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CN 21-1139/TG
Started in 1956

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    REVIEW OF THE INVESTMENT CASTING OF TiAl-BASED INTERMETALLIC ALLOYS
    CHEN Yuyong, JIA Yi, XIAO Shulong, TIAN Jing, XU Lijuan
    Acta Metall Sin, 2013, 49 (11): 1281-1285.  DOI: 10.3724/SP.J.1037.2013.00536
    Abstract   PDF (797KB) ( 1232 )

    TiAl-based intermetallic alloys are emerging as a new generation light—weight, high temperature structural materials and possess wide capacities of engineering applications in aeronautics, space and automobile industries. Near—net—shape casting is a commonly used manufacturing method for TiAl—based alloys. In this paper, the evolution of the alloy design and investment casting technology of TiAl—based alloys are reviewed. The prospects of the investment casting of TiAl—based alloys are also represented.

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    RECENT ADVANCES ON MICROSTRUCTURAL CONTROLLING AND HOT FORMING OF Ti2AlNb-BASED ALLOYS
    SHEN Jun, FENG Aihan
    Acta Metall Sin, 2013, 49 (11): 1286-1294.  DOI: 10.3724/SP.J.1037.2013.00607
    Abstract   PDF (1682KB) ( 1042 )

    Intermetallic alloys based on an ordered orthorhombic phase Ti2AlNb,hereafter referred to as the Ti2AlNb-based alloys, are continually developing as attractive materials for aerospace applications due to their desirable properties like a unique combination of strength and ductility, good creep resistance, and low densities. This review article is thus aimed at summarizing recent advances on the microstructural evolution and mechanical properties of the Ti2AlNb-based alloys. Particular attention is paid to their crystal structure, phase transformations, microstructure and hot forming processing.

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    PHASE SELECTION AND THE SOLIDIFICATION CHARACTERISTICS OF TiAl BASE ALLOYS IN THE NONEQUILIBRIUM SOLIDIFICATION
    HU Rui, LIU Yi, ZHANG Tiebang, KOU Hongchao, LI Jinshan
    Acta Metall Sin, 2013, 49 (11): 1295-1302.  DOI: 10.3724/SP.J.1037.2013.00441
    Abstract   PDF (3386KB) ( 866 )

    The Al content and the β stabilizers in TiAl base alloys were investigated,which have an important effect on the solidification characteristics and phase selection.TiAl base alloys with single β-solidifying have the characteristics of low Al content and narrow crystallization temperature range. It is attributed to forming a homogeneous fine-grained microstructure. Dendrite morphology in TiAl base alloys is investigated by SEM-BSE in the nonequilibrium solidification conditions. The relationships between undercooling and nucleation of two phases are investigated systematically by using the classical nucleation theory in the undercooled Ti48Al2Cr2Nb (atomic fraction, %) peritectic alloy. By calculating the interfacial critical nucleation work and steady state nucleation rates of β phase (bcc) and α phase (hcp), it is obtained that β phase is always prior to nucleate from the Ti48Al2Cr2Nb undercooled melts at the cooling rate about 15 K/s in the achieved undercooling range. Due to narrow crystallization temperature range, microstructures of TiAl base alloys with high Al content as the hyper-peritectic solidification path take on the gradual solidification style and finer dendrite morphology. It is conducive to reducing and eliminating shrinkage porosity and hot cracking caused by mushy solidification. The β stabilizers have an important effect on solidification path depending on its Al equivalent and will change the solidification path of TiAl alloy from hyper--peritectic solidification to hypo-peritectic solidification. The overmuch Al content in TiAl base alloys leads to the formation of large number of massive γ phase impacting on strength and ductility of alloys.

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    OXIDATION BEHAVIOR OF IN SITU SYNTHESIZED MoSi2-SiC COMPOSITES AT 500℃
    ZHANG Laiqi, PAN Kunming, DUAN Lihui, LIN Junpin
    Acta Metall Sin, 2013, 49 (11): 1303-1310.  DOI: 10.3724/SP.J.1037.2013.00527
    Abstract   PDF (4936KB) ( 1138 )

    The oxidation behavior for in situ synthesized MoSi2-SiC composites with different SiC volume fraction at 500℃ was investigated. The results of oxidation for 1000 h indicate that the oxidation resistance for composites is significantly higher than that of monolithic MoSi2, MoSi2-30%SiC composite in situ synthesized possesses higher oxidation resistance compared with the traditional composite which is fabricated by hot-pressing the mixture of commercial powders of MoSi2 and SiC. After oxidation for 1000 h, the composites have not been observed to disintegrate (pest). The oxidation kinetics curves are divided into three stages: incubation period, rapid oxidation period and the steady-state. The oxide scale consists of MoO3, amorphous silica and β-SiC, therefore, the oxidation of the materials is mainly done between MoSi2 and O2, and SiC is not oxidized. The surface morphology observations of samples oxidized for 5, 10 and 20 h show that the phase boundary is the preferential oxidation site, the oxides formed at the initial stage of oxidation are irregular-shaped, and MoO3 whisker and oxide cluster mainly composed of amorphous SiO2 come gradually into being along with the development of oxidization. It is also found that the nucleation and growth of MoO3 whisker preferentially take place at the concave and convex sites of substrate surfaces.

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    HIGH TEMPERATURE CREEP AND FATIGUE BEHAVIOR AND LIFE PREDICTION METHOD  OF A TiAl ALLOY
    YU Huichen, DONG Chengli, JIAO Zehui, KONG Fantao, CHEN Yuyong, SU Yongjun
    Acta Metall Sin, 2013, 49 (11): 1311-1317.  DOI: 10.3724/SP.J.1037.2013.00434
    Abstract   PDF (804KB) ( 677 )

    Creep (at 700℃) and low cycle fatigue (at 700 and 750℃) tests of Ti-43Al-9V-Y alloy with duplex (DP) and fully lamellar (FL) microstructures are carried out to study the creep, fatigue deformation and life prediction. Firstly, Omega method is employed to characterize the creep deformation and to predict the rupture life. Secondly, the proposed fatigue life model based on walker strain is employed to predict the fatigue life for these two types of TiAl alloy. The results show that: (1) high temperature creep curves of the material of both DP and FL microstructure contain steady and accelerated creep stages other than initial creep stage, and Omega method is able to characterize the creep deformation of TiAl alloy with DP and FL microstructure; (2) the rupture time of the material of FL microstructure is longer than that of DP microstructure, and the predicted rupture time by Omega method agrees well with the test data; (3) the fatigue life of DP is longer than that of FL under the same test condition, and the predicted fatigue life is well located in the scatter band of 3 of the test fatigue life.

