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

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    DEVELOPMENT AND APPLICATION OF ATOM PROBE TOMOGRAPHY
    LIU Wenqing, LIU Qingdong, GU Jianfeng
    Acta Metall Sin, 2013, 49 (9): 1025-1031.  DOI: 10.3724/SP.J.1037.2013.00362
    Abstract   PDF (4410KB) ( 3013 )

    The atom probe tomography (APT) is a developing technique to characterize and analyze materials with atomic-scale spatial resolution and high analytical sensitivity. Recently, the substantial progress has been made in technical improvement of APT equipment and data analysis software. In this paper, the new development of APT is introduced, and its unique applications in characterization of traditional structural materials of high-strength low-alloy steel and Al alloy are discussed.

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    MODELING OF MICROPOROSITY FORMATION IN AN Al-7%Si ALLOY
    LI Zhengyang, ZHU Mingfang, DAI Ting
    Acta Metall Sin, 2013, 49 (9): 1032-1040.  DOI: 10.3724/SP.J.1037.2013.00128
    Abstract   PDF (2417KB) ( 825 )

    The performance of castings is primarily dependent on the solidification microstructures and defects. Gas porosity is one of the major casting defects existing in the castings of aluminium and magnesium alloys. In this work, a two-dimensional (2D) cellular automaton (CA) model is proposed to simulate dendrite and microporosity formation during solidification of alloys. The model involves three phases of liquid, gas and solid. The effect of liquid-solid phase transformation on the nucleation and growth of porosity, the redistribution and diffusion of solute and hydrogen, and the effects of surface tension and environmental pressure are taken into account. The growth of both dendrite and porosity is simulated using a CA approach. The diffusion of solute and hydrogen is calculated using the finite difference method (FDM). The simulations can reveal the coupling and competitive growth of dendrites and microporosities, as well as the microsegregation of solute and hydrogen. The model is applied to simulate the microporosity formation during solidification of an Al-7%Si (mass fraction) alloy. The effects of initial hydrogen concentration and cooling rate on microporosity formation are investigated. The results show that the simulated pressure difference between the inside and outside of a porosity as a function of the reciprocal of porosity radius obeys the Laplace law. With the increase of initial hydrogen concentration, porosity volume fraction increases, and the incubation time of microporosity nucleation and growth decreases, while the porosity density does not increase obviously. With cooling rate decreasing, porosity volume fraction and maximum porosity radius increase, as well as porosity nucleates and starts to grow at higher temperatures. However, the porosity density shows a decreasing trend with the decrease of cooling rate. The competitive growth between different microporosity and dendrites is observed. The porosity nuclei with larger size are able to grow preferentially, while the growth of the small porosity nuclei is inhibited. Because of the effect of gas-liquid surface tension, porosity grows spherically when it is enveloped by liquid. After touching with dendrites, the growth space of porosity is restricted by the complex dendrite network, and thus becomes irregular shape. On the other hand, the growth of dendrite might also be influenced by the nearby porosity. With cooling rate decreasing, the competitive growth between porosities and dendrites becomes more evident, leading to non-uniform porosity size, and more irregular morphology of the porosities with larger size. The simulation results are compared reasonably well with the experimental data.

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    ELEMENTS DIFFUSION LAW OF DD407/FGH95 DIFFU-SION BONDING UNDER HOT ISOSTATIC PRESSING: I. Building Diffusion Bonding Model
    YAO Yao, YE Jianshui, DONG Jianxin, YAO Zhihao, ZHANG Maicang,GUO Weimin
    Acta Metall Sin, 2013, 49 (9): 1041-1050.  DOI: 10.3724/SP.J.1037.2013.00105
    Abstract   PDF (1350KB) ( 706 )

