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CN 21-1139/TG
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    COMPETITIVE GROWTH IN BI-CRYSTAL OF NI-BASED SUPERALLOYS DURING DIRECTIONAL SOLIDIFICATION
    Acta Metall Sin, 2011, 47 (6): 641-648. 
    Abstract   PDF (6177KB) ( 2215 )
    Single crystal nickel-based superalloys, which offer improved creep-rupture, fatigue, oxidation and coating properties, have been widely applied in turbine blades and vanes. In general, the single crystal structure is produced through competitive grain growth by directional solidification technology. To understand the competitive grain growth mechanism during directional solidification, the bi-crystal experiment has been employed by many researchers, and several models have been proposed. Up to now, the competitive grain growth mechanism is still not clear yet, which is reflected in two areas, as shown in the follows. First, the influence of the growth rate on the competitive grain growth is not clear yet; second, the illustration of competitive grain growth are based on the two-dimensional simplified models. However, in actual directional solidification process, the orientations of the primary and secondary dendrites are random due to the randomness of the grain nucleation. Therefore, the actual competitive grain growth during directional solidification is a three-dimensional process. However the related literatures are fewer. In the present study, an experimental study has been conducted to investigate the three-dimensional competitive grain growth behavior under different withdrawal velocities by the bi-crystal samples with special orientation relationships. The results show that the whole sample with bi-crystal microstructure can be obtained when the two seeds are with the same primary dendrites orientation paralleled to the heat flow direction but different orientations of the secondary dendrites. And the withdrawal velocity has little influence on the competitive grain growth in this disposition relationship. However, only the favorably oriented grain can be obtained when the two seeds are with the non-coplanar primary dendrites and a group of secondary dendrites with the same orientation. The favorably oriented grain overgrows the misaligned grain in a heliciform way. And the overgrowth velocity is in direct proportion to the withdrawal velocity.
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    NANOCRYSTALLIZATION OF Al–12.7Si ALLOY INDUCED BY SURFACE INTENSE PLASTIC DEFORMATION
    LIU Gang YAN Wencong YU Fuxiao ZHAO Gang ZHAO Xiang ZUO Liang
    Acta Metall Sin, 2011, 47 (6): 649-654.  DOI: 10.3724/SP.J.1037.2011.00030
    Abstract   PDF (1779KB) ( 1441 )
    Gradient nano–micro–structured surface layer was synthesized on Al–12.7Si alloy by means of surface mechanical attrition treatment. The microstructural evolution was examined by XRD and TEM. The surface intense deformation induced nanocrystallization mechanism was summarized as follows: upon the application of repeated loads, dislocation cells/sub–micro–grains form through slips, annihilations and recombinations of high density of dislocations in Al coarse grains. The misorientations between the dislocation cells/sub–micro–grains increased by cotinuously absorbing more dislocatons. High density of dislocations in sub–micro–grains developing in above route led to the redction of grain size and the increment of misorientatins between the refined grains, and finally equiaxed nc grains with random orientations formed. The refinement of Si particles was found to be carried out through single–or multi–slips of internal dislocations and partial dissolution, then the refined Si particles distributed niformly due to the plastic flows in Al matrix.
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    EFFECT OF La AND Hf DOPANT ON THE HIGH TEMPERATURE OXIDATION OF CoNiCrAl ALLOYS
    SONG Peng LU Jiansheng ZHANG Defeng LU Jianguo LI Dejiang
    Acta Metall Sin, 2011, 47 (6): 655-662.  DOI: 10.3724/SP.J.1037.2010.00713
    Abstract   PDF (4924KB) ( 1632 )
    CoNiCrAl alloys are widely used as bond coats for thermal barrier coating (TBC) at high operating temperature for energy conversion systems, especially turbine. However, the effect of reactive element (RE) on the oxidation behavior of CoNiCrAl alloys is still not uniform, thereby limiting the usage of the alloys. To study the effect of co–doped RE, the polished cast CoNiCrAlLa, CoNiCrAlHf and CoNiCrAlLaHf alloys were oxidized at 1100 ℃ in vacuum. Growth kinetics and adherence of oxide scales formed on above alloys were studied through comparing mass dynamics curves and tracer oxidation of the alloys. SEM, TG and XRD as well as secondary neutrals mass spectrometry (SNMS) were employed to study the microstructure and growth mechanism of the oxide layers. The results showed that a protective Al2O3 scale formed on the surface of CoNiCrAlLaHf alloy, and the Al2O3 scale showed a good adherence due to a pinning effect of the internal oxidation and less Ni(Co)Al2O4 formation. The Hf could suppress La enrichment at the oxide/alloy interface for the co–doped alloy. Oxygen inwards–diffusion along the oxide, which mainly controlled oxide scale growth, was observed when these three alloys oxidized in the atmospheres including tracer 18O. Enrichment of Cr, Co and Ni occurred at the outer part of oxide scale, and then decreased to a low level within the oxide layer. These Cr, Co and Ni enrichments could promote the Ni(Co)Al2O4 formation at the top of oxide scale for the single–doped alloys, which was detrimental to Al2O3 adherence properties.
