Papers by Author: Heng Rong Guan

Abstract: a kind of as-cast nickel-base single crystal superalloy was TLP bonded using Ni-Cr-B amorphous foil at different temperatures. Special attention is paid to the formation of boride in diffusion zone of TLP joints at different conditions. The chemical composition and microstructure of borides were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). At different bonding temperature, M3B2 precipitates appear distinct morphologies. At 1200°C, both blocky and plate-like borides formed owing to the diffusion of boron atoms into base metal and precipitation during the cooling process. At 1230°C or above, due to the diffusion of boron atoms the constitutional liquation of original γ/γ′ eutectics in the base metal occurs and borides formed when the system was cooled to room temperature. The analysis of TEM results reveals that M3B2 has a tetragonal structure and is rich in Mo, W, and Cr elements.

Abstract: Effect of different aging temperatures on microstructure and stress rupture properties of DZ951 alloy were investigated in this paper. The results show that the shape of carbide changed from script-like in as-cast alloy to block during different aging treatments. MC carbide degrades into M23C6 at the aging temperature of 970°C, which made alloy have a better combination of strength and ductility than that at other aging temperatures. The size of γ′ phase increases and the shape of γ′ phase changed from sphere (870°C) to quasi-cuboid (920°C) and cuboid (970°C) with the increasing aging temperature. The stress rupture life of DZ951 alloy at 1100°C/60MPa improves with the increased of aging temperature. The fractographs at different conditions showed a ductile fracture mode.

Abstract: Smooth specimens of single crystal (SC) superalloy SRR99 with [001] orientation were subjected to high-cycle fatigue (HCF) loading at temperatures of 700°C, 760°C, 850°C and 900°C in air atmosphere. The results demonstrated that conditional fatigue strength reached the maximum at 760°C and decreases with increasing temperature. Analysis on fracture surface showed a trend for cleavage rupture at 850°C and 900°C and ductile rupture at 700°C and 760°C. Fatigue cracks initiated at the surface or subsurface were primarily responsible for the ultimate failure. The influence of testing temperature on fatigue lifetime was studied by examining evolution of the microstructure through SEM observation. With the process of cyclic loading at elevated temperatures, the primary cuboidal γ′ precipitates tended to agglomerate and spheroidized, meanwhile a larger number of secondary γ′ particles were formed in the γ matrix in specimens fatigue tested at 700°C, which would have a significant effect on the high temperature properties.

Abstract: Stress rupture and tensile properties of a single crystal superalloy DD32 are investigated comparing with the alloy SRR99. It is shown that the alloy DD32 offers an improved creep temperature capability of more than 60°C at higher stresses. The g¢ precipitates in the stress ruptured samples were rafted to P-N type directional coarsening. The fracture mechanism of the stress ruptured samples was initiated from the micropores.

Abstract: The precipitation behavior during ageing treatment of a single crystal nickel-base superalloy was investigated by SEM and TEM. The results showed that tetragonal needle-like σ phase and blocky -W phase precipitated during low temperature ageing treatment after this testing alloy was completely solution heat treated. σ and -W phases robbed of solid solution strengthening alloying element W、Mo in the matrix and degraded high temperature creep rupture property severely. The creep curve of the crystal tested at 1010 °C and 248 MPa exhibited that the steady state creep rate ε was as high as 9.46 × 10-3/h. The creep-rupture life was only 25 hours. -W phase was not formed by decrease of W content properly. A relatively low level of Co could inhibit σ phase precipitation and improve microstructural stability.

Abstract: The tensile deformation behavior of M963 superalloy treated by various heat treatments has been studied at both room and high temperature (1173K). The result shows that the alloy treated by the standard solution treatment at 1483K for 4h followed by air-cooling has low ductility especially at 1173K. The additional aging treatment at 1123K for 16h followed by furnace cooling can recover the ductility of the alloy at 1173K, but further decrease its room temperature ductility. The TEM observation shows that the deformation mechanism varied with both the testing temperature and heat treatment. Finally, the mechanism of the aging treatment on the deformation behavior of the M963 superalloy is discussed.

Abstract: The effect of melt treatment on the microstructure and creep-rupture behavior of M963 superalloy at 1248K under 225MPa has been investigated. The microstructure of the as-cast superalloy without melt treatment consists of γ solid solution matrix, γ´precipitate, coarse blocky MC carbide and (γ+γ´)eutectic. The striking difference in microstructure is that the melt treatment changes the MC carbide from the coarse blocky morphology into the fine script-like morphology. After heat-treated at 1483K for 4h followed by air-cooling, both the creep life and rupture elongation of the melt-treated alloy are all doubled those of the alloy without melt treatment. The mechanism of the melt treatment on the creep-rupture behavior of the M963 superalloy is discussed.

Abstract: The tensile deformation behavior of M963 superalloy treated by various heat treatments has been studied at both room and high temperature (1173K). The result shows that the alloy treated by the standard solution treatment at 1483K for 4h followed by air-cooling has low ductility especially at 1173K. The additional aging treatment at 1123K for 16h followed by furnace cooling can recover the ductility of the alloy at 1173K, but further decrease its room temperature ductility. The TEM observation shows that the deformation mechanism varied with both the testing temperature and heat treatment. Finally, the mechanism of the aging treatment on the deformation behavior of the M963 superalloy is discussed.