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

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    NUMERICAL INVESTIGATION ON ENERGY BALANCE AND DEPOSITION BEHAVIOR DURING COLD SPRAYING
    WU Xiangkun ZHOU Xianglin WANG Jianguo ZHANG Jishan
    Acta Metall Sin, 2010, 46 (4): 385-389.  DOI: 10.3724/SP.J.1037.2009.00620
    Abstract   PDF (619KB) ( 1093 )

    In this paper, the physical process during the impact of a cold spraying spherical particle with an infinite homogencous substrate was simulated using the finite element method (FEM). The effects of the impact vlocity, the particle and substrate materials on thimpact bebvior were invesigated. The results show that a significant amount of the initial total kinetic energy of the particle is dissipated due to the plastc deformation of particle and substrate, finally transforming to ther nternal energies. An energy partition coefficient K, defined as the ratio of the enegies distributed in the particle and substrate, has been introduced, which is closely related to the iniiatotakinetic enery of the particle and the deformability of the particle and substratduring the impactIt is fond that it is difficult to make a particle with hight K and a substrate with low K cohere strongly.

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    SIMULATION OF LEDGEWISE GROWTH KINETICS OF PROEUTECTIOD FERRITE UNDER INTERFACIAL REACTION–DIFFUSION MIXED CONTROL MODEL
    LIU Zhiyuan YANG Zhigang LI Zhaodong LIU Zhenqing ZHANG Chi
    Acta Metall Sin, 2010, 46 (4): 390-395.  DOI: 10.3724/SP.J.1037.2009.00455
    Abstract   PDF (1177KB) ( 1313 )

    A mixed control model considering both carbon diffusion and interfacial reaction is established for ledgewise growth kinetics of proeutectiod ferrite during  γ →α isothermal transformation in low carbon steels. In this model, the growth rate of ledgewise ferrite and the carbon concentration at the ledge riser are determined by both carbon diffusion at ledge riser in austenite and interfacial reaction rate of  γ →α transformation, which is different from the traditional local equilibrium(LE) model. Simulation is done by utlizing this model to analyze the ledgewise growth kinetics of proeutectiod ferrite. And based on analysis of the characteristics of growth kinetics of proeutectiod ferrite obtained by the simulation, a factor S, which includes the effects of carbon diffusion, interface migration, temperature and composition, is proposed to characterize the control mode of growth kinetics. By judging this factor S, we can predict that if the carbon diffusion or interfacial reaction dominates the gowth kinetics of proeutectiod ferrite. In the case that the value S is relatively small, the growth kintics is mainly controlled by interfacial reaction. In the case that the value of S is quite large, the growth kinetics is controlled by carbon diffusion, which returns to the traditional local equilibrium model. The mixed control simulation results for ledgewise growth of proeutectoid ferrite at 720 ℃ in Fe–1%C (atomic fraction) alloy show a good agreement with the experimental results previously reported.

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    THREE DIMENSIONAL FEMSIMULATION OF TITANIUM HOLLOW MONOLITHIC STRUCTURE PROCESS BASED ON VISCO–PLASTIC CONSTITUTIVE
    ZHAO Bing LI Zhiqiang HAN Xiuquan LIAO Jinhua HOU Hongliang BAI Bingzhe
    Acta Metall Sin, 2010, 46 (4): 396-403.  DOI: 10.3724/SP.J.1037.2010.00035
    Abstract   PDF (3314KB) ( 1081 )

    A three dimensional finite element model (FEM) was established to simulate the forming process of titanium alloy hollow monolithic structure. The influence of process parameters on this process was analyzed by finite element method code MSC.Marc in which a rigid–viscoplatic constitutive equation was implemented. The results demonstrated that with the increase of twisting velocity, the torque on the hollow structure changes indistinctively. In case of increasing die velocity and target strain rate, the forming force of hot forming and gas pressure of superplastic forming will increase. On the contrary, along with the increasing of temperature, the forming force decreased. In case of temperature above 900 ℃, the influence of temperature on the forming force and gas pressure was weaken. Based n FEM, at the conditions oprocess paramtrs, like torsions speed of 4.376×10−3 rad/s, hot forming die velocity of 2.12×10−1 mm/starget strain rate o1.0×10−3 s−1, as well as at the temperature of 925 ℃, a simulacrum f hollow mnolithic structure ftitanium alloy was prepared, the thickness distribution of the ace sheet of the obtained part was in good agreement with the simulated results.

