roots.

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    SOLIDIFICATION BEHAVIOR OF A Pt—CONTAININ Ni—BASED SINGLE CRYSTAL SUPERALLOY
    LIN Huiwen, ZHOU Yizhou, ZHANG Xuan, JIN Tao, SUN Xiaofeng
    Acta Metall Sin, 2013, 49 (12): 1567-1572.  DOI: 10.3724/SP.J.1037.2013.00141
    Abstract   PDF (1895KB) ( 563 )

    The Ni—based single crystal superalloy is the primary material for the manufacture of blades of the advanced areoengine and gas turbine. As the alloying design theory improved, the concentration of refractory elements increased and some new elements were introduced. Among these refractory elements, Ru became a fresh and sign element which was introduced into the fourth generation Ni—based single crystal superalloy. A large amount of research had indicated that Ru, as a member of Platinum group metals (PGMs), had played a significant role on the enhancement of phase stability and rupture life for Ni—based single crystal superalloy. Inspired by these works, other PGMs like Pt have been suggested to be the major alloying elements of the next generation advanced Ni—based single crystal superalloy. But the research for the effects of Pt addition on solidification behavior or creep property of the single crystal superalloy is rare. To explore the possibility of Pt using as a major alloying element, the present work investigated the influence of Pt on the solidification behavior of the Ni—based single crystal superalloy. Directional solidification method was used to grow the single crystal. DTA and EPMA were used to determine the effect of Pt on the phase transition temperatures and composition variety, respectively. In addition, OM and SEM were used to show the phases morphology. Some results are described as follow. Firstly, it has been found that the addition of Pt changes the eutectic morphology and increases eutectic fraction. Grid—like eutectic increases with the addition of Pt. Secondly, Pt promotes not only the segregation of refractory elements but also the eutectic—forming element Al. In addition, Pt prefers to segregate to the interdendritic region and is able to form an ordered Pt3Al phase with Al which may be a reason for the increase of eutectic fraction. But Pt reduces the segregation of Mo element whose content is a sensitive factor for the formation of the topological close—packed (TCP) phase. Thirdly, Pt decreases initial melting temperature and enhancesγ′phase precipitation temperature; thereby reduces the solution heat treatment window  of the alloys. The solution heat treatment of the alloys therefore becomes more difficult. Since the element segregation is hard to be eliminated by heat treatment in the Pt—containing alloys, Pt addition may be harmful for the mechanical properties of single crystal superalloy. The effects of Ru addition on the solidification behavior of the Ni—based single crystal superalloy will be also discussed for comparison.

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    EFFECT OF ELECTROMAGNETIC FIELD ON MICRO-STRUCTURE AND MECHANICAL PROPERTY FOR INCONEL 625 SUPERALLOY
    JIA Peng, WANG Engang, LU Hui, HE Jicheng
    Acta Metall Sin, 2013, 49 (12): 1573-1580.  DOI: 10.3724/SP.J.1037.2013.00509
    Abstract   PDF (3187KB) ( 673 )

    Inconel 625 is a Ni—Cr—Mo—Nb alloy which was developed primarily for high turbine applications. The elemental addition of Nb increases the solidification temperature range, which exhibits a strong propensity to form interdendritic segregation. The enrichment of elements Nb and Mo at the terminal stage of solidification leads to the formation of brittle eutectic structure, i.e., γ+Laves phases, which becomes potential crack origin during the subsequent hot processing and application. The present work has demonstrated that, the introduction of electromagnetic field (EMF) to the solidification process of Inconel 625 alloy has the obvious effect on grain refinement. The EMF can also effectively influence the segregation ratio of Nb and Mo. However, the inappropriate application of electric current intensity and frequency will lead to more severe segregation of elements Nb and Mo, which causes the increment of eutectic structure volume fraction. Further analysis illustrates that both of the grain refinement and eutectic volume fraction control the tensile property at room temperature, increasing the yield strength and decreasing the tensile plasticity for Inconel 625 alloy. It has been proven that a proper selection of input current intensity (100 A) and frequency (8 Hz) can effectively dominate the segregation behavior during solidification process under EMF with more than 30% increase of yield strength and a minute loss of plasticity.