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    MICROSTRUCTURE EVOLUTION OF LAVES PHASE Cr2Nb/Cr ALLOYS PREPARED BY ARC MELTING
    LI Kewei LI Shuangming FU Hengzhi
    Acta Metall Sin, 2011, 47 (6): 663-670.  DOI: 10.3724/SP.J.1037.2010.00700
    Abstract   PDF (3562KB) ( 1698 )
    The Cr–12%Nb (atomic fraction) and Cr–20%Nb alloys prepared by vacuum non–consumable arc melting were investigated to understand the  icrostructure evolution of the Laves phase Cr2Nb/Cr alloys. The solidified microstructures including the phase formation and competitive growth were studied using OM, XRD as well as SEM with an EDS. The results showed that different microstructure morphologies were observed in the Cr–12%Nb and Cr–20%Nb alloys. For the Cr–12%Nb alloy, divorced eutectics were grown at the bottom part of the ingot. Primary Cr dendrites and coupled eutectic (Cr2Nb+Cr) appeared in the middle and upper part of the ingot. For the Cr–20%Nb alloy, cellular eutectics of which the lamellar spacing was about 0.3 μm were found at the bottom of the ingot. With the decrease in cooling rate in the middle and upper part of the ingot, plate primary Cr2Nb dendrites and petal–like Cr2Nb dendrites were developedBesides, using the TMK model for rapid utctc solidification and the BCT dendrtc growth model, the inteface growth temperatures between the coupled eutectic (Cr2Nb+Cr) and priary Cr2Nb phase wee computed and compared with the experimental results. Based on the maximum interface growth temperature criteria, the earance of multiple solidified microstructures of the Cr–20%Nb alloy could be explaind  successfully.
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    SIMULATION OF THERMAL BEHAVIOR DURING STEEL SOLIDIFICATION IN SLAB CONTINUOUS CASTING MOLD I. Mathematical Model
    CAI Zhaozhen ZHU Miaoyong
    Acta Metall Sin, 2011, 47 (6): 671-677.  DOI: 10.3724/SP.J.1037.2010.00663
    Abstract   PDF (2637KB) ( 1735 )
    Thermal behavior of the solidifying shell in continuous casting mold is very important to final steel products. In the present work, one two–dimension transient thermal–mechanical coupling finite element model was developed to simulate the thermal behavior of steel solidifying in slab continuous casting mold by using the sequential coupling method. In this model, in order to get the physical properties of steel at high temperature, a microsegregation model which would give the relationship of phase fractions and temperature for acquiring the physical properties with δ/γ  transformation in mushy zone was established. And the heat flux was obtained according to the displacement between the surface of solidifying shell and the hot face of mold as solidification contraction, the liquid/solid structure and distribution of mold flux, the temperature distribution of slab surface and mold hot face, and air gap distribution. In addition, the rate–dependent elastic–viscoplastic constitutive equation was applied to account for the evolution of shell stress in the mold.
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    SIMULATION OF THERMAL BEHAVIOR DURING STEEL SOLIDIFICATION IN SLAB CONTINUOUS CASTING MOLD II. Model Verification and Results Analysis
    CAI Zhaozhen ZHU Miaoyong
    Acta Metall Sin, 2011, 47 (6): 678-687.  DOI: 10.3724/SP.J.1037.2010.00664
    Abstract   PDF (4184KB) ( 1735 )
    Based on the thermal–mechanical coupling finite element model described in Part I of present work, the thermal behavior of one high strength ship plate steel solidifying in slab continuous casting mold was simulated with the practical continuous casting conditions of a steel plant, and the availability of the model was verified by in–plant measurement temperature. With this model, the thickness distributions and thermal behavior of air gap, mold flux (including liquid flux and solidified flux), and the variation characteristics of heat flux and surface temperature, as well as the influence of casting speed, mold taper, and mold flux properties on thermal behavior of the steel solidifying in mold were investigated.