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    MICROSTRUCTURE AND MECHANICAL PROPERTIES OF LAVES PHASE STRENGTHENING NiAl BASE COMPOSITE FABRICATED BY RAPID SOLIDIFICATION
    SHENG Liyuan, ZHANG Wei, LAI Chen, GUO Jianting,XI Tingfei, YE Hengqiang
    Acta Metall Sin, 2013, 49 (11): 1318-1324.  DOI: 10.3724/SP.J.1037.2013.00413
    Abstract   PDF (3924KB) ( 724 )

    The Laves phase strengthening NiAl base composite was fabricated by conventionally casting and rapid solidification, and their microstructure and mechanical properties were investigated together. The results exhibit that the Laves phase in the conventional cast alloy is relative coarse and distributes along the NiAl phase boundary. Moreover, small stick-like Laves phase precipitates in the NiAl phase. Due to the segregation of Ni and Al in Laves phase, it still keeps the C14 crystal structure. The rapid solidification refines the NiAl and Laves phase greatly and promotes the formation of NiAl/Laves phase eutectic structure, which surrounds the NiAl phase and forms the cell-like structure. However, the rapid solidification can not handicap the precipitation of needle-like Laves phase in the NiAl phase. But, the rapid solidification restrains the formation ofα-Cr phase. The compression tests show that the cell-like Laves phase strengthening NiAl base composite has better mechanical properties at ambient and elevated temperature, compared with the conventional cast alloy. The improvement of mechanical properties should be attributed to the cell-like Laves phase and the nanocrystallization of the cell-like Laves phase during high-temperature deformation.

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    MICROSTRUCTURE, MECHANICAL, FRICTION AND WEAR PROPERTIES OF NiAl-2.5Ta-7.5Cr-1B ALLOY
    WANG Zhensheng, ZHANG Meng'en, YANG Shuangshuang, GUO Jianting, ZHOU Lanzhang, CHEN Zhigang
    Acta Metall Sin, 2013, 49 (11): 1325-1332.  DOI: 10.3724/SP.J.1037.2013.00452
    Abstract   PDF (4108KB) ( 1150 )

      The microstructure and properties of NiAl-2.5Ta-7.5Cr-1B alloy are investigated in this work. The microstructure is analyzed by SEM and XRD, the mechanical properties are conducted by a universal mechanical testing machine and the friction and wear features from room temperature to 600℃ are mainly studied using an elevated temperature friction and wear machine. The experimental results show that the NiAl-2.5Ta-7.5Cr-1B alloy consists of NiAl, eutectic (NiAl-Cr), Cr2Ta, (Ta, Cr)3B2 and TaB. The addition of B into the NiAl-2.5Ta-7.5Cr alloy improves the plasticity by 50%. The NiAl-2.5Ta-7.5Cr-1B alloy exhibits excellent self-lubricating and wear-resistant properties from room temperature to 600℃ which could be mainly attributed to the co-existence of B2O3 liquid phase and Al4B2O9 solid particles on the worn surface. The B2O3 liquid phase makes the friction and wear process conform to the mechanism of hydrodynamic lubrication and the Al4B2O9 solid particles have a load-bearing function.

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    APPLICATION OF ARTIFICIAL NEURAL NETWORK FOR PREDICTION OF SUPERPLASTIC  BEHAVIOUR IN NiAl ALLOYS
    HOU Jieshan, ZHOU Lanzhang, GUO Jianting, YUAN Chao
    Acta Metall Sin, 2013, 49 (11): 1333-1338.  DOI: 10.3724/SP.J.1037.2013.00455
    Abstract   PDF (2216KB) ( 649 )

    Chemical composition, grain size, and processing conditions such as temperature and strain rate have important influence on superplasticity of NiAl alloys, which would allow the optimization of these parameters in order to achieve the desired combination of properties. In this work, the optimal superplastic deformation conditions of NiAl alloys were studied by using artificial neural networks (ANN). The standard multilayer feedforward networks were trained and tested using comprehensive datasets from previous experimentally works on the as-extruded NiAl, NiAl-25Cr, NiAl-20Fe-Y(Ce), NiAl-30Fe-Y, NiAl-9Mo, NiAl-P alloys.Different effects are modeled, including the influence of the alloying elements on the superplastic, and the influence of deformation temperature, strain rate and grain size on the elongations during the superplastic tensile tests. The artificial neural network models are combined with computer programmers for optimization of the inputs in order to achieve desirable combination of outputs. Good performances of the neural networks  are achieved. Results of this research propose a range of strain rate and temperature within which the NiAl alloy possesses superplasticity with larger elongations, although the deformation temperature and strain rate of superplastic alloys alternately influence each other within the range. These models are convenient and powerful tools for practical applications in superplastic prediction in NiAl alloys.

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    DISCONTINUOUS YIELDING BEHAVIOR OF β PHASE CONTAINING TiAl ALLOY DURING HIGHTEMPERATURE DEFORMATION PROCESS
    XU Wenchen, SHAN Debin, ZHANG Hao
    Acta Metall Sin, 2013, 49 (11): 1339-1346.  DOI: 10.3724/SP.J.1037.2013.00470
    Abstract   PDF (3030KB) ( 926 )

    γ-TiAl base alloys are promising high-temperature materials for aviation and aerospace applications due to their low density, exceptional high-temperature strength and good oxidation resistance. However, low ductility and poor hot workability limit the use of such alloys. The introduction of β phase appears to be effective to improve the hot workability of TiAl alloys, while the influence of β phase on hot deformation behavior of TiAl alloy has been rarely investigated until now. In this work, high-temperature compression experiments of β phase containing TiAl alloy (Ti-42Al-9V-0.3Y) were conducted on a Gleeble-1500 thermal simulation machine at 1000—1200℃ and strain rates of 0.001—1.0 s-1. The hot deformation behavior of the TiAl alloy was investigated and the discontinuous yielding mechanism was analyzed. The results show that the main deformation softening mechanism was the dynamic recovery (DRV) of β phase and dynamic recrystallization (DRX) of γ phase. The discontinuous yielding behavior was closely related to the DRV in β phase and the multiplication of the superdislocation with Burgers vector $\bm b=1/2〈112〉 in γ phase. The established dislocation dynamics model based on the Orowan equationin the present work could reasonably explain the causes for the discontinuous yielding phenomenon, indicating that the rapid increase of mobile dislocation density and small dislocation motion velocity sensitivity m* could induce the discontinuous yielding of the TiAl alloy. In addition, the fluctuating yielding behavior was attributed to the interaction effect of dislocation slip and twin at lower temperatures of 1100—-1150℃ and higher strain rate of 1 s-1.

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    CHARACTERISTICS OF A NiAl EUTECTIC ALLOY JJ—3 WITH EXCELLENT PROPERTIES
    YUAN Chao, ZHOU Lanzhang, LI Gusong, GUO Jianting
    Acta Metall Sin, 2013, 49 (11): 1347-1355.  DOI: 10.3724/SP.J.1037.2013.00472
    Abstract   PDF (3070KB) ( 889 )

    A new NiAl eutectic alloy JJ—3 with excellent comprehensive properties had been successfully developed in China for the potential vanes materials. In this paper, the study results on alloy JJ—3, including the composition, microstructure characteristics, physical, mechanical and chemical properties, and process technologies have been introduced briefly. On the basis of maintaining the low density, the higher melting point and thermal conductivity, the excellent oxidation resistance being peculiar to NiAl alloy, alloy JJ—3 have good high temperature stress—rupture strength, which are almost as the same as those of the advanced directionally solidified superalloys. Meanwhile, alloy JJ—3 possess good weldability and castability, and had already been cast the vanes with the complicated shape. With the excellent comprehensive properties, alloy JJ—3 is very close to practical engineering application, only if the poor ductility at room temperature must be improved in the future.