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    MICROSTRUCTURE EVOLUTION AND MECHANICAL PROPERTIES OF DD5 SINGLE CRYSTAL SUPERALLOY JOINT BRAZED BY Co—BASED FILLER ALLOY
    SUN Yuan, LIU Jide, LIU Zhongming, YANG Jinxia, LI Jinguo,JIN Tao, SUN Xiaofeng
    Acta Metall Sin, 2013, 49 (12): 1581-1589.  DOI: 10.3724/SP.J.1037.2013.00406
    Abstract   PDF (5874KB) ( 760 )

    The single crystal supperalloy DD5 was brazed using Co—Ni—Si—B filler alloy at  1453 K. The microstructure of joint and effect of brazing time on the microstructure and mechanical properties were investigated by SEM, EPMA and TEM. The formation mechanism of joint was discussed deeply. The result indicates that three distinct regions can be identified in a joint: the filler alloy zone, interfacial bonding zone, element diffusion zone. The filler alloy zone in the center of joint consists of Ni—Co solid solution, M3B2—type boride, CrB and Ni—Si compounds. The interfacial bonding zone adjacent to the filler alloy zone is composed of Ni—Co solid solution formed isothermally at brazing temperature. The element diffusion zone between the interfacial bonding zone and substrate consists of γ phase, γ′ phase and granular M3B2 phase which was formed in element diffusion. With extending brazing time, the brazing defects reduce, the thickness of bonding layer and the size of spherical Ni—Co solid solution in the filler alloy zone increase, and the brittle compounds reduce. The tensile strength at 1143 K increases from 198.5 MPa to 580 MPa. The investigation on fracture of high temperature tensile specimens exhibits that the fracture occurred in the seam and the fractural model is a mixed fracture. Through extending brazing time the bonding strength between seam and substrate can be improved, the brittle compounds can be controlled and the mechanical properties of joint are improved. The analysis results of microstructure and properties also imply that the optimum holding time would be 180 min.

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    STRESS CORROSION CRACKING BEHAVIOR AND MECHANISM OF X65 AND X80 PIPELINE STEELS IN HIGH pH SOLUTION
    ZHU Min, LIU Zhiyong, DU Cuiwei, LI Xiaogang, LI Jiankuan, LI Qiong, JIA Jinghuan
    Acta Metall Sin, 2013, 49 (12): 1590-1596.  DOI: 10.3724/SP.J.1037.2013.00315
    Abstract   PDF (2785KB) ( 690 )

    X80 pipeline steel is a low carbon, micro—alloyed high—grade steel and a fairly new steel used as pipeline material in worldwide. The material has the huge potential to be used widely for building the oil/gas transmission pipelines in the 21st century because of its high intensity and high toughness. X80 steel has been adopted on the second west—east gas transmission pipeline project in China. Whereas, there is a issue, stress corrosion cracking (SCC) is more likely to occur on X80 pipeline steel, because of its high strength and fine microstructure, it will be a vital threat to safe operation of buried oil/gas pipelines. However, the related research about SCC behavior of X80 pipeline steel in high pH carbonate/bicarbonate solution is rarely reported at present. Comparing with X65 pipeline steel, X80 steel has higher strength and finer microstructure, because of these differences, it may have some certain influence on the SCC behavior of X80 steel, and even change the mechanism of high pH SCC. Consequently, it is necessary to study the SCC behavior and mechanism of X80 steel in high pH solution. In this work, the SCC behavior and mechanism of X65 and X80 pipeline steels in high pH concentrated carbonate/bicarbonate solution are investigated by slow strain rate testing (SSRT), electrochemical test and surface analysis technique. The results show that the cracking mode of X65 pipeline steel in carbonate/bicarbonate solution is intergranular SCC (IGSCC). While the mixed cracking mode of X80 pipeline steel in high pH solution is that the crack is intergranular in the early stage of the crack propagation, and transgranular SCC (TGSCC) in the later stage, which is mainly transgranular. The cracking mode of X80 steel is associated with the microstructure and high strength of the steel. The key reason for TGSCC occurring of X80 steel is that the decrease of pH value of the crack tip during the crack propagation process. The SCC mechanism of X65 steel in high pH carbonate/bicarbonate solution is anodic dissolution (AD) mechanism. While the SCC mechanism of X80 steel in high pH solution is mixed controlled by both AD and hydrogen embrittlement (HE) mechanisms, and the HE mechanism may play a significant role in the deep crack propagation at the later stage. The high strength X80 steel consisted of fine acicular ferrite and granular bainite has a higher susceptibility to SCC in high pH solution, comparing with low strength X65 steel composed of ferrite and pearlite.