    Turbine platform and blade are two main parts of aero engines and gas turbines.Due to different requirements in practice, platforms are always fabricated by single crystal superalloys, which have high temperature strength and resistance to hot corrosion and oxidation. The platforms employed at relatively lower temperatures can be made of powder superalloys. Therefore there is a great demand for bonding single crystal superalloys to powder superalloys. Because of high content of γ′ forming elements, traditional fusion welding methods employed in bonding the two materials are high susceptibility to cracking. Hot isostatic pressure (HIP) bonding is a preferable technique now to join nickel base superalloys. However, using experimental methods to explore appropriate HIP bonding parameters is time consuming and costly. This work puts forward a calculated method to simulate diffusion process and phase distribution of diffusion couples obtained by HIP diffusion bonding. In this work, the numerical model of HIP diffusion bonding was built, and distribution of elements and phases of DD407/FGH95 diffusion couples under different HIP temperature and bonding time were calculated  with DICTRA and Thermal—Calc software. The simulated results indicated that the appropriate HIP temperature should be chosen between 1120℃ and 1210℃. γ′ in DD407 and FGH95 kept initial concentration under 1120℃ HIP bonding.γ′ in FGH95 began to entirely solute and γ′ in DD407 partly solute under 1170℃ HIP bonding, and under 1210℃ HIP bonding, γ′ in DD407 could solute completely near the interface and partly solute away from the interface. The simulated results also implied that appropriate time for 1120℃ HIP bonding is 3—5 h, 1—3 h for 1170 and 1210℃ HIP bonding.

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    ELEMENTS DIFFUSION LAW OF DD407/FGH95 DIFFUSION BONDING UNDER HOT ISOSTATIC PRESSURE
    II. Model Verification and Experimental Analysis
    YAO Yao, DONG Jianxin, YAO Zhihao, ZHANG Maicang,GUO Weimin
    Acta Metall Sin, 2013, 49 (9): 1051-1060.  DOI: 10.3724/SP.J.1037.2013.00106
    Abstract   PDF (8217KB) ( 661 )

    Manufacturing dual superalloy turbine rotor is becoming an important direction of development in the field of aviation and aerospace. In order to improve working temperature and service life of turbine rotor, blade materials have been evolved to single crystal superalloys, turbine disc materials have also been developed to powder superalloys. Therefore the connection of these two materials has become key problem in improving turbine performance. Nowadays hot isostatic pressure (HIP) bonding is a preferable bonding technique due to no fusion and macroscopic deformation in bonding area. In HIP bonding process, elements diffusion is the key factor in forming microstructure of interface, and determining interfacial properties. At present, articles about HIP diffusion bonding mainly focused on interfacial microstructure and mechanical test of bonding couples, while few articles explored the relationship between elements diffusion and microstructure forming. In this work, diffusion couples of DD407 and FGH95 alloys were conducted under 1120, 1170, 1210℃ and 120 MPa HIP for 3 h, the microstructure and elements distribution of interface were studied to verify simulated results in the last article, and explore interface formation mechanism. Then the relationship between simulated phase distribution and actual microstructure of interface was built, which can be used to forecast interface morphology. And also the recrystallization in bonding zone was studied. Combining simulated and experimental results, this work put forwards principles for HIP bonding process controlling.

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    DEPENDENCE OF PRIMARY PHASE AND ITS GROWTH DIRECTION ON SOLIDIFICATION PROCESS IN DIRECTIONALLY SOLIDIFIED Ti-46Al-2Cr-2Nb ALLOY
    ZHANG Yuan, LI Xinzhong, LIU Guohuai, SU Yanqing, GUO Jingjie, FU Hengzhi
    Acta Metall Sin, 2013, 49 (9): 1061-1068.  DOI: 10.3724/SP.J.1037.2013.00279
    Abstract   PDF (4912KB) ( 1121 )

