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

About the Journal

  Current Issue
    , Volume 45 Issue 10 Previous Issue    Next Issue
    For Selected: View Abstracts
    论文
    A TWO-PHASE FLOW MODEL FOR SIMULATING AIR ENTRAPMENT DURING MOLD FILLING OF HIGH PRESSURE DIE CASTING PROCESS
    Acta Metall Sin, 2009, 45 (10): 1153-1158. 
    Abstract   PDF (3314KB) ( 1468 )

    The most common defect found in high pressure die casting (HPDC) process is the gas porosity which significantly affects the mechanical roperties of the final components. The generation of gas porosity is known mainly due to the air entrapment in the liquid metal during the mold filling stage. Knowing the trapped–air location and amount could allow for a more accurate and objective analysis of casting quality. In the past few decades, extensive efforts have been made to develop simulation codes of casting flow. Most of these codes solve the velocity, pressure and fluid fraction only in the liqid phase wih he assumption that the effect of air in the die cavity is negligible. As a matter of fact, the air in the die cavity has significant influence on the filling pattern of the molten metal and the gas porosity distribution of the die casts. Recently, following the development of computational fluid dynamics (CFD), two–phase flow models have drawn continuous attention in the numerical simulation of casting processes, but there are still few models and further studies are needed. In this study, the mechanism of the formation of air entrapment defects in the HPDC process was discussed and it turned out that the air flow in the die cavity as well as the interaction between air and liquid metal resulted in the final air entrapment. In order to consider the air flow in the die cavity, an incompressible two–phase flow model was presented to simulate the air bubbles entrapped by the free surface of liquid metal durig the mold filling stage. Two numerical benchmark tests of fluid dynamics were performed to verify thvalidity and stability of the model. Furthermore, a high speed water analog exeriment similar to real die casting process was designed and carried out to compare the exerimental results with the simulation ones. Good agreements obtained demonstrate that the two–phase flow model has acceptable rediction accuracy in modeling the filling behavior of liquid and tracking the entrapped air bubbles.

    References | Related Articles | Metrics
    EVOLUTION OF Σ3 BOUNDARIES DURING RECRYSTALLIZATION OF COLD–ROLLED NICKEL DEFORMED TO HIGH STRAIN
    ZHANG Yubin A. Godfrey LIU Wei LIU Qing
    Acta Metall Sin, 2009, 45 (10): 1159-1165. 
    Abstract   PDF (3519KB) ( 1754 )

    The concept of grain boundary engineering (GBE) has been proposed based on the fact that many studies have demonstrated that boundaries associated with low value coincident site lattice (CSL) misorientations show higher resistance to intergranular fracture and corrosion, reduced susceptibility to impurity segregation and superior ductility. It is commonly accepted that for fcc metals of low to medium stacking fault energy metals, including Ni and many Ni–alloys, the most important CSL boundary for the GBE process is a Σ3 boundary, the occurrence of which is dominated by the formation of annealing twins. Moreover, it has been found that repetitive thermo–mechanical processing can be used to increase further the fraction of Σ3 (and Σ3n (n >1)) boundaries. However, the mechanism for this is not yet clear. Therefore, an investigation on the evolution of Σ3 boundaries during recrystallization is important for understanding the mechanisms of GBE for those materials. In the present paper the evolution of Σ3 boundaries during recrystallization in a 96% cold–rolled sample of pure nickel f 99.996% purity has been explored using orientation maps obtained using electrn backscatter diffraction (EBSD). Each orientation map was taken from the same area after annealing for various times. Based on the EBSD data the Σ3 boundaries can be divided into two groups: "twin" type and "non–twin" type. These groups can be differentiated using a parameter of deviation angle (Δθ) of boundary misorientation to the ideal twin misorientation (60°<111>). During recrystallization incoherent twin boundaries are found to develop from coherent twin boundaries. It is found also that most Σ3n (n >1) boundaries are formed by impingement of a nucles with its n–order twins, and that the chance for such impingement events decreases significantly with increasing n. Most non–twin type Σ3 boundaries arise from impingement of Σ1 and twin type Σ3 boundares. Non–twin type Σ3 boundaries may be more effective than twin type Σ3 boundaries to develop a beneficial grain boundary network.

