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

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    EFFECT OF GRAIN SIZE ON THE GRAIN BOUNDARY CHARACTER DISTRIBUTIONS OF COLD ROLLED AND ANNEALED PURE COPPER
    CAI Zhengxu WANG Weiguo FANG Xiaoying GUO Hong
    Acta Metall Sin, 2010, 46 (7): 769-774.  DOI: 10.3724/SP.J.1037.2010.00079
    Abstract   PDF (1054KB) ( 1563 )

    Pure copper (99.97%) samples with varied grain sizes were cold rolled and annealed at first and then their grain boundary character distributions (GBCDs) were characterized by electron back scatter diffraction (EBSD) techniques. The results showed that the sample with finer initial grain size (12 μm) appeared to be largely populated by the so-called special boundaries such as Σ3n (n=1, 2, 3) after cold rolling and annealing, the fraction of Σ3n boundaries reached 75.7% and the averaged size of clusters of grains with Σ3n (n=1, 2, 3) orientation relationships (Σ3n CG) were as large as 200 $\mu$m in this sample. However, with initial grain size increasing, the fraction of special grain boundaries as well as the size of Σ3n CG in the samples were decreasing dramatically. In situ EBSD observation, twin-filtering and five parameter method (FPM) analyses indicated that triple-junctions are the preferred sites for the nucleation of incoherent Σ3 boundaries which play a important role in enhancing the fraction of special boundaries.It accounts for primarily the desired GBCD results as obtained in the sample with finer initial grain size.

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    THREE-DIMENSIONAL PHASE FIELD STUDY ON STRAIN SELF-ACCOMMODATION IN MARTENSTIC TRANSFORMATION
    MAN Jiao ZHANG Jihua RONG Yonghua
    Acta Metall Sin, 2010, 46 (7): 775-780.  DOI: 10.3724/SP.J.1037.2010.00051
    Abstract   PDF (736KB) ( 1274 )

    The number of martensitic variants depends on the grain size and orientation of parent phase. Although the growth of polytwin structure with the combination of different martensitic variants was studied by in situ dynamic observation, the energy change during the growth of martensite cannot be revealed, and in turn the strain self-accommodation of martensitic transformation (MT) cannot be confirmed. Based on the recent study of the phase field model, describing proper martensitic transformation for a single-crystal system under a fully constrained boundary, a three-dimensional (3D) phase field simulation is performed to demonstrate the microstructure evolution of plolytwin structure in this paper, meanwhile, the energy change during the growth of martensite is calculated. The results indicate that during the growth of two martensitic variants the total strain energy increases with prolonging microstructural evolution time, however, the opposite change occurs during the growth of three martensite variants. The 3D phase field simulation reveals that the growth of three martensitic variants possesses the best strain self-accommodation effect.

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    THREE DIMENSIONAL MULTI-PHASE FIELD SIMULATION OF GROWTH OF EUTECTIC CBr4-C2Cl6 ALLOY
    I. Modeling and Testing
    YANG Yujuan YAN Biao
    Acta Metall Sin, 2010, 46 (7): 781-786.  DOI: 10.3724/SP.J.1037.2010.00091
    Abstract   PDF (928KB) ( 1102 )

    The KKSO multi-phase field model is extended to three dimensions (3D). The modeling and the choice of parameters are confirmed to be correct by tests with low lamellar thickness. With the KKSO multi-phase field model, the morphology evolution of hypoeutectic, eutectic and hypereutectic CBr4-C2Cl6  alloy is studied with low lamellar< thickness. The simulated results showed that the morphology evolution in 3D is similar to that in two dimensions (2D). With the increase of the dimensionless initial lamellar spacings Λ in the range of 0.598-2.336, the sequence of morphology evolution is: lamellar merges→stable growth→lamellar oscillation→lamellar branching. But the critical value of morphology evolution in 3D is smaller than that in 2D due to effect of the third dimension.