    GE alloy Ti-(46--48)Al-2Cr-2Nb (atomic fraction) is well known for its high strength and improved ductility. The primary phase and its growth direction are important in controlling the lamellar direction of GE alloys. However it is greatly affected by solidification conditions. In this work, primary phase and its growth direction have been investigated by carrying out Bridgman-type directional solidification with different growth lengths ranging from 5 to 30 mm on Ti-46Al-2Cr-2Nb alloy. It is found that the primary phase is β at the beginning of directional solidification with constant temperature gradient (G=18 K/mm) and growth rate (v=20 μm/s). With the increase of growth length, Al gradually concentrates in the liquid between primary dendrites, which leads to the peritectic reaction L+β→α. With further increase of the growth length, growth competition between primary β phase and peritectic α phase is promoted, leading to gradual transition of primary phases from β  phase to α phase. The growth direction of primary phase in different stages of solidification has been characterized by EBSD analysis. The results indicate that primary β phase has a growth direction parallel to its preferential growth direction <100>β at the initial stage of solidification. By comparing the growth directions of the α2 grains formed from primary β phase and peritectic α phase,it is found that peritectic α phase related to primary β phase by the {110}β //{0001}α orientation relationship. Therefore, as the primary phase has transformed to α phase, the growth direction deviates from its preferential growth direction <0001>α at an angle of 45.9°. The growth direction of α  phase formed after the primary phase transformation is determined not only by the kinetic factors of solidification, but also by the β phase exiting at the beginning of directional solidification. These results provide fundamental references for understanding and controlling the lamellar orientation of GE alloys.

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    CHANGE OF THERMOELECTRIC POWER OF LIQUID Al ALLOYS CAUSED BY DC MAGNETIC FIELD
    ZHANG Jianfeng, WANG Hongling, LE Qichi, BA Qixian, CUI Jianzhong
    Acta Metall Sin, 2013, 49 (9): 1069-1074.  DOI: 10.3724/SP.J.1037.2013.00231
    Abstract   PDF (800KB) ( 350 )

    The effect of DC magnetic field on the thermoelectric power of Al alloys at different temperatures was investigated in this work. The results showed that if the magnetic field intensity difference between the two ends of the sample was large, it would lead to a decrease in thermoelectric power of liquid Al-2.89% Fe Al alloys. When the magneticfield was removed, it took a period of time for the thermoelectric power to increase gradually to the initial value. This phenomenon had been observed in other Al alloys melt at different temperature. The higher the square difference of magnetic induction was, the more significant thermoelectric power changed. The approximate homogeneous magnetic field had no effect on the thermoelectric power. According to the theory of weak localization, the magnetic field can change the distribution of extended states electrons which contributes to the change of thermoelectric power.

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    OPTIMIZING CONTROL OF PRECIPITATES IN T91 FERRITIC HEAT-RESISTAN STEEL
    ZHANG Ruihui, ZHANG Chi, XIA Zhixin, YANG Zhigang
    Acta Metall Sin, 2013, 49 (9): 1075-1080.  DOI: 10.3724/SP.J.1037.2013.00178
    Abstract   PDF (2084KB) ( 717 )

    T91 steel is one representative of (9%-12%)Cr (mass fraction) ferritic heat-resistant steel, in which MX carbonitrides and M23C6 carbides are two major strengthened precipitates for long-term creep under high temperature. This work attempted to control the precipitation of MX carbonitrides and M23C6 carbides by applying new heat treatment procedures. With the assist of Thermal-Calc software calculation, two new heat treatment procedures have been designed for T91 steel based on its traditional normalized-tempered treatment, in which an isothermal treatment at 850℃ was introduced between normalized treatment and tempered treatment. The mean size of M23C6 carbides decreased from 350 nm to 250 nm and the number density of MX carbonitrides increased due to the new heat treatment procedures. The calculated results of Thermal-Calc software showed that the nucleation rate of M23C6 carbides at 750℃ increased with the decrease of carbon content in the matrix, which might be the major reason why the mean size of M23C6 carbides decreased.

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    PRECIPITATION BEHAVIOR OF M23C6 AND ITS EFFECTS ON DUCTILITY AND TOUGHNESS OF A NOVEL Cr-Mn-N AUSTENITIC HEAT RESISTANT STEEL
    ZHENG Leigang, HU Xiaoqiang, KANG Xiuhong, LI Dianzhong
    Acta Metall Sin, 2013, 49 (9): 1081-1088.  DOI: 10.3724/SP.J.1037.2013.00198
    Abstract   PDF (2297KB) ( 971 )