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    GRAIN BOUNDARY PLANE DISTRIBUTIONS IN THE COLD–ROLLED AND ANNEALED FERRITIC STAINLESS STEEL
    FANG Xiaoying WANG Weiguo Rohrer G S ZHOU Bangxin
    Acta Metall Sin, 2010, 46 (4): 404-410.  DOI: 10.3724/SP.J.1037.2009.00650
    Abstract   PDF (4936KB) ( 1756 )

    The grain boundary plane distribution in the cold–rolled and annealed ferritic stainless steel has been analyzed statistically by a five–parameter method based on the mis–orientations of adjacent grains and the orientations of grain boundary traces determined by electron backscatter diffraction (EBSD) attached to a field emission scanning electron microscope (FE–SEM). The results show that no preferred texture occurrs either in grain orientations or grain boundary mis–orientations characterized by axis–angle pairs in the specimens which were cold rolled by 85% reduction in thickness and subsequently annealed at 780 ℃. However, there are developed textures for the orientations of grain boundary planes. And also, the preferred planes vary with averaged grain size levels. At the level of 9 μm, grain boundary planes favor the {100} orientation, with their density distribution 12% higher than a random distribution; Whereas at the level of 15 μm, {111} planes are preferred and {100} and {112} planes are sub–preferred, with the density distribution on {111} about 10% higher than a random distribution. Moreover, the anisotropy of grain boundary plane distributions is larger at specific mis–orientations. The discussion points out the preferred grain boundary planes might possess lwer energes and tend to being preserved due to their lower growth rates during annealing.

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    PRECISE DETERMINATION OF THE IRRATIONAL PREFERRED INTERFACE ORIENTATION BY TEM
    MENG Yang GU Lin ZHANG Wenzheng
    Acta Metall Sin, 2010, 46 (4): 411-417.  DOI: 10.3724/SP.J.1037.2009.00577
    Abstract   PDF (3664KB) ( 1447 )

    This paper presents a method for precise determination of the orientation of the irrational preferred interface by using TEM. The interface trace and the edge–on orientations are measured careflly and separately to minimize systematic error of thrsults. Tis method was developed according to the error analysis and the fact that te accuracy of the measurement is higher when the measurement is made directly on an interface trace tan on its project on an edge–on orientation, and whn the angle between the trace and the beam direction along the an edge–on orientation is larger. Compard with xisting methods, the present method has minimized the measurement errors and its resuts show better convergence.

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    A MODIFIED MODEL USED TO DESCRIBE AUSTENITE/MARTENSITE INTERFACE
    CHENG Ning GUO Zhenghong MENG Qingping
    Acta Metall Sin, 2010, 46 (4): 418-422.  DOI: 10.3724/SP.J.1037.2009.00800
    Abstract   PDF (451KB) ( 1011 )

    A modified Landau polynomial was proposed to describe the structure features of austenite/martensite interface under the existence of an intrfacial preferrd state. When the elastic modulus of austenite, the energy of preferred state and thdriving force for martensitic transformation were selected as the variables, a transitional platform with a slow change in order parameter, i.e. scaled shear strain whose structure is neither austenite nor martensite, was found using Ginzburg–Landau theory. The energ and width of austenite/martensite interface were calculated. Based on these calculated results, some experimental phenomena during martensitic transformatin can be explaned perfectly. It is demonstrated that the current model is more universal.

