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CN 21-1139/TG
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    YANG Chenggong, SHAN Jiguo, REN Jialie
    Acta Metall Sin, 2012, 48 (5): 513-518.  DOI: 10.3724/SP.J.1037.2011.00683
    Abstract   PDF (2800KB) ( 796 )
    In order to control the shape memory function of TiNi alloy weld joint, it is necessary to clarify the effect of the three different parts (weld metal, heat-affected zone (HAZ) and base metal) on the shape recovery temperatures of the whole weld joint, but few reports are available on this aspect. In this work, the microstructure in the HAZ was studied by Gleeble thermal-simulation test. Phase transformation temperatures of weld joint, weld metal, HAZ and base metal were measured by differential thermal analysis. The inverse phase transformation temperature was analyzed. The microstructure, distribution of precipitation and crystal structure were investigated by using OM, SEM and XRD. The weld joint shows the similar shape recovery ratio to the base metal, but the shape recovery temperature range is significantly different. The start recovery temperature of the weld joint is lower about 40 ℃ than that of the base metal. Both of the austenite start temperature (As) and finish temperature (Af) of the weld metal and HAZ vary much compared with the base metal. The change in the weld metal is attributed to the fusion-solidification process, in which the preferred crystal orientation is lost. The newly formed precipitation phases show a small size and an uneven distribution. The change in the HAZ refers to the drop of As and Af, which is possibly caused by the solution of minor precipitation phase in the matrix. The Asand Af of the laser weld joint are quite the same as those of the weld metal for TiNi shape memory alloy, which indicates that the key to guarantee the shape memory function lies in controlling the phase transformation temperatures of the weld metal.
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    LIU Dahai,YU Haiping, LI Chunfeng
    Acta Metall Sin, 2012, 48 (5): 519-525.  DOI: 10.3724/SP.J.1037.2011.00741
    Abstract   PDF (4053KB) ( 1125 )
    In order to reveal the hyperplasticity mechanism of electromagnetic forming (EMF), the plastic instability mechanisms and formation behaviors of 5052 aluminum alloy sheets in pulsed magnetic tension process were investigated by theoretical analysis and microanalysis. Results show that inertia force plays an important role in dynamic forming, which has the suppression effect on structural instability and thus improves the formability of sheet and spreads instability. The nature of dynamic formation is much similar with that of quasi--static formation and no special formation structures arise in dynamic process for 5052 aluminum alloy sheets. The formation mechanism of both processes is dislocation slip mechanism. For quasi--static formation, the dislocations show a uniform single--slip pattern, fracture combined with dislocation tangling and climbing. While for dynamic formation, dislocation system tends to more slips, large areas showing clear cross-slip structures. The dislocation bands are narrower and much denser than those shown in the quasi--static process, and a much more uniform dislocation configuration is also exhibited after pulsed magnetic loadings. The characteristics of multi--slips and uniform effect of dislocations under pulsed magnetic loading conditions will result in much higher plasticity and strength of materials.
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    GANG Jianwei,SHI Binqing, CHEN Rongshi, KE Wei
    Acta Metall Sin, 2012, 48 (5): 526-533.  DOI: 10.3724/SP.J.1037.2012.00079
    Abstract   PDF (6302KB) ( 1103 )
    Wrought Mg alloys alloyed with rare elements (RE) addition were deemed to be one of the most promising Mg alloys in industrial application, due to the formation of weakened texture and refined microstructure. Generally, the wrought Mg alloys with RE addition after normal thermal-mechanical processing possessed incompletely recrystallized microstructure, so it was necessary to research the subsequent annealing treatment for controlling the microstructure. Unfortunately, the corresponding investigations on the mentioned above were still limited. In this study, hot-rolled Mg-1.02Zn and Mg-0.76Y (mass fraction, %) alloys were selected to investigate the microstructure evolution, static recrystallization behavior and grain growth kinetics under different annealing treatments. The microstructure examination showed that hot-rolled Mg-1Zn alloy was composed of shear bands and twins with the occurrence of dynamic recrystallization; whereas only twins were observed in the hot-rolled Mg-1Y alloy, no shear bands and recrystallization were detected. That should be attributed to the difference in the deformation modes during rolling processing. After isothermal annealing for the two alloys, recrystallization occurred in some remaining twins, whereas no recrystallization took place in others. EBSD analysis revealed that low angle grain boundaries or orientation differences were observed in the remaining twins with recrystallization, suggesting that recrystallization should be associated with the levels of stored deformation energy. The process of static recrystallization and grain growth kinetics were described by the JMAK model and grain growth model, respectively. The process of static recrystallization for the Mg-1Zn alloy was mainly dominated by the process of nucleation; while that for the Mg-1Y alloy was both controlled by the process of nucleation and growth, resulting in finer grain size. Moreover, the results showed that the Avrami exponent of recrystallization n≈1 deviated from the expected value in theory n=4, which could be due to the non-random recrystallization sites in the deformed alloys. Lower value in the grain growth exponent n' was obtained for Mg-1Y alloy than that for Mg-1Zn alloy, which may be ascribed to the stronger dragging effect of the solute Y element on the grain boundaries than that of solute Zn element.