    Cr-Mn-N austenitic heat resistant steels have bright future in wide applications due to their lower cost, excellent mechanical properties, and outstanding local corrosion resistance after substituting the nickel partly or completely by the Mn, N and C elements. As the content of C or N elements increasing in Cr-Mn-N steels, a large quantity of precipitates form during hot working, resulting in embrittlement of these steels. In the present study, a new Cr-Mn-N austenitic heat resistant steel was aged at temperatures from 600℃ to 1000℃ in duration from 10 min up to 6000 min. By OM, SEM and XRD, the microstructure evolution of precipitates and their effects on ductility and toughness of the studied steel were investigated. The results indicated that precipitates formed during aging were mainly Cr-rich M23C6 carbides, whose morphologies changed in a sequence of intergranular films, lamellae in cellular microstructure and intragranular rods or particles with the increase of aging time and/or temperature. The time-temperature-precipitation (TTP) curves for M23C6 carbides were determined, which have a typical C-shaped profile with a nose temperature between 850℃ and 900℃, and an incubation period not more than 1 min. In addition, it was found out that the aging embrittlement of the studied steel is strongly dependent on the morphology of M23C6 carbides. The intergranular film-shaped M23C6 carbides were considered as the main factor to favor cracks rapidly expanding along grain boundaries, finally resulting in brittle intergranular fracture.

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    EFFECT OF CATHODIC POLARIZATION ON HYDROGEN EMBRITTLEMENT SUSCEPTIBILITY OF X80 PIPELINE STEEL IN SIMULATED DEEP SEA ENVIRONMENT
    LIU Yu, LI Yan, LI Qiang
    Acta Metall Sin, 2013, 49 (9): 1089-1097.  DOI: 10.3724/SP.J.1037.2013.00271
    Abstract   PDF (3498KB) ( 934 )

    Pipeline steels utilized in deep seawater are usually protected cathodically. However, inappropriate operations of cathodic protection systems cause hydrogen embrittlement failures to these high strength steels in seawater which result from the application of excessive negative potentials, leading to massive generation of hydrogen at the protected pipelines' surface. With high strength steels increasingly widely used in the deep sea environment, the basic research to the cathodic protection and susceptibility to hydrogen embrittlement of high strength steels under such a circumstance is still unfortunately relatively lack and urgently needed to supplement. Electrochemical measurement, hydrogen permeation current detection, slow strain rate tensile test (SSRT) and fracture morphology analysis, therefore, were employed to investigate effect of cathodic polarization level on the susceptibility of API X80 pipeline steels to hydrogen embrittlement in simulated deep seawater in the present work. The results showed that the applied cathodic polarization potentials significantly affected hydrogen permeation and hydrogen--induced cracking behavior of X80 steels immersed in deep seawater. A linear relationship was found between the hydrogen permeation current densities and cathodic polarization potentials applied according to the findings of the potential dynamic polarization and hydrogen permeation current measurements. SSRT tests suggested that X80 pipeline steels immersed in simulated deep seawater didn't show susceptibility to hydrogen embrittlement at open circuit potential, and thus the optimum cathodic protection potential range was supposed to be above -900 mV (vs saturated calomel electrode). Under such a cathodic polarization potential, the hydrogen permeation current densities of X80 pipeline steel specimens were less than 0.1157 μA/cm2 andtheir mechanical properties didn't decrease remarkably. Once the cathodic polarization potentials lower than -900 mV, however, hydrogen permeation current densities and calculated hydrogen embrittlement coefficients ψ of X80 pipeline steels increased significantly, exhibiting higher susceptibility to hydrogen embrittlement in simulated deep seawater. Furthermore, macro-and micro-morphologies of fracture surface of X80 pipeline steels after SSRT test indicated that the fracture morphology transformed from a dimpled pattern with ductile fracture to a quasi—cleavage pattern when cathodically polarized to lower than -900 mV, i.e. showing obvious brittle failure characteristics.