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    THREE DIMENSIONAL MULTI-PHASE FIELD SIMULATION OF GROWTH OF EUTECTIC CBr4-C2Cl6  ALLOY
    II. Effect of Lamellar Spacing on Morphology Evolution
    YANG Yujuan YAN Biao
    Acta Metall Sin, 2010, 46 (7): 787-793.  DOI: 10.3724/SP.J.1037.2010.00092
    Abstract   PDF (873KB) ( 1285 )

    Using KKSO multi-phase field model, with equal lamellar width and thickness, three dimensional (3D) morphology evolution, lamellar-rod transition and the mechanism of adjustment of lamellar spacing of CBr4-C2Cl6   alloys are investigated at different initial lamellar spacings. It's found that, as for the hypoeutectic and eutectic CBr4-C2Cl6   alloy, different initial lamellar spacings may lead to lamellar-rod transition, which is related to the initial lamellar spacings. The CBr4-C2Cl6   hypereutectic lamellar alloy can't transit to rod-like eutectic, with the increase of the dimensionless initial lamellar spacings Λ in the range of 0.598-2.336, the sequence of morphology evolution is: lamellar merges to form 1λO→T-xλO→1λO→2λO→lamellar branching→zigzag bifurcation$\rightarrow$lamellar destabilizes to form the disordered pattern. The simulated results also showed that the mechanism of adjustment of lamellar spacing in 3D of the CBr4-C2Cl6   hypereutectic alloy is similar to that in two dimensions (2D), which is lamellar annihilation and branching on the whole, the lamellar annihilation takes place with a smaller initial lamellar spacing while the lamellar branching takes place with a bigger initial lamellar spacing. The adjustment mechanism of the lamellar spacing in 3D is more complex than that in 2D because of the additional effect of the third dimension.

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    ANALYSIS OF ELECTROMAGNETIC FIELD AND JOULE HEATING OF ELECTROSLAG REMELTING PROCESSES
    WANG Fang LI Baokuan
    Acta Metall Sin, 2010, 46 (7): 794-799.  DOI: 10.3724/SP.J.1037.2010.00080
    Abstract   PDF (1016KB) ( 1481 )

    A 3D finite element model was developed to simulate the magnetic field, electromagnetic force, current density and Joule heating of electroslag remelting< processes (electrode, slag and ingot) by using of Maxwell equation, Lorentz law and Joule law. Especially, the skin effect is shown and discussed based on numerical results. The results show that the current distribute on the external surfaces of slag and ingot. In slag, due to the low electrical conductivity of slag, the current distribution changes, gathering on the end of electrode. The maximum electromagnetic force is at the upper surface nearby the electrode and the maximum joule heating is at the interface of electrode and slag. When current frequency is greater than 35 Hz, the vortex flow  will occur in interior of electrode and ingot. When electrode immersion depth increase and slag cap thickness decrease, the maximum of the joule heat in slag will increase.

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    EFFECTS OF ANNEALING TEMPERATURE AND STRESS-STRAIN CYCLE ON SUPERELASTICITY  OF Ti-Ni-Cr SHAPE MEMORY ALLOY
    WANG Qi HE Zhirong WANG Yongshan YANG Jun
    Acta Metall Sin, 2010, 46 (7): 800-804.  DOI: 10.3724/SP.J.1037.2010.00047
    Abstract   PDF (1029KB) ( 1233 )

    Effects of annealing temperature (θan) and stress-strain cycle on superelasticity (SE) of Ti-50.8Ni-0.3Cr alloy were investigated by tensile test. The results show that with increasing θan, critical stress for stress-induced martensite (σM) descends firstly and then rise. The σM reaches its minimum value (311 MPa) and maximum value (583 MPa) when annealing temperature is 450 and 600℃,\linebreak respectively. When θan=350-590℃, the residual strain (εR) has no obvious change, keeps at a low value all the time. When θan>590℃, the εR rises rapidly. With increasing stress-strain cycle number (N), the alloy annealed at 350 and 590℃ transform from incompletely nonlinear superelasticity to completely nonlinear superelasticity. The alloy annealed at 650℃ transform from incompletely superelasticity to linear-like superelasticity. Moreover, with increasing N, the energy dissipation ability of the alloy annealed at 590 and 650℃ descend gradually, and tend to stability. The energy dissipation ability of the alloy annealed at 350℃ nearly keeps constant. In order to get excellent SE, the annealing temperature of Ti-50.8Ni-0.3Cr alloy should be below recrystallization temperature.

