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

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    论文
    EFFECT OF NITROGEN ON MARTENSITIC TRANSFORMATION AND MECHANICAL PROPERTIES OF TWIP STEEL
    HUANG Baoxu WANG Changzheng WANG Xiaodong RONG Yonghua
    Acta Metall Sin, 2012, 48 (7): 769-774.  DOI: 10.3724/SP.J.1037.2012.00047
    Abstract   PDF (1183KB) ( 1073 )
    Twinning induced plasticity (TWIP) steels show large elongation and high tensile strength,
    exhibiting a super balance between strength and plasticity. Until now, the effects of Mn, Si and Al on stacking
    fault energy (SFE) and phase transformation of TWIP steel had been investigated, but the effect of N on phase
    transformation, especially martensitc transformation in TWIP steel has not been reported. In the present paper, the
    mechanical properties of TWIP steel with the addition of N were tested. The phases were analyzed by XRD and
    the microstructure was characterized by TEM. The average and local probabilities of stacking faults were also
    calculated by using shifts of X-ray peak and electron diffraction spot, respectively. Compared with the
    conventional TWIP steel, the results showed that at a lower SFE level, when fcc austensite or hcp martensite
    transformed to bcc martensite, the largest interstice decreased from 0.1047 to 0.0725 nm. The lattice distortion
    energy of bcc martensite was greatly enlarged by N, which situated in the interstices, leading to the suppression of the bcc martensitic transformation. As a result, the content of hcp martensite increased, causing the increase of
    strength and decrease of plasticity. Besides, the results also showed that deformation increased stacking faults and
    hcp or bcc martensitic transformation consumed a large number of stacking faults.
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    EFFECTS OF Nb ON RECOVERY OF HOT-DEFORMED AUSTENITE
    NIE Wenjin SHANG Chengjia WU Shengjie SHI Peijian CHENG Junjie ZHANG Xiaobing
    Acta Metall Sin, 2012, 48 (7): 775-781.  DOI: 10.3724/SP.J.1037.2012.00189
    Abstract   PDF (4877KB) ( 1269 )
    Solute and precipitates of Nb can effectively affect stastic recrystallization and recoverry of austnite in steels during hot rolling process. However, more research is concerned about the role of Nb precipitation on the strain accumulation in finish rolling process, the solute drag effect of Nb is neglected comparing with precipitates. In this paper, the stress-relaxation curves of the low C high Mn steels with different Nb content were investigated by thermal simulation test, the evolution of dislocation and its interaction with Nb solute and precipitate during recovery process of deformed autentie in a Fe-40%Ni-0.1%Nb (mass fraction) modle steel was also studied by transmission electron microscopy (TEM). Thereby, a theoretical model about recovery of deformed austenite was developed according to the slip of dislocations and the solute drag. The values calculated by the model are consistent with the experimental results and the metallurgic principles. It is shown that both solute and precipitation of Nb can slow down the recovery and enhance the strain accumulation. The Nb solute drag can increase the activation free energy of the recovery U0 and decrease the activation length. It is believed that for Nb micro-alloyed steels with low C and high Mn, the strain accumulation during finish rolling process would be relied on the Nb solute drag effect in hot-strip mill, and both solute drag and precipitation pin effects in heavy plate mill.
