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

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    论文
    PHASE–FIELD SIMULATION OF SOLUTE PRECIPITATIONS AROUND THE  γ PHASE IN Al–Ag ALLOY
    GAO Yingjun LUO Zhirong ZHANG Shaoyi HUANG Chuanggao
    Acta Metall Sin, 2010, 46 (12): 1473-1480.  DOI: 10.3724/SP.J.1037.2010.00216
    Abstract   PDF (4609KB) ( 2720 )
    Interactions between different precipitation products during phase decomposition for different alloys have been observed. Spinodal decomposition and intragranular precipitation are the two well–know mechanisms for explaining phase decomposition, both of which interaction mechanisms have been investigated experimentally in recently. A local free energy density function depending on aging temperature and composition has been proposed to describe the interaction between the Ag solute field and γ precipitates in phase–field simulation of spinodal decomposition in Al–Ag alloy. The evolution of spinodal decomposition in Al–Ag alloy with 4.2%Ag and 22%Ag has been simulated by the phase–field method using this function to represent numerically the precipitated Guinier–Preston zones (GPZ) around a γ phase. The simulated results show that PFZ around a precipitated phase is an elliptical and its width is about two times the width of γ phase. In the region far from PFZ, a pattern of Ag solute field appears due to spinodal decomposition. When Ag–depleted zones are relatively far apart with each other, spinodal decomposition is strongly affected by them. The formation of PFZ resulting from spinodal decomposition has been initiated in the central region of the supersaturated α matrix before the modulation effect of Ag solute field at edge reachs here. It is found that two or three Ag–rich bands appear around the PFZ. During aging, Ag diffuses from not only the α matrix but also the edge of Ag–depleted zones, where an accumulation of Ag occurs. After long time aging, many droplet–like Ag solute bands are formed near PFZ around γ phase. These simulated results are in beter agreement with the experimental results.
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    STUDY OF THE AGING PRECIPITATION AND HARDENING BEHAVIOR OF 6005A ALLOY SHEET FOR RAIL TRAFFIC VEHICLE
    YANG Wenchao WANG Mingpu SHENG Xiaofei ZHANG Qian WANG Zheng’an
    Acta Metall Sin, 2010, 46 (12): 1481-1487.  DOI: 10.3724/SP.J.1037.2010.00223
    Abstract   PDF (3443KB) ( 1803 )
    The single–stage aging process, microstructure evolution during the precipitation process and crystal structure and orientation relationships between the β′′ phase and matrix were studied by means of conventional transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and hardness testing in 6005A aluminum alloy used for rail traffic vehicle. The results show that 6005A alloy can reach its peak hardness after aging at 175 for 12 h, and maintain it for a longer time. It is found that there exist clusters or GP zones in this alloy at early stage of aging and the β′′  phase at the peak–aging stage. After the peak–aging, the β′′  phase as a main strengthening phase in 6005A aluminum alloy continues being precipitated from matrix for a long time. β′′  phase has a C–centered monoclinic lattice with the lattice parameters a=(1.52±0.04) nm, b=0.405 nm, c=(0.67±0.04) nm, β=105.26?. The crystallographic orientation relationship between the β′′  phase and matrix is found as follows: (010)β′′ //(001)Al, [200]β′′ //[230]Al, [002]β′′ //[¯310]Al. And, the precipitation sequence with aging time in 6005A aluminum alloy can be described: super saturated solid solution (SSS) →clusters→GP zone→ metastable β′′  phase→ metastable β′  phase+Q′ phase→ stable β(Mg2Si) phase+Q phase.
