Superalloy 2015

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Xuewei YAN,Ning TANG,Xiaofu LIU,Guoyan SHUI,Qingyan XU,Baicheng LIU
Acta Metall Sin    2015, 51 (10): 1288-1296.   doi:10.11900/0412.1961.2015.00338
Accepted: 09 September 2015

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Haigen ZHAO,Shusuo LI,Yanling PEI,Shengkai GONG,Huibin XU
Acta Metall Sin    2015, 51 (10): 1279-1287.   doi:10.11900/0412.1961.2015.00434
Accepted: 08 September 2015

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According to the requirement of high-pressure turbine guide vane during service, the aim of this work is to design a single crystal Ni3Al-based alloy named IC21 with low density, low cost, and high strength which can be used as high-pressure turbine guide vane material. The mass fraction of the Re has been limited less than 1.5% on purpose. The single crystal bars of IC21 were prepared by high rate solidification method. The density of IC21 is 8.0 g/cm3 and the incipient melting temperature was identified by metallography. After standard heat treatment, the distribution of the g' precipitates is uniform with the average size of about 420 nm, and volume fraction of 80%. The tensile and yield strengths at 1100 ℃ are 490 and 470 MPa, respectively. Moreover, IC21 shows superior creep properties, the stress-rupture life at 1100 ℃,140 MPa is 170.5 h and at 1150 ℃,100 MPa still remains 110.0 h. The microstructure stability of IC21 alloy at 1080 ℃ for as long as 1000 h were evaluated. The results show that no precipitated phase exists during thermal exposure at 1080 ℃, which exhibits good stability. The oxidation kinetic curves of IC21 alloy follows a parabolic rate law in different oxidation stage during cycle oxidation for 100 h in air. IC21 alloy has a good high temperature oxidation resistance, the strengthening mechanism are attributed to high volume fraction of g' phase, large negative misfit and well-established interface networks.

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Xiaoguang WANG,Jiarong LI,Jian YU,Shizhong LIU,Zhenxue SHI,Xiaodai YUE
Acta Metall Sin    2015, 51 (10): 1253-1260.   doi:10.11900/0412.1961.2015.00369
Accepted: 26 August 2015

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The Ni-based single crystal superalloys are widely used in key hot section parts of advanced aero engine due to the superior high temperature mechanical properties. Multi-axial stresses resulting from complex temperature and stress state happen frequently in blades during service, thus the mechanical properties of three orientations need to be studied. However, most of these works are conducted in the first and second single crystal superalloys and there is rare report concerning the third single superalloys. Therefore, in this work the microstructures and tensile properties of the third generation single crystal superalloy DD9 with [001], [011] and [111] orientations were investigated by OM, SEM, TEM and tensile testing machine at 760 and 1100 ℃. The results show that as-cast dendritic structures and heat treated γ’ of DD9 alloy with three orientations are different on the section perpendicular to the crystal growth direction. With rising of temperature, the ultimate tensile strength and yield strength decrease and tensile anisotropy drops obviously. The ultimate tensile strength and yield strength of DD9 alloy with [001] orientation are higher than those with [011] and [111] orientation except that the yield strength with [001] orientation is slightly lower than that with [011] orientation. With temperature increasing, the fracture characteristic transforms from quasi-cleavage at 760 ℃ to dimple at 1100 ℃. At 760 ℃, very high density dislocations appear in the matrix channels with [001], [011] and [111] orientations, but some stacking faults are present only in γ’ particles with [001] orientation. At 1100 ℃, the high density dislocation networks resulted in the matrix channels and particles of the alloy with [001] and [111] orientations, while a large number of deformation twins are found in samples with [011] orientation.

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Jinyan TONG,Wei FENG,Chao FU,Yunrong ZHENG,Qiang FENG
Acta Metall Sin    2015, 51 (10): 1242-1252.   doi:10.11900/0412.1961.2015.00265
Accepted: 26 August 2015

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Service safety of turbine blades in aircraft engines are threatened by microstructural and property degradation instantly caused by overheating during service. Systematic investigations about microstructural degradation during overheating exposures and its influence on mechanical properties of turbine blades during service are limitedly reported. In this work, microstructure and mechanical properties of GH4033 alloy, which was sectioned from the shank of a serviced 2nd stage turbine blade in an aircraft engine, were studied after overheating at 900~1100 ℃ for 3 min. Microstructural degradation during overheating exposures as well as its influence on room temperature hardness and stress rupture life at 700 ℃, 430 MPa were analyzed. The results of microstructural characterization indicated that the coarsening and dissolution of γ’ precipitates were introduced by overheating exposures, and all of the γ’ precipitates dissolved at 980 ℃ for 3 min. Gradual dissolution of grain boundary (GB) carbides was observed with the increase of overheating temperature. Complete dissolution of GB carbides at 1100 ℃ resulted in grain growth. The room temperature hardness after overheating exposures decreased grossly with the dissolution of γ’ phase. Due to the dissolution and re-precipitation of γ’ phase as well as the dissolution of GB carbides, the stress rupture life under 700 ℃, 430 MPa of GH4033 alloy was initially increased and then decreased significantly.