    References | Related Articles | Metrics
    EFFECT OF Ti ON THE ANTI-CORROSION PROPERTY OF Zn-Al ALLOY FILMS
    ZHANG Jingyu LIU Qingfeng LIU Qian
    Acta Metall Sin, 2009, 45 (10): 1166-1170. 
    Abstract   PDF (1643KB) ( 894 )

    Zinc coatings have been widely used to provide corrosion protection for metal materials because they act as barriers and sacrificial anodes to prevent their protected substrates from becoming rust. However, it is seldom satisfactory for zinc coatings to meet the more demanding anti–corrosion needs in severer atmosphere. The increased requirements for enhancing anti–corrosion properties have led to the industrial production of zinc alloy coatings. A range of Zn–Al coatings were thus developed as replacements for zinc coatings. Further researches on Zn–Al coatings indicated that the anti–corrosion properties of the binary system can be improved by doping other elements. Besides, metal titanium could exhibit outstanding anti–corrosion properties under a wide variety of environments. Therefore, Zn–Al–Ti thin films or coatings are strongly supposed to be the promising materials for improving anti–corrosion properties. Using the combinatorial material chip technology, Zn–Al–Ti thin films with different Ti contents (where the mass fraction of Al to Zn are 55% and 45%, respectively) were synthesized on the low–carbon substrate by an ion beam sputtering method. The as–deposited multilayer films were trnsformed into alloy films after a two–step annealing: diffusion at lower temperature and crystallization at higher temperature. The anti–corrosion behavior of the alloy films in 3.5% (mass fraction) neutral NaCl aqueous solution was determined by the electrochemical methods. Further experiments were conducted to investigate the corrosion properties of the optimized composition. The results indicate that the doping of Ti obviously improves the anti–corrosion properties of the Zn–Al films, where the optimal content of Ti doping is around 6% (mass fraction). The structure and morphology of the optimal alloy film were characterized using XRD and SEM, respectively. Besides, the anti–corrosion mechnism of the ternary Zn–Al–Ti alloy films was analyzed, which provides some useful results for the further research on the alloy film systems.

    References | Related Articles | Metrics
    HOT CORROSION BEHAVIOR OF ARC-ION PLATING Ti-Al-Cr(Si, Y) COATINGS ON Ti60 ALLOY
    YAN Wei SUN Fengjiu WANG Qingjiang LIU Jianrong CHEN Zhiyong LI Shaoqiang
    Acta Metall Sin, 2009, 45 (10): 1171-1178. 
    Abstract   PDF (5808KB) ( 1046 )

    High–temperature titanium alloys intended for aero engine compressor applications suffer from high–temperature oxidation and environmental corrosion, which prohibit their long–term service at temperatures above 600 ℃. In an attempt to improve the oxidation resistance and corrosion resistance, Ti–48%Al–12%Cr (0.2%Si, 0.1%Y, atomic fraction) protective coatings were plated on the substrate of alloy Ti60 by arc ion plating (AIP) method. The corrosion behavior of the bare alloys and the protective coatings in Na2SO4 and 75%Na2SO4+ 25%K2SO4 (mass fraction) in air was investigated by XRD, SEM and EDS. The results indicate that Ti60 alloy shows a poor corrosion resistance in the hot corrosion process at 800 and 850 ℃due to corrosion product scales spalling. Ti–Al–Cr(Si, Y) coated specimens, however exhibited good hot corrosion resistance at 800 and 850 ℃ in sulfate. Corrosion in 75%Na2SO4+25%K2SO4 is more severe than that in Na2SO4. Ti60 with Ti–Al–Cr–Si coating or Ti–Al–Cr–Si–Y coating has better hot corrosion resistance than that with Ti–Al–Cr coating.

    References | Related Articles | Metrics
    A CONSTITUTIVE MODEL FOR SHAPE MEMORY ALLOY IN PURE SHEAR STATE
    ZHOU Bo LIU Yanju LENG Jinsong
    Acta Metall Sin, 2009, 45 (10): 1179-1184. 
    Abstract   PDF (436KB) ( 1025 )

    It is of engineering interest to establish a constitutive model which includes the equations describing the phase transformation and mechanical behaviors of shape memory alloys (SMA) in pure shear state. In this study, such a shape memory evolution equation is established using the shape memory factor and Brinson’s relationship of phase transformation critical stress and temperature. A mechanical constitutive equation is also developed from 3D micro–mechanical constitutive equation based on the assumption that SMA is isotropic material to express the mechanical behaviors of SMA in pure shear state. All material constants in the shape memory evolution equation and mechanical constitutive equation can be determined through macroscopic experiments, so that they are moe easily sed in practical applications. Numerical simulation results show that this shape memory evolution equation cold simulate truly the processes of phase transformations in austenite, twinned and detwinned martensites, and the mechanical constitutive equation could predict reasonably the mechanical behaviors of SMA in pure shear state.

    References | Related Articles | Metrics
    FIRST-PRINCIPLES CALCULATION OF ELECTRONIC STRUCTURE, BONDING CHARACTERISTIC AND BONDING STRENGTH OF TiN(111)/BN/TiN(111) INTERFACE
    NIU Jiangang; WANG Baojun; WANG Cuibiao; TIAN Xiao
    Acta Metall Sin, 2009, 45 (10): 1185-1189. 
    Abstract   PDF (684KB) ( 1122 )

    The nanocomposite 'nc–TiN/a–BN' as a representation of the family of superhard nitride–based nanocomposites, which is a nanocomposite thin film material, exhibits a significant hardness enhancement as compared with the pure constituents. In this paper, first–principles calculations were performed to investigate the role of interfaces in the nanocomposite 'nc–TiN/a–BN', to which less attention has been paid up to now. In order to determine theoretically the stable interface configuration in 'nc–TiN/a–BN', 16 possible theoretical TiN(111)/BN/TiN(111) sandwich interface cnfigurations have been constructed based on the stucture characteristic of 'nc–TiN/a–BN'. It is found in this calculation that the mst favorable interface configuration istop–top–BN, which is closely related to each B atom covalently bonding to its tetahedrally coordinated N atoms in it. ts electronic structure is calculated. The calculated results show that the bnds at the interface in 'top–top–BN'configuration are covalent. Its interface bonding strength is higher than that between two 111 crystalline planes in slab TiN or bulk TiN.