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    STUDY ON PRECIPITATION BEHAVIOR OF PHASES CONTAINING Cu IN THE Cu–BEARING STEEL IN CONTINUOUS COOLING PROCESS
    LI Chuang WANG Xuemin SHANG Chengjia ZHENG Chang’an HE Xinlai
    Acta Metall Sin, 2010, 46 (12): 1488-1494.  DOI: 10.3724/SP.J.1037.2010.00227
    Abstract   PDF (3364KB) ( 1223 )
    The hardening behavior of five Cu–bearing steels during continuous cooling has been studied with the aid of thermo–simulation technique. Optical microscope (OM) and high resolution transmission electron microscopy (HRTEM) were employed to investigate the influence of cooling rate on the precipitation behavior in these steels and their hardness. The results show that during the continuous cooling the second phase precipitates occur in these steels and cause the precipitation hardening. These precipitates are proved to be Cu–rich phases and formed by the way of inter–phase precipitation. The precipitation behavior and hardening effect could be affected by cooling rate and copper content in these steels. When the steels are cooled at a cooling rate between 0.1—1 ℃/s, the second phase precipitates become finer and denser with the increase of cooling rate. Only when the cooling rate is 1 ℃/s the density of the second phase precipitates is the largest. When the cooling rate is quicker than 1 ℃/s, increasing the cooling rate leads to the precipitates being finer and fewer. When the samples are cooled at a rate of 10 ℃/s, there are few precipitates in samples. The Cu–rich phase is the main cause to strengthen these steels. It is also found that when the copper content is less than 1%, the precipitation behavior is unobvious.
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    EFFECTS OF Al–Al4C3 REFINER AND ULTRASONIC FIELD ON MICROSTRUCTURES OF PURE Mg
    GAO Shengyuan LE Qichi ZHANG Zhiqiang CUI Jianzhong
    Acta Metall Sin, 2010, 46 (12): 1495-1500.  DOI: 10.3724/SP.J.1037.2010.00230
    Abstract   PDF (3260KB) ( 1604 )
    The melt of pure Mg was inoculated at the conditions of different addition amounts of Al–Al4C3 refiner. When adding 1.0%Al–Al4C3  refiner, the refining effect is the best and the average grain size of α–Mg is reduced from millimeter level to 106 μm. Ultrasound fields with different power levels were applied on the melt of pure magnesium and the best refining effect was obtained when the power was 600 W. The results show that the undercooling formed by breakdown of cavitation bubbles may have a promoting effect to nucleate. The breakdown of cavitation bubbles and the agitations of acoustic stream on the melt cause the fragmentation of fine dendritic crystal which could form new crystallization nuclei, this process plays the main role for nucleating in the melt. The combined application of ultrasound and Al–Al4C3  refiner to the melt of pure Mg could significantly refine its grain. The mechanism would be an the activation effect of ultrasound on the impurities in the melt.
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    THE EFFECTS OF V ON PHASE TRANSFORMATION OF HIGH CARBON STEEL DURING CONTINUOUS COOLING
    LI Yi YANG Zhongmin
    Acta Metall Sin, 2010, 46 (12): 1501-1510.  DOI: 10.3724/SP.J.1037.2010.00284
    Abstract   PDF (5493KB) ( 1881 )
    Interlamellar spacing is one of the most fundamental parameters to characterize the pearlite microstructure and its mechanical properties, the influence of some alloying elements on interlamellar spacing in high carbon steels has been extensively studied in the past. Unfortunately, few studies refer to the influence of vanadium. In this paper, the effects of different amounts of vanadium on phase transformation in high carbon steels during continuous cooling has been investigated on Gleeble1500 thermo–mechanical simulator, and the mechanical properties relevant to these effects have been measured. The results show that by the way of adding vanadium to the high carbon steel, the pearlite transformation temperature is depressed, the CCT curves of this transformation are postponed so that the interlamellar spacing is reduced, leading to the increase of the pearlite hardness and strength in the high carbon steel. There are two reasons why vanadium can depress the average temperature of pearlite transformation: firstly, vanadium depresses the start temperature of this transformation, secondl, it lowers the velocity of pearlite nodule growing. It is alsfund that ferrite appears on the austenite grain boundaries of the high carbon steel, introducin 0.1% vanadium into the high carbon stecan somewhat retrain the ferrite formation, nevertheless by the way adding 0.2% or 0.3% vanadium, the ferrite formation on the austenite grain boundaries is apparently prooted. Evidently, the ferrite formation on austenite grain boundaries is beneficial to the improvement of ductility of high carbon steels as confirmed in this paper.