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Lei WANG,Jinlan AN,Yang LIU,Xiu SONG,Guohua XU,Guangpu ZHAO
Acta Metall Sin    2015, 51 (10): 1235-1241.   doi:10.11900/0412.1961.2015.00375
Accepted: 27 August 2015

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GH4169 alloy is widely used to make aero engine, gas turbine as it is one of the most important superalloy. γ’' phase is the main strengthen phase, however, the metastable γ’' phase will transform to stable d phase during aging or servicing for certain time. d phase is significance in the alloy, its precipitating and distributing behavior have an effect on the properties of the alloy. In recent year, researchers pay more attention on electric field treatment (EFT), this is because of high energy density, accurate controlling, clean and safety. EFT is one of the most important energy field except temperature field and stress field. In this work, EFT was performed on GH4169 superalloy to investigate the influence of EFT on precipitation behavior of d phase in the alloy, and the mechanism of the effect of EFT on the phase transformation was also discussed. The results show that d phases precipitate on the grain boundaries after EFT with 8 kV/cm at 850 ℃ for 15 min, and large amounts of γ’' phases precipitate inside the grains. With the increasing of EFT time, both the volume fraction and the size of d phase increase, at the same time the size of γ’' phase increases. The volume fraction of d phase is less and the size of d phase is smaller, and the volume fraction of γ’' phase is higher by EFT, compared with that by aging treatment (AT) for the same time. In addition, the Nb content on the grain boundary decreases and both Fe and Cr content increase, meanwhile the lattice parameters of c decreases and a, b increase. The vacancy concentrations can be accelerated by EFT, so that the diffusion of Fe and Cr atoms can be promoted. Meanwhile, the Nb atoms in d phases on the grain boundaries can be displaced by Fe atoms and Cr atoms, therefore the Nb atoms are dissolved into the grain. The nucleation rate of γ’' phases increases with the increasing of vacancy concentrations. The vacancies relax coherent distortion between γ phases and γ’' phases, and suppress γ’' phases to transform to d phases. Thus the stabilization of γ’' phases is enhanced.

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Beijiang ZHANG,Guangpu ZHAO,Wenyun ZHANG,Shuo HUANG,Shifu CHEN
Acta Metall Sin    2015, 51 (10): 1227-1234.   doi:10.11900/0412.1961.2015.00368
Accepted: 25 August 2015

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Much attention has been paid to the development of more advanced materials for high-pressure compressor and turbine discs of gas turbine engines. A high performance wrought superalloy GH4065 for disc applications has been recently developed based on the comprehensive evaluation of a series of model alloys with characteristic chemical composition, lattice parameter, particularly γ’ volume fraction. The concentration of major alloying elements of GH4065 is closely similar with René 88 DT and specifically optimized considering the demands of ingot metallurgy technologies. Therefore, GH4065 can be considered as an ingot metallurgy version of powder metallurgy René 88 DT. Large scale vacuum arc remelting (VAR) ingots of GH4065 alloy with diameter up to 508 mm have been produced via standard triple melting techniques. Micro-scale segregation of alloying elements on large VAR ingot has been effectively suppressed due both to optimized alloying elements concentration and to improved melting techniques. Ultra-low carbon content (less than 0.02% in mass fraction) significantly decreases the dendritic segregation tendency of certain alloying elements and promotes the uniformity of microstructures. VAR ingot of GH4065 exhibits extraordinary hot plasticity, ingot conversion can be accomplished using conventional open die forging procedure. Fine and uniform γ+γ’ duplex structures can be obtained on billets and disc forgings via a newly developed multi-cycle thermomechanical processing method. The flow stress data show that the formation of γ+γ’ microduplex results in a significant decrease of flow stress in comparison with γ’ dispersion structures under exactly the same deformation conditions. The distribution of strain rate sensitivity m in relationship with temperature and strain rate accurately identifies a specific domain within which γ+γ’ microduplex exhibits superplasticity. Full-scale turbine discs of GH4065 alloy with diameter of 630 mm achieve an optimal combination of creep resistance, fatigue lifetime and ductility. GH4065 discs exhibit extraordinary microstructural and property stability during prolonged thermal exposure, which means that dendritic segregation has been successfully restricted to an acceptable level. The results reveal that highly alloyed disc alloys produced via ingot metallurgy techniques exhibit lower costs and higher productivity, and can still meet the ever increasing demand of high performance gas turbine engines.