    References | Related Articles | Metrics
    PHASE-FIELD SIMULATION OF TWO-PHASE GRAIN GROWTH WITH HARD PARTICLES
    GAO Yingjun ZHANG Hailin JIN Xing HUANG Chuanggao LUO Zhirong
    Acta Metall Sin, 2009, 45 (10): 1190-1198. 
    Abstract   PDF (3474KB) ( 1423 )

    Grain growth, due to its importance in controlling the physical properties of a wide variety of materials, has been extensively investigated. Second–phase particles have the capacity to "pin" grain boundaries and therefore affect the grain growth behavior of polycrystalline materials profoundly. They reduce the mobility of grain boundaries and eventually, when a critical grain size is reached, arrest grain growth. Based on a diffuse–interface description, a computer simulation model for studying the microstructural evolution in two–phase solid has been developed. For a grain system with hard particles, the kinetics of two–phase grain growth with the third hard particles was investigated by phase field model with a continuum diffuse–interface field. A polycrystalline microstructure of temporal and spatial evolution of the three–phase–solid system was obtained by solving three kinetics equations. It is found that the pinning effect is enhanced with the increase of the size and the volume fraction of third–phase particles. The greater the volume fraction and size of third–phase particles are, the smaller the limited sizes of grain growth are. If the volume fraction of third–phase particle maintains a constant and the size of third–phase particles is smaller, then the pinning effect of third–phase particles is stronger. When third particles with two different sizes under the same volume fraction are introduced in the system of grain growth, the pinning effect of the particles is the best. The power growth law, grain morphology, critical grain size, grain growth dynamics and topology structure of two–phase polycrystalline materials simuated by phase–fielmodel are in well accordnce with the experimental results and theoretical results of other simulations.

    References | Related Articles | Metrics
    MOLECULAR DYNAMICS SIMULATIONS OF NANOMACHINING MECHANISM AND THERMAL EFFECTS OF SINGLE CRYSTAL Cu
    GUO Yongbo LIANG Yingchun CHEN Mingjun LU Lihua
    Acta Metall Sin, 2009, 45 (10): 1199-1204. 
    Abstract   PDF (3275KB) ( 1265 )

    In recent years, nanomachining has received an increasing attention because of the remarkable advancement in sciences and technologies. In nanomachining process, the atomic interaction in surface and subsurface layers plays an important role. At such a small nanoscale, the traditional continuum representation method, such as finite element method, becomes questionable. This difficulty can be solved in general by molecular dynamics (MD). MD provides the necessary insight into nanomachining process and allows researching local material properties and behaviors in detail. Based on the large scale parallel algorithm, a new three–dimensional molecular dynamics simulation model was established for nanomachining of single crystal Cu. The interactions between workpiece atoms (Cu—Cu), copper and diamond atoms (Cu—C), and diamond atoms (C—C) were described by embedded atom method (EAM), Morse and Tersoff potentials, respectively. The temperature distribution and thermal effects during nanomachining were investigated. The chip formation and nanomachining mechanism were analyzed from the point of view of dislocation theory and thermal effects. The simulation results demonstrate that both the dislocation emission and chip pilep direction are along the h110i orientation. Temperature ditribution presents a roughly concentric shape and a steep temperature gradient lies in diamond tool, and the highest temperature is found in chip. The workpiece material becomes soft as the system temperature increases. Cutting speed and cutting edge radius have a significant effect on the system temperature distribution.

    References | Related Articles | Metrics
    MOLECULAR DYNAMICS SIMULATION OF NANOMETRIC CUTTING CHARACTERISTICS OF SINGLE CRYSTAL Cu
    LIANG Yingchun PEN Hongmin BAI Qingshun
    Acta Metall Sin, 2009, 45 (10): 1205-1210. 
    Abstract   PDF (6160KB) ( 1289 )