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    HAO Xuehui, DONG Junhua, WEI Jie, KE Wei, WANG Changgang,XU Xiaolian, YE Qibin
    Acta Metall Sin, 2012, 48 (5): 534-540.  DOI: 10.3724/SP.J.1037.2012.00105
    Abstract   PDF (3747KB) ( 953 )
    International martime organization (IMO) has approvedand considered corrosion resistant steel as the only alternative for anti-corrosion coating since May 2010. The implementation of the standard will have a profound impact on ship building, steel, shipping and other industries. At present, Japan has a relatively mature technology, South Korea has completed the pre-development work, while China has only carried out some preliminary studies. If the technology is blocked, a large number of steel needs to be imported which would push up the cost of construction of the shipbuilding industry, and has a direct impact on the orders of shipping enterprises. The amount of steel for cargo oil tank in China is more than two million tons each year, and therefore, the localization of research and application of corrosion resistant steel for cargo oil tanks has become an urgent task. The impact of microstructure of the existing shipbuilding steel on corrosion behavior in simulated corrosion environment is studied based on the standard in this paper to develop our own corrosion resistant steel. According to the standard, immersion test was used to measure the corrosion process of AH32 corrosion resistant steel in the bottom simulated environment of cargo oil tanks. Using gravimetric measurement, electrochemical polarization and impedance methods, scanning electron microscopy and electron probe et al, the influence of microstructure of AH32 corrosion resistant steel on its corrosion behaviors was analysed. The experimental results showed that: during the test simulating the corrosion of the bottom plate of cargo oil tanks, corrosion rate of the rolling surface of AH32 corrosion resistant steel was low with small area fraction of pearlite, corrosion rate of the cross section was fast due to the big area fraction of pearlite, and both corrosion rate increased with the immersion time. In addition, there were uniform corrosion and the pits formed by dissolved inclusions in rolling surface, and corrosion of the cross-section was selected along the banded pearlite. The carbon enriched in pearlite area of the sample after immersion, which caused corrosion rate increased with the immersion time.
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    FENG Shaobo, ZHANG Nannan, LUO Xinghong
    Acta Metall Sin, 2012, 48 (5): 541-546.  DOI: 10.3724/SP.J.1037.2012.00037
    Abstract   PDF (2246KB) ( 767 )
    Ni-based superalloys have been widely applied in advanced aeroengine as gas turbine blades and vanes. The freckles in superalloys formed during directional solidification have deleterious influence on the properties of the alloys. The generation of freckles is associated with the local liquid density gradient in the mushy zone, which is obviously influenced by microsegregation of alloy elements. However, the individual contributions of the various elements to the total density variation are still not well known. Therefore, the effect of microsegregation on the liquid density variation in DZ483 Ni-based superalloy was investigated by isothermal solidification together with liquid quench method. Solidification microstructures were observed by optical microscope and SEM, and the compositions of the solids and the residual liquid were determined by EDS. Based on the compositions of residual liquids, the densities of liquids at different temperatures were calculated. The results show that the onset solidification temperature of DZ483 alloy is a little bit below 1335℃, and MC, which is enriched with Ta and Ti, formed at about 1325℃. The segregation coefficients of different elements show that W and Co are negative segregation elements, Ta and Ti positive segregation elements, while Al and Cr show little segregation. The density of the residual liquid generally decreases as the decrease of temperature, with the exception that it increases somewhat from 1325 to 1315℃. Calculation results show that temperature has insignificant influence on liquid density, and variation of density is mainly due to microsegregation. Segregations of Mo and Ta lead to the increase of density, but segregations of Ti and W present opposite effect. Contribution of each element to the variation of the liquid density is analyzed. The sequence of contributions of alloy elements to the variation of total liquid density is TiTa>W>Cr>Mo>Al>Co. The formation of MC consumes an abundant of Ti and Ta, resulting in the increase of liquid density from 1325 to 1315 ℃.