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    MICROSTRUCTURE EVOLUTION OF Ti44Al6Nb ALLOY DIRECTIONALLY SOLIDIFIED WITH COLD CRUCIBLE
    CHEN Ruirun, WANG Jichao, MA Tengfei,GUO Jingjie, DING Hongsheng,SU Yanqing, FU Hengzhi
    Acta Metall Sin, 2013, 49 (11): 1356-1362.  DOI: 10.3724/SP.J.1037.2013.00550
    Abstract   PDF (3637KB) ( 633 )

    Ti44Al6Nb (atomic fraction, %) ingots with industry size were directionally solidified without contamination with electromagnetic cold crucible. Effects of pulling rates and powers on the surface quality, the S/L interface, grain morphology and phase selection were studied. The results show that the surface quality are improved by decreasing pulling rate or increasing power, they influence the surface quality by changing the superheat degree of the melt or the volume of the mushy zone. The S/L interface becomes concave and the grain width becomes big with increasing of pulling rate, the columnar grains grow discontinuously when the pulling rate is increased. The grain width decreases with the increase of the power. Columnar grain to equiaxed grain transition (CET) is easy to occurred when the pulling rate is lower or the power is higher. At the beginning of solidification, the initial phase is αphase when the pulling rate is 0.5 mm/min, whereas, it is β phase when the pulling rates are others.

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    THE HOT DEFORMATION BEHAVIOR OF Ti-43Al-9V-Y ALLOY
    KONG Fantao, CUI Ning, CHEN Yuyong, XIONG Ningning
    Acta Metall Sin, 2013, 49 (11): 1363-1368.  DOI: 10.3724/SP.J.1037.2013.00513
    Abstract   PDF (1628KB) ( 678 )

    The hot deformation behavior of as-cast Ti-43Al-9V-Y alloy has been studied using thermo-simulator system Gleeble-1500D in the temperature range 1100-1225℃ and strain rate range 0.01-1 s-1. It was found that the flow behavior of Ti-43Al-9V-Y alloy depends on the deformation temperature and strain rate. The flow stress-strain curves exhibit oscillations in the stage of flow softening. According to principles of dynamic materials modeling (DMM), processing maps were constructed on the basis of flow stress data. Wedge cracking was expected to be observed in the region of high strain rates with low peak efficiency (<40%). The α+β+γ→β+γ phase transformation occurring at 1150℃ and 0.01 s-1 contributes to the peak efficiency of around 60 %. The dynamic recrystallization occurred at 1200-1225℃ and 0.01-0.05 s-1 with a peak efficiency of around 80%, which is the optimum condition for the hot deformation of Ti-43Al-9V-Y alloy. Combining with the titanium and aluminum phase graphs, it could be found that 1225℃ located at the single-phase region ofα phase. The excellent hot deformability of disorderedαphase is considered to be the main reason for the improvement of hot workability of TiAl alloys. As-forged alloys have good room-temperature ductility and fracture toughness simultaneously. In addition, it has been found that B2 phase could hinder the propagation of crack without decreasing the plasticity at room temperature.

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    STUDY ON CYCLIC OXIDATION RESISTANCE OF  HIGH NIOBIUM CONTAINING TiAl BASE ALLOY WITH ERBIUM
    GONG Ziqi, CHEN Ziyong, CHAI Lihua, XIANG Zhilei, NIE Zuoren
    Acta Metall Sin, 2013, 49 (11): 1369-1373.  DOI: 10.3724/SP.J.1037.2013.00394
    Abstract   PDF (1130KB) ( 636 )

    Ti-46Al-8Nb and Ti-46Al-8Nb-0.1Er alloys were prepared by the cold crucible induction levitation melting method. The oxidation behavior experiment was done in air at 900℃ up to one hundred times. Based on the oxidation kinetic analysis and the phase constitution, microstructure and the interface of the oxidation film and the matrix were investigated by means of XRD and SEM equipped with EDS, the cyclic oxidation resistance of TiAl based alloys was explored. The results showed that no spallation of layer occurs in both alloys, and Ti-46Al-8Nb-0.1Er comparing with Ti-46Al-8Nb exhibits more excellent oxidation resistance, oxides formed on the surface consist of mainly Al2O3, the continuous compact oxidation film with a good combination with the matrix significantly decreases the oxidation rate, the mass gain and the oxide film thickness. The addition Er purifies the alloy matrix and prevents the inward diffusion of oxygen atoms, thereby improves the cyclic oxidation resistance of TiAl based alloy.

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    EFFECTS OF GROWTH RATE ON PRIMARY PHASE AND MICROSTRUCTURES OF DIRECTIONALLY SOLIDIFIED Ti-46Al-2Cr-2Nb ALLOY
    ZHANG Yuan, LIU Guohuai, LI Xinzhong, CHEN Ruirun, SU Yanqing, GUO Jingjie, FU Hengzhi
    Acta Metall Sin, 2013, 49 (11): 1374-1380.  DOI: 10.3724/SP.J.1037.2013.00398
    Abstract   PDF (1592KB) ( 571 )

    TiAl alloys with fully lamellar structure has been intensively studied for excellent fracture toughness and creep properties. Directional solidification is an effective way to control the lamellar structure. Thus it is important to investigate the effect of solidification parameters on the structure of TiAl alloys during directional solidification. In this work, microstructures of directionally solidifiedTi-46Al-2Cr-2Nb (atomic fraction, %) alloy has been investigated throughBridgman-type directional solidification experiment at constant temperaturegradient and wide range of growth rates (v=10-120 μm/s). The type of primary phase,the primary dendritic arm spacing (λ1), the secondary dendritic arm spacing (λ2) and the lamellar spacing (λe) are investigated. It is found that: the primary phase of the specimenstransformed from β phase to α phase with the increase of growth rates; the transformation trend of primary phases with growth rates can be predicted by the phase selection model based on the highest interface temperature criterion. λ1, λ2, andλe all decrease with the increasing of v. The relationship between λ1 and v is not affected by the transformation of primary phases, which follows λ1=700.6v-0.24 relationship. However the relationship between λ2 and v is associated with the type of primary phases, λ2 and v followsλ2=44.0v-0.10 relationship when β is the primary phases, while follows λ2=57.3v-0.23 relationship when the primary phase has transformed to α phase. The relationship between λe and v can be expressed by λe=16.4v-0.76.Compared with TiAl binary alloys, lamellar spacing is more effectively refined by increasing the growth rates in Ti-46Al-2Cr-2Nb alloy.