    The increasing demand for designing and manufacturing micro parts with high quality comes from the high speed development of micro electromechanical systems (MEMS) and nano electromechanical systems (NEMS) in recent years. Nanometric cutting as an important machining way of micro parts has become a hot spot in machining field. Some main issues in nanometric cutting such as chip formation, machined surface quantity and diamond tool wear etc., have been investigated by molecular dynamics. Previous researchers have pointed out that the generation and evolution of defects are mainly responsible for causing plastic deformation of machined workpiece in nanometric cutting of plastic materials and a high compressive stress remaining in shear zone is considered beneficial to ductile–mode machining of brittle materials. Up to now, however, the influence of cutting thickness on defect behaviors and stress distribution in a workpiece and the relationship between them for single crystal materials are still unclear. In the present study, molecular dynamics simulations of nanometric cutting of single crystal Cu were performed. The simulation results show that stacking fault and partial dislocation are two main types of the defects in workpieces. A high surface stress at the atomc scale was observed in workpieces and there exist the compressive stress in shear zones and tensile stresses in the machined surfaces. It is found that the stress–distance curves of workpieces present a clear periodicity corresponding to the generation and evolution of dislocations in them. At he beginning of cutting (a small cutting thickness), no apparent stacking faults inside workpieces have been found, but with the increase of cutting thickness, the defects on surfaces and subsurfaces increase significantly and the thicker the cutting thickness, the smaller the corresponding stress components.

    References | Related Articles | Metrics
    MICROSTRUCTURE ANALYSIS OF INTERFACIAL LAYER WITH TUNGSTEN INERT GAS WELDING–BRAZING JOINT OF ALUMINUM ALLOY/STAINLESS STEEL
    LIN Sanbao SONG Jianling YANG Chunli MA Guangchao
    Acta Metall Sin, 2009, 45 (10): 1211-1216. 
    Abstract   PDF (1624KB) ( 1193 )

    Against the background of the required weight reduction in transportation through lightweight construction, the application of hybrid structures, where aluminum alloy and steel are jointed together, has a high technical and economical potential. But jointing of material combinations of aluminum alloy and steel is problematic by fusion welding since brittle intermetallic compounds (IMCs) are formed between aluminum alloy and steel. Nowadays, tungsten inert gas (TIG) welding–brazing offers a great potential for aluminum alloy and steel jointing. In this process, the sheet and filler metal are heated or melted by TIG heat, and the joint has a dual characteristic: in aluminum alloy side it is a welding joint, while in steel side it is a brazing joint. However, in the dynamic heating process, the heating temperature changes so quickly and the reaction time between the liquid filler metal and solid steel is so short that it is more difficult to control the IMC layer’s growth, predominantly its thickness and microstructures. Most of past reports about the brazing of aluminum alloy and steel indicate Al–Fe binary IMC layers, e.g., Fe2Al5 and FeAl3, formed in the brazing joint, which are detrimental to the mechanical properties of the joint. Si additions are used to limit the growth of the brittle Al–Fe IMC layer between aluminum alloy and steel by replacing Al–Fe phases with less detrimental Al–Fe–Si phases in aluminizing and furnace brazing of aluminum alloy and steel. By now, there have been few reports of investigating the interfacial layer of TIG welding–brazing joint of aluminum alloy and stainless steel. In this paper, a butt TIG welding–brazing joint of aluminum alloy/stainless steel was formed using Al–Si eutectic filler wire with modified Noclock flux precoated on a steel surface. The microstructure characteristics of the welded seam–steel interfacial layer were analyzed by OM, SEM and EDS and its mechanical properties were measured by dynamic ultra–microhardness tester and SEM in situ tensile tester. The results show that a nonuniform and sawtooth IMC layer forms at the seam–steel interface and its thickness changes from 4 to 9 μm, less than the maximum permissible value (about 10 μm). The interfacial layer is composed of two types of IMC layers, which are τ5 IMC layer on the seam side  and θ+η+τ5 IMC layer on the steel side. τ5 phase forms preceding θ+η+τ5 due to its lower growth energy than Al–Fe phases and the primary τ5 layer inhibits the growth of rough dendritic θ+η+τ5 phases. The ultra–microhardness test results show the microhardnesses of  θτ5 and θ+η+τ5 layers reach HV1025 and HV835, respectively. Indentation cracking of τ5 layer at higher loads indicates that τ5 is a type of hard brittle phase. SEM in situ tensile test results confirm that cracking initiates from θ+η phases and then fracture rapidly generates along θ+η+τ5 layer while suffering external force. The tensile strength of IMC layer reaches 120 MPa.

    References | Related Articles | Metrics
    STUDY OF A NEW-TYPE HIGH STRENGTH Ni--BASED SUPERALLOY DZ68 WITH GOOD HOT CORROSION RESISTANCE
    LIU Enze SUN Shunchen TU Ganfeng ZHENG Zhi NING Likui ZHANG Lingfeng
    Acta Metall Sin, 2009, 45 (10): 1217-1224. 
    Abstract   PDF (3055KB) ( 1175 )