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    DEFORMATION BEHAVIOR AND MICROSTRUCTURE EVOLUTION OF 7050 ALUMINUM ALLOY DURING SEMI—SOLID STATE COMPRESSION PROCESS
    LIU Yunzhong, LI Zhilong, GU Caixin
    Acta Metall Sin, 2013, 49 (12): 1597-1603.  DOI: 10.3724/SP.J.1037.2013.00329
    Abstract   PDF (5423KB) ( 384 )

    The compression characters of the semi—solid slurries are the key to semi—solid processing such as semi—solid rolling which has high strain rates during the deformation. The deformation mechanism of semi—solid alloy can be understood well only after the relationship between stress and strain is obtained. Among many relevant research works done up to now, very few studies focus on the 7050 aluminum alloy. In order to study the sensibility of 7050 aluminum alloy to the strain rates, temperatures and reductions, the deformation behavior and microstructure evolution of 7050 aluminum alloy under different compression parameters were studied in this work. The grain coarsening of 7050 aluminum alloy prepared by the strain induced melt activation (SIMA) method during the isothermal heating process was studied firstly. Then the compression tests, within the semi—solid temperature range, on conventional cast alloy and semi—solid alloy were carried out respectively by using a Gleeble—3500 material thermo—simulation machine with the strain rates from 0.1 s-1 to 10 s-1.The relationship between stress and strain was analyzed subsequently. The synergistic effect between liquid and solid was analyzed in—depth as well. In addition, the differences of cracks propagation between conventional cast alloy and semi—solid state alloy during compression were discussed. Experimental results show that the stress of conventional cast alloy has a higher level than that of semi—solid alloy, which is 12 MPa higher at the peak position and 9 MPa higher during the stabilization stage. Reductions, deformation temperatures and strain rates during compression have remarkable effects on the microstructure evolution and the liquid phase distribution. The high reduction leads to the sharp deformation of the grain shape. The deformation has an obvious transition region in the middle which can be clearly seen that elongated grains have a deflection toward the edge. The lower the temperature, the smaller the liquid fraction is. This leads to recrystallization during the compression. The strain rates contribute to the flowing and distribution of liquid phase. The liquid phase transfers hardly when consisting with the solid phase under the high strain rate (10 s-1), which results in a uniform deformation in different regions. Because of the remarkable differences in microstructures between conventional cast alloy and semi—solid alloy, the evolution of cracks propagation is also different, which corresponds to a solid—liquid separation mechanism and a mixed separation mechanism respectively. Semi—solid alloy has spherical crystals that can slide easily when comparing with conventional cast alloy with dendritic crystals. This makes a further explanation for the lower stress of the semi—solid alloy.

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    STUDY ON KINETICS OF PRECIPITATION IN Al—0.6Mg—0.9Si—0.2Cu ALLOY FOR AUTOMOTIVE APPLICATION
    ZHANG Qiaoxia, GUO Mingxing, HU Xiaoqian, CAO Lingyong, ZHUANG Linzhong, ZHANG Jishan
    Acta Metall Sin, 2013, 49 (12): 1604-1610.  DOI: 10.3724/SP.J.1037.2013.00330
    Abstract   PDF (541KB) ( 840 )

    The kinetics of precipitation in Al—0.6Mg—0.9Si—0.2Cu (mass fraction, %) alloy under different conditions was investigated by differential scanning calorimetric analyses (DSC) and microhardness measurements. The results show that, an exothermic peak at about 100—-150℃ can be observed in the DSC curve for the nature aged (T4) alloy, but no peak for the pre—aged (T4P) alloy, and the peak corresponding with β″ phase moves toward the low temperature zone for T4P alloy. The fraction of transformation Y, the rate of transformation dY/dT, and the kinetic parameters such as activation energy Q and frequency factor k0 for GP zones dissolution and β″phase transformation were calculated by Avrami—Johnson—Mehl equation. The values of activation energy for GP zones dissolution after natural aging and pre—aging were 66 and 119 kJ/mol, respectively, and the values forβ″phase formation after natural aging and pre—aging were 114 and 60 kJ/mol, respectively. The kinetics expressions were obtained as follows, for T4 alloy:YGP=1-exp[-1.3×107t exp(-7977/T)],Yβ=1-exp[-4.7×1022t2 exp(-27484/T)];for T4P alloy: YGP=1-exp[-2.4×1013 t exp(-14345/T)],Yβ=1-exp[-2.9×1011t2 exp(-14392/T)].In addition, with the increasing of aging time, the wholetrend of hardness changing for pre—aged alloy is increasing at first, and then keeps constant basically,but for the nature aged alloy, the hardness decreases after aging at 185℃ for 20 min, which was explained by the kinetics obtained above.