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    EFFECT OF Ti AND Nb MICRO–ALLOYING ON THE MICROSTRUCTURE OF THE ULTRA–PURIFIED 11%Cr FERRITE STAINLESS STEELS
    LIU Jing LUO Xinghong HU Xiaoqiang LIU Shi
    Acta Metall Sin, 2011, 47 (6): 688-696.  DOI: 10.3724/SP.J.1037.2011.00059
    Abstract   PDF (4086KB) ( 1770 )
    Ultra–purified ferrite stainless steels (UP–FSS) are widely used in the fields of automobile, household appliances etc. Much better performance than conventional ferrite stainless steels is obtained by minimizing the interstitial elements level in UP–FSS. However, some negative effects, such as degradation of the formability and ridging–resistance, are also brought by purifying the steels, which lead to abnormally growth of the columnar grains. Moreover, the grains in heat affected zone (HAZ) are apt to coarsen during welding process. One of the effective ways to resolve such problems is to increase the equiaxial grain ratio in as–cast microstructure and refine the grain size. Micro–alloying of steels with strong carbide and nitride former, such as Ti and Nb, is a way to do that. In this work, the effects of Ti and Nb micro–alloying on the as–cast, as–rolled, and HAZ microstructures of the ultrapurified 11%Cr ferrite stainless steels with different interstitial element levels were investigated by both experimental research and thermodynamic calculation. The results indicated that the effect of Ti and Nb micro–alloying was better when the content of C and N in the steel was 0.0163%, which showed the minimum grain size in as–cast, as–rolled, and HAZ microstructures, and the equiaxial grain ratio in as–cast microstructure was evidently improved as well. It was found by calculation that, with increasing of C and N content and addition of Ti and Nb to the steels, the solid–liquid two–phase region was broadened. This was beneficial to increase the undercooling in front of the liquid–solid interface and the probability of heterogeneous nucleation, and consequently, increase the equiaxial grain ratio and decrease te mean grain size in as–cast microstructure. On the other hand, with addition of Ti and increase of N conent, TiN type particles might precipitate in olid–liquid two–phase regio, which was helpful for promoting the heterogeneous nucleation of δ ferrite.
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    EFFECT OF HYDROSTATIC PRESSURE ON THE CORROSION BEHAVIORS OF TWO LOW ALLOY STEELS
    LIU Jie LI Xiangbo WANG Jia
    Acta Metall Sin, 2011, 47 (6): 697-705.  DOI: 10.3724/SP.J.1037.2010.00705
    Abstract   PDF (2693KB) ( 1390 )
    In this paper, the effect of hydrostatic pressure on the corrosion behaviors of two steels was investigated by weight loss test, electrochemical measurements, rust layer analysis and morphology observation in natural seawater environment with high–pressure in lab. The results showed that X steel was inclined to being corroded. With the increase of hydrostatic pressure, the corrosion resistance of two steels were deteriorated, which were attributable to the increase of the anodic reaction rate. Hydrostatic pressure had little effect on oxygen diffusion reductin process of X steel. Howeve, the cathodic current density of Y steel under higher hydrostatic pressure decreased because the coosion products were directly involved in cathodic reduction reaction. And with the increase of hydrostatic pressure, the morphology of two steels changed significantly and dfferently. Under high pressure, some shallo–dish shape localized corrosion appeared on the surface of X steel whereas on the surface of Y steel some tunnel localized crrsion appeared.