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Yiwen ZHANG,Shoubo HAN,Jian JIA,Jiantao LIU,Benfu HU
Acta Metall Sin    2015, 51 (10): 1219-1226.   doi:10.11900/0412.1961.2015.00384
Accepted: 19 August 2015

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Microelement Hf added in Ni-based powder metallurgy (PM) superalloy can modify microstructure and improve mechanical properties, such as stress-rupture life, creep resistance and crack growth resistance, and also benefit to eliminate notch sensitivity. So systematically studying the effect of microelement Hf on PM superalloy microctructure will help to comprehend its corresponding mechanism. The effects of microelement Hf on the morphologies, chemical compositions and content of γ’ phase and MC carbide in FGH97 PM Superalloy were investigated by means of SEM and physiochemical phase analysis. The results showed that Hf facilitated the precipitations of γ’ phase and MC carbide, and changed chemical compositions of γ’ phase and MC carbide, the effect of Hf on the size and morphology of MC carbide was not obvious, while Hf greatly affected the size and morphology of γ’ phase and accelerated the splitting of γ’ phase from one instable cubic γ’ particle to stable octet of cubes. As Hf affected the lattice misfit of γ’/γ phase (d), modifying Hf content changed the critical splitting size of γ’ phase (Dc). The relationship between Dc and Hf content (w(Hf)) was found to be Dc=315.4+640.2w(Hf)-358.2[w(Hf)]2. With Hf content increased, the absolute value of d decreased and Dc increased. Cubic γ’ particle split into an octet of cubes when γ’ phase grew up to the critical splitting size.

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Jianxin DONG,Linhan LI,Haoyu LI,Maicang ZHANG,Zhihao YAO
Acta Metall Sin    2015, 51 (10): 1207-1218.   doi:10.11900/0412.1961.2015.00419
Accepted: 02 September 2015

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The elimination of the segregation improves the thermo plasticity of superalloy ingot during the homogenization process, but coarser grain structure and high-temperature oxidation caused in further homogenization have an adverse impact on the thermo plasticity. The inheritance of coarse grain structure in the followed hot working process increases the tendency of cogging crack and makes the grain refining harder, leading to a lower yield of the final workpiece. The microstructure characteristics and their hot deformation behaviors of GH4740H, GH4738, GH3625 and 690 alloys under different homogenizations were investigated by means of microstructure analysis methods and crack propagation testing. The experimental results show that the reasonable homogenization processing needs to take into account the segregation elimination arising thermo plasticity addition, more to consider grain coarsing and severe oxidation leading to decrease plasticity. Based on the residue dendrites can provide more recrystalazation nucleation sites, the partial homogenization possessing probably exists rationality. This research work provides an exploratory study for the improvement of the homogenization-cogging process of superalloy.

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Yuefeng GU,Chuanyong CUI,Yong YUAN,Zhihong ZHONG
Acta Metall Sin    2015, 51 (10): 1191-1206.   doi:10.11900/0412.1961.2015.00442
Accepted: 08 September 2015

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Acta Metall Sin    2015, 51 (10): 1179-1190.   doi:10.11900/0412.1961.2015.00380
Accepted: 01 September 2015

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Based on the analysis of solidification processing in complex turbine blades, a new idea of 3-dimensional and precise control of single crystal (SC) growth was proposed. A series of new techniques were presented,exhibiting the new development in the production of SC blades of superalloys. The heat conductor (HC)technique was developed to minimize the hot barrier effect which hindered the lateral SC growth. This method promotes the successful transition of SC growth from the blade body into the platform extremity prior to the nucleation of stray grains. To achieve symmetric thermal conditions for solidifying the SC blades, the PHC (parallel heating and cooling) system has been employed. With this technique, both sides of a shell mold can be both symmetrically heated in the heating zone as well as cooled in the cooling zone. The negative shadow effect in the current Bridgman process and the related defects are hence removed. With the H&D (dipping and heaving) technique using thin shell, the main problems of the Bridgman process, such as the ineffective radiative heat exchange and the large thermal resistance in thick ceramic molds, can be effectively resolved. This technique enables the establishment of a high temperature gradient at solidification front. By combining targeted cooling and heating technique, a 3-dimensionalcontrol of SC growth in large components can be achieved.