    In order to meet the requirements of marine gas turbine blade materials, a new–type directional solidification Ni–based superalloy named DZ68 was developed. The alloy composition was designed by low segregation technology. Its nominal chemical composition (mass fraction, %) is C
    0.05, Cr 12.0, Mo 1.0, W 5.0, Co 8.5, Al 5.3, Ti 0.5, Ta 5.0, Re 2.0, B 0.01, and balance is Ni. The microstructures of as–cast DZ68 alloy and after heat treatment states were analyzed by OM, SEM and XRD. The tensile, rupture and hot corrosion resistance properties of DZ68 alloy were compared with
    DZ125 and IN738 alloys. Results show that the microstructure of as–cast DZ68 alloy is composed of  γ, γ', (γ+γ') eutectics, MC type carbides and a few borides. After heat treatment, the microstructureof DZ68 alloy is composed of  γ, γ' and carbides. The carbides are mainly MC and M23C6. The tensile strength of DZ68 alloy decreases slightly with the increase of temperature, and reaches its minimum value at 700℃. When the temperature is higher than 700℃, the tensile stregth increases so evidently that reaches its maximum t oce at 760 ℃, But whethe temperature is higher than 760 ℃its tensile strength decreases obviously. It is well recognized the relatioship of the tensile strength of DZ68 alloy with temperature is abnormal, similar to that of its yield strength but oposite to that of its plasticity.  The tensile and rupture properties of DZ68 alloy are nearly the same as those of DZ125 alloy and its hot corrosion resistance property is nearly the same as that of IN738 alloy under the same conditions.

    References | Related Articles | Metrics
    INFLUENCE OF WITHDRAWING RATE TRANSITION ON THE PRIMARY DENDRITE ARM SPACING AND MICROSEGREGATION OF DIRECTIONALLY SOLIDIFIED SINGLE CRYSTAL SUPERALLOY DD3
    HUANG Taiwen LIU Lin ZHANG Weiguo ZHANG Jun FU Hengzhi
    Acta Metall Sin, 2009, 45 (10): 1225-1231. 
    Abstract   PDF (1328KB) ( 1231 )

    The microstructure of single crystal Ni–based superalloys is virtually determined by both processing parameters of growth rate v and thermal gradient G ahead of the solidification front. Previous researches have established the relationship between dendrite spacings and G or v under
    supposing those parameters to be constant from beginning to end of solidification, resulting in the disaccord between the predicted and experimental results for actual blade productions. Furthermore, previous experimental and theoretical works seldom involve the influence of processing parameter
    change on dendrite growth for multi–component industrial alloys. Therefore, investigation of directional solidified microstructure varying with processing parameter is the focus of present study. The influence of the withdrawing rae ransition during directional solidification on the primary dendrite arm spacing PDAS) and microsegregtion for single crystal superalloy DD3 has been studied by liquid metal cooled (LMC) directional solidification method. A wide range of withdrawing rate variations from 50 to 600 μm/s is allowable under G high to 250 K/cm. The results indicate that the average PDAS is remarkably dependent on the history of growth rate variation. The smaller PDASs of 56.5 and 86 μm can be obtained under withdrawing rate transition from higher values of 600 and 300 μm/s into 100 μm/s, while they are kept being 111.5 μm if withdrawing rate is 100 μm/s all the time. In contrast, PDASs increase to 109 and 93 μm under withdrawing rate transition from lower values of 50 and 100 μm/s into 300 μm/s, while tey are kept being a small value of 70 μm if growth rate is 300 μm/s at beginning. These experimental results agree approximately with those from Hunt–Lu model, but the maximum to miimum ratio of PDASs at a given solidification paameter, i.e.,  λ1max1min , is roved to be more than 2It is also shown that the denrite microsegreation will be lessened with the decrease of PDASs at the same current soldfication parameters. 

    References | Related Articles | Metrics
    CHARACTERISTIC AND FORMATION MECHANISM OF PRECIPITATES AT RECRYSTALLIZATION GRAIN BOUNDARIES OF SINGLE CRYSTAL
    SUPERALLOY DD6
    XIONG Jichun LI Jiarong ZHAO Jinqian LIU Shizhong DONG Jianxin
    Acta Metall Sin, 2009, 45 (10): 1232-1236. 
    Abstract   PDF (3171KB) ( 1077 )

    Single crystal superalloys have extremely good elevated temperature capability in advanced gas turbine aero engines due to no highly stressed grain boundaries in them. With the removal of grain boundary strengthening elements such as C, B and Zr, the occurrence of recrystallization may be detrimental to their performance. Therefore, recrystallization becomes critical in industrial manufacture of single crystal superalloy blades. In the present study, specimens of single crystal superalloy DD6 were grit blasted, solution treated and aged at vacuum atmosphere, and then the
    precipiaes at recrystallization grain boundaries were invstigated by SE, TE, EPMA and Themo–Calc. The results show that a few of M6C carbides preciptate at recrystallization grain boundaries, and their size is about 0.5 μm. These M6C carbides are rch in W, Re and Mo, but poor in A, Ta, Ni,
    the contents of Cr, Nb, Co in them are almost the same as the nominal composition of DD6 alloy. The carbon accumulation arecrystallization boundaries and combination with μ phase forming elements such as W and Mo restrain the μ phase forming. M23C6 phase hardly forms in DD6 alloy due to its high W and low Cr content.