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    EFFECTS OF MELTING TREATMENTS ON THE MECHANICAL PROPERTIES OF REVERTED DZ40M ALLOY
    YANG Jinxia ZHENG Qi SANG Zhiru LANG Haixia SUN Xiaofeng HU Zhuangqi
    Acta Metall Sin, 2010, 46 (12): 1511-1516.  DOI: 10.3724/SP.J.1037.2010.00142
    Abstract   PDF (2708KB) ( 1159 )
    Reverted DZ40M alloy was refined by melting treatment (MT) which tensile, stress–rupture and creep properties were measured. MT can reduce the content of N from 0.0078% to 0.0024%, as well as the content of S from 0.0015% to 0.0007%. It makes impurity elements such as N, O and S being removed, and the shrinkage area being decreased. MT improves the tensile strength and ductility of reverted DZ40M alloy, especially elongations at room temperature and 1000 ℃ are increased about twice as high as those by regular melting (RM). In addition MT prolongs the stress–rupture lives, and improves the creep properties of reverted DZ40M alloy. Therefore stress–rupture and creep properties of reverted DZ40M alloys are upgraded greatly.
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    INFLUENCES ON MECHANICAL PROPERTIES OF FRICTION STIR WELDED JOINTS BY Al CLAD ACCUMULATION
    KANG Xu SHI Qingyu SUN Kai WANG Xin
    Acta Metall Sin, 2010, 46 (12): 1517-1521.  DOI: 10.3724/SP.J.1037.2010.00276
    Abstract   PDF (1582KB) ( 1168 )
    Friction stir welding (FSW) is increasingly used in joining the high strength aluminum alloys. Compared to traditional fusion welding methods, one of advantages of FSW is to obtain defect–free joints more easily. Besides the forms of FSW tools and welding parameters, certain characteristics of the base material can also affect the quality of welded joints. There is usually a protective layer on aluminum alloy surface to prevent corrosion, called Al clad. Generally the mechanical properties of Al clad are significantly lower than those of the base metal. In this paper, tensile and three–point bending tests were applied to investigate the influence of accumulated Al clad on mechanical properties of joints. The mechanism of reducing the mechanical properties of joints, caused by the accumulated Al clad was analyzed by OM, SEM and EDS. The results show that the mechanical properties of the welded joints are lower than those of the joint with which the bottom Al clad on the base material is removed before or after welding. The Al clad tiled on bottom of the base material is squeezed to both sides of the onion ring in the role of the FSW tool and gathered in the bottom after welding. Due to the poorer mechanical properties of Al clad than the base metal, the Al clad aggregation positions become the weak areas in the joints, leading to the formation of the micro cracks in the joints under tensile stress where the micro cracks continue extending to generate macro cracks under the continuous tensile stress. This is the main eason of joint breaking more easily under low strss.