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    EFFECT OF OXYGEN ON MICROSTRUCTURE AND PHASE TRANSFORMATION OF HIGH Nb CONTAINING TiAl ALLOYS
    WU Zeen, HU Rui, ZHANG Tiebang, ZHOU Huan, KOU Hongchao, LI Jinshan
    Acta Metall Sin, 2013, 49 (11): 1381-1386.  DOI: 10.3724/SP.J.1037.2013.00401
    Abstract   PDF (1490KB) ( 801 )

    Due to the low density, high specific strength, elastic modulus and oxidation resistance at high temperature, TiAl-based alloys have attracted much attention as a candidate of the next generation high temperature materials in aerospace and automobile application. Meanwhile, the excellent properties oxidation resistance, creep strength and tensile strength at the elevated temperature make the high Nb containing TiAl alloys be one of the promising development directions of future TiAl alloys. During the studies about alloying which is an efficient way to improve the performance of TiAl alloys, researchers have found that interstitial atoms B, C and N notably refine the grains and then improve mechanical properties including yield strength, micro-hardness, and tensile ductility of TiAl alloys. During the melting, casting, forging and the application environment, the TiAl alloys also are always inevitable to be contaminated by the O. In this work, the high Nb containing Ti-46Al-8Nb-xO alloys (atomic fraction) were prepared by non-consumable vacuum arc remelting under the protection of Ar atmosphere. The aim of the present work is to study the influence and the corresponding mechanism of oxygen atoms on the microstructure evolution and phase transformation of high Nb-TiAl alloys. The results indicate that oxygen atoms in Ti-46Al-8Nb-xO alloys remarkably increase the amount of α2 phase. The increasing oxygen content leads to the grain refinement. Meanwhile, the duplex microstructures translate into fully lamellar. It indicates that the interstitial oxygen essentially reduces the kinetics of α→γ. Consequently, the fully lamellar is easier formation than the duplex microstructures. It is found that interstitial oxygen atoms preserve significantly influence on the microstructure of Ti-46Al-8Nb-xO alloys. With the increase of oxygen content, the β solidification translates into αsolidification and the peritectic reaction α+L→γ moves to a lower Al content. At the same time, the eutectoid reactionα→α2+β shifts to a higher Al content which extends the area of α $ phase. The DSC results show the effect of oxygen on the phase transformation of Ti-46Al-8Nb. The DSC curves indicate that the addition of oxygen increases the eutectoid reaction temperature of Ti-46Al-8Nb-xO alloys, but this effect can be gradually reduced with the further increase of oxygen content.

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    FIRST-PRINCIPLES STUDY OF EFFECT OF POINT DEFECT ON ADSORPTION AND DIFFUSION OF OXYGEN AT γ-TiAl (100) SURFACE
    ZHOU Liying, WANG Fuhe
    Acta Metall Sin, 2013, 49 (11): 1387-1391.  DOI: 10.3724/SP.J.1037.2013.00408
    Abstract   PDF (711KB) ( 488 )

    γTiAl intermetallics have attracted much interest in the last decade as prospective structural materials for aerospace applications, since they maintain a large number of outstanding properties, such as high melting point, low density and high-temperature strength. However, the oxidation resistance of TiAl alloys is inadequate at high temperature, hindering their practical applications. In order to improve the oxidation resistance, an understanding of the growth mechanisms of these surface oxides is of great importance. The adsorption and diffusion of oxygen atom at the surface are the important processes in the oxidation. In this work, the effects of point defects on the adsorption and diffusion of oxygen at γ-TiAl (100) surface are studied by the means of first-principles calculations. The calculated results of the formation energies of the point defects show that in the doped γ-TiAl (100), the alloying Si atom prefers occupying the first surface layer Al site, while W prefers to occupy the second sublayer Ti site at theγ-TiAl (100) surface, and both of them increase the absorption energies of oxygen atom, which is the near neighbour of them. It indicates that Si prefers to segregate at the first surface layer and W prefers to segregate at the second sublayer, and both of them can suppress the adsorption of oxygen atoms at the γ-TiAl (100) surface. In the defect of vacancy system, the Ti vacancy is easier to be formed than Al vacancy on the first surface layer. Furthermore, the diffusion of oxygen atom at the γ-TiAl (100) surface are studied by the method of Climbing-image Nudge Elastic Band. The calculated diffusion barriers of oxygen atom from the position on the surface to the second sublayer in these systems of perfect, Ti vacancy, Si and W doped are 1.98, 1.34, 2.53 and 2.69 eV, respectively. It indicates that the diffusion of oxygen atom is made easier by the Ti vacancy, while it is more difficult by the dope of Si and W.

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    BRITTLE-TO-DUCTILE TRANSITION FOR T2 IN THE Mo-Si-B SYSTEM
    PAN Kunming, ZHANG Laiqi, WANG Meng, LIN Junpin
    Acta Metall Sin, 2013, 49 (11): 1392-1398.  DOI: 10.3724/SP.J.1037.2013.00410
    Abstract   PDF (3804KB) ( 528 )

    The ordered intermetallic Mo5SiB2 (T2 phase) is a promising elevated-temperature structural material because of its high melting temperature, and excellent resistance to oxidation and creep. T2 phase has a body-centered tetragonal D81 structure (I4/mcm),with 20 Mo, 4 Si and 8 B atoms situating in layered arrangements along the c axis.This structure has been regarded as the obstacle to plastic deformation or dislocation activity especially at ambient temperature. However, like most intermetallic compounds, it undergoes a brittle-to-ductile transition (BDT) up to high temperatures, where the increased ductility is companied by microstructure changes. The feature of high-temperature applications makes it necessary to investigate the BDT behavior. The analysis of microstructure, dislocation configuration and fracture surfaces were carried out by compression and three-point bending tests, coupled with XRD, SEM-EDS and TEM methods. The result suggested that BDT happened over a wide temperature range of 1000-1200℃. At the BDT temperature above, multiple slip systems such as the [010](001) and <110>{110} could be activated under an applied stress, resulting in obvious plastic deformation. Since the edge dislocation displayed more activity than screw and mixed ones, the edge dislocation with b=[010] interacted with the <110>-type dislocation via slipping into its plane of {110}. Therefore, the work hardening was caused by the formation of lots of dislocation nodal points during the subsequent compression process. With temperature increasing, the fracture mode underwent a conversion from transgranular cleavage at ambient temperature to a mix of transgranular cleavage and intergranular failure.