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    PREDICTION OF CRITICAL TEMPERATURE FOR DELAYED HYDRIDE CRACKING IN IRRADIATED N18 ZIRCONIUM ALLOY
    SUN Chao TAN Jun YING Shihao LI Cong PENG Qian ZHAO Suqiong
    Acta Metall Sin, 2010, 46 (7): 805-809.  DOI: 10.3724/SP.J.1037.2009.00868
    Abstract   PDF (757KB) ( 1286 )

    Delayed hydride cracking (DHC) has been recognized as a potential threat to the structural integrity of zirconium alloy components in water-cooled nuclear reactors since the early 1970 s. Although DHC has been studied for a long time, most of the investigations were focused on unirradiated Zr-Nb and Zr-Sn alloys, no information was found on Zr-Sn-Nb alloy. Currently, several new Zr-based alloys have been developed from the point of view of enhancement of corrosion resistance and hydrogen pickup properties. In China, N18 (Zr-Sn-Nb) alloy has been developed based on Zr-Sn and Zr-Nb systems to meet the requirements of higher fuel burn-up. Therefore, it is necessary to investigate the DHC behavior of N18 alloy systematically. The objective of this study is to predict the critical temperatures for initiating and arresting DHC in irradiated N18 alloy. A model used to forecast DHC critical temperature of irradiated zirconium alloys was established based on the terminal solid solubility of hydrogen in irradiated zirconium alloys and stress-induced hydrogen diffusion. By using the model, the critical temperatures for irradiated N18 alloy were predicted. The result showed that the irradiated N18 alloy has high sensitivity for DHC initiation, because neutron irradiation increased the solubility of hydrogen and yield strength of N18 alloy. Compared with unirradiated N18 alloy, the critical temperature of irradiated N18 alloy rise nearly 20℃. The variation of critical temperatures of irradiated N18 alloy is similar to the unirradiated one. Tc is slightly less than Th.

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    EFFECT OF D+ BEAM IRRADIATION ON Ti FILM
    WANG Boyu XIANG Wei TAN Xiaohua DAI Jingyi CHENG Liang QIN Xiubo
    Acta Metall Sin, 2010, 46 (7): 810-813.  DOI: 10.3724/SP.J.1037.2010.00017
    Abstract   PDF (769KB) ( 995 )

    Effect of the D+ ion beam on Ti film is studied by radiating the Ti films with different energy D+ beams on electrostatic accelerator. Slow positron annihilation spectroscopy and SEM are used to characterize the films. With the D+ beam energy increases, the extent of radiation damage and the surface ablation of Ti films increase. The maximum of radiation damage locates at 0.3 μm depth. The geometrical non-uniformity on original surface is the main reason for the ablation behavior on Ti films by inducing a selective ablation under radiation. Radiation damage and energy loss of D+ in Ti film are discussed in detail by the numerical simulation of the beam and the solid interaction. It shows that the maximum radiation damage is at 0.65 $\mu$m depth and energy loss of D+ beam in Ti films accords with the experimental observation.

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    HOT DEFORMATION BEHAVIOR AND MICROSTRUCTURAL STABILITY OF SPRAY FORMED Al-22Si-5Fe ALLOY
    CAI Yuanhua LIANG Ruiguang SU Zhanpei ZHANG Jishan
    Acta Metall Sin, 2010, 46 (7): 814-820.  DOI: 10.3724/SP.J.1037.2010.00119
    Abstract   PDF (1075KB) ( 953 )

    Hot deformation behavior of spray forming Al-22Si-5Fe alloy at different deformation temperatures with isothermal constant strain rates of 0.005, 0.01, 0.03, and 0.05 s-1 was investigated by using Gleeble-1500 thermo-mechanical simulator, with maximum strain of 30%, the microstructures were studied by using of OM and XRD method. The experimental results showed that the hot deformation behavior of spray formed alloy could also be described by a model containing Z parameter (Zener-Hollomon parameter). The calculated deformation activation energy of the studied alloy was consistent with the self-diffusion activation energy of Al atoms at lower deformation temperature, and much higher than the self-diffusion activation energy of Al atoms at higher temperature because of the influences of large content hard constitutes such as prime Si and Fe-bearing intermetallics. The Si particles changed little when deformed at low temperature, but coarsened obviously when deformed at elevated temperature of 698 K or above. The heating temperature had no obvious effect on the sizes of Fe-bearing intermetallics.

