Materials Science Forum Vols. 747-748

Abstract: The phase equilibrium and the precipitation behavior of the γ' phase in the Co-Al-W-Ni-Cr superalloys were studied by thermodynamic method. The phase equilibrium of the Co-Al-W-15Ni-5Cr alloy at 900 was calculated, and it was found that there were a γ' single-phase region and a γ+γ' two-phase region. The effects of Al/W ratio on the precipitation of the γ' phase were studied, and it was clear that higher mole fraction of the γ' phase can be obtained with the Al/W ratio of 2.5. The stability of the γ' phase can be improved by adding of Ni, as well as the servicing temperature of the Co-Al-W based superalloys. The results obtained in this work can be used for the developing of the new-type Co-based superalloys.

Abstract: The influence of three different ageing treatments (R1:1100/2h,air cooling+870/32h,air cooling, R2: 870/32h,air cooling and R3: 1060/2h,air cooling+870/32h,air cooling) on stress rupture properties of Ni₃Al-base single-crystal alloy IC21 was investigated. The results indicate that ageing heat treatments have obvious effects on stress rupture properties of IC21 at 850/500Mpa. After R3 ageing treatment, IC21 alloy presents the longest rupture life and the smallest ellipticity and elongation compared to those after the other two ageing treatments. Microstructure examination shows that the mean size of γ precipitate is about 0.44μm after R3 ageing heat treatment (1060/2h, ac + 870/32h, ac). Transmission electron microscope (TEM) study on the rupture samples illustrates that after R1 and R2 ageing heat treatments, the density of stacking faults increases and the length is larger compared to that after R3 heat treatment. Meanwhile the shearings of γ precipitates are more severe. The appropriate γ phase size and γ channel width after R3 treatment promote homogenous deformation by <110>{111} slip in the matrix, and facilitate the formation of finer dislocation networks on the γ/γ interface, which can restrain the shearing of γ phase by dislocations.

Abstract: C32 is a new type of Ni₃Al-based single crystal alloy, which has been designed to serve at 1200. The effect of heat treatment on the microstructure of single crystal superalloy IC32 was investigated. The microstructure after heat treatment under different solution and aging temperature was examined by using optical microscope (OM), scanning electron microscopy (SEM) and electron probe microscopic analyzer (EPMA). The as-cast microstructure of IC32 consisted of a two-phase γ/γ microstructure, and primary bulky γ phase which priciptated in the interdendritic region. The incipient melting temperature of IC32 is 1360, and the heat treatment window is about 40. Considering high contents of refractory elements Re and Ta, a complicated 18 hours solution heat treatment at different temperatures was conducted to elimate the segregation and dissolove the primary bulky γ phase. The optimum heat treatment of IC32 alloy is 1325/4h+1335/4h+1345/10h, ac + 1165/2h, ac + 870/32h, ac.

Abstract: By means of creep properties measurement and microstructure observation, the influence of the traces P, B on creep behaviors and fracture mechanism of GH4169 alloy was investigated. Results showed that during creep the twinning was thought to be the main deformation mechanism of GH4169 alloy, however the deformation mechanisms of alloy containing P, B were the twinning and slipping of dislocations activated within the grains. And the fact that the slipping of dislocations activated within the grain can delay the stress concentration in the local regions to restrain the initiation of the cracks to improve the creep resistance of the alloy. Compared to GH4169 alloy, the fracture of GH4169G alloy displayed the non-smooth surface. The adding traces P, B were segregated in the region near the boundary, which may promote the particle-like δ phase precipitated along the boundary to restrain the boundaries slipping and the cracks propagation. This was thought to be the main reason for enhancing the strength of the boundary and prolonging the creep life.

Abstract: The effect of long-term aging on stress rupture properties was investigated by means of microstructure observation and properties measurement. The results show that, after long-term aging for 500 h at 723 K, the coarser particle-like γ phase is precipitated along the grain boundaries of alloy, significant amount of γ particles are regularly distributed within the grain. Moreover, some carbide particles precipitated depressively along the boundaries and within the grain. As the aged time and temperature increase, the size of fine γ particles in the alloy increases slightly, and the stress rupture property of the alloy decreases under the condition of 923K and 1034MPa. The deformation features of the alloy during creep are that the dislocations slip in the matrix or shear into the γ phase, the dislocations shearing into the γ phase may be decomposed to form the configuration of the two partial-dislocations plus stacking fault. As the creep goes on, the slipping traces with different orientations appear in the surface of the specimen, and some slipping steps are formed in the region near the boundaries, which may cause the stress concentration to promote the initiation and propagation of the cracks along the boundaries up to creep fracture.