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    MICROSTRUCTURE AND PROPERTY OF ISOTHERMAL FORGED SPRAY FORMING FGH4095 SUPERALLOY
    XU Yi, HUANG Peng, SHU Qin, GUO Biao, SUN Chuanshui
    Acta Metall Sin, 2013, 49 (11): 1399-1405.  DOI: 10.3724/SP.J.1037.2013.00442
    Abstract   PDF (4125KB) ( 742 )

    Spray forming is a rapid solidification technology.The metal melts are atomized by inert gas into droplets of 10—100 μm in size, and fly at subsonic speed onto a deposition substrate during the spray forming process. The main features of spray formed material are free from macro-segregation, the well fine grain size and uniform distribution of the strengthening phase, leading to improvement of mechanical property at acceptable costs compared to the conventional techniques. Spray forming is an inexpensive alternative to powder metallurgy (PM) for the production of High-alloyed materials, finer microstructures and better mechanical properties than conventional materials. The spray forming route is trying to manufacture superalloy turbine components in the aerospace field. FGH4095 superalloy is generally used to manufacture powder metallurgy turbine disc for high working temperature of 650℃ in aerospace industries of China. The microstructure features of spray formed FGH4095 superalloy after isothermal forging and heat treatment process are studied in thework. Spray formed FGH4095 superalloy with grain size of 8 μm was prepared by 75% engineering deformation near isothermal forging under beginning temperature 1120℃ and finish temperature 1050℃. The tensile strengths (σb) of isothermal forged FGH4095 superalloy at room temperature and 650℃ reach 1565 and 1552 MPa, respectively, and the yield strengths (σ0.2) at room temperature and 650℃ reach 1231 and 1130 MPa, respectively. Meanwhile, the microstructure evolution was observed by OM, SEM and TEM. The results indicate that alloy microstructure are composed of fine recrystallization grains and larger deformed grains. Fine recrystallization grains  provide nucleation core, and then small—angle grain boundaries provide lattice distortion energy for static recrystallization. Compared with spray formed material, isothermal forged superalloy possesses fine tertiary γ′ phases dispersed in the matrix, cleaner boundary, crushed carbides, and less defects such as micropores, which has better mechanical properties.

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    FRICTION AND WEAR PROPERTIES OF SURFACE PLASMA Cr-W ALLOYING LAYER OFγ-TiAl ALLOY
    WEI Xiangfei, ZHANG Pingze, WEI Dongbo, CHEN Xiaohu, WANG Qiong, WANG Ruonan
    Acta Metall Sin, 2013, 49 (11): 1406-1410.  DOI: 10.3724/SP.J.1037.2013.00475
    Abstract   PDF (2633KB) ( 947 )

    Cr-W alloying layer was prepared on the TiAl alloy by using the double glow plasma surface alloying technology (DGP). The hardness tests show that the surface micro-hardness of Cr-W alloying layer is greatly increased and the micro-hardness of diffusion layer is 648.8 HV0.1, increased two times more than the matrix. The scratch tests show that the bond between TiAl alloy and Cr-W alloying layer is very good and the layer at least can withstand 70 N vertical load. The friction and wear at room temperature tests show that the friction coefficient of Cr-W alloying layer is about 0.43, that of the matrix is 0.70. The wear scar is narrow, and the depth is small. The wear weight loss and wear rate relative to the matrix decrease greatly. The friction and wear at 500℃ tests show that the friction coefficient of Cr-W alloying layer is about 0.50, that of the body is 0.75. The wear weight loss and wear rate of Cr-W alloying layer are all less than the matrix.

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    TEXTURE EVOLUTION OF Nb MICRO-ALLOYED Fe14Si2 HIGH SILICON STEEL DURING WARM ROLLING
    YANG Kun, LIANG Yongfeng, YE Feng, LIN Junpin
    Acta Metall Sin, 2013, 49 (11): 1411-1415.  DOI: 10.3724/SP.J.1037.2013.00492
    Abstract   PDF (2634KB) ( 741 )

    Fe-6.5%Si (mass fraction) alloy possess perfect magnetic properties,though intermetallics of Fe14Si2 phase brought 6.5%Si leads to room temperature brittleness and hinder this significant materials industrialization. Nb was adopted into micro-alloying of Fe-6.5%Si high silicon steel. OM, thermal simulated test machine and XRD were employed to study the influence of Nb on high silicon steel in processing stages. Textures of warm-rolled high silicon steel strips were determined by XRD. The results indicate that Nb could refine the grains during fabrication of high silicon steel. Comparing to the non-Nb micro-alloyed high silicon steel, the grain size were reduced 17.50%, 24.51% and 30.13% in as-cast, forged and hot-rolled microstructure. Compression strength of as-cast specimen was enhanced from 1365 MPa to 1520 MPa with elongation rate from 0.225% to 0.400%. Tensile strength of warm-rolled strip was increased from 573 MPa to 621 MPa with elongation rate from 0.018% to 0.026%. XRD was carried out to indicate the surface texture evolution of warm-rolled high silicon steel with thickness from 0.30-1.68 mm during the warm rolling. The original (011)<100> Goss texture totally transformed to (100)>011> rotating cubetexture with single pass deformation of 26.2%. Then, rotating cube texture completely transformed to {111} fiber texture which remains until the thickness of 0.30 mm with single pass deformation of 22.6%.

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    MICROSTRUCTURE STABILITY IN A FULLY LAMELLAR HIGH Nb-TiAl ALLOY AFTER LONG-TERM THERMAL CYCLING
    FANG Lu,DING Xianfei, ZHANG Laiqi, HAO Guojian, LIN Junpin
    Acta Metall Sin, 2013, 49 (11): 1416-1422.  DOI: 10.3724/SP.J.1037.2013.00495
    Abstract   PDF (5483KB) ( 711 )

    Microstructure stability in the fully lamellar Ti-45Al-8.5Nb(W, B, Y) alloy were investigated by OM, SEM and TEM after long-term thermal cycling (500 and 1000 cyc) at 900 and 1000℃. The results showed that Al-segregation could not be eliminated completely after the heat treatment. After long-term thermal cycling at 900℃, the discontinuous coarsening was inclined to occur in the Al-segregation region in the alloy. And almost no spheroidized precipitates ofα2 were observed even after 1000 thermal cycles. After long-term thermal cycling at 1000℃, the massive γ grains were generated in the Al-segregation region. After 500 thermal cycles, the spherodized α2 precipitates were produced within γ grains which were found at colony boundaries. After 1000 thermal cycles, however, the large equiaxed γ grains containing different orientation of plate-shaped precipitates of theα2 phase were observed within the lamellar structure or at colony boundaries. After long-term thermal cycling at 1000℃, the plate-shaped or particle-shapedα2, which is coherent with the γ matrix, precipitates on the {111}γ plane in the γ grain interior.

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    HIGH TEMPERATURE STRENGTH AND AMBIENT DUCTILITY DEPENDENCES ON Al CONTENTS OF HIGH Nb CONTAINING TiAl ALLOYS
    LI Haizhao, ZHANG Ji
    Acta Metall Sin, 2013, 49 (11): 1423-1427.  DOI: 10.3724/SP.J.1037.2013.00499
    Abstract   PDF (2891KB) ( 749 )

    The influences of Al contents on the high temperature strength and ambient ductility of cast high Nb containing TiAl alloys were investigated with 7%Nb (mole fraction) addition and Al contents varying from 46%-49%. The macro- and micro-structures were examined. Due to the Nb solid solution strengthening effect, the cast alloy with lowest Al content consisting of a refined lamellar microstructure in equiaxed macro-grains exhibits excellent strength at 900℃ that is comparable with the well developed wrought alloys. The alloys with Al contents higher than 47% contains mostly or fully lamellar colonies with little orientation differences in macro-columnar crystals. Probably because of the lamellar structures with orientation hard to deform and the twining reinforcement, those alloys exhibit the more preponderant strength at 900℃ that can be as high as 595 MPa in the alloy with 47.5%Al. While, the effect of Al contents on ambient ductility becomes reversed as the Hard-to deform lamellar structures present very poor plastic elongation after tensile fractures.