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Jun ZHANG,Taiwen HUANG,Lin LIU,Hengzhi FU
Acta Metall Sin    2015, 51 (10): 1163-1178.   doi:10.11900/0412.1961.2015.00448
Accepted: 07 September 2015

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Single crystal (SC) superalloy is a kind of complex structure and multi phase materials. With the increase of the degree of alloying and the content of refractory elements, or the more complicated structure and larger size of the casting made of SC superalloy, it is essential important to suppress the formation of solidification defects to improve the quality and performance of the blades. The microstructure and solidification defects of single crystal alloy are not only related to the composition of the alloy, but also depend on its solidification characteristics and technological conditions. The paper first summarizes the research progress of the solidification characteristics for advanced SC superalloys, focusing on analysis of the effects of solidification characteristics and processing parameters on the formation and its mechanics for two typical directional solidification defects, crystallographic orientation deviation and stray grains. Then some methods and approaches to suppress such defect formation for complex single crystal blade have been reviewed.

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Tao JIN,Yizhou ZHOU,Xinguang WANG,Jinlai LIU,Xiaofeng SUN,Zhuangqi HU
Acta Metall Sin    2015, 51 (10): 1153-1162.   doi:10.11900/0412.1961.2015.00429
Accepted: 27 August 2015

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Ni-based single crystal superalloys have been widely used to produce turbine blades for advanced aero-engines because of the super temperature-related microstructural stability and comprehensive mechanical properties. However, due to effects of the high temperature and complicated stresses in service, the microstructures of superalloys might gradually evolve and fail in different modes. The present paper reviews the progress of microstructural stability and mechanical behavior including the γ’ phase rafting, TCP phase precipitation, high temperature creep, low cycle fatigue and thermomechanical fatigue of single crystal superalloys. The addition of Ru improves the creep life of superalloys, but also indirectly promotes the occurrence of “topological inversion”. On the other hand, with the increase of aging temperature and time, the contents of refractory elements in m phase rise significantly. With the increase of applied tension stress, more m phase precipitate from the γ matrix, whereas inverse tendency is shown under compression stress. Numerous planar defects are formed during precipitation of m phase, and these defects promote the nucleation of P and R phases. During high temperature and low stress creep, an important dislocation a<010> superdislocation is observed, which moves in the γ’ phase slowly by a combination of slide and climb. Under very high temperature, incubation with accelerating creep rate occurs before the primary stage, which relates to the extending process of the γ width. At last, the stacking fault energy is significantly reduced after Ru additions, and thus a series of complex deformation mechanisms occur during low cycle fatigue, e.g. stacking faults penetrating γ/γ’ interface, trailing a/6<112> Shockley dislocations shearing into the γ’ phase. During thermomechanical fatigue, the life of superalloys is influenced by the site of crack initiation, microstructural evolution and oxidation resistance.

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Yuxiang ZENG,Xiping GUO,Yanqiang QIAO,Zhongyi NIE
Acta Metall Sin    2015, 51 (9): 1049-1058.   doi:10.11900/0412.1961.2015.00092
Accepted: 06 July 2015

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Nb-Ti-Si base in situ composites which consist of Nb solid solution (Nbss) and silicides (a-Nb5Si3, b-Nb5Si3, g-Nb5Si3 and/or Nb3Si) phases, have shown great potential as alternative materials to Ni-based superalloys due to their high melting points (beyond 1700 ℃), good formability, low density (6.6~7.2 g/cm3) and high strength. However, a major hindrance to the applications of these alloys at elevated temperatures is their poor oxidation resistance. Alloying is an effective method to improve the integrated properties of the alloys, especially for the oxidation resistance. Up to now, many beneficial elements such as Ti, Al, Cr and Sn have been employed to ameliorate their oxidation resistance. Nevertheless, there is no systematic and comprehensive investigation on the effect of Zr contents on the microstructure and oxidation behavior of the alloys based on Nb-Ti-Si system. The aim of this work is to clarify the effects of Zr contents on phase selection, microstructure and high temperature oxidation resistance of Nb-Ti-Si based alloys in detail. The constituent phases, microstructure and composition of the alloys under as-cast state and after oxidation were investigated by OM, XRD, SEM and EDS. Thus, six Nb-Ti-Si base ultrahigh-temperature alloys with compositions of Nb-22Ti-15Si-5Cr-3Hf-3Al-xZr (x=0, 0.5, 1, 2, 4, 8, atomic fraction, %) were prepared by vacuum non-consumable arc-melting. The results show that the alloys with different Zr contents are mainly composed of Nbss and g-(Nb, X)5Si3 (X represents Ti, Hf, Cr and Zr). However, the addition of Zr has an obvious affect on the microstructure of Nb-Ti-Si base alloys. Both the sizes and amounts of primary g-(Nb, X)5Si3 increase with increase in Zr contents. Alloys with different Zr contents were oxidized at 1250 ℃ for 1~50 h, respectively. It is found that both adhesion and compactness of the scales are improved effectively by increase in Zr contents. The scales of alloys with higher Zr contents (x=4 and 8) after oxidation for 50 h show an obvious layered structure: the outmost layer is only composed of TiO2, the middle layer mainly consists of ZrO2, TiNb2O7 and TiO2, and the inner layer is mainly comprised of Si-rich oxides. The mass gain per unit area and the thickness of the scale after oxidation decrease with increase in Zr contents in the alloys, indicating that the addition of Zr can improve the oxidation resistance of the alloys significantly.