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    INFLUENCE OF COLUMNAR GRAINS ON THE COLD ROLLING TEXTURE EVOLUTION IN Fe-3%Si\ ELECTRICAL STEEL
    ZHANG Ning YANG Ping MAO Weimin
    Acta Metall Sin, 2012, 48 (7): 782-788.  DOI: 10.3724/SP.J.1037.2011.00659
    Abstract   PDF (6037KB) ( 1135 )
    Columnar grains exist in nearly all cast slabs of Fe-3%Si electrical steels, their morphological and crystallographic anisotropies strongly influence the microstructure and texture evolution during following hot rolling, cold rolling and final annealing. This work investigates the rolling texture and microstructure of the specimens with columnar grains initially arranged along different directions by means of XRD and EBSD techniques, and the effects of columnar grain boundaries are analyzed. The results show that at intermediate rolling reduction, all three types of specimens show to different extents the inheritance of {001} texture, or in other words, they prevent effectively the formation of {111} texture. The {001} texture is strongly retained in ND specimen (with initial columnar grains' longitudinal axis being along normal direction) and TD specimen (with initial columnar grains' longitudinal axis being along transverse direction), whereas {111}<112> texture is the strongest in RD specimen(with initial columnar grains' longitudinal axis being along rolling direction). In addition, rotated cube and {111}<110> textures develop in TD specimen. All three types of specimens demonstrate an orientation rotation path from cube {001}<100> over 20o rotated cube {001}<130> to {113}<251>, which is in contrast to the conventional rotation path from α fiber to $\gamma$ fiber in equiaxed polycrystalline electrical steels. In spite of the significant difference in grain boundary arrangement in the three types of specimens, their influence on texture formation is limited because of the large grain sizes and the influence is mainly related with initial orientations. To understand rolling texture evolution of columnar grain specimens is of great significance in the development of new type electrical steels with strong {001} texture.
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    SEGREGATION OF ATOMS ON THE INTERFACES IN THE RPV MODEL STEEL STUDIED BY APT
    XU Gang CAI Linling FENG Liu ZHOU Bangxin LIU Wenqing WANG Junan
    Acta Metall Sin, 2012, 48 (7): 789-796.  DOI: 10.3724/SP.J.1037.2011.00717
    Abstract   PDF (3189KB) ( 1251 )
    The segregation of impurity or solute atoms to grain boundaries as well as phase interfaces can either improve or degrade the chemical, physical and mechanical properties of alloys. This phenomenon has been studied widely for iron based alloys, and the analysis method by an atom probe tomography (APT) is a powerful tool for better understanding this problem. The resulting composition changes of grain boundaries and phase interfaces, as well as the precipitation of Cu-rich nanophases, are frequently associated with the phenomenon of embrittlement in ferritic reactor pressure vessel (RPV) steels. The present work was carried out to study the segregation of impurity or solute atoms to grain boundaries as well as phase interfaces in a RPV model steel with higher content of Cu (0.53%, atomic fraction) than commercially available one. The RPV model steel was prepared by vacuum induction melting. The specimens were further heat treated by water quenching at 880 ℃ for 30 min and tempering at 660 ℃ for 10 h, and finally aged at 370 ℃ for 3000 h. The results show that the segregation amount of Ni, Mn, Si, C, P and Mo atoms on grain boundaries are varied. The sequence of segregation tendency for different atoms from strong to weak is C, P, Mo, Si, Mn and Ni, whilst Cu atoms were clearly depleted at the grain boundaries. Si atoms also segregate to the grain boundaries, but it depends on the characteristic of the grain boundaries. The C segregation range at grain boundaries is the widest. According to the width of the composition profiles at the half intensity for different atoms at the grain boundaries, the segregation range of C atoms is 1.5 times wider than that of Mn, Ni and Mo atoms. Furthermore, Ni and Mn atoms evidently segregate to the interfaces between the Cu-rich phase and the α-Fe matrix, while C, P, Mo, Si atoms prefer to segregate towards the α-Fe matrix near the interfaces, but their segregation amount at the interfaces of Cu-rich phase and the α-Fe matrix is less than that at the grain boundaries.
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    MICROSTRUCTURE AND TOUGHNESS OF THE SIMULATED WELDING HEAT AFFECTED ZONE IN X100 PIPELINE STEEL WITH HIGH DEFORMATION RESISTANCE
    NIE Wenjin SHANG Chengjia YOU Yang ZHANG Xiaobing Sundaresa Subramanian
    Acta Metall Sin, 2012, 48 (7): 797-806.  DOI: 10.3724/SP.J.1037.2012.00215
    Abstract   PDF (9618KB) ( 1645 )
    A single welding thermal-cycles with different heat inputs (8, 16, 20, 25, 30 and 50 kJ/cm) were simulated by Gleeble 3800 to study the correlation of toughness, hardness and microstructure in heat affect zone (HAZ) of the X100 pipeline steel with multi-phases and 0.10\%Nb (mass fraction). The microstructures of the CGHAZ in HAZ were characterized by means of OM, SEM and EBSD, and mechanical properties were tested. The results show that for a low heat input of less than 20 kJ/cm, the microstructure is lath bainite or acicular ferrite structure with high-density of large-angle boundaries (≧15o), which exhibits good Charpy impact toughness. However, for a large heat input over 25 kJ/cm, the uniformity of prior austenite grains becomes worse, the M/A constituents and the granular bainite (GB) are coarsening, and the amount of large-angle boundaries decreases with the increase of heat input. The results of the instrumented Charpy impact test and the observation of fracture surfaces on the specimens indicate that the cracks are induced near the coarse M/A constituents and the large-angle boundaries can remarkably restrict crack propagations. Therefore, in order to ensure a strong match between the HAZ and the base metal, and the resistance to hydrogen induced delayed damage because of high hardness of HAZ, the heat input energy should be about between 15 and 20  kJ/cm.