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    NUMERICAL SIMULATION OF KEYHOLE SHAPE AND TRANSFORMATION FROM PARTIAL TO OPEN STATES IN PLASMA ARC WELDING
    HUO Yushuang, WU Chuansong, CHEN Maoai
    Acta Metall Sin, 2011, 47 (6): 706-712.  DOI: 10.3724/SP.J.1037.2011.00012
    Abstract   PDF (1265KB) ( 1217 )
    It is of great significance to develop a mathematical model of keyhole shape and dimension in order to widen the process parameter window and improve the process stability in keyhole plasma arc welding (PAW). In this study, a keyhole model was developed according to the force–balance conditions on the keyhole wall. The establishing process of quasi–steady state keyhole was numerically simulated for stainless steel plates of 6 mm thickness, and the keyhole shapes and dimensions were obtained under different welding process parameters. The transformation mechanism of the keyhole from blind (partial) to open (complete) states in PAW process was analyzed based on the calculated action forces on the keyhole wall. The values of action forces at different locations on the keyhole wall were calculated. With increasing of welding current, the keyhole depth rised in a nonlinear way. There existed a critical value of welding current, i.e., if welding current was a little bit hgher than this value, the keyhole inside the weld pool would suddenly transform from partial state (blind keyhole) ino complete state (open keyhole). The fast centralization of the plasma arc force at the keyhole bottom region resulted in the sudden transformation from a partial keyhole to an open keyhole. The keyhole PAW experiments were conducted to validate the numerical analyss results.
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    EFFECT OF HIGH MAGNETIC FIELD ON PRECIPITATION BEHAVIORS AND MECHANICAL PROPERTIES IN REDUCED ACTIVATION STEELS
    XIA Zhixin ZHANG Chi YANG Zhigang
    Acta Metall Sin, 2011, 47 (6): 713-719.  DOI: 10.3724/SP.J.1037.2011.00035
    Abstract   PDF (2642KB) ( 1214 )
    The long–term exposition of reduced activation steels under high temperature and high magnetic field leads to the microstructural changes. And the microstructure evolution will damage the safety of fusion reactors. This work investigated the influence of high magnetic field on precipitation behavior and mechanical properties in reduced activation steels. As–quenched steels were tempered at 923 K for 3 h with and without a 10 T magnetic field. Tensile strength of the specimens tempered with a 10 T magnetic field decreased in comparison with the specimens tempered without magnetic field. The precipitation behaviors in reduced activation steels were also studied. The results indicated that the applied field could effectively prevent the directional growth of rod–shaped M23C6(M=Cr, W and Fe) carbides along martensite packet boundaries. The aspect ratio of M23C6 carbides decreased due to the increasing of the carbide/ferrite interfacial energy under the high magnetic field. Application of the Laner–Schwartz theoy to model metal carbide precipitation behavior under the magnetic field was described. The results indicated that the density of precipitates decreased and its mean size increased owing to an increase of the precipitate/ferrite interfacial energy. The model could predict the coarsening process of precipitates in reduced activation steels. Moreover, an improvement of the formula between yield strength and mean size of precipitates was also made.
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    STUDY ON THE MARTENSITE IN LOW CARBON CrNi3Si2MoV STEEL TREATED BY Q&P PROCESS
    WANG Cunyu SHI Jie CAO Wenquan HUI Weijun WANG Maoqiu DONG Han
    Acta Metall Sin, 2011, 47 (6): 720-726.  DOI: 10.3724/SP.J.1037.2010.00695
    Abstract   PDF (3218KB) ( 1638 )
    The martensite in low carbon CrNi3Si2MoV steel treated by Q&P (quenching and partitioning) process was characterized by means of SEM, TEM, EBSD and nano intender. The effect of martensite on uniaxial tension behaviors was discussed. The results showed that initial martensite phase formed at the first quenching step, whose carbon content was lowered due to its carbon diffusion into untransformed austenite during partitioning step. However, the martensite phase formed at the final quenching steconsisted of only one single set of packet with lath thicknes about 0.1—0.2 μm, which was thinner than that of the initial martensie lath. It was found that the carbon content and hardness of the martenite formed in the final quenching step were higher than thoe of initial martensite, which deformed cooperatively with other phases and played a role of strengthening phase during deformation process. In addition, large sized carbonitride and oxide pecipitations induced nucleation and exansion of crack during deformation rocess.