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    CHANG Zhengkai, XIAO Jinquan, CHEN Yuqiu, LIU Shanchuan, GONG Jun, SUN Chao
    Acta Metall Sin, 2012, 48 (5): 547-554.  DOI: 10.3724/SP.J.1037.2012.00056
    Abstract   PDF (7062KB) ( 635 )
    Arc ion plating (AIP) has been widely used for depositing various kinds of coatings due to the excellent characteristics of high deposition rate, convenient parameter control, high degree of ionization in the target material, good coating--substrate adhesion, flexibility of target arrangements and merits of producing coatings with high packing density. Magnetic films, with a few micrometers or less, could be utilized in the electronics industry, such as magnetic recording, magnetic microelectromechanical systems, magneto optical modulator, and so on. In AIP process, due to magnetic shielding and self--induced magnetic field, arc spot on the surface of the magnetic target moved outside all the time, and the erosion of the magnetic target could not be stable. In this study, arc spot outside moving and unstable erosion of the magnetic target in arc ion plating have been investigated. The distribution of the magnetic field of the nonmagnetic target and the magnetic target under an additional magnetic field was simulated by the finite element method (FEM). The effect of magnetic field on the arc spot movement was researched. With the physical mechanism of the arc spot discharge, the feasibility on the solution of the application problem of the magnetic target has been discussed by the program of the composited structure target, which were composed of magnetic target materials and target shell of low saturation vapor pressure metal, target shell of insulating ceramics, or target shell of soft magnetic metal. The results showed that all these solutions could solve the problem of arc spot outside moving efficiently. In the study, the transition temperature is (136.6±23.0) ℃ in the solutions of the target shell of low saturation vapor pressure metal or insulating ceramics, during which the arc striking frequently transformed to the controlled movement of arc spot.
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    XU Junhua, CAO Jun, YU Lihua
    Acta Metall Sin, 2012, 48 (5): 555-560.  DOI: 10.3724/SP.J.1037.2011.00724
    Abstract   PDF (3024KB) ( 1297 )
    A series of TiVN and TiVCN nano-composite films with different V and C contents were synthesized using a multi-target magnetron sputtering technique. The microstructures, mechanical properties and friction properties at different test temperatures were investigated by X-ray diffraction, nano-indentation, CSM high-temperature ball-on-disc tribo-meter and SEM-EDS analysis. The results showed that the hardness values of the TiVN and TiVCN films reached maximum of 25.02 GPa and 28.51 GPa at a V target power of 60 W and C target power of 20 W, respectively. With the further increase of C, however, the hardness of TiVCN films decreased gradually. At room temperature, the friction coefficient of the TiVCN film decreased with the increase power of C target. At high temperature, the friction coefficient of the TiVCN showed a up-down-up curve with the increase of test temperature. The effects and adaptive mechanisms of Magneli phase formed in the TiVCN films at high temperature were discussed.
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    ZHANG Shu, TIAN Sugui, YU Huichen, YU Lili, YU Xingfu
    Acta Metall Sin, 2012, 48 (5): 561-568.  DOI: 10.3724/SP.J.1037.2011.00536
    Abstract   PDF (3795KB) ( 669 )
    The distribution of von Mises stress and strain energy density in regions near interfaces of γ/γ' phases was calculated by an elastic--plastic stress--strain finite element method (FEM), and the influences of applied stress on the von Mises stress distribution and coarsening regularity of γ' phase in a [111] oriented single crystal nickel--based superalloy were also investigated. The results show that, after heat treated, the microstructure of the [111] oriented single crystal superalloy consists of a cubical γ' phase embedded coherently in γ matrix, and the cubical γ' phase is regularly arranged along <100> direction. When tensile stress is applied along [111] direction, compared to (010)γ' plane, larger expanding lattice strain occurs on (100)γ' and (001)γ' planes under the action of principal stress component, which may trap Al, Ti atoms with bigger radius to promote γ' phase directionally growing along [010] and [100] orientations on (010) plane, this is thought to be the main reason of γ' phase grown directionally into a mesh--like rafted structure along (010) plane.