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    LIQUID PHASE SYNTHESIS OF FLOWER CLUSTER-LIKE ZnO VIA LIGNOSITE TEMPLATE AND ITS PHOTOCATALYTIC PROPERTY
    WANG Xiaohong, XIE Wenjing, HAO Chen, ZHANG Pengfei, FU Xiaoqi, SI Naichao
    Acta Metall Sin, 2013, 49 (9): 1098-1104.  DOI: 10.3724/SP.J.1037.2013.00273
    Abstract   PDF (1302KB) ( 456 )

    In recent years, increasing interest has been concentrated in the treatment of organic pollutants in wastewater or sewage by means of photocatalysis based on semiconductor nano-catalysts. Photocatalysis is a very potential method for the complete destruction of contaminants and wide selection of target polluted compounds. ZnO as a very efficient metal oxide semiconductor photocatalyst has been studied extensively since found. Owing to its abundant availability, cost effectiveness, non-toxicity, reusability, high photo-sensitivity and excellent chemical stability, wide band gap and large excitation binding energy at room temperature, ZnO displays a promising application. Organic pollution in water can be significantly reduced when ZnO is used as photocatalyst and secondary pollution could be avoided by the mineralization of inorganic ions, which is quite attractive in the field of environmental protection technology. In this work, the purpose was to use inexpensive lignosite from low-priced resources of lignin as surfactant, template and nucleating promoter to prepare nanocrystalline ZnO to cut down the synthesis cost. Flower cluster-like nanometer ZnO photocatalysts doped with different amounts of lignosite were prepared by liquid-phase precipitation. The prepared pure ZnO and lignosite-doped ZnO(ZnO-LS) samples were characterized by FT-IR, UV-Vis diffusion refraction spectroscopy (UV-DRS), XRD, SEM/EDS and BET. The photocatalytic activity of the samples was evaluated by the photodegradation of methyl orange under UV light irradiation. The results showed that lignosite doping significantly altered the morphology of ZnO, improved zinc oxide surface state, increased the specific surface area, made ZnO produce more hydroxyl on the surface, and was helpful to obtain petal-like ZnO. On the other hand, the calcinations temperatures influenced the crystallinity and crystal size of the photocatalysts. The tests indicated that the sample calcined at 300℃ had good crystallinity and a small crystal size. At the optimal calcinations temperature of 300℃ and when LS-doping quality is 2 g, the petal ZnO composite photocatalysts had smaller band gap width and much higher photocatalytic activity than the pure ZnO and those ZnO-LS (LS was other quality). The high photocatalytic performance of ZnO doped with the lignosite could be attributed to an increase in surface hydroxyl groups and high crystallinity.The obtained results show that the photocatalytic efficiency of the optimal ZnO-LS was superior to commercially available TiO2 Degussa P-25 for the photodegradation of methyl orange.

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    EFFECT OF HEAT TREATMENT PROCESS ON MICROSTRUCTURE AND HARDNESS OF V55Ti 30Ni 15 ALLOY FOR HYDROGEN PERMEATION
    JIANG Peng, YU Yandong
    Acta Metall Sin, 2013, 49 (9): 1105-1112.  DOI: 10.3724/SP.J.1037.2013.00184
    Abstract   PDF (4098KB) ( 537 )

    V55Ti30Ni15 alloy seems to be a promising alloy as a potential replacement for Pd-based alloy, which may reach a good combination of hydrogen permeability and mechanical stability in hydrogen. Metal rolling process is most widely used for sheet fabrication and may make multiphase V-Ti-Ni metallic membranes being thinner for increasing hydrogen transport flux. Different heat treatment processes have been carried out on a selected V55Ti30Ni15 alloy to soften for improved workability in this work. The effect of heat treated process on microstructure and hardness have been investigated for V55Ti30Ni15 alloy by using hardness measurement, OM, XRD, SEM, EDS and TEM. The microstructures resulting from different heat treatment temperatures and time have a great influence on hardness. Fine NiTi particles precipitate from the V-based supersaturate solid solution when the alloys were heat treated at 750 and 800℃, and the amount of NiTi particles increases with time. Fine NiTi particles precipitate from the V-based supersaturate solid solution results in the decrease in hardness. With increasing temperatures, fine NiTi particles re-dissolve into V-based solid solution, and the alloys hardness increase instead. Solid solution hardening of Ni and Ti elements is greater than precipitation strengthening of fine NiTi particles formed by atomic binding of Ni and Ti in V55Ti30Ni15 alloy. When the alloys were heat treated at 900℃, casting stress and lattice distortion decrease at heat treatment earlier stage, and then NiTi2 phases start to aggregate and spheroidize on phase boundaries with extended time. This causes the phenomenon that the hardness first decrease and then increase.