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    EFFECT OF 0.037%H ON HIGH TEMPERATURE DEFORMATION BEHAVIOR OF A CAST Ti-45Al-5Nb-0.8Mo-0.3Y ALLOY
    WEN Daosheng, ZONG Yingying, XU Wenchen, YANG Danmei, SHAN Debin
    Acta Metall Sin, 2013, 49 (11): 1428-1432.  DOI: 10.3724/SP.J.1037.2013.00507
    Abstract   PDF (1641KB) ( 744 )

    TiAl-based alloys have been regarded as potential structural materials for the aerospace industry because of their high specific strength at elevated temperature, good creep properties and excellent resistance to oxidation. However, their low room temperature ductility and bad high temperature deformability are the considerable barriers to wide application of such alloys. It has been reported that thermo hydrogen treatment (THT) was an effective way to improve the formability of Ti-Al binary alloys. However, up to now few works have been done about the effect of THT on the mechanical properties of utilizable TiAl-based intermetallic compounds. So, the effect of hydrogen on high temperature deformation behaviors of a utilizable TiAl-based intermetallic compound, Ti-45Al-5Nb-0.8Mo-0.3Y alloy, was investigated. Isothermal hot compression tests of the cast Ti-45Al-5Nb-0.8Mo-0.3Y alloy containing 0 and 0.037%H (mass fraction) were carried out by use of a hot simulator at 1150 and 1200℃, with strain rates of 0.1 and 0.01 s-1, and their microstructures were studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that hydrogen was able to decrease high temperature deformation flow stress and refine microstructure, which led to an improvement of the hot formability. Comparing with the unhydrogenated alloy, the hydrogenated alloy presented a reduction of peak flow stress, approximately 25% when deformed at 1200℃ and strain rate of 0.01 s-1.The lamellar spacing of Hydrogenated Ti-45Al-5Nb-0.8Mo-0.3Y alloy was 265 nm, 77 nm less than that of the unhydrogenated alloy, which was mainly owing to hydrogen-increased β phase content. Besides, the increase of β phase with good high temperature plasticity was one of the main reasons for the decrease of flow stress of the hydrogenated alloy. Hydrogen made the peak strain of true stress and strain curves of the hydrogenated alloy occur earlier than that of the unhydrogenated alloy, which indicated that hydrogen promoted dynamic recrystallization of the hydrogenated alloy to decrease the flow stress. The dynamic recrystallization of β phase was found in the hydrogenated alloy, but not in the unhydrogenated alloy. Comparing with the unhydrogenated alloy, the density of dislocation of the hydrogenated alloy was lower because hydrogen induced the movement of dislocation. More twinning of γ phase occurred in the hydrogenated alloy, which assisted the alloy in high temperature deformation to some extent. In summary, hydrogen-induced dislocation movement, hydrogen-promoted dynamic recrystallization and twinning, and hydrogen-increased β phase content were the main reasons for the decrease of high temperature deformation flow stress of the hydrogenated Ti-45Al-5Nb-0.8Mo-0.3Y alloy.

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    FORMATION OF Ge-Y MODIFIED SILICIDE COATINGS ON Nb-Ti-Si BASE ULTRAHIGH TEMPERATURE ALLOY
    YANG Lingxiao, GUO Xiping, QIAO Yanqiang, PAN Ruobing
    Acta Metall Sin, 2013, 49 (11): 1433-1438.  DOI: 10.3724/SP.J.1037.2013.00517
    Abstract   PDF (1735KB) ( 407 )

    Nb-Si base ultrahigh temperature alloy is a promising candidate for high temperature applications due to its high melting point, high strength and relatively low density at elevated temperatures. However, the poor oxidation resistance of this alloy has still be a major barrier to its high temperature applications. Ge and Y modified silicide coatings on an Nb-Ti-Si based ultrahigh temperature alloy have been prepared successfully by co-depositing Si, Ge and Y in order to improve its high temperature oxidation resistance. The structure, constituent phases and formation process of the coatings have been revealed using XRD, SEM and EDS. The results showed that the coatings had obvious layered structure which was composed of a (Nb, X)(Si, Ge)2 (X represents Ti, Cr and Hf elements) outer layer and a (Ti, Nb)5(Si, Ge)4 inner layer. The concentration of Ge in the coating increased with increase in the distance away from the coating surface. Compared with the single Y modified silicide coating, the addition of Ge did not change the coating structrue, but reduced the absorption and deposition of Si and retarded the coating growth. The coatings prepared at 1250℃ for different time (0-8 h) had similar structures and the coating growth kinetics followed a parabolic law.

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    QUASI ISOTHERMAL FORGING SIMULATION OF β-γ TiAl ALLOY CONTAINING HIGH CONTENT OF Nb
    ZHENG Junzi, ZHANG Laiqi, HOU Yongming, MA Xiangling, LIN Junpin
    Acta Metall Sin, 2013, 49 (11): 1439-1444.  DOI: 10.3724/SP.J.1037.2013.00518
    Abstract   PDF (3138KB) ( 498 )

    TiAl-based alloys containing high content of Nb are significantly promising for high-temperature structural applications in aerospace and automotive industries, due to their low density, excellent high temperature strength, high resistance to oxidization and creep resistance. However, poor hot workability limits their extensive applications. Owing to sufficient number of independent slip system, small deformation resistance, apt to plastic forming of disordered bccβ phase at elevated temperature, the novel β-γ TiAl with high content of Nb alloys exhibit excellent hot deformability. The quasi isothermal forging process of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy is investigated by using a Deform-3D software. The effective stress, effective strain, temperature distribution of the alloy are analyzed. In order to obtain the critical damage factor of the alloy under the condition of 1150℃ and strain rate 5×10-2s-1, the hot compression physical simulation experiment of the alloy is performed to achieve true stress-strain data. By inputting these data into the Deform-3D software to simulate the true compression process, the critical damage factor is obtained. The results demonstrate that with increase of strain rate, the temperature loss becomes less obvious, the effective stress and the maximum effective strain rises up, the deformation uniformity ratio decreases and the deformation becomes more uniform. Combined with the simulation and experiment results, the critical damage factor of the β-γ TiAl alloy containing high content of Nb is 0.206 during the quasi isothermal forging process at\linebreak 1150℃ and strain rate 5×10-2s-1.