    References | Related Articles | Metrics
    PHASE–FIELD SIMULATION OF THE EFFECT OF KINETIC ANISOTROPY ON CRYSTAL GROWTH IN UNDERCOOLED MELTS
    ZHAO DaWen LI Jinfu
    Acta Metall Sin, 2009, 45 (10): 1237-1241. 
    Abstract   PDF (1300KB) ( 934 )

    Solidification is generally determined by a complex interplay of heat and/or solute diffusion processes, capillary and/or kinetic effects of the solid/liquid interface. Theoretical analysis indicates that the crystal growth morphology and behavior both depend sensitively on the degree of the capillary and kinetic anisotropy. In present, the influences of capillary anisotropy on crystal growth process have been extensively studied, especially simulated by the phase–field model. Unfortunately, the influences of kinetic anisotropy on the crystal growth morphology and behavior are seldom researched. In this paper, the phase–field model was employed to quantitatively simulate the effects of kinetic anisotropy on the crystal growth in undercooled melts. It is illustrated that the selection of the solid/liquid interface morphology is determined by the kinetic anisotropic parameter if the capillary anisotropy s set as zero. With a wak kinetc anisotropy, the melt solidifies in a fractal pattern, during which there s no obvious preferrd growth drection, and any steady–state gowth cannot be detected. As a stong anisotropy becomes greater than 0.02, the interface morphology changes to a dendritic pattern growth along the h110i orientation. Further analysis indicates that the stability parameter ncreases inearly with the increase of inetic anisotropic parameter and is independent of the kinetic coefficient.

    References | Related Articles | Metrics
    GRAIN GROWTH MODEL OF INCONEL 718 ALLOY FORGED SLAB IN REHEATING PROCESS PRIOR TO ROUGH ROLLING
    CHEN Liqing SUI Fengli LIU Xianghua
    Acta Metall Sin, 2009, 45 (10): 1242-1248. 
    Abstract   PDF (4410KB) ( 1995 )

    The Inconel 718 superalloy is extensively used to manufacture critical parts in aeronautical, astronautical, oil and chemical industries due to its excellent mechanical, physical and anti–corrosion behavior. Usually, these parts are shaped by hot forging or rolling in open–train mills.
    Recently, the tandem hot rolling has been applied to form superalloy bar products. In some cases, it can replace the traditional rolling, since it has higher productivity and product quality. In order to obtain the most favorable microstructure and the best mechanical properties of Inconel 718 alloy in
    tandem hot rolling, it is necessary to control its microstructural evolution in every step of the whole rolling process. With the aid of computer modeling, it is possible to make such a controlling process possible. As the first step in tandem hot rolling, reheating process of a forged slab prior to rough
    rolling plays a predominant role in predicting the grain size change or even the microstructural evolution. Thus, in this study, an Inconel 718 alloy forged slab was used as the experimental material and the effects of reheating temperature and holding time on its grain growth were investigated. A
    universal model was developed and verified for the grain growth of Inconel 718 alloy forged slab in reheating process prior to rough rolling. With the increase of holding time, the grain size shows no remarkable change up to 1173 K. The grain growth presents a linear trend in the range from 1173 to
    1323 K. A parabolic trend of gain growth can be observed when reheating temperature is higher than 1323 KThe established grain growth model of Inconel 718 alloy would be suitable to calculate the grn size evolution under the both isothermal and non–isothermal reheating conditions. This could also provide a basis in formulating the technological parameters for tandem hot rolling of Inconel 718 superalloy.

    References | Related Articles | Metrics
    AGING PROCESS OPTIMIZATION FOR A HIGH STRENGTH AND TOUGHNESS OF FV520B MARTENSITIC STEEL
    ZHOU Qianqing ZHAI Yuchun
    Acta Metall Sin, 2009, 45 (10): 1249-1254. 
    Abstract   PDF (2845KB) ( 2880 )

    Low carbon martensitic precipitation hardening stainless steels are widely utilized in many engineering applications due to their high strength with reasonable toughness, ductility and corrosion resistance. However, those properties and their combinations are not always satisfactory to their users. For further improvement of the mechanical properties of these types of steels, a fundamental understanding of the detailed microstructural features with various aging conditions is necessary. Therefore, the effects of aging temperature, aging time and cooling rate on the microstructure and mechanical poperties of a martesitic precipitation hardening stainless steel FV520B were investigated by OM, SEM, TEM and XRD methods. The results show that the steel aged at 630℃ for a short time and then furnace cooled, in which a typical lath martensitic with the proper amounts of everse austenite and fined dispersed precipitates was bservedhas a good combination of high strength and high toughness. It could be an optimized ang process for FV520B steel.