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    EFFECT OF HIGH MAGNETIC FIELD ON THE TRANSITION BEHAVIOR OF Cu–RICH PARTICLES IN Cu–80%Pb HYPERMONOTECTIC ALLOY
    ZHANG Lin WANG Engang ZUO Xiaowei HE Jicheng
    Acta Metall Sin, 2010, 46 (4): 423-428.  DOI: 10.3724/SP.J.1037.2009.00629
    Abstract   PDF (2323KB) ( 1476 )

    The Cu–80%Pb(mass fraction) hypermonotectic alloy with a dense distribution of Cu–rich particles was annealed at different temperatures under high magnetic field, the transition behavior of Cu–rich particles in solid–liquid mixture zone was investigated. The results show that, the transition behavior of Cu–rich particles occurs above 800 ℃, a compact segregation layer forms in the sample top above 900 ℃, the particle size and transition velocity increase with annealing temperature increasing. The Cu–rich particles grow and coalesce with each other during transiting, the coalescence mechanism is different from Cu–rich droplets. The clster of Cu–rich particles float up wholly, leave a clear boundary between Cu–rich particle and Cu–rich dendrite zones, the mutual interaction between the prticles makes the trnsition velocity decreased. The high magnetic field has effect on decreasing the transition velocity of Cu–rich particles, inhibiting the coarsening and coalescing of Cu–rich particles, which could decrease the segregation of Cu–rich particles, and inhibit the formation of compact Cu–rich segregation layers. Based on the mutual interaction between the particles, the acting forces on the Cu–rich particles and the final velocity have been analyzed and calculated to show the influence of magnetic field.

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    STUDY ON THE MULTI–SEGMENT FEATURE OF 625 ℃ CREEP–RUPTURE PROPERTY AND THE QUATITATIVE CHANGE OF PHASE PARAMETERS OF M23C6 AND LAVES PHASES IN EACH SEGMENT OF P92 STEEL
    PENG Zhifang CAI Lisheng PENG Fangfang HU Yongping CHEN Fangyu
    Acta Metall Sin, 2010, 46 (4): 429-434.  DOI: 10.3724/SP.J.1037.2009.00653
    Abstract   PDF (1623KB) ( 1703 )

    In the present study, the multi–segment feature of stress vs rupture–time plot and the corresponding quatitative change of phase parameters of M23C6 and Laves in each segment were investigated for P92 steel samples subjected to 625 ℃ creep–rupture tests. The results indicate that the stress vs rupture–time plot obtained from the test results can be divided into two segments (higher stress level and shorter rupture time segment: 180—150 MPa/30—454 h, and lower stress level and longer rupture time segment: 140—110 MPa/2881—10122 h) respectively, and the microstructural evolution is closely related to the feature of each segment. In the higher stress level and shorter rupture time segment the M23C6  particles are coarsened, while in the lower stress level and longer rupture time segment both M23C6  and Laves phases are coarsened, but the latter is predominantly. The average composition, the amount and the elemental partitioning of each alloy phase called phase parameters can be determined by a multiphase separation method developed.

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    STUDY ON CREEP–RUPTURE PROPERTY ASSESSMENT METHOD FOR 9%—12%Cr FERRITIC HEAT–RESISTANT STEELS
    PENG Zhifang DANG Yingying PENG Fangfang
    Acta Metall Sin, 2010, 46 (4): 435-443.  DOI: 10.3724/SP.J.1037.2009.00652
    Abstract   PDF (626KB) ( 1487 )

    Long–term creep–rupture properties are usually evaluated from short–term data by time–temperature parameter (TTP) method, such as arson–miller parameter (LMP) and Orr–Sherby–Dorn (OSD) methods. However, the conventional TTP methods sometimes overestimate long–term creep rupture properties if the prediction is based on their short–term test data for 9%—12%Cr ferritic steels. The following concepts/methods are thus proposed in this paper in order to reduce the property overestimation tendency caused by the conventional TTP methods and to obtain a better agreement of the predicted property values with the observed ones. They include the C–value optimization and the multi–C region analysis, the long–term (5×103—1×105 h) creep rupture property prediction using short term test data (≤5×103 h), the optimization of function used for property prediction, and the effect of d[g(σ)]/d(P) vs P on the stability of steel properties based on the improved LMP method. All the data sets for the 9%—12%Cr steels are from NIMS database for the related calculations and analyses. The results show that the C value in LMP is not only different from steel to steel type but also varies with the multi–region stress levels, and the new approach to rupture life prediction proposes procedures for extrapolations of the short–term results, with rupture time measurements from tests lasting up to only 5×103 h providing reasonable estimates of 105 h rupture strengths, as well as the variation tendency of d[f(σ)]/d(P) vs P can reflect directly the long–term property stability of the steels investigated. Therefore, the concepts/methods proposed could improve effectively the accordance of predicted property values with observed ones and overcome obviously the overestimation tendency of 105 h strengths, which are more suitable and easily realized to assess the long–term creep–rupture properties of the advanced high Cr ferritic steels.