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    PITTING CORROSION DYNAMICS AND MECHANISMS OF 304 STAINLESS STEEL IN 3.5%NaCl SOLUTION
    DU Nan TIAN Wenming ZHAO Qing CHEN Sibing
    Acta Metall Sin, 2012, 48 (7): 807-814.  DOI: 10.3724/SP.J.1037.2012.00005
    Abstract   PDF (2593KB) ( 1489 )
    Pitting rate of a single pit and pitting mechanisms of the 304 stainless steel in 3.5%NaCl solution were investigated by utilizing electronic speckle pattern interferometer (ESPI), electrochemical noise (EN) and three-dimensional video microscope. The results show that under 0.05 V polarization, the pitting corrosion process can be divided into four stages: drastic fluctuations of the current noise occured at 740 s, which means the passivation film was breaking, thus it can be concluded that the span of pitting incubation period is 740 s; a speckle occurred on the ESPI image at 750 s, thus the span of initiation period of the steady pitting is about 10 s; the growth rate of the pit increased during 750-780 s, which indicates that the pit corrosion is in active dissolution period; since then, the growth rate of the pit declined rapidly which means the pit was repassivated. After\linebreak 793 s, the growth rate of the pit raised again as secondary pits emerged. The pit image was observed and its volume was measured by three-dimensional video microscope, and the results were in agreement with those which were obtained by corrosion product concentration analysis. Some secondary pits were found in the bottom of the pit in three-dimensional reconstruction images.
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    STUDY ON THE CORROSION MECHANISM OF T2 COPPER CONDENSER TUBE UNDER THE HUMID ENVIRONMENT
    WANG Changgang DENG Weimin ZHAO Guangyu DONG Junhua KE Wei CHEN Xuebin
    Acta Metall Sin, 2012, 48 (7): 815-821.  DOI: 10.3724/SP.J.1037.2012.00114
    Abstract   PDF (3420KB) ( 938 )
    During the maintenance of two years, T2 copper tube in a heat exchanger has leaked, it can be deduced that the residual water and volatile water vapor would play an important role in leakage. By SEM, OM, stereo microscope observation, it was found that serious corrosion happened on the surface of copper in the gaps constituted by copper tube and stainless steel baffle holes, and perforation occurred in a few locations. Wetting experiments show that the gap formed between the T2 copper tube and the baffle hole wall is small enough that it could produce siphon liquid film, which could connect the copper tube surface and baffle hole. Therefore there is a difference of oxygen supply between the copper tube outer surface and the copper tube in the gap site, the outer surface of copper tube becomes oxygen-rich zone and the copper tube in the gap site oxygen-poor zone. Potential monitoring results show that the potential of the external surface of copper tube with an oxide is higher than that of copper in the gap site leading to a galvanic cell formed between them. The surface of copper in the gap site is anode region and the external copper tube surface is cathode region. The differences of oxygen supply combined the effect of the galvanic cell leads to the severe localized corrosion of copper tube in the gap site.