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    EFFECTS OF Al ON Si-Al-Y2O3 CO-DEPOSITION COATINGS ON Nb-Ti-Si BASE ULTRAHIGH TEMPERATURE ALLOY
    ZHANG Ping GUO Xiping
    Acta Metall Sin, 2010, 46 (7): 821-831.  DOI: 10.3724/SP.J.1037.2010.00044
    Abstract   PDF (1680KB) ( 1389 )

    Si-Al-Y2O3 co-deposition coatings on an Nb-Ti-Si base ultrahigh temperature alloy were prepared by pack cementation processes at 1050℃. The effects of Al content (mass fraction) in pack mixtures on the microstructure and constituent phases of the co-deposition coatings were studied, and the oxidation-resistant performance of the coating was investigated. The results show that all coatings prepared at 1050℃with pack mixtures containing different Al contents have a multiple layer structure, and the inner layers of all coatings are composed of Al3(Nb, X) (X represents Ti, Cr and Hf) and (Cr, Al)2(Nb, Ti) phases. The outer layer of the coating prepared at 1050℃ for 10 h with the pack mixture containing 10% Al is composed of (Nb, X)Si2, and the middle layer is composed of (Nb, X)5Si3. However, the constituent phases change into Al3(Nb, X) and (Nb, X)5Si3 in the middle layer of the coating prepared with the pack mixture containing 15%Al. (Nb, X)3Si5Al2 and (Nb, X)Si2 phases have been detected in the outer layer of the coating prepared with the pack mixture containing 20%Al, and Al3(Nb, X) is the main phase constituent in its middle layer with dispersed (Nb, X)5Si3 particles in it. EDS analysis reveals that the distribution of Y in the coatings is not uniform, with its content increasing with the rising of Al content in the pack mixtures. Co-deposition of Al and Si occurs in a sequential manner during the deposition process and the growth of the coatings obeys a parabolic kinetics. After oxidation at 1250℃ for 5 h, dense Al2O3 scales have formed on the coatings, which prevent the oxygen from diffusion into the substrate effectively.

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    EFFECT OF Nb SOLUTE DRAG AND NbC PRECIPITATE PINNING ON THE RECRYSTALLIZATION GRAIN GROWTH IN LOW CARBON Nb-MICROACLOYED STEEL
    FU Liming SHAN Aidang WANG Wei
    Acta Metall Sin, 2010, 46 (7): 832-837.  DOI: 10.3724/SP.J.1037.2010.00110
    Abstract   PDF (896KB) ( 1971 )

    A kinetic equation for austenite grain growth has been derived concerning the mutual effect of NbC and Nb solute in low carbon Nb-microalloyed steels. It is shown that both solute drag of Nb in solid solution and pinning of NbC particles inhibit the grain boundary migration during grain growth after recrystallization in low carbon Nb-microalloyed steels. At high temperatures the NbC pinning plays a dominate role for retarding the austenite grain growth with less Nb solute drag effect. An obvious Nb solute drag restraint was, however, observed at relatively low temperatures. Also, the theoretical calculations are in good agreement with experimental results. The effectiveness of drag effect of soluble atoms and pinning effect of precipitates can be characterized by a p factor. The pinning of precipitates and solute drag of soluble atoms are more effective for suppressing grain growth as p>0 and p<0, respectively. And the ratio of Nb in solute and Nb in precipitate as p=0 reaches the priority and most effectively retards the grain growth. In the traditional hot rolling or< austenitizing temperature range, a strong suppression for grain growth after recrystallization could be obtained due to those fine NbC particles smaller than 10 nm in Nb-microalloyed steels.