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Yumin WANG,Shuangming LI,Hong ZHONG,Hengzhi FU
Acta Metall Sin    2015, 51 (9): 1038-1048.   doi:10.11900/0412.1961.2015.00035
Accepted: 16 June 2015

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Homogeneous distribution of primary dendritic arm spacing (PDAS) is required to achieve uniform mechanical properties in final product of single-crystal superalloys. In this work, the dendrite characterization and orientation of Ni-based single-crystal DD6 superalloy have been deeply investigated using different methods, which include minimum spanning tree (MST), Voronoi polygon-based approach, fast Fourier transform (FFT), as well as EBSD and RO-XRD. The investigation results indicate that the mean PDAS of DD6 superalloy is about 325.7 mm and its variation ratio is 7.38%. The measured Voronoi polygon parameters suggest that the number of nearest-neighbor dendrite ranges from 5.87 to 5.93, approximating six nearest neighbors in the spatial distribution of dendrite microstructures. However, the change in ratio of six nearest number proportion has exceeded 30% for the twenty specimens. The MST method shows that the change in branch length measured from the twenty specimens achieves 26.95%. Also, the analysis results of FFT imply that the dendrite microstructures of DD6 superalloy evolve apparently. These results give the proof that the dendrite microstructures of DD6 superalloy vary with the solidified distance. Additionally, the deviation angles between preferential orientations of DD6 with the axial direction of specimen were measured by EBSD and RO-XRD, respectively. The deviation angle values of DD6 superalloy in this experiment are both within 10°. The reason for the deviation angle measured by RO-XRD being smaller is well explained due to the fact of selecting the diffraction intensity maximum angles. Furthermore, the EBSD results indicate that the orientations of DD6 superalloy prepared by grain selector can be well controlled along the Z-axial direction, but do not work in other two X and Y directions.

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Yiwen ZHANG,Benfu HU
Acta Metall Sin    2015, 51 (8): 967-975.   doi:10.11900/0412.1961.2014.00704
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Hafnium (Hf) is one of the most important microelements in powder metallurgy (P/M) superalloy. Hf modifies the microstructure and drastically improves mechanical properties in P/M superalloy. The effect of Hf in a nickel-based P/M superalloy was systematically studied by means of FEG-SEM, TEM, AES, EDS and physical and chemical phase analysis. Hf mainly distributes at interdendritic region of the solidification powder in form of solid solution, which is helpful to reduce prior particle boundary (PPB). Hf facilitates morphology of g′ phase to be unstable and enhances the large cubic g phase to split into smaller ones, so the g′ phase turns into a stable state with a lower energy faster. Hf is mainly distributed in g′ phase and MC carbides, which changes the distribution of element between the g′ phase, MC and g solid solution, which is beneficial to eliminate notch sensitivity and improves overall mechanical properties of the alloy.

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Suigeng DU,Xifeng WANG,Man GAO
Acta Metall Sin    2015, 51 (8): 951-956.   doi:10.11900/0412.1961.2014.00686
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Due to high demand of welded turbine blisk, a fine grain superalloy, GH4169, has been widely used to make the disk and a single crystal superalloy, DD3, has been used for blades. In this work, the joint micro-structure and the mechanism of friction welding between the GH4169 and DD3 have been investigated by using the SEM and TEM equipped with EDS. The research results show that there is a friction deformed band of the GH4169 in the weld zone. The heat and mechanical affected zones of the two alloys form dynamic recrystallization grains. The bonding interface is between their dynamic recrystallization grains. The two alloys bond with the common grains and the common grain boundaries. Their compositional change mainly occurs within the common grains and the common grain boundaries at bonding interface. The common grain (C2) at the viewpoint of TEM analysis and adjacent dynamic recrystallization grain (C3) of the GH4169 has a specific orientation relationship, [1ˉ14]C2 ∥ [1ˉ10]C3 , (220)C2 ∥ (220)C3. With the friction welding thermal cycles and post weld heat treatment, the g' phase precipitates with tiny spherical distribution at the common grain and the two sides of dynamic recrystallization grains, and coherent with g matrix. But no g" phase precipitates.