    References | Related Articles | Metrics
    DIRECT ELECTROCHEMICAL REDUCTION OF NiO–CeO2 POWDER FOR PREPARATION OF CeNi5 ALLOY BY SOLID–OXYGEN–ION
    CONDUCTING MEMBRANE PROCESS
    ZHAO Bingjian LU Xionggang LI Chonghe ZHONG Qingdong
    Acta Metall Sin, 2009, 45 (10): 1255-1260. 
    Abstract   PDF (1732KB) ( 1126 )

    Ce–Ni base alloy CeNi5 is often used as the hydrogen storage alloy in Ni–H batteries. Its application is more or less limited by the high cost in the traditional preparing process. Therefore, lots of researchers have paid more attention to develop a novel process with high production efficiency and low cost. The goal of the present research was to demonstrate the technical viability of a new process (solid–oxygen–ion conducting membrane process, i.e., SOM process) for the production of  CeNi5 alloy directly from its oxide precursors. This process was improved on the basis of FFC process (Fray–Farthing–Chen Cambridge process): (1) the preparation of cathode was he same as that in FFC process, (2) Cu (oSn) liquid saturated with carbon was used as anode separated from the melt ba yttria–stabilized zirconia tube in which only oxygen–ion was permeated to prevent the side reactions and decomposition of molten salts taking place until a voltage as high as 3.5 V. This paper was focused on the preparation of hydrogen storage alloy CeNi5 by SOM process, some parameters such as molten salt temperature, electrolytic time, configurations and phase compositions of products were investigated. The results show that NiO–CeO2 pellets can be completely reduced to CeNi5 alloy by  SOM process. The analysis of phase compositions of intermediate products indicates that the reduction of NiO–CeO2 starts from NiO, it reduces firstly into Ni, then reacts with newly–formed CeOCl and finally forms CeNi5. The comparison of FFC and SOM processes shows that for SOM, NiO–CeO2 pellet (2.5 g) can be completely reuced to CeNi5 after electrolyzed for 3 h, and the current efficiency is 75.5%, the electrolysis energy consumption is only as low as 4.03 kW·h/kg; while for FFC, it takes 12 h for the same pellet to be reduced to pure CeNi5, and the current efficiency is 26.1% but the electrolysis energy consumption is 10.27 kW·h/kg. It could be concluded that SOM process has a bight future for industrial application.

    References | Related Articles | Metrics
    FABRICATION OF Cu/Si COMPOSITES ON SOL–GEL PRETREATED Si POWDERS
    CAI Hui WANG Fei WANG Yaping SONG Xiaoping DING Bingjun
    Acta Metall Sin, 2009, 45 (10): 1261-1266. 
    Abstract   PDF (2621KB) ( 1131 )

    Cu/Si composite is an ideal material for electronic packaging owing to its excellent thermophysical and mechanical properties. Especially, its high thermal conductivity can fulfill the requirements of quick elimination of heat of high power devices. However, because of the severe diffusion and reaction between Cu and Si, the Cu–Si compound replaces the Cu and Si phases during the powder metallurgy fabrication at elevated temperature. Therefore, the crucial issue of Cu/Si composite fabrication is to control the Cu–Si diffusion and reaction. In this paper, the Cu/Si composites were fabricated using pretreated films on Si powder formed in Al2O3/TiO2 sol as a diffusion barrier to prevent Cu–Si reaction. The phases, microstructures and properties of Cu/Si composites were investigated. The results indicate that Cu/Si composites on which Si powders are pretreated by sol–gel are primarily composed of Cu, Si, and a few Cu3Si phases. The hardness of the composite is 147HV0.1, and the thermal diffusivity at room temperature is 26.4 mm2/s. Cu and i atoms diffuse via. the defects ifilm and react to form Cu–Si compound in local regions at Cu/Si interface during sintering. However, oly Cu3Si phase is detected in the composite on which the Si powder s not pretreated, and no Cu or Si trace is found. The hardnss is as high as 399HV0.1, but the thermal diffusiviy at room temperature is only 3.0 mm2/s. Thrfore, sol–gel pretreatment on Si powders can effecively reduce the Cu–Si reaction and protect the Cu and Si phases in composites so as to elevate the thermal conductivity.

    References | Related Articles | Metrics
    MICROSTRUCTURE AND MAGNETOSTRICTION OF Fe-Ga POWDERS PREPARED BY GAS ATOMIZATION
    GAO Xuexu LI Jiheng ZHU Jie BAO Xiaoqian JIA Juncheng ZHANG Maocai
    Acta Metall Sin, 2009, 45 (10): 1267-1271. 
    Abstract   PDF (1390KB) ( 1175 )