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    FRACTURE CHARACTER OF LOW CYCLE FATIGUE OF P/M SUPERALLOY FGH97
    ZHANG Ying ZHANG Yiwen ZHANG Na LIU Mingdong LIU Jiantao
    Acta Metall Sin, 2010, 46 (4): 444-450.  DOI: 10.3724/SP.J.1037.2009.00626
    Abstract   PDF (4463KB) ( 1515 )

    Powder metallurgy (P/M) nickel–based superalloy FGH97 has been researched in recent 10 years in China which is particularly suitable for producing aircraft engine disks and shafts etc.. In the range of operating temperature, the resistance to low cycle fatigue (LCF) of P/M nickel– based superalloy is one of the most important performances. In this paper the fracture features of LCF on P/M nickel–based superalloy FGH97 were investigated. The study focused on macroscopic and microscopic features of LCF fractures under 650 ℃, 980—30 MPa, 1 Hz test conditions by optical microscope and SEM. The types and morphologies of failure origins were observed. The effects of failure origin location and type, shape and size of defects located in fracture on LCF life were discussed. The results show that LCF life of all the specimens of FGH97 is over 5000 cyc under above test conditions. Statistically, 23% of LCF failure origins are on sample surface, 47% near surface and 30% in interior of samples. There are statistically three types of the LCF failure origins in this alloy: 5% of grain facet, 15% of powder particle and 80% of inclusion. Based on the statistical analysis and theoretical calculation of the influences of three types of LCF failure origins on LCF life, it can be concluded that the LCF life can be more severely affected by the inclusion than by the abnormal powder particle and partial plastic deformation.

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    MORPHOLOGY AND CRYSTALLOGRAPHIC CHARACTERISTICS OF DEFORMATION BANDS IN Mg ALLOY UNDER HOT DEFORMATION
    YANG Xuyue JIANG Yupei
    Acta Metall Sin, 2010, 46 (4): 451-457.  DOI: 10.3724/SP.J.1037.2009.00581
    Abstract   PDF (3586KB) ( 2007 )

    When metals and alloys are subjected to considerable plastic deformation such as cold-or warm–rolling, characteristic inhomogeneities generally appear as a form of deformation or shear bands. Recently much attention has been given to the deformation mechanism and morphology of fcc and bcc materials. It is well known that such inhomogeneities play an essential role in the process of work–hardening, recrystallization, metal fatigue and fracture. Therefore, in order to obtain the guiding principle for controlling those processes in Mg based alloys, it is indispensable to make clear the details of those inhomogeneous deformation structures in hcp materials. In this work, the inhomogeneous deformation and microscopic features of AZ31 Mg based alloy were studied under compression at temperature ranging from 250 to 400℃ and at a strain rate of 3×10−3 s−1. The analysis of experimental data shows that such inhomogeneities depend on deformation temperature and strain sensitively. At 250 ℃, the {1012} c–axis extension twins and deformation bands appear at around 45° with the compression axis in grain interior when straining to ε=0.1, the {1012} twins continue to grow until they impinge each other and finally most of the original grains are replaced by twinned grains at a strain of about ε=0.2. The boundaries between {1012} twins and their neighbors disappear during twinning. At 300 ℃, in contrast, the non–basal slips are activated, the kink bands with low misorientation angles are frequently evolved in grain interior and they are roughly perpendicular to the (0001) basal plane, further deformation leads to an increase in the number and misorientation angle of the kink bands. The initial grains are fragmented by kinking. With temperature increasing, the spacing of kink bands increase rapidly. The difference between kinking and other deformation bands was discussed in some detail.