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Xiaoyan CHEN,Zhe JIN,Xuefeng BAI,Yizhou ZHOU,Tao JIN,Xiaofeng SUN
Acta Metall Sin    2015, 51 (7): 853-858.   doi:10.11900/0412.1961.2015.00007
Accepted: 08 April 2015

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Superalloy components are always produced by the way of investment casing. During investment casting, interfacial reactions may take place and bring about metal contamination and defect formation on the surface of the components. The influence of C content on the interfacial reaction and wettability between a Ni-based superalloy and ceramic mould was investigated by using a sessile drop method. The interfacial morphology and elements distribution were studied by SEM and EPMA. Activities of C, Cr and Al were calculated by using Thermo-Calc software. The relationship between interfacial reaction and wettability was discussed. It was found that when C content was higher than 0.1%, activity of C increased greatly and interfacial reaction took place. The wettability varied from non-reactive wetting to reactive wetting. In the reactive wetting systems, sand adhesions appeared and Al and Cr diffused to the ceramic surface.

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Yuefei TENG,Yingju LI,Xiaohui FENG,Yuansheng YANG
Acta Metall Sin    2015, 51 (7): 844-852.   doi:10.11900/0412.1961.2014.00692
Accepted: 17 March 2015

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The researches on the grain refinement by applied pulsed magnetic field (PMF) during solidification have received much attention in recent years and lots of positive experimental results indicate that it is a potential method for controlling solidification process. Various grain refinement mechanisms under PMF are proposed and most of them are considered to be relevant to the convection of melt driven by the electromagnetic force. An obvious fact is that the forced convection caused by PMF is strongly limited by the shape of the melt. However, most of previous studies were focused on the cylindrical samples rather than rectangular ones, and actually the later one was widely used in industry. The aim of this work is to investigate the influence of PMF on the grain refinement of K4169 superalloy rectangular samples with various aspect ratios. Grain refinement of K4169 superalloy under PMF was experimentally investigated in the rectangular samples with the aspect ratios of 1.0, 2.0, 4.5 and 5.5 on the transverse section. In order to study the influence of aspect ratio on the forced convection, the distributions of the electromagnetic field, electromagnetic force and melt flow caused by PMF were numerically simulated by finite element software ANSYS. The experimental results show that the grains of the K4169 rectangular samples are coarse equaxied grains without PMF and the grain size slightly decreases with the increase of aspect ratio . Under the PMF with same excitation voltage and frequency, the grains are refined remarkably in the sample with the aspect ratio of 1.0. As the aspect ratio is increased, the grain refinement effect can still be observed but not such obvious. The numerical simulation results indicate that the periodic pushing-pulling electromagnetic force is induced by the PMF, which drives the melt to vibrate and flow circularly. Under the same PMF, the electromagnetic force and fluid rate decreases with the increase of aspect ratio. When the aspect ratio increases from 1.0 to 5.5, the average electromagnetic force and fluid rate in the melt is reduced to 40% and 60%, respectively. The strongest fluid flow and vibration occur in the sample with section aspect ratio 1.0 in the present experiment, which is beneficial for grain refinement due to detachment of the solidified nuclei from mould wall and the break of dendrite arms from dendrite trunks.

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Xuan LI, Xiping GUO, Yanqiang QIAO
Acta Metall Sin    2015, 51 (6): 693-700.   doi:10.11900/0412.1961.2014.00498
Accepted: 20 April 2015