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    HYDROGEN STORAGE PROPERTIES OF ScMn2 ALLOY
    LI Wuhui TIAN Baohong MA Ping WU Erdong
    Acta Metall Sin, 2012, 48 (7): 822-829.  DOI: 10.3724/SP.J.1037.2012.00109
    Abstract   PDF (1185KB) ( 1172 )
    As an important rare--earth type Laves phase compound, ScMn2 alloy is endowed certain significance in the viewpoint of either theoretical or applicable investigation. In this study, the structures of ScMn2 alloy and its hydride (deuteride) are characterized by XRD. The hydrogen activation properties, pressure-concentration-temperature (P-C-T) curves and absorption kinetic curves of ScMn2 alloy are measured using Sieverts-type hydrogenator. The desorption kinetics of the passivated hydride are determined by TG-DSC. The results show that the hydride and deuteride of the alloy retain the C14 type Laves phase structure of the parent alloy, with the volume expansions of about 25%. ScMn2 possesses outstanding activation properties and can react quickly with hydrogen (deuterium) at room temperature and atmospheric pressure. The hydrogen and deuterium storage capacities of 1 mol ScMn2 are about 3.7 mol H and 3.6 mol D at 100 kPa and 298 K. ScMn2 has low hysteresis critical temperature for absorption and desorption, good plateau characteristics and relatively low plateau pressure, hence it is suitable for the storage of hydrogen isotopes. The enthalpy and entropy for formation of ScMn2 hydride at concentration corresponding to room temperature plateau pressure are -45 kJ/mol and -80 J/(K?mol), respectively. The hydriding kinetics of the alloy can be interpreted by Johnson-Mehl-Avrami (JMA) model, with the estimated reaction order of 0.4. The apparent activation energies for hydriding and deteuriding process are estimated to be  (16±0.3) and (19±1.7) kJ/mol, respectively, the observed isotope effect on kinetics can possibly be applied to separation of hydrogen isotope. The passivated hydride can release completely at 639 K and the corresponding apparent activation energy is (144±14) kJ/mol.
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    MORPHOLOGICAL CHANGES BEHAVIOR OF FAN-TYPE STRUCTURES OF γ' PRECIPITATES IN NICKEL-BASED POWDER METALLURGY SUPERALLOYS
    HU Benfu LIU Guoquan WU Kai HU Penghui
    Acta Metall Sin, 2012, 48 (7): 830-836.  DOI: 10.3724/SP.J.1037.2012.00226
    Abstract   PDF (5041KB) ( 763 )
    The variety and complexity of γ'  phase morphological changes in heat treatment process of nickel-based powder metallurgy (P/M) superalloy, which contains high volume percentage γ'  strengthening phase, is one of hot issues which material researchers focused on. γ'  phase morphological changes have important effect on the strength, toughness, high-temperature creep and fatigue property of alloy. Based on the research of heat treameat process in the thirdly nickel-based P/M superalloy (FGH98I), the researcher find that there are γ'  fan-structure in solution heat treatment at different cooling rates, and follow-up treatment have obvious influence on γ'  fan-structure morphology. Therefore, it is necessary to research scientifically and penetratingly on the formation conditions, formation mechanism, and the effects of different heat treatment process of γ'  fan-structure which exist as a kind of special organization morphology. The formation and evolution of fan-type structure in a new type nickel-based P/M superalloy FGH98I was studied by means of FESEM and TEM. The results show that the fan-type structure in alloy FGH98I consists of finger-shaped γ'  dendrites and the $\gamma$ matrix between them. It forms only in a selection area characteristic, nucleates inhomogeneously in the chemical segregation area at different scales on highly supersaturated grain boundary and develops by own concentration gradient diffusion. The standard aging makes the fan-type structure growing up and coarsening. The γ'  fingers become unstable, transforming into stable cubic shape γ'  in low-energy state after high-temperature aging.