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    ANALYSIS OF THE THERMAL STABILITY OF COPPER SPECIMENS DEFORMED BY HIGH–PRESSURE TORSION
    XIE Ziling WU Xiaolei XIE Jijia HONG Youshi
    Acta Metall Sin, 2010, 46 (4): 458-465.  DOI: 10.3724/SP.J.1037.2009.00673
    Abstract   PDF (3519KB) ( 1460 )

    The thermal stability of Cu specimens subjected to high–pressure torsion (HPT) deformation with varying strains was studied by optical icroscope (OM), differential scanning calorimetry (DSC) and transmission electron microscope (TEM). It is found that cellular subgrains with high dislocation densities are firstly formed at a low strain level, some of the cellular subgrains are transformed into dislocation–free equiaxed grains at larger strains. A single exothermal peak between 150 and 250 ℃ is shown in DSC curves, corresponding to the heat release due to recrystallization and subsequent grain growth. With the increase of strain, the peak position is shifted to a lower temperature and then is leveled off, but the stored energy of cold work, calculated according to the area under a peak, increases with strain at relatively low strain level and reaches its maximum value of 0.91 J/mol at strain of 13. Further deformation induces the stored energy of cold work to decrease due to the dynamic recovery of microstructure. A large drop in hardness appears in as–deformed samples at a temperature 45 ℃ lower than the start temperature of the exothermal peak after isochronal annealing, indicating that the recrystallization and grain growth process is closely relatine to annealing time and temperatur.

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    CRYSTAL PLASTICITY NUMERICAL ANALYSIS ON YIELD AND SUBSEQUENT YIELD OF POLYCRYS-TALLINE COPPER UNDER COMBINED TENSION–TORSION LOADING
    HU Guijuan ZHANG Keshi SHI Yanke SU Li
    Acta Metall Sin, 2010, 46 (4): 466-472.  DOI: 10.3724/SP.J.1037.2009.00752
    Abstract   PDF (865KB) ( 952 )

    Combined tension–torsion test under pre–tension and pre–torsion deformation, the yield characteristics of polycrystalline copper in grain scale was investigated by crystal plasticity theory associated with polycrystalline aggregate model. Through a sub–model method, the cross–scale analyses of mechanical behavior of polycrystalline copper by the calculations using a representative volume element (RVE) and a specimen under tension and torsion were carried out. Based on the research on the shape and the evolution of a subsequent yield surface, the effects of different loading paths and yield definitions on the subsequent yield surface were explored. The heterogeneous statistical nlysis of the polycrystalline copper under different loading paths was also performed. And further more, the effects of loading history on the subsequent yield surface, and on the micro heterogeneous distribution were estimated. The numerical results show that the shape of the subsequent yield surface and the appearance of yield surface corner are related to the pre–loading direction and the different yield definitions, the heterogeneous deformation in polycrystal under different loading paths is very various. The results by the analyibased on crystal plasticity calculation combined with the sub–model method are compared with experimental results and they are in reasonable agreement.

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    PHASE TRANSFORMATION AND STRENGTH–TOUGHNESS OF A FGBA/BG DIPHASE STEEL CONTAINING 0.02%Nb
    FENG Chun FANG Hongsheng BAI Bingzhe ZHENG Yankang
    Acta Metall Sin, 2010, 46 (4): 473-478.  DOI: 10.3724/SP.J.1037.2009.00646
    Abstract   PDF (3019KB) ( 1668 )