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    PREPARATION AND CHARACTERIZATION OF ANTICORROSION SELF-HEALING AND HYDROPHOBIC COATING ON ALUMINUM ALLOY
    PAN Xiaoming, WU Junsheng, XIAO Kui, GAO Shujun, PEI Lihong, TIAN Ran, LI Xiaogang
    Acta Metall Sin, 2013, 49 (9): 1113-1120.  DOI: 10.3724/SP.J.1037.2013.00268
    Abstract   PDF (2397KB) ( 682 )

    Aluminum alloy has been widely used as the basic material in many industries because of its low density and high strength to weight ratio. Poor resistance to corrosion, however, limits the variety of practical applications of aluminum alloy. Traditionally, this problem was faced with the use of chromate conversion coatings which result in effective corrosion protection because of the self-healing property. However, the presence of toxic hexavalent chromium compounds makes those coatings very hazardous to the environment. Consequently, it is necessary to develop new environmentally compliant, chromate (VI)-free alternatives. Recently, sol-gel technology provides a new promising approach to prepare protective coatings on aluminum alloy. It has many advantages, such as the simple operation and environmental protection, etc. In this work, an Ormosil coating was developed on aluminum alloy through the sol-gel method using triethoxyoctylsilane (TEOCS) and tetraethylorthosilicate (TEOS) as the precursors. The sol-gel coatings were deposited by dip-coating method on aluminum alloy substrate. The sol-gel coatings doped with different concentrations of cerium salt (Ce(NO3)3•6H2O) were investigated. Surface morphology, wettability, anti-corrosion and self-healing of the sol-gel coatings non-doped and doped cerium salt were characterized by using atomic force microscopy, contact angle measurements, electrochemical impedance spectroscopy and scanning electrochemical microscopy. The hydrophobic of the coatings was evaluated by means of contact angle measurements. The corrosion resistance of the coatings was investigated by means of electrochemical impedance spectroscopy measurements, and the anti-corrosion, self-healing properties were discussed based on equivalent circuit fitting. The corrosion behavior of the damaged sol-gel coatings was studied by scanning electrochemical microscopy. The results indicated that it had a marked effect on the surface morphology, corrosion resistance and hydrophobicity when cerium salt was added to the Ormosil sol-gel coating. The contact angle of the Ormosil sol-gel coating is about 92.0°. Cerium salt doped coatings have a better hydrophobicity due to marked improvement of the surface morphology. This positive effect was more evident when the concentration of the doped cerium salt is 0.005 mol/L in the silane solution. It was found that cerium salt dopedcoatings were less resistant to corrosion than non-doped coating at initial immersion. However, the coating doped with 0.005 mol/L cerium salt rendered improved protection after longer time immersion because of the inhibitive action of Ce3+. It can be released at the defects, hindering the further corrosion reactions at defective sites and showing the self-healing ability of the doped with cerium salt Ormosil sol-gel coating.

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    CORROSION BEHAVIOR AND EIS STUDY OF NANOCRYSTALLINE Ni-CeO2 COATINGS IN AN ACID NaCl SOLUTION
    ZHOU Xiaowei,SHEN Yifu
    Acta Metall Sin, 2013, 49 (9): 1121-1130.  DOI: 10.3724/SP.J.1037.2013.00101
    Abstract   PDF (2717KB) ( 556 )