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    PREPARATION AND CHARACTERIZATION OF ORGANIC HYBRID FILMS ON THE SURFACE OF Al TUBES
    LIANG Yonghuang MAN Ruilin
    Acta Metall Sin, 2010, 46 (12): 1522-1528.  DOI: 10.3724/SP.J.1037.2010.00258
    Abstract   PDF (1661KB) ( 1232 )
    Dense and hydrophobic organic hybrid films were deposited on the surface of aluminum tubes by the method of chemical immersion. Corrosion resistance of hybrid films was examined by bluestone drips, hydrogen evolution and copper accelerated salt spray tests, and the results show that the hybrid film significantly increases the corrosion resistance of aluminum tubes, with better protection than that of chromate passivation films. Tafel polarization curves and electrochemical impedance spectroscopy (EIS) were employed to investigate the electrochemical properties of hybrid films, the results indicate that the hybrid film greatly enhances the resistance of Al and reduces obviously the corrosion current density, which results in effective decrease to the corrosion rate of Al. Adhesion and bending experiments show that hybrid films have good adhesion and play a good performance on bending resistance. The surface structure and composition of hybrid films are analyzed using SEM, AFM and EDS. It is found that the surface of hybrid films, mainly consisting of Al, C, O, Si and P elements, is smooth, uniform and dense. The film surface is deposited a large number of amorphous solid granules with high coverage. Mechanism analysis of hybrid films’ formation and anticorrosion indicates that the reactions are taken place in the components of the organic hybrid passivation solution, synergistic passivation is carried out on the surface of aluminum tubes, and then uniform films are obtained, which effectively enhance the corrosion resistance of aluminum tubes.
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    CHARACTERIZATION OF THE SURFACE FILM FORMED ON MOLTEN AZ91D MAGNESIUM ALLOY PROTECTED BY SO2 MIXTURES
    WANG Xianfei XIONG Shoumei
    Acta Metall Sin, 2010, 46 (12): 1529-1533.  DOI: 10.3724/SP.J.1037.2010.00303
    Abstract   PDF (7507KB) ( 1690 )
    Molten magnesium alloy oxidizes rapidly during casting and handling processes, sulfur dioxide (SO2 ) mixed with carrier gases can be used to protect molten magnesium alloy by reacting with the melt to form a coherent protective film on the melt surface. In this paper, the films formed in controlled atmospheres with SO2  gas were characterized by X–ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and Auger electron spectroscopy (AES), the formation process and the protective mechanism of the surface film were also discussed. The results show that the film is a network structure with similar microstructure characteristics and composition in the boundary area and the interior area. AES depth profiling and SEM observation both indicate that the film thickness is to be in a range of 0.5—3.0 μm, and the film is composed of two layers with an outer layer of MgO and a composite layer of MgO and MgS beneath the outer layer. MgS increases the pilling and bedworth ratio of the surface film and enhances its protective capability.
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    MICROSTRUCTURE SIMULATION OF HIGH PRESSURE DIE CAST MAGNESIUM ALLOY BASED ON MODIFIED CA METHOD
    WU Mengwu XIONG Shoumei
    Acta Metall Sin, 2010, 46 (12): 1534-1542.  DOI: 10.3724/SP.J.1037.2010.00279
    Abstract   PDF (7228KB) ( 1463 )
    As the lightest structural material, magnesium alloy has been widely used in the automotive, aerospace and electronic industries. High pressure die casting (HPDC) process is the dominant process for magnesium alloy products. The microstructure of die cast magnesium alloy has a great influence on the final performance of the castings. Numerical simulation provides a way to predict the solidification structure and the corresponding mechanical properties. However, as one of the most widely used methods in microstructure simulation, the cellular automaton (CA) method has difficulties in simulating the solidification structure of magnesium alloy with hcp crystal structure, though simulations of solidification structure for bcc and fcc metals have been widely reported. Besides, for the microstructure simulation of magnesium alloys by HPDC process, accurate nucleation model has to be considered, and by far little report was found on it. In the present paper, based on the accurate temperature field of die castings obtained by an inverse heat transfer model, analysis of the temperature curves during solidification was made to establish a nucleation model that correlated the cooling rate with the nucleation density of magnesium alloys during solidification of HPDC process. A modified CA model was also developed to simulate the crystal growth of magnesium alloys. It takes account of the solute diffusion,  constitutional undercooling, curvature undercooling, and anisotropy etc. Validations were made to the model, and the results show that the model has the capability to simulate the dendrite growth of magnesium alloy with different growth orientations. Besides, the model can also reveal the dendrite morphology with features of secondary and ternary dendrite branches, the dendrite competition growth under different temperature gradients and solidification rates, and the three dimensional morphology of the dendrite growth. To validate the nucleation and growth model established for magnesium alloy under HPDC process, "step–shape" die castings of AM50 magnesium alloy were produced at different process parameters. The average grain size prediction results are in good agreement with the experimental ones.