    In order to produce Mn–series low carbon steels using alloying elements with low cost and following the conventional production line without controlled cooling facilities and not needing special heat–treatment, Mn–series low carbon air cooling grain boundary allotriomorphic ferrite (FGBA)/granular bainite (BG) steels, alloying with Si–Mn–Cr, have been developed. The effect of 0.02%Nb on the phase transformation and strength–toughness of a FGBA/BG steel have been investigated by using Gleeble–1500D machine and hot rolling test in this paper. Two specimens with and without Nb addition were deformed by 21% at 850  and then air cooling to room temperature. For Nb free steel, the average size of FGBA is 25 μm in length and 10 μm in width. The average size of intragranular ferrite is about 6—8 μm. For FGBA/BG steel with 0.02%Nb, the average size of FGBA is 10 μm in lengtand 4 μm in width. The average size of intragranular ferrite is less than 4 μm. Compared with Nb free steel, the size of bainitic ferrite and martensite/austenite (M–A) island decease and their corresponding volume fractions increase in the FGBA/BG steel with 0.02%Nb. The experimental results indicate that the addition of 0.02%Nb increases the hardenability of the FGBA/BG steel, suppresses the transformation of  γ→α, refines the size of grain boundary allotriomorphic ferrite, promotes the granular bainitic transformation, lowers the bainitic start temperature (Bs), and refines the granular bainite including its bainitic ferrite and M–A islands. It is suggested that the effects of structure refinement and increase of strengtening phases improve the strength of the FGBA/BG steel with 0.02%Nb. Compared wih ypicaNb ree FGBA/BG steel after hot rolling and air cooling, it is found that the tensile strengtand yielstrength of the FGBA/BG steel wit0.02Nb% rise to 157 MPa and 97 MPa respectively. Moreover, some possible reasons of structure refinement and increase of stregthening phases induced by 0.02%Nb addition have been disscussed in this paper.

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    NUMERICAL SIMULATION OF SWIRLING FLOW IN IMMERSION NOZZLE INDUCED BY A ROTATING ELECTROMAGNETIC FIELD IN ROUND BILLET CONTINUOUS CASTING OF STEEL
    SU Zhijian LI Dewei SUN Liwei MARUKAWA Katsukiyo HE Jicheng
    Acta Metall Sin, 2010, 46 (4): 479-486.  DOI: 10.3724/SP.J.1037.2009.00713
    Abstract   PDF (5148KB) ( 1569 )

    Swirling flow in an immersion nozzle generated with a swirl blade in it has been proved to be effective to reduce the meniscus fluctuation and homogenize the distribution of temperature in a mold during continuous casting of steel. However, this process has insuperable limitations: the swirling flow intensity can not be regulated to meet the process operation needs; the immersion nozzle with blade is liable to clog, leading to its low life span; and frequent replace of a nozzle in casting may cause operational difficulties. In this study a new process that a rotating electromagnetic field was set up around the immersion nozzle to induce a swirling flow in it by Lorentz force, has been proposed. In this case, te same effects as the swirl blade can be achieved without the above limitations. Four types electromagnetic stirrers, such as round, half round, U–shaped and modified U–shaped, were used in the simulation and their effects of structure, coil current intensity and magnetic field frequency on the magnetic field distribution and the flow filed in the immersion nozzle and mold during the round billet continuous casting of steel were numerical simulated and analyzed. The simulated results show that the magnetic flux density is the largest and magnetic field distribution is most uniform under the round electromagnetic stirrer. By using round electromagnetic stirrer, 500 A coil current intensity and 50 Hz frequency will induce a strong swirling flow and reversing flow in the mold. The distribution of flow field under the modified U–shaped stirrer is better than that under the U–shaped, and closer to that uner the round one. Considering the operational difficulty to replace the nozzle etc., the modified U–shaped electromagnetic stirrer is a better alternative to the round stirrer. The numerical simulation method has been proven to be sound by the swirling experiment of a low melting point alloy in an immersion nozzle surrounded by a round stirrer.

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    STUDY ON GROWTH MECHANISM OF LANTHANUM SALT CONVERSION COATING ON GALVANIZED STEEL
    KONG Gang LIU Renbin LU Jintang CHE Chunshan ZHONG Zheng
    Acta Metall Sin, 2010, 46 (4): 487-493.  DOI: 10.3724/SP.J.1037.2009.00546
    Abstract   PDF (2103KB) ( 1417 )