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    HORIZONTAL ELECTROREFINING OF Al IN Na3AlF6–LiF MOLTEN SALT
    JIA Ming TIAN Zhongliang LAI Yanqing LIU Fangyang LI Jie LIU Yexiang
    Acta Metall Sin, 2011, 47 (6): 727-734.  DOI: 10.3724/SP.J.1037.2011.00028
    Abstract   PDF (2434KB) ( 1217 )
    A creative and simple horizontal electrorefining method had been developed for high pure aluminum production in fluoride molten salt. Impure primary aluminum was purified due to the different electronegativity between Al and impurities during electrorefining. Pre–electrolysis was effective in the purification of electrolyte, concentration of impurities such as Cu, Si and P were reduced to 3.2×10−6, 14×10−6 and 1.5×10−6, respectively. Aluminum electrorefining could run steady with current density as high as 800 mA/cm2, hich indicated that the uneven current density istribution had no bad effect on the electrochemical process. The anode nd cathode were both covered by Na3AlF6–LiF electrolyte, hich argely reduced the oxidation of aluminum at hih temperature and yieded a igh curent efficiency of 98.6%. The analysis of the anode feed and refined aluminum showed a remarkabe reduction of the mass fraction of Cu, from 14.5×10−6 to 0.9×10−6. Besides, particular mention should be made of efficient removaof impuritis likF, Si and Zn, the purity of electrorefined aluminum was above 99.99%.
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    CORRELATION BETWEEN COMPOSITION OF REINFORCING STEEL SURFACE FILM AND STEEL CORROSION BEHAVIOR IN SIMULATED CONCRETE
    PORE SOLUTIONS
    CHEN Wen DU Ronggui HU Ronggang SHI Haiyan ZHU Yanfeng LIN Changjian
    Acta Metall Sin, 2011, 47 (6): 735-742.  DOI: 10.3724/SP.J.1037.2011.00057
    Abstract   PDF (565KB) ( 998 )
    It is well known that reinforcing steel in concrete can be protected from corrosion by forming a compact passive film on its surface in a concrete pore solution with high alkalinity. The corrosion behavior of the steel is related to its passivation and depassivation. The pH value and the Cl− concentration in a concrete pore solution are of the most important parameters affecting the passivity of reinforcing steel in concrete. In the present work, XPS analysis, linear polarization and potentiodynamic anodic polarization tests were used to study the relationship between the electrochemical corrosion behavior of reinforcing steel immersed in a simulated concrete pore solution and the chemical composition of the steel surface film. The results showed ht with the increase of the Cl− concentration or the decrease of the pH value in the solution, the corrosion potential of the steel shifted negatively and its corrosion current density rose, and the Fe2+ content of the steel surface film increased and the Fe3+ content decreased. The steel was in an active state in the solution with Cl− concentration higher than 0.6 mol/L or pH lower than 11.31, and the addition of 0.24 mol/L NaNO2 to the solution could protect steel from corrosion.
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    EFFECT OF AGING TREATMENT AND La CONTENT ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF AZ91 ALLOY
    WANG Shebin QI Xiaoye ZHANG Jinling LIU Lu XU Bingshe
    Acta Metall Sin, 2011, 47 (6): 743-750.  DOI: 10.3724/SP.J.1037.2010.00560
    Abstract   PDF (2923KB) ( 1792 )
    The effects of aging treatment and La content on the microstructures and mechanical properties of AZ91 magnesium alloys were investigated by XRD, SEM, EDS and microhardness meter. The results showed that a great number of β–phases were precipitated and distributed along the grain boundaries after aging treatment. As the content of La increased, the grain size of β–phase decreased gradually, while α–phase decreased first and then increased. The mechanical properties of as–cast and aged specimens increased first and then decreased as the cotet of La increased. The tensile strength, yield strength, elongatin and Vickers hardness of aged magesium alloys were increased by 12.65%, 16.85%, 13.71% and 37.24%, espectively. When the content of La was 0.1648%, the tensile strength, yield strength, elongation and Vickers hardness achieved 276 MPa, 208 MPa, 13.85% and 132 HV, respectively.