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    TAN Xipeng, LIU Jinlai, SONG Xiaoping, JIN Tao, SUN Xiaofeng, HU Zhuangqi
    Acta Metall Sin, 2012, 48 (5): 569-574.  DOI: 10.3724/SP.J.1037.2011.00629
    Abstract   PDF (2414KB) ( 1101 )
    The microstructure of a Ru--containing single crystal nickel-based superalloy, which is a potential fourth generation single crystal nickel-based superalloy heat treated, has been investigated. The X--ray diffraction (XRD) results indicate that this single crystal superalloy is not a perfect monocrystal, which is comprised of many subgrains with an average neighboring misorientation difference about 0.5° and the maximum misorientation deviation about 3°; and each subgrain includes large numbers of mosaic structures with misorientation difference of 0.1°---0.2° between each other. It is found that the microstructure of the heat treated alloy is of heterogeneity, where the volume fraction of γ' phase increases and the γ' lattice misfit decreases. It is shown that the average size of γ' phase is 0.39 μm, the average γ' volume fraction 70% and the average γ' lattice misfit --2.0×10$-3. The evolution of this microstructure was also briefly described.
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    LI Ying, DING Yushi, CUI Shaogang, WANG Changzhen
    Acta Metall Sin, 2012, 48 (5): 575-578.  DOI: 10.3724/SP.J.1037.2011.00776
    Abstract   PDF (1051KB) ( 679 )
    CaZrO3 solid state electrolyte displays proton conductivity and high chemical stability. The Sc doped CaZrO3 proton conductors were prepared by solid state reaction in order to improve the conductivity in this study. XRD analysis suggest that CaZr1-xScxO3-α(x=0, 0.1, 0.15) samples were synthesized completely. The electrochemical impedance spectra were applied to study the conductivities and the activation energy for proton diffusion of the CaZr1-xScxO3-α(x=0, 0.1, 0.15) in the temperature range of 610---850℃, and the electric conductivities of CaZr1-xScxO3-α were compared with those of CaZr1-xInxO3-α. The experiment results show that the electric conductivities of CaZrO3, CaZr0.9Sc0.1O3-α, CaZr0.85Sc0.15O3-α, CaZr0.9In0.1O3-α and CaZr0.85In0.15O3-α are 4.3×10-19---1.4×10-6 S/cm (610---850℃), 1.16×10-4---4.6×10-4 S/cm (690---850℃), 1.8×10-4---1.4×10-3 S/cm (610---850℃), 0.34×10-4---4.30×10-4 S/cm (741---847℃) and 0.57×10-4---4.33×10-4 S/cm (585---814℃), respectively. These conductivities results show that the conductivity of CaZrO3 proton conductor can be significantly improved by doping. The conductivities of CaZrO3 increase with the Sc doping content and temperature increasing. The results reveal that the conductivities of CaZr1-xScxO3-α are higher than those of CaZr1-xInxO3-α. Sc--doping is more beneficial for increasing the conductivity of CaZrO3 solid state electrolyte.
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    Acta Metall Sin, 2012, 48 (5): 579-586.  DOI: 10.3724/SP.J.1037.2012.00087
    Abstract   PDF (2832KB) ( 823 )
    The orthorhombic Ti2AlNb alloys have received significant attentions because of their good physical and mechanical properties. However, these orthorhombic alloys face problems of oxidation at high temperature, especially above 700 ℃. To solve these problems, the use of surface coatings is an efficient way. However, when single coating was applied on the orthorhombic Ti2AlNb alloys, problems of serious interdiffusion and interfacial reaction were encountered, which resulted in worse oxidation behavior and deteriorated mechanical properties. To obtain good oxidation protection of NiCrAlY coating on the orthorhombic Ti2AlNb alloy, an efficient diffusion barrier should be added. In this study, NiCrAlY/Al--Al2O3 double--coatings were deposited on the orthhombic--Ti2AlNb alloy by arc ion plating. NiCrAlY coating acted as oxidation resistance coating and Al--Al2O3 coating acted as diffusion barrier. By introducing metallic Al in the Al2O3 film, the problem of coefficient of thermal expansion (CTE) mismatch between film and alloy substrate might be mitigate. Also metallic Al in the Al2O3 film can act as diffusion path which permits proper interdiffusion to improve the interface adhesion. The oxidation and interdiffusion behavior of specimens with and without diffusion barriers were investigated by oxidation tests at 900 ℃. The results indicated that substantial interdiffusion and rapid oxidation degradation occurred in the coated specimens without diffusion barrier. With Al--Al2O3 diffusion barriers, deferred interdiffusion and improved oxidation resistance were observed. Different contents of metallic Al in the Al2O3 coatings had different efficiency of diffusion barrier, and also affected interfacial mechanical properties. Among these NiCrAlY/Al--Al2O3 coatings, double--coating containing 1Al--Al2O3 diffusion barrier exhibited best performance. Coefficient of diffusion hindering was used to compare and quantify the efficiency of the diffusion barriers.