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    EFFECT OF Nb, Ti AND V ON THE HOT DEFORMATION BEHAVIOR OF LOW CARBON Nb MICROALLOYED STEELS
    WU Jinbin LIU Guoquan WANG Hao
    Acta Metall Sin, 2010, 46 (7): 838-843.  DOI: 10.3724/SP.J.1037.2009.00858
    Abstract   PDF (910KB) ( 1363 )

    The characteristic values such as critical strain εc, peak strain εp, critical stress σc, peak stress σp and steady-state stress σss were obtained from work-hardening rate-flow stress (θ-σ) curves based on the thermal/mechanical simulation data of 6 microalloyed steels with different contents of Nb, Ti and V. The results show that the peak stress σp was raised with the increasing contents of Nb, Ti and V, while the εc/εp value for all the experimental steels are in the range of 0.50-0.65. Then the Avrami kinetic curves of dynamic recrystallization were calculated from σ-ε curves for the 6 steels, which are very similar to those curves obtained by metallographic methods. With the increase of content of Nb and Ti, the corresponding Avrami curve shifted to the right and the dynamic recrystallization kinetics were hindered; while simultaneously adding Ti and V into Nb microalloyed steel would lead to the dynamic recrystallization to be initiated at much lower strain rate.

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    PRECIPITATES IN HR3C STEEL AGED AT HIGH TEMPERATURE
    FANG Yuanyuan ZHAO Jie LI Xiaona
    Acta Metall Sin, 2010, 46 (7): 844-849.  DOI: 10.3724/SP.J.1037.2010.00037
    Abstract   PDF (1062KB) ( 2946 )

    HR3C steel (Fe-25Cr-20Ni-Nb-N) is a new type of austenitic heat-resistant steel which has been widely used for super-heater and re-heater tubes in the ultra supercritical pressure (USC) boiler. The mechanical properties of HR3C steel were dependent on the stability of the microstructure, particularly the large amount of precipitates formed during service. This work was focused on the change of microstructure and the precipitations of HR3C steel after thermal aging for 500 h at 750℃. The results indicated that the major precipitates were variform M23C6 carbides and NbCrN nitrides. M23C6 carbides precipitated on grain boundaries and in intragranular. Fine and dispersed NbCrN nitrides were found to precipitate in intragranular. The hardness of HR3C steel had been improved through dispersed M23C6 and fine NbCrN precipitates after thermal aging for 500 h at\linebreak 750℃. All the M23C6 carbides, with lattice parameter of three times of the austenite matrix,  grown in a cube to cube orientation relationship with the matrix. Some of the M23C6 carbides which precipitated on grain boundaries were transformed into σ phase.

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    EVOLUTION OF NANOSCALE Al3(ZrxEr1-x) PRECIPITATES IN Al-6Mg-0.7Mn-0.1Zr-0.3Er ALLOY ALLOY DURING ANNEALING
    Gong Bo WEN Shengping HUANG Hui NIE Zuoren
    Acta Metall Sin, 2010, 46 (7): 850-856.  DOI: 10.3724/SP.J.1037.2010.00036
    Abstract   PDF (910KB) ( 1339 )

    Erbium (Er) has attracted considerable attention as a substitute to scandium (Sc) for microalloying. The solid solution of Er and Zr decomposes to form a dispersion of secondary Al3(ZrxEr1-x) during annealing. This secondary Al3(ZrxEr1-x) precipitates improve strength, especially the elevated temperature strength. In the present paper we investigate the evolution of Al3(ZrxEr1-x) precipitate during annealing by transmission electron microscopy (TEM). The Al3(ZrxEr1-x) precipitates nucleate homogeneously at lower temperatures (280℃ and 470 ℃). The transition from homogeneous to heterogeneous nucleation at higher aging temperature (510℃) is a natural consequence of the reduced driving force for nucleus formation as the temperature increases and the Er supersaturation in the α-Al matrix decreases. The change in morphology of the Al3(ZrxEr1-x) precipitates from faceted to approximately spheroidal is associated with a change in the value of interfacial free energies. The diameter and the ratio of Zr/Er increase with increasing aging time at constant temperature. The coarsening of Al3(ZrxEr1-x) is dependent on the diffusivity of Er, Zr in α-Al.