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    FABRICATION AND THERMAL STABILITY OF AlCrTaTiNi/(AlCrTaTiNi)N BILAYER DIFFUSION BARRIER
    ZHANG Lidong, WANG Fei, CHEN Shunli, WANG Yuan
    Acta Metall Sin, 2013, 49 (12): 1611-1616.  DOI: 10.3724/SP.J.1037.2013.00207
    Abstract   PDF (1875KB) ( 772 )

    The (AlCrTaTiNi)N and AlCrTaTiNi/(AlCrTaTiNi)N thin films were deposited on Si substrates as copper interconnects diffusion barriers by magnetron sputtering methods. The phase structure, phase composition and thermal stability at high temperature were investigated by XRD, EDS and SEM in this work, respectively. The results indicate that the gap could be observed between Cu and (AlCrTaTiNi)N layers when the samples were annealed at 500℃ for 30 min, and the Cu film fall off at 700℃ under visual conditions. The bonding properties of Cu and (AlCrTaTiNi)N layers can be improved by inserting a AlCrTaTiNi layer. The XRD patterns, SEM cross-sectional micrographs and sheet resistance data show that the AlCrTaTiNi/(AlCrTaTiNi)N bilayers is stable up to 800℃.

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    PREPARATION AND PROPERTIES OF NANOSIZED MUL-TILAYERED Ni COATINGS BY ULTRASOUND-ASSISTED ELECTRODEPOSITION
    NIU Yunsong, WEI Jie, ZHAO Jian, HU Jiaxiu, YU Zhiming
    Acta Metall Sin, 2013, 49 (12): 1617-1622.  DOI: 10.3724/SP.J.1037.2013.00303
    Abstract   PDF (1686KB) ( 704 )

    Nanosized multilayered Ni coatings were deposited on Q235 steel substrates by ultrasound-assisted electrodeposition. The multilayered Ni coating has a laminated structure of ordinary Ni layer/ultrasonic Ni layer. In order to determine the corrosion and wear property of this novel multilayered Ni coating, conventional Ni coating and bilayered Ni coating were prepared. The phase structure and average grain size of the Ni coatings were determined by X—ray diffraction analysis. Cross section morphologies were observed by scanning electron microscope. Corrosion properties of the Ni coatings were investigated by the immersion test in 7%HCl solution at room temperature, and then their corrosion morphologies were examined by scanning electron microscope. Another way to evaluate the corrosion resistance of the Ni coatings is to calculate the porosity of the Ni coatings using porosity analysis software on an optical microscope. Wear resistance of the Ni coatings were evaluated by rubber wheel test method. The test condition was 400# SiC emery paper and the applied load on the samples is 6.3 N. Moreover, after 1500 cycles, cross sections of the worn Ni coating samples along the wear direction were observed by means of scanning electron microscope. The results indicate that the growth orientation of the multilayered Ni coating is (111) preferably. Its microstructure is obviously a smooth stacker--up laminated structure. Hence, the epitaxial columnar growth of the conventional Ni coating is annihilated by the formation of the multilayered structure. Compared with the conventional Ni coating, not only the wear resistance of the multilayered Ni coating is improved obviously, but also the corrosion resistance of the multilayered Ni coating is enhanced dramatically. With multilayered structure, the pinholes were obviously inhibited. That is because the multilayered Ni coating can cover the whole surface of the substrate without pinhole interruption. The multilayered structure can extend the pinhole passages extremely for the multilayered Ni coating, unlike the direct and straight pinhole passages for the conventional Ni coating. Therefore, its corrosion mechanism is the uniform corrosion, instead of the hydrogen blistering and Fe/Ni galvanic couples for the ordinary Ni coating. The wear mechanism of the multilayered Ni coating is the adhesive wear, which is a kind of the so-calledmicro-area detachment; while that of the ordinary Ni coating consists of adhesive wear, which can lead to the wear behavior of the bulk damage due to the columnar microstructure.

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