    Rare earth conversion coating was proposed as one of the most prospective alternatives to toxic chromate conversion coatings on some metals and alloys. In this work, the lanthanum salt conversion coating was obtained by immersing a galvanized steel in a lanthanum salt passivation solution modified with citric acid. Scanning electron microscopy (SEM), X–ray energy dispersive spectrometer (EDS), X–ray photoelectron spectroscopy (XPS) and infrared spectroscopy (IR) were used to investigate its structure and composition. Neutral salt spray (NSS) tests were used to eval- uate the anti–corrosive performance of the coating and the optimum range of treatment time. The results show that the lanthanum salt conversion coating modified with citric acid mainly consists of LaCit, La(OH)3/La2O3 and a trace of Zn(OH)2/ZnO. When treatment time is more than 10 min, the modified coating on the samples consists of two layers: the inner layer is compact and the outer lanthanum–riched layer is thin and poriferous. Cracks occur in the coating when treatment time within 10 s and are evolved during further treatment. The thickness of the coating obtained on galvanized steel that the immersion time is 1 min is approximately 250 nm, and increasing with treatment time within 60 min. The growing rate of the coating is faster than that of conversion coatings without modified with citric acid. The growth process of the coating can be divided into three stages: at the initial time complexes ions of lanthanum citrate were adsorbed on the whole surface of galvanized steel, then lanthanum citrate and lanthanum hydroxide are co–deposited on surface, and in the final stage there is lanthanum hydroxide mainly deposited on surface of the coating. The corrosion resistance of the coating is close to that of a chromate conversion coating. The optimum treatment time is 10 to 30 min.

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    INFLUENCE OF T6I6 TEMPER ON TENSILE AND INTERGRANULAR CORROSION PROPERTIES OF 6061 ALUMINUM ALLOY
    LI Hai PAN Daozhao WANG Zhixiu ZHENG Ziqiao
    Acta Metall Sin, 2010, 46 (4): 494-499.  DOI: 10.3724/SP.J.1037.2009.00570
    Abstract   PDF (1900KB) ( 2004 )

    It is important to achieve a good combinization of high tensile properties and high intergranular corrosion resistance for 6000 series alloys in wider applications. In this paper, the effect of T6I6 temper on tensile and intergranular corrosion properties of 6061 aluminum alloy was investigated by tensile test, intergranular corrosion test, OM and TEM. The experimental results show that after T6 temper the ultimate strength and yield strength of 6061 alloy are 356.0 and 331.6 MPa respectively, but it has a serious tendency to intergranular corrosion sensitivity. It is found that pre–ageing time of T6I6 temper has no remarked effect on tensile properties of the alloy, but interrupted temperature and interrupted time have obvious effects on its tensile and intergranular corrosion properties. After T6I6 treatment, the tensile strength of 6061 aluminium alloy reaches its peak strength firstly and then decreases to a small value with the increase of interrupted time after pre–ageing at interrupted temperature of 150 ℃, and the corrosion mode also changes from intergranular to uniform corrosion. Higher interrupted temperature and longer interrupted time are beneficial to achieve a good combinization of tensile properties and intergranular corrosion resistance, which results from the high density of precipitates inside grains and discontinuously distributed precipitates on grain boundaries. After an optimum T6I6 treatment of 180 ℃×2 h+150 ℃×2160 h+180 ℃×8 h, the ultimate strength and yield strength are 348.5 and 326.9 MPa respectively, close to those after T6 temper. The corrosion mode is changed from the intergranular corrosion of 6061 Al alloy after T6 temper to a uniform etching with the etching depth about 30 μm.

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    EFFECT OF AN ABRUPT GROWTH RATE ON PRIMARY Al2Cu PHASE IN DIRECTIONAL SOLIDIFICATION OF Al–40%Cu HYPEREUTECTIC ALLOY
    QUAN Qiongrui LI Shuangming FU Hengzhi
    Acta Metall Sin, 2010, 46 (4): 500-505.  DOI: 10.3724/SP.J.1037.2009.00706
    Abstract   PDF (1270KB) ( 2023 )