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    OPTIMIZATION OF HEAT TREATMENT PROCESS OF Fe-6.5%Si SHEET
    ZHANG Hao, LI Hui, YANG Kun, LIANG Yongfeng, YE Feng
    Acta Metall Sin, 2013, 49 (11): 1445-1451.  DOI: 10.3724/SP.J.1037.2013.00520
    Abstract   PDF (2425KB) ( 502 )

    Fe-6.5%Si (mass fraction) alloy exhibits excellent soft magnetic properties and therefore is very suitable to be used as iron core in high frequency electromotor. However, the room-temperature embrittlement and poor workability limit the practical applications of the alloy and it is hard to be fabricated to thin sheet. It is reported that ultra-thin sheet with 0.05 mm in thickness has been successfully obtained by an advanced technique of stepwise ductilization, including hot, warm and cold rolling processes with intermediate heat treatment. And suitable heat treatment can improve the ductility of this alloy sheet, therefore it plays an important role in this technique. However, the existing heat treatment is time consuming and not suitable for industry. In this work, effects of heat treatment on mechanical properties of the hot-rolled sheet have been investigated in dependence of annealingtemperature and time. The experimental results show that heat treated Fe-6.5%Si sheets have better mechanical properties than those of as hot-rolled sheets. Elongation of heat treated sample is twice as hot-rolled sample's and hardness decreases significantly. Also it is found that heat treatment at  850℃ for 1 min and then quenching in brine provides good ductility and low hardness, which is benefit to the subsequent processes and increases the productivity substantially.

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    ORDERED STRUCTURE AND MECHANICAL PROPERTIES  OF  Fe-6.5%Si ALLOY FABRICATED BY RAPID QUENCHING
    LI Hui, LIANG Yongfeng, HE Ruiqi, LIN Junpin, YE Feng
    Acta Metall Sin, 2013, 49 (11): 1452-1456.  DOI: 10.3724/SP.J.1037.2013.00521
    Abstract   PDF (1889KB) ( 862 )

    Fe-6.5%Si alloy is one excellent soft magnetic material. However, due to the appearance of ordered phases, this alloy is very brittle at room temperature and is very hard to be fabricated into thin sheet by conventional rolling process. In this work, continuous Fe-6.5%Si ribbons of 20 mm in width, and 35-50 μm in thickness were fabricated by rapid quenching technique. The ribbons exhibited good ductility compared with ordinary cast ingots. TEM and XRD were employed to analyze the ordering and microstructure of the ordinary cast ingots and the ribbons. The long range order parameters of B2 ordered phase in these two alloys were calculated. The results showed that the cast ingot contained amounts of B2 and D03 ordered phases, and had high degree of order. While in rapid quenched ribbons, the grains were extremely fine, the formation of D03 ordering was suppressed effectively and the growth of B2 was largely decreased. The decreased ordering content and degree of order in rapid quenched ribbons lowered the anti-phase boundary energy, leading to the reduction in slip resistance, which improved the mobility of superlattice dislocations, and then obviously enhanced the ductility of the alloy.

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    RELATIONSHIP BETWEEN MECHANICAL PROPERTIES AND LAMELLAR ORIENTATION OF PST CRYSTALS IN Ti-45Al-8Nb ALLOY
    PENG Yingbo, CHEN Feng, WANG Minzhi, SU Xiang, CHEN Guang
    Acta Metall Sin, 2013, 49 (11): 1457-1461.  DOI: 10.3724/SP.J.1037.2013.00535
    Abstract   PDF (1495KB) ( 907 )

    Room temperature compression performance of polysynthetic twinned crystal (PST crystal) in Ti-45Al-8Nb (atomic fraction, %) alloys with different angles between lamellar orientation and uniaxial stress direction was studied. The causes of the fracture failure were investigated by the fracture surface and the effects of the Nb element on strengthening were also studied. It was found that when the lamellar orientation in the PST crystal was paralleled or vertical to the stress direction, shear deformation was vertical to lamellar interface, and when the angle of lamellar orientation to the stress direction was 45°, the shear deformation was paralleled to lamellar interface. The yield strength of the 0° PST crystal was the maximum which was up to 1296 MPa. The PST crystal of 45° had the best compression plastic strain of approximately 1.1%, but its yield strength was the minimum of only 847 MPa. Nb element could refine lamellar structure and improve the room temperature yield strength significantly through increasing shear stress of dislocation motion and reducing the stacking fault energy.

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    TiAl-BASED COMPOSITE SHEET WITH MULTI-LAYER DISTRIBUTED REINFORCEMENT PREPARED BY SOLID-LIQUID REACTION
    CUI Xiping, GENG Lin, FANG Kun, FAN Guohua, ZHENG Zhenzhu, WANG Guisong
    Acta Metall Sin, 2013, 49 (11): 1462-1466.  DOI: 10.3724/SP.J.1037.2013.00546
    Abstract   PDF (1836KB) ( 649 )

    TiAl-based alloy have potential as high temperature structural materials for aerospace applications, especially for thermal protection systems in aerospace vehicles including skin materials. Unfortunately, the ductility and formability of TiAl-based alloys are rather poor and thus γ-TiAl sheets or foils are quite difficult to be manufactured by traditional methods and still far from practical applications. In the present work, commercial pure Ti foils and in-house fabricated TiB2/Al composite foils were alternately stacked and rolled to prepare Ti-(TiB2/Al) laminates, and then heat treatment was utilized to make liquid Al react with solid Ti,and finally TiAl-based composite sheet with multi-layer distributed reinforcement TiB2 particles were achieved. This method avoided the direct deformation of brittle TiAl billets and thus the near-net-shape processing of TiAl-based alloys sheets was feasible. Phase transformation and microstructure evolution during heat treatment were investigated and mechanical properties of the resulting micro-laminated TiB2-TiAl composite sheets were evaluated. The results showed that porous TiAl3 layers were produced by the reaction between liquid Al and solid Ti because of Kirkendall effect, and the following densification treatment under 50 MPa at 1300℃ for 2 h significantly improved the relative density of material. Subsequently, the reaction diffusion between TiAl3 and residual Ti proceeded in the following heat treatment, and finally fully lamellar (α2-Ti3Al+γ-TiAl) layers were obtained. TiB2 particles did not participate in any reaction and remained, and displayed multi-layered distribution in the matrix(α2-Ti3Al+γ-TiAl) layers. TiB2-rich layer hindered the coarsening of lamellar colony of (α2-Ti3Al+γ-TiAl) layer. Tensile properties at 800℃ of multi-layered TiB2-TiAl composite sheets remarkably increased because of an increase of energy dissipation caused by plastic deformation.