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    PREPARATION OF NANOSTRUCTURE MnO$_{\bf 2}$ SINGLE CRYSTAL IN VARIOUS ACID SOLUTION
    CHEN Yong HONG Yuzhen MA Yanping YANG Hao LI Jianbao
    Acta Metall Sin, 2010, 46 (7): 857-861.  DOI: 10.3724/SP.J.1037.2010.00088
    Abstract   PDF (782KB) ( 1283 )

    Nanostructure MnO2 single crystal was prepared through redox reactions of potassium permanganate in different inorganic acid (hydrochloric acid, sulfuric acid and nitric acid) and organic acid (acetate). The products were characterized by TEM and XRD. It indicated that the crystal structure and morphology of the synthesized MnO2 can be tailored by adjusting the pH value in solution and reaction temperature. It was also found that layer folded δ-MnO2 microspheres were obtained at low reaction temperature and low hydrochloric acid concentration, whereas α-MnO2 single-crystal nanorods were fabricated with increased reaction temperature and hydrochloric acid concentration. The possible formation mechanism of δ-MnO2 microspheres and α-MnO2 nanorods is also discussed.

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    PREPARATION OF LUMINESCENT MATERIALS BaMoO4:Eu3+ BY HIGH TEMPERATURE BALL MILLING
    CHEN Xuebing SHAO ZhongbaoTIAN Yanwen
    Acta Metall Sin, 2010, 46 (7): 862-866.  DOI: 10.3724/SP.J.1037.2010.00183
    Abstract   PDF (860KB) ( 1358 )

    BaMoO4:Eu3+ phosphors were synthesized by high temperature ball milling method and using (NH4)6Mo7O24?4H2O, BaCO3 and Eu2O3 as raw materials. Crystalline phases, morphology and luminescent properties of sample were characterized by XRD, SEM and emission spectra. XRD patterns indicated that a single phase of BaMoO4:Eu3+ can be obtained by 4 h ball milling at\linebreak 600℃, with 15% Eu3+ (mole fraction) dopped concentration. Photolumincence experiment results showed that the emission is a line spectrum and the main peak is located at visible red light\linebreak (616 nm). The synthesized temperature was decreased about 400℃ while the dopped Eu3+ concentration was increased about 7\% than those of chemical deposited method. The BaMoO4:Eu3+ powders has considerable luminescent intensity prepared under the optimum technological conditions. In the high temperature ball milling process, the cooperation action of milling and temperature, significantly increased the activation energy of the reaction system.

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    EFFECTS OF HEAT-TREAT TEMPERATURE ON MICROSTRUCTURE OF ELECTRO-BRUSH PLATING Ni-P COATINGS
    LI Zhiming QIAN Shiqiang WANG Wei LIU Jihua
    Acta Metall Sin, 2010, 46 (7): 867-872.  DOI: 10.3724/SP.J.1037.2010.00086
    Abstract   PDF (958KB) ( 1396 )

    The effects of heat-treat temperature on the structural of electro-brush plating Ni-P coatings were investigated using X-ray diffraction (XRD) analysis. The results show that the eletro-brush plating Ni-P coating is mainly in amorphous state. When heat-treat temperature above 350 ℃, Ni and Ni3P crystal appear, NiO is generated as the heat-treat temperature is above 500 ℃. There is a typical S-shaped relationship between the crystallinity and heat-treat temperature. The activation energy of crystallization is (237.9±10.46) kJ±mol-1. As the heat-treat temperature rises, Ni crystal cell parameter a increases firstly and then decreases, Ni3P crystal cell parameter a decreases but $c$ has a little increase. The crystal size of both Ni and Ni3P has some increase with the rise of heat-treat temperature. The activation energies for the growth of Ni and Ni3P are (66.85$\pm$3.15) kJ?mol-1 and (133.41±4.69) kJ?mol-1, respectively, and the great disparity imply that Ni crystal has dynamic for preferred growth.\par

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    EFFECTS OF P CONTENT ON (LAVES+γ) EUTECTIC REACTION DURING INCONEL 706 ALLOY SOLIDIFICATION
    XIN Xin HUANG Aihua QI Feng ZHANG Weihong LIU Fang YANG Hongcai SUN Wenru HU Zhuangqi
    Acta Metall Sin, 2010, 46 (7): 873-878.  DOI: 10.3724/SP.J.1037.2010.00038
    Abstract   PDF (871KB) ( 1485 )