    Intermetallics combining metal and ceramic properties exhibit colorfully complicated growth morphologies in solidification. Different from the solidus phase solidification, intermetallic solidification remains less understanding of the correlation between processing parameters and microstructure morphologies. In this paper, considering that the intermetallic compound Al2Cu phase has well–known thermophysical properties available for the theoretical and experimental research, we carried out the directional solidification experiments and focused on an Al–Al2Cu hypereutectic alloy, where the Al2Cu phase was solidified as a primary phase. The primary Al2Cu phase growth behavior in the experiments included phase competition growth between coupled eutectic and primary phase, faceted phase transition and its change in growth morphology. By using a high thermal gradient directional solidification apparatus, the directionally solidified microstructures of Al–40%Cu (mass fraction) hyereutectic alloy were investigated and discussed based on the competition growth model. As the growth rate was changed abruptly from 10 μm/s to 2 μm/s, the microstructure transition from a primary Al2Cu dendrite plus interdendritic eutectic to an entirely coupled eutectic occurred due to the interface growth temperature of the coupled eutectic exceeding that of the primary Al2Cu dendrite. Also, the alloy liquid composition ahead of the solid/liquid interface approaching the eutectic point caused this microstructure transition. Simultaneously, in the changing–growth rate experiments, the primary Al2Cu dendrites were firstly broken into small ones and then became the eutectic microstructure; it was interpreted rather by the decrease in liquid solute concentration ahead of the solid/liquid interface tan by te effect of the thermal–solutal convection. Moreover, the morphology change iphase growth from faceted primary Al2Cu phase to non–faceted phase was observed by reducing abruptly the growtrate from 10 μm/s to 2 μm/s, which can be explained by the Jackson fctor α decreasing with increasig the interface growth temperature of primary Al2Cu pase.

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    EFFECT OF SOLIDIFYING RATE ON INTEGRALLY DIRECTIONALLY SOLIDIFIED MICROSTRUCTURE AND SOLID/LIQUID INTERFACE MORPHOLOGY OF AN Nb–Ti–Si BASE ALLOY
    WANG Yong GUO Xiping
    Acta Metall Sin, 2010, 46 (4): 506-512.  DOI: 10.3724/SP.J.1037.2009.00696
    Abstract   PDF (4027KB) ( 3367 )

    Since temperatures of airfoil surfaces in advanced turbine engines are approaching the limit of nickel base superalloys, Nb–Ti–Si base alloys as their potential materials have attracted much attention recently. Nb–Ti–Si base alloys have high melting temperature, suitable densities, good
    elevated temperature creep strength and acceptable room temperature fracture toughness, therefore, they are expected to be employed in the temperature range of 1200—1450 ℃as structural materials. Alloying and directional solidification are generally used to obtain a better combination of room temperaure fracture toughness with high temperature creep strength and oxidation resistance for an elevated–temperature alloy. In this paper, the master alloy ingot with a nominal composition of Nb–20Ti–16Si–6Cr–5Hf–4Al–2B–0.06Y (atomic fraction, %) was prepared by using vacuum consumable arc–melting. The integrally directional solidification of this alloy was conducted in a high vacuum and ultrahigh temperature directional solidification furnace with the use of a ceramic crucible at melt temperature of 2050. The integrally directionally solidified microstructure, preferred orientation of constituent phases and solid/liquid (S/L) interface morphology at different solidifying rate (2.5, 5, 10, 20, 50 and 100 μm/s) for this alloy have been investigated by XRD, SEM and EDS, and the growth mechanism of Nbss/(Nb, X)5Si3 (where Nbss denotes Nb solid solution, X represents Ti, Hf and Cr elements) eutectic in it has been discussed. The results show that the directionally solidified microstructure of the alloy is mainly composed of hexagonally cross–sectioned primary (Nb, X)5Si3 columns and coupled grown lamellar Nbss/(Nb, X)5Si3 eutectic colonies both aligned straight and uprightly along the growth direction. When the solidifying rate varies from 2.5 μm/s to 100 μm/s, the
    solid/liqid interface of the alloy undergoes an evolution from coarse cellular, fine cellulaand finally to cellular dendrite morphologies. Both the average diameter of eutectic cells and lamellar spacing in hem decrease with the increase in solidifying rate. The formatin of a regular bss/(Nb, X)5Si3 euecic morphology is attributable to a large kinetic undercoling and a low fusion entropies of alloy phases.

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