Abstract: The effect of long-term aging on stress rupture properties was investigated by means of microstructure observation and properties measurement. The results showed that, after long-term aging for 500 h at 723 K, the coarser particle-like γ phase was precipitated along the grain boundaries of alloy, significant amount of γ particles were regularly distributed within the grain. Moreover, some carbide particles precipitated depressively along the boundaries and within the grain. As the aged time and temperature increased, the size of fine γ particles in the alloy increased slightly, and the stress rupture property of the alloy decreased under the condition of 923K and 1034MPa. The deformation features of the alloy during creep were that the dislocations slip in the matrix or shear into the γ phase, the dislocations shearing into the γ phase may be decomposed to form the configuration of the two partial-dislocations plus stacking fault. As the creep went on, the slipping traces with different orientations appeared in the surface of the specimen, and some slipping steps were formed in the region near the boundaries, which may cause the stress concentration to promote the initiation and propagation of the cracks along the boundaries up to creep fracture.

Abstract: By means of solution treatment at various temperatures, creep properties measurement and microstructure observation, the effects of heat treatment on composition segregation and creep properties were investigated. Results show that the various segregation extents of the elements are displayed in the alloys solution treated at different temperatures, and the segregation extent of the elements is improved with the solution temperature elevated, which may obvious improve the creep resistance of the alloy. And no rafted structure of the γ phase is detected in the alloy during creep at medium temperature. The deformation features of the alloy during creep at medium temperature are that the slipping of dislocations is activated in the γ matrix channels, and dislocations shearing into the γ phase may be decomposed to form the configuration of partials + stacking faults, which may hinder the cross-slipping of the dislocations to improve the creep resistance of the alloy.

Abstract: By means of creep property measurement and microstructure observation, an investigation has been made into the creep behaviors of DZ125 superalloy at high temperature and low stress. Results showed that the superalloy under the applied stress of 137MPa at 1293 K displayed a better creep resistance, and the apparent creep activation energy of the alloy during steady state creep was measured to be Q = 325.57 kJ/mol. The various microstructures were displayed in different regions of the sample, thereinto, the rafted γ phase was uniformly distributed in the regions far away from the fracture, but the twisted and coarser rafted γ phase appeared in the region near the fracture. The deformation mechanism of the alloy during steady state creep was the dislocations climbing over the rafted γ phase. In the later stage of creep, significant amount of dislocations shearing into the rafted γ-phase promoted the initiation and propagation of the cracks along the boundaries up to the occurrence of fracture, which was though to the fracture mechanism of the alloy during creep.

Abstract: Hot compressive deformation of Udimet720Li alloy was carried out on Gleeble-3500 thermal mechanical simulator. The flow stress behavior of Udimet720Li alloy during hot compression was studied in the temperature range of 1100-1160 and at a strain rate of 0.001-1s ^-1. The results showed that the flow stress was controlled by both strain rate and deforming temperature. The flow stress decreased with the increase of deforming temperature, while increased with the increase of strain rate. The change of flow stress with deformation thermal parameters was revealed from true stress-true strain curves, and constitutive relationship of Udimet720Li alloy was obtained on the base of Arrhenius equations and the deformation activation energy was calculated.

Abstract: The T15 high speed steel (HSS) spray formed (SF) billets were manufactured using a twin-atomizer spray forming facility at Beijing Institute of Aeronautical Materials (BIAM) AVIC, China. The billets were then hot isostatically pressed (HIP) following hot forge (HF) and heat treatment (HT). The as-spray formed billet is integrally dense and has a high relative density of >98%, and the macrostructure is free of macrosegregation. The microstructure consists of polygonal grains of 20µm with tempered martensite inside the grains and refined carbides uniformly distributed along the grain boundaries. Hardness and three-point bending properties were also measured. The spray formed materials was comparable to those made by powder metallurgy in terms of microstructure uniformity, hardness and bending property.