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    HIGH TEMPERATURE DEFORMATION BEHAVIOR AND MICROSTRUCTURE EVOLUTION MECHANISM TRANSFORMATION IN Ti2448 ALLOY
    TIAN Yuxing LI Shujun HAO Yulin YANG Rui
    Acta Metall Sin, 2012, 48 (7): 837-844.  DOI: 10.3724/SP.J.1037.2012.00007
    Abstract   PDF (4535KB) ( 989 )
    Ti2448 (Ti-24Nb-4Zr-8Sn, mass fraction, %) is a multifunctional β-type biomedical titanium alloy with low elastic modulus, high strength and good biocompatibility. The alloy exhibits a peculiar plastic deformation behavior at room temperature called highly localized plastic deformation. With aid of such mechanism, the initial microstructure with coarse grains can be easily refined to homogenous equiaxed microstructure with nano-sized grains by the conventional cold processing such as rolling. In the paper, its high temperature plastic deformation behavior and the corresponding microstructure evolution were investigated in the single $\beta$ phase field by varying the strain rates in the ranges of 0.001-70 s-1. The results showed that the true stress and strain rate can be described by a bilinear relation, which is in sharp contrast with the conventional Sigmoidal relation found in other β-type titanium alloys. As the strain rates less than 0.1 s-1, the alloy follows the conventional β-type titanium alloys with a high average value of strain rate sensitivity being 0.265. As the strain rates higher than 1 s-1, the true stress and strain rate can be described by another linear relation with a much small average value of strain rate sensitivity being 0.032. This is different from other alloys exhibiting gradual decrease of strain hardening with the increase of the strain rates. Microstructure observations and kinetic analyses revealed that such bilinear relation would be related to its highly localized plastic deformation behavior and dynamic recrystallization (DRX), which are triggered and enhanced at higher strain rates over 1 s-1. Although dynamic recovery (DRV) is still a key microstructure evolution mechanism of the alloy during plastic deformation in single β phase field, the increase of strain rate induces a transformation from DRV to DRX, resulting in significant grain refinement from the initial coarse grains about 80 μm to refined grains less than 3 μm. Thus, the DRX is a crucial mechanism of the Ti2448 alloy to achieve significant grain refinement during hot processing.
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    EFFECTS OF Re AND Ru ON MICROSTRUCTURE AND SEGREGATION OF Ni-BASED SINGLE-CRYSTAL SUPERALLOYS
    LIU Gang LIU Lin ZHANG Shengxia YANG Chubin ZHANG Jun FU Hengzhi
    Acta Metall Sin, 2012, 48 (7): 845-852.  DOI: 10.3724/SP.J.1037.2012.00001
    Abstract   PDF (4494KB) ( 958 )
    The influence of Re and Ru on segregation and microstructure evolution during heat treatment has been investigated in four Ni-based single-crystal superalloys with varied contents of Re (3%-6%, mass fractions) and Ru (0% and 3%). The additions of Re and Ru lead to the more severe segregation of alloying elements in as-cast structures. However, the effects of Ru on as-cast segregation can be neglected after the stepwise solution heat treatments. The additions of Re and Ru lead to the lower γ'-coarsening rate, more cuboidal γ'-morphology and reduced γ'-size. Electron microprobe analysis (EPMA) indicates that Al, Ta and Ni partition to the γ'-precipitates, whereas Re and Cr strongly partition to the γ-matrix. In comparison to Re and Cr, Ru and W show the less tendency to partition to the γ-matrix. Additionally, Re increases the supersaturation of Re, Cr, Co and W in the γ-matrix, whereas Ru only slightly suppresses the partition of these TCP-forming elements to the γ-matrix.
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    PREDICTED CONSTITUTIVE MODELING OF HOT DEFORMATION FOR AZ31 MAGNESIUM ALLOY
    SUN Chaoyang LUAN Jingdong LIU Geng LI Rui ZHANG Qingdong
    Acta Metall Sin, 2012, 48 (7): 853-860.  DOI: 10.3724/SP.J.1037.2011.00641
    Abstract   PDF (1593KB) ( 997 )
    The uniaxial compression tests of AZ31 magnesium alloy at different strain rates of 0.01-10 s-1 and different deformation temperatures of 523-723 K were performed by using Gleeble-1500 simulator with a maximum strain of 0.916. The influences of deformation temperature and strain rate on the flow stress were investigated. The fine microstructure is attributed to dynamic recrystallization during compression process at 523 K. The stretched grains of dynamic recrystallization and growth up along radial direction were founded in microscopic observation at 723 K. Considering plasticity deformation and friction induced temperature rise, the flow stress was corrected at high strain rate by using temperature compensation. The peak flow stress and unified constitutive model were established based on hyperbolic sine model. Strain sensitivity of flow stress was studied to describe the coupling of materials parameters on the strain, and then the relationship between deformation temperature, strain rate and strain during hot deformation was obtained. Comparing with experimental results, the correlation coefficient and average relative error of predicted and measured values are 0.976 and 5.07\% respectively, it is proved that the model reflects the real deformation feature of the AZ31 magnesium alloy.