    The effects of P on elemental segregation and as-cast microstructure in Inconel 706 alloy were investigated in this paper. The results showed that P was markedly enriched in residual liquid and enhanced the segregation of Nb and Ti, which promoted the precipitation of Nb-enriched Laves phase and Ti-enriched η-Ni3Ti phase. In the as-cast alloy doped with 0.004% P (mass fraction), fine (Laves+γ) eutectic and needle like $\eta$ phase were formed in interdendritic region. As the P dopant was higher than 0.016%, blocky Laves phase was formed and lath η particles were precipitated among Laves particles. Moreover, with P increasing from 0.004\% to 0.016%, the incipient melting-point was found to drop from 1170-1180 ℃ to 1160-1170 ℃. In high P-doped alloys, the enrichment of P in the residual liquid was believed to hinder the solidification of γ phase, as a result, the final solidification temperature was found to be decreased and the solidification was terminated by the solidification of γ phase prior to the formation of P-bearing Laves phase rather than forming the (Laves+γ) eutectic.

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    EFFECT OF WITHDRAWAL RATE ON THE MICROSTRUCTURE OF DIRECTIONAL SOLIDIFIED Ti-47Al-2Cr-2Nb-0.8B ALLOYS
    YANG Lili ZHENG Lijing XIAO Zhixia YAN Jie ZHANG Hu
    Acta Metall Sin, 2010, 46 (7): 879-884.  DOI: 10.3724/SP.J.1037.2010.00081
    Abstract   PDF (894KB) ( 1049 )

    The microstructures, morphology of boride and grain refinement in directional solidified Ti-47Al-2Cr-2Nb-0.8B alloy with different withdrawal rates have been investigated. The results indicated that the primary phase, borides and their precipitation consequence varied with the increasing of withdrawal rates (0.36, 5, 10 and 20 mm/min). The morphologies of borides and grain refinement were affected by the solidification paths due to the different withdrawal rates. The alloy with fine grains and borides were obtained at the withdrawal rate of 5 mm/min.

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    EFFECT OF ULTRASONIC VIBRATION ON THE DIRECTIONAL SOLIDIFICATION OF SCN-3%ETH MODEL ALLOY
    DUAN Mengmeng CHEN Changle
    Acta Metall Sin, 2010, 46 (7): 885-889.  DOI: 10.3724/SP.J.1037.2010.00095
    Abstract   PDF (786KB) ( 1084 )

    The directional solidification process of SCN-3%ETH model alloy under 27.5 kHz ultrasonic vibration is studied in this paper. The results show that the interface growth is restrained because of the effect of ultrasonic vibration. The radius of curvature on the interface increases as the ultrasonic action time increase. When the ultrasonic vibration is removed from the melt, the refined cellular structure is formed. Because the impact of transient high temperature and pressure generated by the cavitation effect and stirring action of the acoustic streaming and shearing action caused by forced vibration, part of the dendrite tips are fractured. The interface growth rate decreases as the ultrasonic action time increase. Cavitation effects and high-frequency forced vibration caused by ultrasonic vibration in the melt are main reasons for interface growth rate change.

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    INTERACTION BETWEEN Ti-47Al-2Cr-2Nb ALLOY AND Y2O3 CERAMIC DURING DIRECTIONAL SOLIDIFICATION
    ZHANG Huarui GAO Ming TANG Xiaoxia ZHANG Hu
    Acta Metall Sin, 2010, 46 (7): 890-896.  DOI: 10.3724/SP.J.1037.2010.00099
    Abstract   PDF (1088KB) ( 1428 )

    Interaction between Ti-47Al-2Cr-2Nb alloy and inner layer Y2O3 of Y2O3/Al2O3 double-layer ceramic tube has been investigated during directional solidification. The results show that the Y2O3 layer effectively prevents the chemical reactions between TiAl alloy melt and exterior layer Al2O3 with lower chemical stability by blocking their direct contact. The DS alloy ingot contains Y2O3 inclusions which mainly come from the shedding of Y2O3 from ceramic tube inwall by being penetrated and eroded by fluid alloy melt in the initial phase of the directional solidification test. The average volume fraction of the inclusions is only about 0.14% and is insensitive to time. The re-sintering of the Y2O3 ceramic surface layer occurs and a 2-10 μm dense barrier layer is formed by dint of alloy melt at high temperature. It prevents further pervasion of alloy melt into the Y2O3 ceramic and shedding of imperfectly sintered Y2O3 particles into the alloy melt.

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