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    WANG Songlin, FENG Yi, WANG Dongsheng, WANG Jingwen
    Acta Metall Sin, 2012, 48 (5): 587-592.  DOI: 10.3724/SP.J.1037.2012.00097
    Abstract   PDF (2736KB) ( 1092 )
    Developing cost-effective methods to prepare dense ceramic interconnect membrane for solid oxide fuel cell (SOFC) stacks is currently considered as a major technical obstacle. In order to improve the co-firing compatibility of LaCrO3-based interconnects with the traditional YSZ-based SOFC anode support NiO/YSZ, interconnect material of  La0.7Ca0.3Cr0.97O3-δ(LCC) was introduced to NiO/YSZ anode. Triple-phase composite NiO/YSZ/LCC was prepared, and then examined as novel anode support. Sintering character, microstructure, electrical conductivity, and thermal expansion coefficient of the composite anode were investigated in detail as a function of LCC addition, respectively. Results indicated that the NiO/YSZ/LCC composite anode had excellent overall performance. Furthermore, by using a simple drop-coating process, LCC and YSZ wet membranes were prepared on the opposite surfaces of NiO/YSZ/LCC support, respectively. Followed by three-layer co-firing at 1400℃ in air for 4 h, dense La0.7Ca0.3Cr0.97O3-δ interconnect and YSZ electrolyte thin membranes were both successfully prepared on the porous NiO/YSZ/LCC anode support. This work presents a simple drop-coating/three-layer co-firing technical route for developing dense interconnect and electrolyte membranes for YSZ-based SOFC stacks.
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    WU Zhiqiang, TANG Zhengyou, LI Huaying,ZHANG Haidong
    Acta Metall Sin, 2012, 48 (5): 593-600.  DOI: 10.3724/SP.J.1037.2011.00590
    Abstract   PDF (3123KB) ( 2554 )
    The microstructure and mechanical properties of Fe--18Mn low carbon high manganese TRIP/TWIP steels during tensile tests in the range of initial strain rate of 1.67×10-4---103 s-1 at room temperature were studied. The inverse effect of strain rate on strength of steel was produced, the strength and ductility of steels decreased with increasing strain rate in the range of quasi--static tensile strain rate of 1.67×10-4---1.67×10-1 s-1. While inverse effect of strain rate on ductility of steels was produced in the range of dynamic tensile strain rate of 101---103 s-1, the strength and ductility of materials increased significantly with increasing strain rate. The tensile strength of high manganese TRIP/TWIP steels was 957 MPa and their elongation was 55.8%. These results indicated that Fe--18Mn steel had excellent mechanical properties and good fracture resistance. The higher the strain rates applied, the less martensite, the more directions of deformation twins. The microstructure evolution of the specimen was analyzed by SEM, TEM and XRD, martensitic transformation and deformation twins were produced during the tensile deformation, and adiabatic temperature rise effect made the matrix softening during the high--speed deformation.