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    Ti-BASED AMORPHOUS COMPOSITES WITH QUANTITATIVELY CONTROLLED IN--SITU FORMATION OF DENDRITES
    TANG Mingqiang ZHU Zhengwang FU Huameng WANG Aimin LI Hong ZHANG Hongwei MA Guofeng ZHANG Haifeng HU Zhuangqi
    Acta Metall Sin, 2012, 48 (7): 861-866.  DOI: 10.3724/SP.J.1037.2012.00198
    Abstract   PDF (2831KB) ( 828 )
    A series of  (Ti32.8Zr30.2Ni5.3Cu9Be22.7)100-x (Ti61.5Zr36.4Cu2.1)x (x=10-95) bulk amorphous composites with quantitatively controlled in-situ formation of bcc-dendrites were prepared. The microstructures were analyzed by SEM, XRD and TEM. Thermal behaviors were examined using DSC. The results show that the size and volume fraction of dendrites depend on x in the composites. With increasing x, the volume fraction of dendrites rises monotonically from 20% to 90%. The microstructural transition between the matrix and the dendrites is smooth and successive with no phases observed at the interface. Compression tests show the size and volume fraction of dendrites largely influence the mechanical properties of composites. When the volume fraction is larger than the critical value (approximately 30% in the present work), the plasticity of composites cannot be improved. When the volume fraction is over the critical value, the higher the volume fraction of dendrites, the larger the plastic strain, the stronger the work hardening capacity. It implies that the properties of composite can be mediated by the tunable volume fraction of dendrites. These findings are significant to develop the controllable microstructure and performance materials. When x=90, the plastic strain and the compressive strength of the composites reach 14.4% and 1917 MPa, respectively.
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    MICROSTRUCTURE AND MECHANICAL PROPERTIES EVOLUTIONS OF CONTINUOUS COLUMNAR-GRAINED CuAlNi ALLOY WIRES DURING DIELESS DRAWING PROCESS
    WANG Zhen LIU Xuefeng XIE Jianxin
    Acta Metall Sin, 2012, 48 (7): 867-874.  DOI: 10.3724/SP.J.1037.2012.00187
    Abstract   PDF (5699KB) ( 1315 )
    Dieless drawing experiments of continuous columnar-grained Cu-14.0%Al-3.8%Ni (mass fraction) alloy wires were carried out at drawing speeds of 0.8-1.1 mm/s and deformation temperatures of 650-900 ℃, when the feeding speed and distance between the heating and the cooling sources kept invariant at 0.50 mm/s and 15 mm, respectively. Effects of dieless drawing parameters on the microstructure and mechanical properties of the alloy were investigated, and the mechanism of microstructure and mechanical properties evolutions of the deformed alloy was discussed. It was found that the straight continuous columnar-grained boundaries of the alloy wires evolved into regular small zigzag, disordered large zigzag and recrystallized boundaries. When the deformation temperature was 650 ℃ and the drawing speed was 0.8 and 0.9 mm/s, the alloy remained continuous columnar grains with straight boundaries after dieless drawing, while the straight columnar-grained boundaries gradually transformed into zigzag boundaries with the deformation temperature and drawing speed increasing. When the drawing speed was 0.9 mm/s and the deformation temperature up to 850 ℃, the alloy exhibited obviously incomplete dynamic recrystallized microstructure characteristic, $i.e$. the original columnar grains were elongated along the deformation direction, and small dynamic recrystallized grains generated at parts of zigzag grain boundaries. The alloy occurred complete dynamic recrystallization at the deformation temperature of 900 ℃, and the deformed columnar grains were completely replaced by numerous equiaxed dynamic recrystallized grains with larger size. The tensile strengths of the alloy after dieless drawing first showed a very modest increase trend and then decreased greatly, while the elongations kept decrease.