    The magnetostrictive composite material such as Terfenol–D, is composed of magnetostrictive particles dispersed within a polymer matrix, which is used to bind these particles to a relatively tough material, and the binder creates an insulating layer between the particles which increases the receptivity and reduces eddy current losses at high frequencies operation. Hong et al. reported that the maximum magnetostriction of 5.4×10−5 was obtained in a composite made by mixing the spherical Fe–Ga particles prepared by spark erosion in liquid Ar with epoxy of 48% volume fraction and curing in a magnetic field. Gaudet et al. investigated firstly the Fe–Ga powders prepared by mechanical alloying. Their results suggested that a disordered bcc A2 phase with no indication of any ordered DO3 phase was observed in these powders. Unfortunately, in their report, they also did not describe how to bond powders into a composite and how its magnetostrictive performance was. In present study, the spherical Fe–Ga particles were prepared by gas atomization and their microstructures were investigated by XRD, DTA, SEM and EDS. The results demonstrate that the Ga concentration of gas–atomized particles is near the nominal composition of Fe81Ga19 and most of particles are polycrystallne mainly composed of A2 phse and a small amount of ordered DO3 phase. It is found that L12 phase appeared in the Fe81Ga19 annealed powders is detimental to improvement of magnetostriction. However, many single crystals were obtained due to crystallization during annealing, which is beneficial to increasing the magnetostriction. The bonded magnetostrictive composite was prepareby magneticallaligning compression molding Fe81Ga19 powders and epoy. The maximum saturation magnetostriction of 6.4×10−5 is obtained in the composite containing annealed powdes.

    References | Related Articles | Metrics
    EFFECT OF Pd SUBSTITUTION ON THE STRUCTURES OF Ti-Zr-Ni QUASICRYSTALLINE ALLOYS
    HUANG Huogen JIA Jianping LI Rong
    Acta Metall Sin, 2009, 45 (10): 1272-1276. 
    Abstract   PDF (899KB) ( 1037 )

    Quasicrystals are a kind of special crystals, combining aperiodicity with long–range translational order and displaying a strong potential of applications. However, the forming law of quasicrystals, especially icosahedral, which have drawn a lot of attention since the discovery of the first quasicrystal, are not clearly known. A general knowledge, derived from the past work, is that the quasicrystal belongs to Hume–Rothery phase with special electron concentration per atom (e/a), and furthermore needs meeting the topological packing of atoms. Based on this, certain criteria named by e/a–constant line, e/a–variant line and cluster line, et al., have been used to design novel quasicrystals, especially in ternary alloy systems. But, as indicated in the previous work, these rules would not work very well in pseudo–ternary or quaternary systems built on Ti–Zr–Ni quasicrystals with the addition of Co or Cu. In order to further specify the quasicrystal formation in multi–component systems, in the present study an investigation was conducted in an alloy system, based on Ti45Zr38Ni17 and Ti40Zr40Ni20 alloys with the addtion of Pd, because Pd is the neighbourship of Ni in the element table like Co and Cu and has the atom size and e/a between Ti (or Zr) and Ni. Using XRD, TEM and OM, it is found that in the suction cast alloys, the icosahedral quasicrystal (IQC) phase is formed in the coexistence of a little bcc β–(Ti,Zr) in Ti45Zr38Ni17 alloy and of a bit hexagonal α–(Ti,Zr) in Ti40Zr40Ni20 alloy. And after 2%Pd (atomic fraction) substitution for Ti and Zr, a MgZn2–type Laves C14 hase with coordination nuber of 14 is precipitated along with the disappearance of β–(Ti,Zr) with coordination number of 8 in the former alloy, while only a complete IQC phase with coordination number of 12 is yielded in the latter alloy. However, as the subtitution is increased up to 6.7% or 13.3% in Ti40Zr40Ni20 alloy, the formation of IQC is prevented and the C14 phase occurs instead till a single C14 phase formin Ti40Zr26.7Ni20Pd13.3 alloy. The results show that the Pd addition in Ti–Zr–Ni allos would promte closer atom packing, being beneficial to the formation of Laves phase because of the weak interaction between Fermi surface and Brillouin zone in Hume-Rthery phase like IQC.

    References | Related Articles | Metrics
    AGING EFFECTS OF TRITIUM ON Pd POWDERS AND Pd91.31Y8.50Ru0.19 ALLOY FILM
    CHEN Miao LU Guangda ZHANG Guikai ZHANG Yanzhi WANG Xiaoying REN Dapeng
    Acta Metall Sin, 2009, 45 (10): 1277-1280. 
    Abstract   PDF (1208KB) ( 1502 )

    During tritium (T) treatment, Pd and its alloys will be aged due to solution of 3He, a product of T decay, in lattices, which changes the microstructure and properties of the materials. The existing and distributing states of 3He in Pd and Pd91.31Y8.50Ru0.19 alloy during T aging were studied by XRD and TEM. XRD results for Pd powders aged up to 1.6 and 3.5 a show that the peaks widen, intensities of peaks reduce and lattice constants incease by 0.095% and 0.11%, respectivey. TEM observtions show thaabout 1 nm sized He bubbls appear n Pd91.31Y8.50Ru0.19 alloy aged for 41 d, nd more dislocaions and dislocation loops arobserved; after 295 d aging, the He bubbles grow slightly and reach 1.2—1.4 nm in diameter, but the amounts of dislocations nd dslocation loops decrease.

    References | Related Articles | Metrics