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    WANG Bing, LIU Qinyou, WANG Xiangdong
    Acta Metall Sin, 2012, 48 (5): 601-606.  DOI: 10.3724/SP.J.1037.2012.00108
    Abstract   PDF (2438KB) ( 922 )
    Three kinds of ultra-low carbon IF steel with different grain sizes, and same chemical composition were prepared by different rolling and heat treat process. The relationship between grain size and atmospheric corrosion resistance of IF steel was investigated by immersion corrosion test, cyclic immersion corrosion test, AFM/SEM micro-analysis and electrochemical test. The results show that the local corrosion in grain boundary increases after immersion corrosion test, the depth of crack in grain boundary becomes deeper and the width of crack becomes wider with grain sizes of IF steel increase from 15 μm to 220 μm. The crack and cavity in the rust after cycle immersion corrosion test are increased and the atmospheric corrosion resistance is decreased with IF steel grain size coarsing from 15 μm to 46 μm. As grain size increase from 15 μm to\linebreak 220 μm, the whole compactness of rust are increased, the rust resistance and the atmospheric corrosion resistance are increased. The effect of grain size on the corrosion current density of local grain boundary was analysed and the mechanics of corrosion was discussed. The total quantity of corrosion surface defect is decreased due to the decrease of grain boundary energy with the increase of grain size and the atmospheric corrosion resistance is increased. Meanwhile, the local corrosion near the grain boundary is increased duo to the increase of local corrosion current density with the increase of grain size and the atmospheric corrosion resistance is decreased. The atmospheric corrosion resistance is influenced by the two factors simultaneously.
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    ZHANG Jinling1,2,3, FENG Zhiyong1, HU Lanqing1,2,3,WANG Shebin1,2,3, XU Bingshe1,2,3
    Acta Metall Sin, 2012, 48 (5): 607-614.  DOI: 10.3724/SP.J.1037.2012.00014
    Abstract   PDF (3322KB) ( 1386 )
    The changes of microhardness, microstructures and precipitation phase morphologies of as--cast and aged AZ91 alloys with different La were characterized by TEM, OM, SEM and Vickers. The influences of discontinuous β phase and Al11La3 on AZ91 magnesium alloy were discussed and the diffusion coefficient of Al atom in the aging process was calculated. The results showed that the alloy with 0.16%La aged at 170 ℃ for 24 h has the largest hardness (138 HV). By extending aging time and adding La element, the discontinuous precipitation phases was transformed into continuous lath--sharped phases and the diffusion coefficient of Al atom was decreased, the volume and distribution of β phase were changed. During the process of heat treatment, the topography of Al11La3 in grain boundary was not changed, but the anchoring effect of Al11La3 restricted the growth of β phase. By controlling the content of La and parameters of aging treatment, the volume and size of β phase were changed, in the mean time the strengthening and toughness properties of alloys can been improved effectively.
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    Acta Metall Sin, 2012, 48 (5): 615-620.  DOI: 10.3724/SP.J.1037.2012.00069
    Abstract   PDF (1657KB) ( 902 )
    The dendrite morphology is determined by the interaction between the capillarity effect and the transports of heat and solute, and is significantly altered by the presence of fluid flow during solidification. A lot of numerical models have been developed to investigate the effect of fluid flow on the dendritic growth of pure materials. But up to date, only a few researches were carried out on the effect of fluid flow on the dendritic growth of alloys. The effect of fluid flow on three dimensional (3D) dendrite tip selection parameter of alloys remains an unsolved scientific problem. A 3D cellular automaton (CA) model for dendritic growth of alloys was developed in this paper. 3D CA is solved in coupling with a momentum transport model in order to predict the evolution of dendritic morphology during solidification of alloys in the presence of flow. The dendrite growth with a forced flow in an undercooled melt of an Al-4%Cu (mass fraction) alloy was simulated. The effect of forced flow on dendritic growth was investigated. The results show that a forced flow affect the three dimensional dendritic growth of an alloy significantly. The growth of the primary and secondary arm in the upstream direction is much greater than that in the downstream direction. The growth direction of the primary arm perpendicular to the flow direction tilted into the upstream direction. The dendrite tip of the primary arm perpendicular to the flow direction shows an asymmetric morphology. The degree of the tilt and the asymmetry of the tip become stronger with the increase of the forced flow velocity. With the increase of the flow velocity the growth velocity of the upstream dendrite tip increases, the radius and the selection parameter of the upstream dendrite tip decrease. For a given undercooling, the effect of forced flow on the selection parameter of the upstream dendrite tip becomes stronger with the increase of the anisotropy of the interfacial energy. For a given  alloy, the effect of forced flow on the selection parameter of the upstream dendrite tip also becomes stronger with the increase of undercooling.