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    SOLIDIFICATION BEHAVIORS OF HIGHLY UNDERCOOLED Ni-21.4%Si EUTECTIC ALLOY
    CHANG Fang'e ZHAO Zhiwei ZHU Man LI Na FANG Wen DONG Guangzhi JIAN Zengyun
    Acta Metall Sin, 2012, 48 (7): 875-881.  DOI: 10.3724/SP.J.1037.2012.00006
    Abstract   PDF (5333KB) ( 1153 )
    The solidification behaviors of highly undercooled Ni-21.4%Si eutectic alloy molten under a slag composed of B2O3 and soda lime glass were investigated and the undercooling for the alloy to nucleate homogenously was predicted theoretically. It is found that a undercooling of 318 K can be achieved in Ni-21.4%Si eutectic alloy by using slag technique. Theoretical calculation shows that the maximum undercooling obtained in Ni-21.4%Si eutectic alloy has reached the homogeneous nucleation undercooling for the alloy. The solidification behavior and structure of undercooled Ni-21.4%Si eutectic alloy depend on the undercooling. When the undercooling is lower than 250 K, there are two recalescence peaks on the cooling curve. The solidified microstructure is composed of primary Ni3Si phase and regular eutectic as the undercooling is lower than 206 K, while primary α-Ni phase and regular eutectic structure are obtained when the undercooling is in the region from 206 K to 250 K. When the undercooling is greater than 250 K, only one recalescence peak is observed in the cooling curve and anomalous eutectic structure is obtained. Undercooling can influence the growth mode of primary Ni3Si. The primary Ni$_{3}$Si transforms from lateral growth to non-lateral growth with increasing the undercooling.
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    MICROSTRUCTURES AND MECHANICAL PROPERTIES OF CNT/Al COMPOSITES FABRICATED BY HIGH ENERGY BALL-MILLING METHOD
    XU Shijiao XIAO Bolu LIU Zhenyu WANG Wenguang MA Zongyi
    Acta Metall Sin, 2012, 48 (7): 882-888.  DOI: 10.3724/SP.J.1037.2012.00140
    Abstract   PDF (3132KB) ( 1296 )
    The mixture powders of carbon nanotubes (CNTs) and aluminum were high-energy ball-milled, and then the CNT/Al composites with different CNT contents were fabricated using a power metallurgy method. Microstructure examinations show that a certain volume of CNTs can be uniformly dispersed in the Al matrix by ball-milling and the CNTs have a close bonding with the Al matrix. By using an appropriate ball-milling process flow, the CNTs suffer no serious damage. Tensile tests indicate that the composite reinforced by 1.5% (volume fraction) CNTs exhibits the best mechanical performance, and the yield strength is improved by 53.6% compared with the Al matrix. When the CNT volume fraction reaches 3%, lots of clusters are formed in the composite, and therefore the tensile properties are significantly reduced. Both grain refinement and load transfer are proved to be the strengthening mechanisms of the CNT/Al composites.
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    FIRST-PRINCIPLES STUDY ON THE PHASE STABILITY OF Mg-La AND Mg-Nd BINARY ALLOYS
    ZHANG Hui WANG Shaoqing
    Acta Metall Sin, 2012, 48 (7): 889-894.  DOI: 10.3724/SP.J.1037.2012.00089
    Abstract   PDF (724KB) ( 959 )
    The applications of magnesium alloys in automobile industry are limited by their poor high-temperature creep resistance, which can be effectively improved by rare earth (RE) elements addition. In general, rare earth elements are introduced as mixtures of various kinds of metals, and all of the elements are considered to behave in the same way. In practice, various rare earth elements may act differently in magnesium alloys. La and Nd are the two elements usually added to magnesium alloys. In this paper, first-principles calculations are made to investigate the phase stability of Mg-La and Mg-Nd binary alloy systems. In addition, the solubility of La and Nd in Mg is discussed and the elastic constants of the strengthening phases in Mg-La and Mg-Nd alloys are calculated. The equilibrium phases between Mg12RE and Mg3RE are Mg17La2 and Mg41Nd5 for Mg-La and Mg-Nd systems respectively. The difference of calculated solubility for La and Nd in Mg indicates the distinct strengthening mechanism for these two alloy systems. Mg3Nd is predicted to have larger elastic moduli and a better strengthening effect than Mg$_{12}$La.
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