    Pure Ni and Ni-CeO2 nanocrystalline coatings were prepared from a Watts-nickel electrolyte using ultrasonic-assisted pulse electro deposition. Effect of incorporating CeO2 addition on their corrosion behavior in a 3.5%NaCl+1.5%HCl (mass fraction) solution was evaluated by electrochemical impedance spectroscopy (EIS) and equivalent electric circuits (EECs). Meanwhile, static immersion tests and the analysis of corrosion products were carried out. Experimental results indicated that the existence of CeO2 phase in the Ni coatings promoted an effective dispersion strengthening after aged at 600℃ for 4 h in air, and also produced a continuous Ce-rich oxide sale acted as the passive layer covered on the coating surface to reduce pinholes or micro cracks that generated from hydrogen evolution reactions during electrochemical deposition, hence resulting in the decrease of localized corrosion or intergranular attack by Cl-. Based on the observed results from potentiodynamic polarization curves, they exhibited an obvious passivation transition attached with a reducing corrosion current density by lower than 1.5 orders of magnitude of Ni-CeO2 coatings (600℃) as relative to pure Ni. EIS measurements for pure Ni displayed a high frequency capacitive arc together with a middle-low frequency diffusion arc. While for the Ni-CeO2 coatings, they displayed module of capacitive arc with a wide frequency area of phase angle close to 90° so as to confirm superior anti-corrosion property. During long-term static immersion tests in an acid NaCl solution, a small amount of Ce3+ released from CeO2 phase were served as the corrosion inhibitors by means of making strong adsorption onto the exposed area for increasingcorrosion resistance. With the presence of CeO2 addition, slightly uniform corrosion was observed on the surface of Ni-CeO2 coatings as compared to severely pitting corrosion of pure Ni. According to the determination of corrosion products by XRD and XPS, both of their corrosion products were mainly composed of NiCl2 and Ni(OH)2, while some insoluble products of CeCl3 and CeO2 phase generated on the surface of Ni-CeO2 coatings, thereby achieving such insoluble corrosion products covered on pitting holes of localized corrosion to preclude the exposed area from corrosive attack and diffusion behavior by Cl-.

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    EFFECTS OF Cu CONTENT ON THE WEAR PROPERTIES OF Al2O3•SiO2sf/Al-Cu COMPOSITES
    XU Hongyu, HUANG Lujun, GENG Lin, ZHANG Jie, HUANG Yudong
    Acta Metall Sin, 2013, 49 (9): 1131-1136.  DOI: 10.3724/SP.J.1037.2013.00001
    Abstract   PDF (1689KB) ( 569 )

    Al2O3•SiO2sf/Al-Cu composites with different Cu contents were successfully fabricated by squeeze casting. The wear test results show that friction coefficient of the composites decreases with increasing Cu contents. Moreover, the wear resisting property of the prepared composites firstly increases and then decreases with increasing Cu contents. In wear testing process, Al2O3•SiO2 fibers are firmly fixed in the matrix and formed the framework which can protect Al matrix from grinding. Therefore, the wear resistance of the composites is increased by addition of Al2O3•SiO2 fiber reinforcement. The main wear mechanism of Al2O3•SiO2sf/pure Al composite is adherence abrasion, while that of Al2O3•SiO2sf/Al-Cu composite is adhesive wear accompanied by abrasive wear.

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    MICROSTRUCTURE EVOLUTION AND MECHANICAL BEHAVIOR OF INTERMETALLIC COMPOUNDS IN SOLDER JOINT
    AN Tong, QIN Fei, WANG Xiaoliang
    Acta Metall Sin, 2013, 49 (9): 1137-1142.  DOI: 10.3724/SP.J.1037.2013.00186
    Abstract   PDF (2615KB) ( 550 )

    Thickness and roughness evolution of the intermetallic compounds (IMC) layer between Sn3.0Ag0.5Cu and Cu substrate are examined under various isothermal aging times at 150℃.The effect of the microstructure evolution of the IMC on the mechanical behavior of solder joints is experimentally investigated. The results show that the growth of the IMC follows the Fick's law that predicts the total mean thickness increases linearly with the square root of the aging time. With the increase of the isothermal aging time, the initial scallop morphology of the solder/IMC interface changes to a more planar type. Both the thickness and roughness of the IMC layer affect the tensile strength and fracture mode of the solder joints. The tensile strength decreases with the increase of either the IMC thickness or the solder/IMC interfacial roughness. With the increase of the isothermal aging time, the IMC thickness increases and the solder/IMC interfacial roughness decreases, and the fracture mode migrates from ductile fracture in bulk solder to brittle fracture in the IMC layer.