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The rather poor oxidation resistance of Nb-Si base ultrahigh temperature alloys has seriously limited their practical applications at high temperatures. Niobium disilicide coatings, especially those modified by reactive elements (RE) such as Zr and Y, have been shown to possess good anti-oxidation properties at high temperatures due to the formation of a protective RE-containing SiO2 scale. Halide activated pack cementation (HAPC) is one of the most widely used techniques for preparing protective coatings on Nb-Si base ultrahigh temperature alloys, because compact coatings and metallurgical substrate/coating bonds can be obtained with using this technique. However, only a very limited amount of Zr and Y can be diffused into the coatings by a single co-deposition pack cementation process as a result of their large atomic radii and high melting points. To solve this problem, a method such as magnetron sputtering, which can be used for producing overlay coatings with different composition ratios of coating elements, seems to be feasible. In the present study, ZrSi2-NbSi2 multilayer coatings were prepared on an Nb-Ti-Si-Cr base ultrahigh temperature alloy by first magnetron sputtering 2 μm thick Zr-film, and then Si-Y co-deposition at respectively 1150, 1250 and 1350 ℃ by HAPC process. The structures and formation processes, as well as the static oxidation behavior of the coatings were investigated. The results show that the coating prepared at respectively 1150, 1250 and 1350 ℃ had similar structures, consisting of a ZrSi2 outer layer, a (Nb, X)Si2 (X=Ti, Cr, Zr and Hf) middle layer and a (Ti, Nb)5Si4 inner layer. However, the higher co-deposition temperature (1350 ℃) could cause cracks at the interfaces between the constituent layers of the coatings. The formation of the coating was dominated by inward diffusion of Si, accompanied with a certain degree of outward diffusion of Nb, Ti and Cr from the base alloy during the second Si-Y co-deposition process. The oxidation tests demonstrated that the ZrSi2-NbSi2 multilayer coating possessed excellent oxidation resistance. After oxidation, a dense scale consisting of SiO2, TiO2, ZrSiO4 and Cr2O3 formed on the coating, which can protect the base alloy from oxidation at least for 100 h at 1250 ℃ in air.

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PU Sheng, WANG Li, XIE Guang, DING Xianfei, LOU Langhong, FENG Qiang
Acta Metall Sin    2015, 51 (4): 449-457.   doi:10.11900/0412.1961.2014.00425
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The directionally solidificated (DS) Co-based superalloys are widely used in aircraft turbine vanes due to the good stress-rupture parameters and excellent hot corrosion resistance. The cyclic change of temperatures and complex stress state thermal fatigue (TF) cracks happen frequently in vanes during service. However, most of the work are conducted in Ni-based superalloys and there is rare report concerning the TF behavior of DS Co-based superalloys. Furthermore, due to the residual strain accumulated during processing, shot peening, grinding and recrystallization (RX) frequently occur when the DS components are exposed to high temperatures. It is believed that RX may change the microstructure, especially adding more grain boundaries to DS alloys, and result in the reduction of the mechanical properties of DS superalloys. Therefore, in this work, V-notch plate specimens with notch direction perpendicular and parallel to the DS orientation are machined from the DS plate. Local RX grains are prepared (local indented and then heat treated) in the notch areas of some samples. TF test is conducted between 1000 ℃ to room temperature. The effect of DS orientation and RX on TF properties of a DS Co-based superalloy is investigated. The results indicate that the cracks propagate along the interdendritic regions in the samples with notches parallel to the DS direction, which exhibites lower TF properties than samples with notches vertical to the DS direction. TF cracks initiate and propagate along RX boundaries in samples containing RX grains. Precipitation of M23C6 carbides is found along the RX boundaries during TF tests. Due to the oxidation at the tip of crack, M23C6 desquamates and leads to the formation of micro voids, which accelerates the crack propagation and decreases TF properties of tested alloy. In samples with notches parallel to the DS direction, cracks preferentially propagate along the RX grain boundaries.
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PU Sheng, XIE Guang, ZHENG Wei, WANG Dong, LU Yuzhang, LOU Langhong, FENG Qiang
Acta Metall Sin    2015, 51 (2): 239-248.   doi:10.11900/0412.1961.2014.00292
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Ni-based single crystal superalloys have been widely used for blades and vanes in gas turbine. However, recrystallization (RX) induced by residual strain has been a serious problem for the application of single crystal superalloys. In previous work, effect of microstructure, such as ϒ', g/g' eutectics and carbides, as well as heat treatment parameters, on the RX behavior have been studied. However, the effect of alloy elements on the RX behavior has rarely been reported. Therefore, in this work, the effect of the important solution strengthening elements, W and Re, on the deformation and RX of solution heat treated Ni-based single crystal superalloys was investigated. At first, two single crystal superalloys were prepared, and W and Re were added into one alloy among them. After solution heat treatment, these two single crystal superalloys were deformed by shot-peening or Brinell indentation. Then these deformed samples were heat treated to observe the microstructure of RX. It indicated that RX depth decreased with the addition of W and Re irrespective of deformation mode and heat treatment temperature. Short time heat treatment experiment of indented and shot-peened samples both indicated that incubation period of RX was prolonged and nucleation of RX was slowed with the addition of W and Re, which verified that RX was suppressed by W and Re. After shot-peening, micro-hardness of the alloy with W and Re increased, but the depth of deformation zone was obviously reduced. Higher density of dislocation was found in the single crystal superalloy with W and Re, and also lots of dislocation tangles were observed. So, in this alloy, dislocation annihilated slowly, that is, recovery was slowed down, which prolonged the incubation period of RX. During the process of RX grain growth, the maximum RX grain boundary migration velocity was reduced with the addition of W and Re. Moreover, the change of mean RX grain boundary migration velocity showed the same trend with the micro-hardness along the direction of RX depth。
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Huiwen LIN,Jide LIU,Yizhou ZHOU,Tao JIN,Xiaofeng SUN
Acta Metall Sin    2015, 51 (1): 77-84.   doi:10.11900/0412.1961.2014.00364
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The Ni-based single crystal superalloys are considered to be the major materials for advanced areo-engine blades. In order to improve the high temperature properties of Ni-based single crystal superalloys, many refractory elements are introduced into this kind of alloys. Recently Pt has been suggested to be the alloying elements of advanced Ni-based single crystal superalloys. However, there are no researches for the effects of Pt on creep rupture properties of advanced single crystal superalloys. In this work, the influence of Pt element on the creep rupture properties of a Re-containing single crystal superalloy was investigated. The high-temperature creep rupture properties of the Pt-containing Ni-based single crystal superalloy at 1100 ℃, 180 MPa and 1000 ℃, 310 MPa were investigated. The deformation microstructure and the morphology of dislocations were studied by SEM and TEM. The results show that the creep rupture life of Pt-containing superalloy decrease slightly at 1100 ℃, 180 MPa and decreased obviously at 1000 ℃, 310 MPa. The fracture models of different alloys are all ductile fracture, and many irregular microviods and microcracks can be observed in the fracture surfaces. After high temperature creep deformation, regular dislocation networks formed at the g/g' interfaces. The differences of creep rupture properties among those alloys are that Pt element may promote the formation of TCP phase, and the interface between the TCP phase and g matrix may be favorite sites of the initiation of microvoids and microcracks.