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    STUDY ON PARTITION RATIO AND SEGREGATION BEHAVIOR OF DZ125 ALLOY DURING DIRECTIONAL SOLIDIFICATION
    MIN Zhixian SHEN Jun FENG Zhourong WANG Lingshui LIU Lin FU Hengzhi
    Acta Metall Sin, 2010, 46 (12): 1543-1548.  DOI: 10.3724/SP.J.1037.2010.00305
    Abstract   PDF (1231KB) ( 1479 )
    Segregation behavior is an important problem in solidification process of alloys, which determines their growth morphologies, phase distributions and concentration segregations so that it affects the mechanical properties of alloys, such as fatigue lives, creep properies, etc.. However, the segregation behaviors of Ni–based superalloys are very complex that contain multi–components and consist of multi–phases. The solute partition coefficient is a key characteristic parameter to express the solute segregation level and tendency in a solidification process. In the present paper, the solute profiles in directionally solidified DZ125 alloy with planar interface growth, which was obtained through experiments of quenching during directional solidification, have been measured by electron probe micro analysis (EPMA). The steady state of solute redistribution hardly reaches due to melt flow during solidification process. The contents of γ′ phase forming elements, Al, Ti, Ta, Mo and Ni, along longitudinal direction increase with the increase of solidified fraction, while those of W, Cr and Co decrease. The results show that the solute prtition coefficients of Al, Ti, Ta, Mo and Ni are less than unit, those of W, Cr and Co are more than unit. In addition, the solute distribution in solid phase with dendrite growth is similar to that with planar interface growth. Compared the experimental rsults of microsegregation with the calculated ones of Scheil model and Brody–Flemings model, the Brody–Flmings model fits better with the experimental results. It indicates that the back–diffusion in solid phase would reduce the microsegregation during dendrite growth.
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    FABRICATION PROCESS, MICROSTRUCTURE AND MECHANICAL PROPERTIES OF BFe10–1–1 ALLOY TUBES BY CONTINUOUS UNIDIRECTIONAL SOLIDIFICATION
    GAN Chunlei LIU Xuefeng HUANG Haiyou XIE Jianxin
    Acta Metall Sin, 2010, 46 (12): 1549-1556.  DOI: 10.3724/SP.J.1037.2010.00349
    Abstract   PDF (2172KB) ( 1896 )
    BFe10–1–1 cupronickel alloy tubes were fabricated by the vertical downward continuous unidirectional solidification with vacuum melting and argon–shield. The effects of processing parameters on the surface quality of alloy tubes have been investigated, and the microstructure and mechanical properties of cupronickel alloy tubes were characterized. The results show that BFe10–1–1 cupronickel alloy tubes having smooth surface and columnar crystal structure, with a diameter of 10 mm and thickness of 1.7 mm could be fabricated at a range of melting temperatures from 1250℃ to 1280 ℃ matching well with the withdrawal speed range from 0.15 mm/s to 0.45 mm/s on the conditions of coolnwater flow volume 900 L/h, cooling dstance 8 mm and cooling water temperature 25 ℃. The segregation ratio of Mn is larger than one, indicating the Mn tends to partition preferentially to the interdendritic region, whereas Ni and Fe to dendritic structure. Comparing with alloys fabricated by traditional casting, the degree of microsegregation of the alloy fabricated by continuous unidirectional solidification is reduced. The elongation of 49% at room temperature for the fabricated cupronickel alloy tubes exhibits excellent plastic deformation property. BFe10–1–1 cupronickel alloy tubes fabricated by the continuous unidirectional solidification have the tensile strength of 212 MPa which is markedly reduced compared wth cupronickel alloys produced by conventional casting, and are particularly advantageous to subseqent processing.
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