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    EXPLORATION AND RESEARCH OF A NEW Re-FREE Ni-BASED SINGLE CRYSTAL SUPERALLOY
    ZHOU Xuefeng, CHEN Guang, YAN Shitan, ZHENG Gong, LI Pei, CHEN Feng
    Acta Metall Sin, 2013, 49 (11): 1467-1472.  DOI: 10.3724/SP.J.1037.2013.00539
    Abstract   PDF (1574KB) ( 606 )

    Ni-based single crystal superalloy has not only high temperature creep and fatigue resistance, but also excellent oxidation and corrosion resistance,which becomes a main selection of the advanced aero engine turbine blades. In order to enhance high temperature properties, Re is added into the superalloy, however, high density and high cost of the Re, especially promote the precipitation of harmful phases at high service temperature, which limit the use of Re. Therefore, how to reduce or even abolish the use of Re in the single crystal superalloy is the main trend to develop a new generation turbine engine material. A new Re-free Ni-based single crystal superalloy, 7.5Cr-5Co-2Mo-6.1Al-8W-6.5Ta-0.15Hf-0.05C-0.004B-0.015Y (mass fraction, %), Ni balance, has been designed by using the average electron vacancy number theory and the d-electrons concept. The microstructures of the as-cast, solution and aging treated specimens were observed by optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy. The mechanical behavior of the fully heat treated single crystal superalloy and the appearance of fracture at 760℃ were studied. The calculation results indicate that the microstructure of the designed alloy is stable and the main performance criteria, such as Bot (the bond order between alloying elements and Ni atoms),Tγ′solvus (γ′ solvus temperature),P (the parameter which predicts the merit of the composition), etc.,are comparable to those of the second generation of the Ni-based single crystal superalloy.The experimental results indicate that W and Mo enriched in the dendrite cores,while Al and Ta enriched in the interdendritic region. The size and volume fraction of γ′ phase in the dendrite cores is smaller than that in the interdendritic region.After solution heat treatment at 1300℃, 3 h, air cooling,γ/γ′ eutectics are dissolved and composition segregation is significantly improved. After fully heat treatment at 1100℃, 4 h,air cooling and 870℃, 24 h, air cooling, γ′ phase with cube-shaped distributes in theγmatrix channels uniformly, whose ultimate tensile strength at 760℃ is 1009 MPa, comparable to the second generation of Re-containing Ni-based single crystal superalloy considerably.

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    PROCESSING AND PROPERTIES OF Al3CrCuFeNi2 SINGLE CRYSTAL HIGH--ENTROPY ALLOY
    ZHANG Sufang, YANG Xiao, ZHANG Yong
    Acta Metall Sin, 2013, 49 (11): 1473-1480.  DOI: 10.3724/SP.J.1037.2013.00540
    Abstract   PDF (5837KB) ( 1261 )

    The high-entropy alloy is a class of new metal material,its dominant element is generally more than three, and its mixing entropy is high and easy to form a solid solution structure. A large number of investigations show that high-entropy alloy possess excellent high temperature phase stability and softening resistance, which is superior to traditional superalloy. The Al3CrCuFeNi2 high-entropy alloy is studied by using once Bridgman solidification at different draw rates (10, 30 and 150 μm/s) and twice Bridgman solidification at 10 μm/s draw rate, the corresponding microstructure are analyzed. The EBSD technique is applied to detect the alloy growth orientation and grain boundary misorientation angle of alloys by once and twice Bridgman solidification at 10 μm/s draw rate. The room temperature tensile mechanical properties of Al3CrCuFeNi2 high-entropy alloy by twice Bridgman solidification at 10 μm/s draw rate and suction casting are studied. The results show that the primary spacing of dendritic of Al3CrCuFeNi2 high-entropy alloy by Bridgman solidification at 10 μm/s draw rate is larger than other draw rates and suction casting, while the growth orientation angle of dendrite dry is smaller than others. The orientation is close to the <001> direction after twice Bridgman solidification at 10 μm/s draw rate, and most of grain boundary misorientation angle are less than 5°. Compared with suction casting samples, yield strength, tensile strength and elongation of Al3CrCuFeNi2 high-entropy alloy by using twice Bridgman solidification increased by 34.6%, 10.2% and 40%.

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    INFLUENCE OF MELT HOLDING TEMPERATURES ON MECHANICAL PROPERTIES AT ROOM TEMPERATURE OF Wf/Zr-BASED METALLIC GLASS COMPOSITES
    GAO Du, CHEN Guang, FAN Cang
    Acta Metall Sin, 2013, 49 (11): 1481-1486.  DOI: 10.3724/SP.J.1037.2013.00541
    Abstract   PDF (1506KB) ( 2299 )

    The effect of the molten temperatures on the preparation of the Wf/Zr-based metallic glass composites by infiltration casting rapid quenching technique was well studied. The results of the mechanical properties at ambient temperature indicate that the composite has an optimum melt holding temperature in the preparation process. Lower or higher than the optimum temperature, the compressive plasticity of the composite deteriorated. The optical microstructure and EDS analyses indicate that the reason for the deterioration of the compressive plasticity was the resulted microstructures, which contains different precipitates with different molten temperatures. At the optimum preparation condition, a Wf/Zr--based metallic glass composite with 75% volume fraction of tungsten fiber was successfully produced with significantly improved compressive plasticity of 19.6%.

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    EFFECT OF COOLING RATE ON MICROSTRUCTURE AND TENSILE PROPERTIES IN Ti3Al ALLOY
    WANG Zhen, CAO Lei, LIU Renci, LIU Dong, CUI Yuyou, YANG Rui
    Acta Metall Sin, 2013, 49 (11): 1487-1492.  DOI: 10.3724/SP.J.1037.2013.00572
    Abstract   PDF (4537KB) ( 675 )

    The microstructure and tensile properties in Ti3Al alloy rod by forging in the (B2+α2) region after heating at 1020 and 1150℃ for 30 min and cooling to room temperature at the cooling rates of 10, 1.5 and 0.3℃/s were investigated. The results showed that Ti3Al alloy structure was mainly composed of B2 phase and α2 phase at the cooling rate of 10℃/s after heating at 1020℃. With the decrease of cooling rate, the acicular α2/O phase homogeneously precipitated in the matrix on slow cooling. The volume fraction of equiaxed α2/O phase increased and the acicular α2/O phase was coarsed under the cooling rate of 0.3℃/s. For room temperature tensile tests, the data indicated that the strength initially increased and then decreased and the ductility firstly decreased and then increased with reducing cooling rate. During the 600℃ tensile tests, the strength decreased monotonically and ductility rose as reducing cooling rate. When solid solution temperature was 1150℃, Ti3Al alloy was mainly composed of B2 phase and the size of grain was 440 $\mu$m by fast cooling. Basketweave structure was obtained in the grain at the cooling rate of 1.5℃/s. The α2/O phase colony precipitated in the B2 phase with further reducing the cooling rate. With the decrease of cooling rate, the strength decreased gradually and ductility initially increased and then decreased at the room temperature, the 600℃ tensile strength firstly went up and then down and ductility increased. Through microstructure analysis, strength in Ti3Al alloy is controlled by boundary and the size of α2/O phase, and ductility is decided by the number of B2 phase and α2/O phase and the shape of α2/O phase. The good comprehensive properties were obtained at the cooling rate of 1.5℃/s.

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