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    WANG Xiaonan, DI Hongshuang, DU Linxiu
    Acta Metall Sin, 2012, 48 (5): 621-628.  DOI: 10.3724/SP.J.1037.2012.00082
    Abstract   PDF (4244KB) ( 861 )
    In order to control nano-scale precipitation (Nb, Ti)C in hot-rolled 780 MPa grade C-Mn steel micro-alloyed with niobium and titanium for automobile frames, the effects of deformation and cooling rate on nano-scale precipitation were investigated by using the thermal simulation experiment technology, then through the transmission electron microscopy observation and statistical analysis. The result indicated, deformation could significantly improve density of dislocation, subgrain boundary and vacancy etc in microstructure, and promote heterogeneous nucleation of precipitation, and improve nucleation rate of precipitation and decrease the average diameter of precipitation. Deformation could improve vacancy concentration and promoted vacancy nucleation. The induction period of precipitation nucleation decrease with the increase of deformation amount and strain rate, and precipitation more easily to nucleate. Precipitation nucleation driving force was mainly supersaturation of microalloy in undeformed experimental steel, and the nucleation mechanism was mainly homogeneous nucleation. However, the nucleation mechanism was mainly heterogeneous nucleation in deformed experimental steel. In one fixed experimental deformation condition, when the cooling rate below 5℃/s, there was (Nb, Ti)C-PFZ (precipitate free zone) nearby original austenitic grain boundaries or subgrain boundaries, and the width of PFZ at cooling rate of 0.5, 1, 2 and 5℃/s were 46.9, 30.2, 28.1 and 0 nm, respectively, so the width of PFZ decreased with the cooling rate increasing. When the cooling rate reached 15℃/s, the nucleation of precipitation was totally inhibited during cooling process. The number of precipitation along with the cooling rate increases gradually decreases. With the increase of cooling rates, the nucleation zone of precipitation was transferred from austenite to ferrite or bainite, and the average diameter of precipitation was refined. Due to grain boundaries or the subgrain boundaries were main traps for supersaturated vacancy, but the diffusivity of vacancy was high, which made the vacancy concentration nearby grain boundaries or the subgrain boundaries lower than critical vacancy concentration for precipitation nucleation, so precipitate could not nucleate nearby grain boundaries or subgrain boundaries. Due to the diffusivity of vacancy was affected by temperature, when the cooling rate was slow, vacancy had enough time to diffuse and annihilate, which made wide PFZ formed. Whereas, when the cooling rate was high, the diffusivity of vacancy was reducing or disappearing, so the width of PFZ was small. In orde to ensure experimental steel had higher yield strength, austenite zone precipitation and (Nb, Ti)C-PFZ nearby boundaries should be inhabited, so the cooling rate should be more than 15 ℃/s in the practical rolling process.
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    XUAN Weidong, REN Zhongming, LI Chuanjun, REN Weili, CHEN Chao, YU Zhan
    Acta Metall Sin, 2012, 48 (5): 629-635.  DOI: 10.3724/SP.J.1037.2011.00621
    Abstract   PDF (6218KB) ( 868 )
    In order to understand the influence of a longitudinal magnetic field on the microstructures of the superalloy DZ417G with different sample sizes, the superalloy DZ417G was directionally solidified under a longitudinal magnetic field. The solidification structures of the alloy with various dimensions of specimen have been investigated. The results showed that the columnar dendrites on the edge of samples were broken and changed into equiaxed dendrites in the magnetic field at the withdrawal velocity of 5 $\mu$m/s and temperature gradient of 70℃/cm. Additionally, it was found that equiaxed dendrites area expanded from the edge to the center of sample and the number of freckle-like macrosegregation increased with the size of sample. The columnar to equiaxed transition and freckle-like macrosegregation can be attributed to the thermoelectric magnetic convection (TEMC) induced by the magnetic field in the front of solid/liquid interface during directional solidification.
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    WANG Jie, ZHANG Hongwei, WANG Aimin, LI Hong, FU Huameng, ZHU Zhengwang,ZHANG Haifeng
    Acta Metall Sin, 2012, 48 (5): 636-640.  DOI: 10.3724/SP.J.1037.2011.00808
    Abstract   PDF (2024KB) ( 870 )
    Zirconium based alloy is a new type structural material with high specific strength and good comprehensive properties. In this article, quasi-static compression at room temperature and dynamic compression under various temperatures were performed to a new zirconium-titanium alloy. It was found that this alloy showed good strength and plasticity, and the strength increased and plasticity decreased with increasing strain rate or decreasing temperature. A constitutive model under dynamic compression was established based on Johnson-Cook model, and accorded well with experimental results.
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