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SUN Wen, QIN Xuezhi, GUO Jianting, LOU Langhong, ZHOU Lanzhang
Acta Metall Sin    2015, 51 (1): 67-76.   doi:10.11900/0412.1961.2014.00281
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The Ni-based superalloys are widely used as microstructural components of modern turbine engines due to its good high temperature strength, good fatigue and creep property and excellent hot-corrosion resistance. In order to increase their high temperature strength, more and more refractory elements, such as W and Mo, are added into these alloys while Cr content gradually decreases. During long-term aging, these alloys generally experience various microstructural changes, including coarsening of g' phase coarsening, formation of a continuous grain boundary (GB) carbide network, precipitation topologically close-packed (TCP) phase, and degeneration of MC carbide. However, there is limited available data about the effect of (W+Mo)/Cr ratios on the microstructural evolution of Ni-based superalloys. In this work, the cast Ni-based superalloys with different (W+Mo)/Cr ratios (mass ratios) are fabricated by vacuum induction furnace. After standard heat treated (1110 ℃, 4.5 h, air cooling+750 ℃, 10.5 h, air cooling), they are thermally exposed at 850 ℃ for different times. The stress-rupture tests are operated under the condition of 800 ℃, 294 MPa. Effects of (W+Mo)/Cr ratios on the microstructure evolutions and mechanical properties are investigated by the combination of OM, SEM, TEM and stress-rupture tests. The experiment results show that the (W+Mo)/Cr ratio has no obvious influence on the standard heat treated microstructure, which is mainly composed of g matrix, g' phase, MC carbide and secondary carbides distributing at grain boundaries. During long-term thermal exposure, the microstructure evolutions occur by g' phase coarsening, TCP phases formation, MC degeneration and grain boundary coarsening. The g' phase coarsening behavior is not affected obviously by the (W+Mo)/Cr ratio. However, the amount of TCP phases decreases significantly with decreasing of (W+Mo)/Cr ratio and the type of TCP phases transforms from m phase to coexist of m and s phases when (W+Mo)/Cr ratio decreases from 0.55 to 0.37. There are no TCP phases observed in the sample with (W+Mo)/Cr ratio of 0.22. The thermal stability of MC carbide is reduced obviously and the grain boundaries coarsen more severely by the decrease of (W+Mo)/Cr ratio. The degradation of stress-rupture property is attributed to the coarsening of g' phase and grain boundaries and the formation of TCP phases. Combined with the effect of (W+Mo)/Cr ratio on the solid solution strengthening, microstructure evolution and stress-rupture property, it can be concluded that the optimum stress-rupture property can be obtained when the (W+Mo)/Cr ratio is about 0.37.
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