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    INFLUENCES OF PHASE PRECIPITATIONS OF TERNARY βTi-Mo-Zr(Sn) ALLOYS ON YOUNG'S MODULUS AND MECHANICAL PROPERTIES
    LI Qun, WANG Qing, LI Xiaona, GAO Xiaoxia, DONG Chuang
    Acta Metall Sin, 2013, 49 (9): 1143-1147.  DOI: 10.3724/SP.J.1037.2013.00179
    Abstract   PDF (2606KB) ( 912 )

    The influences of second phase precipitations of low Young's modulus (E) βTi alloys [MoTi14]Zr1 (Ti78.2Mo11.2Zr10.6) and [SnTi14]Mo1(Ti75.7Mo10.9Sn13.4) on mechanical properties as well as the variation of phase constitutions of alloys during tensile test were investigated. Alloy rods with 6 mm diameter were prepared by copper-mould suction-cast method. Phase precipitations and microstructures of alloys before and after tension were analyzed by XRD and TEM techniques. The experimental results showed that only a minor amount of   α〞martensite precipitated on β matrix makes the suction-cast [SnTi14]Mo1 alloy possess lower E with a value of 70 GPa, while ω precipitation increases the E (80 GPa) of [MoTi14]Zr1 alloy. It is due to the thinner β plate twins that makes [SnTi14]Mo1 alloy have better mechanical properties. During tensile testing, the amount of  α〞martensite induced by stress increases in [MoTi14]Zr1 alloy while β grains in [SnTi14]Mo1 alloy are easier to be rotated for deformation, which favors to enhance the ductility of alloys.

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    EFFECT OF HIGH MAGNETIC FIELD ON CRYSTAL ORIENTATION, MORPHOLOGY AND MAGNETOSTRICTION OF TbFe2 AND Tb0.27Dy0.73Fe1.95 ALLOYS DURING HEAT TREATMENT PROCESS
    LIU Yin, LIU Tie, WANG Qiang, WANG Huimin, WANG Li, HE Jicheng
    Acta Metall Sin, 2013, 49 (9): 1148-1152.  DOI: 10.3724/SP.J.1037.2013.00289
    Abstract   PDF (1007KB) ( 892 )

    The rare-earth giant magnetostrictive materials are one kind of the most important functional materials now. The rare-earth giant magnetostrictive materials of TbFe2 and Tb0.27Dy0.73Fe1.95(R-Fe) alloys were firstly used in sonar system in military. Now, the applications of these two alloys have been developed widely in both industry and civil fields, such as magnetomechanical transducers, actuators, adaptive vibration control systems, and so on. The TbFe2 and Tb0.27Dy0.73Fe1.95 alloys both have the C15-type cubic Laves phase structure, which has strong magnetic anisotropy. This causes that the alloys exhibit different magnetostrictive properties along different crystal orientations. So, the preparation of high orientation degree magnetostrictive materials has a great of importance. In this work, it is predicted that the magnetostricive property of these two alloys can be enhanced with high magnetic field during heat treatment around their Curie and eutectic points. Therefore, a high magnetic field up to 11.5 T was imposed on these two kinds of R-Fe alloys during heat treatment at different temperatures in the experiments. Then the effects of high magnetic field on the crystal orientation, morphology and magnetostriction were analyzed in detail by XRD, metallographic microscope, and static resistance strain gauge. Also, the relationship between structure and property is discussed. The results showed that high magnetic field didn't change the orientation of the R-Fe alloys, but the orientation degree had a great improvement after heat treatment. The phase composition of the two alloys had no change. However, the heat treatment with a magnetic field up to 11.5 T distinctly reduced the rare-earth phase of TbFe2 and obviously increased the magnetic phase. In addition, the magnetostrictive property of TbFe2 alloy increased after the heat treatment with high magnetic field. But for Tb0.27Dy0.73Fe1.95 alloy, the magnetostriction was not obviously enhanced. The reason for the improvement of the magnetostriction is that the Zeeman energy caused by high magnetic field increased the total Gibbs free energy of the system, which made the random orientation in crystals orient along the <113> direction, and enhanced the magnetostriction of the R-Fe alloys.

     
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