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    EFFECTS OF THERMAL AGING ON MICRO–MECHANICAL PROPERTIES AND IMPACT FRACTURE BEHAVIOR OF Z3CN20–09M STAINLESS STEELS
    LI Shilei WANG Xitao WANG Yanli LI Shuxiao
    Acta Metall Sin, 2011, 47 (6): 751-756.  DOI: 10.3724/SP.J.1037.2011.00023
    Abstract   PDF (1388KB) ( 1402 )
    Cast austenite stainless steels (CASS) were used in the primary circuit piping of pressurized water nuclear reactors (PWRs), because of their excellent strength, corrosion resistance and good weldability. However, after long–term service at mid–temperature, CASS would suffer a loss of toughness and Charpy impact energy due to thermal aging. The micro–mechanical properties and impact fracture behavior of Z3CN20–09M stainless steel after long–term thermal aging at intermediate temperature were studied in this paper. A nano–indenter was used to study the changes of mechanical properties in the ferrite phases during the aging process. The impact behavior of the aged material was investigated by an instrumented impact tester, and the impact fractures were observed with SEM. The results indicated that long–term thermal aging caused the declining plastic deformation ability of the ferrite phases and the decrease of the impact toughness. The loss of impact energy in the aging process was mainly due to the reduction of stable crack propagation energy. The impact fracture morphology chnged from ductile dimple in the initial stage into a mixture of cleavage in ferrite alonwith tearing in austenite in the later stage. In the impact fracture process, cracks initiated firstly in the errite phases and fast propagated under impact loading, and then the ferrite phases fractured along the cleavage planes. At last, the cracks extended to the austenite phases and the cracks connected through the specimen.
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    MECHANICAL PROPERTIES OF SUPER–HIGH STRENGTH Al ALLOY 7A04 AT TRANSIENT HEATING
    WU Dafang PAN Bing WANG Yuewu ZHAO Shougen YANG Hongyuan HUANG Liang
    Acta Metall Sin, 2011, 47 (6): 757-762.  DOI: 10.3724/SP.J.1037.2010.00660
    Abstract   PDF (1661KB) ( 1136 )
    Super–hard aluminum alloy 7A04 (Al–Zn–Mg–Cu), whose ultimate strength is higher than that of duralumin, belongs to a class of aluminum alloys with highest ultimate strength tested at room temperature. As it can be utilized as structural material of various heating components such as rocket liquid storage tank and missile wing, super–hard aluminum alloy 7A04 has been widely used in the field of aerospace engineering. However, the ultimate strength and other token mechanical parameters of aluminum alloy 7A04 at transient high–temperature heating environment are still unclear to us, as these key mechanical parameters are lacking in existing strength design handbook. Experimental characterization of these critical parameters of aluminum alloy 7A04 is undoubtedly meaningful to the reliability estimation, life prediction and security design of the high–speed flight vehicle. In this paper, by combining transient aerodynamic heating simulation system and material testing machine, the high–temperature ultimate strength, loading time and oher mechanical properties of super–hard aluminum alloy 7A04 undedifferent transient heating temperature and loading conditions were investigated Experimental results revealed that the ultimate strength and loading capability of aluminum alloy 7A04 subjected to transient thermal heating were much higher than those teted in a long–time stable high–temperature environmen. The results provided substantial basis fr the loading capability improvement nd optmal design of erospace materials and stuctures subject to transient heating.
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    PHYSICAL SIMULATION OF Ar BUBBLE BEHAVIOR IN THE SOLID/LIQUID INTERFACE OF CONTINUOUS CASTING BILLET
    JIN Xiaoli LEI Zuosheng YU Zhan ZHANG Haobin DENG Kang REN Zhongming
    Acta Metall Sin, 2011, 47 (6): 763-768.  DOI: 10.3724/SP.J.1037.2010.00690
    Abstract   PDF (2990KB) ( 1419 )
    Ar gas was injected to the nozzle during continuous casting to prevent from nozzle clogging caused by solid inclusions, however, some gas bubbles were captured by the initial solidified billet shell, and subsequently billet defects were formed in final product. In present work, a physical model applying mold simulator was designed and used to investigate the bubble behavior in the solid/liquid interface. Different mold simulator angels and bubble diameters were considered, three typical behaviors were observed, some bubbles were captured by the solidified shell, several bubbles coalescence in the solid/liquid interface, and other bubbles stayed in the interface for a moment, and then left. The possible mechaism of gas bubble entrapped in solidified hell was anayzed. Besides, industial billets were studied, many dendites wee found around the hole, it was consdered to be a key factor for gas bubble engulfed in the solidification billets in the meniscus area. Further morethe effective technologes, such as electrical brake near the submerged entry nozzle and electrical stirring in the mold narrow face, can improve billet quality and reduce gas bubbles in solidified shewere imposed
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