Papers by Author: Xiao Feng Sun

Abstract: A model for diffusion is applied to the nickel-chromium-aluminum (Ni-Cr-Al) system to optimize the mobility parameters for the B2 phase by fitting the experimentally obtained inter-diffusivities. In the model, the activation energy for diffusion is expressed as a function of the degree of ordering which is evaluated from thermodynamic data. These data are used to evaluate the thermodynamic factors of the B2 phase in the Ni-Cr-Al ternary system. The interdiffusivities from the model are used to simulate diffusion experiments. The results are generally agreed with the experimental data.

Abstract: A single crystal Ni-base superalloy was prepared with five withdrawal rates: 2, 4, 6, 7 and 10 mm/min. Microstructures including dendrite arm spacing, element segregation, and porosity of the as-cast superalloy were investigated. The results showed that as the withdrawal rate increasing, the primary and secondary dendrite arm spacing decreased markedly, the γ/γ′ eutectic became smaller and more dispersive. Meanwhile, when withdrawal rate was higher, W, Ti, Ta and Al segregated in comparatively larger extent. Furthermore, as the withdrawal rate increasing, the amount of alloy microporosity increased, though the size of which decreased gradually. It can be concluded that the withdrawal rate of 4 mm/min and 6 mm/min are optimal for the experimental alloy.

Abstract: The effect of rhenium on the microstructure and mechanical properties of single crystal superalloys with a nominal composition of Ni-3Cr-12Co-1Mo-6W-6Al-8Ta-0.1Hf-(0, 2, 4) Re has been studied. With the rise of rhenium content, the size of as-cast  particles becomes smaller. Rhenium addition elevates the incipient melting temperature and slows down the solid solution process. Even after 2000h prolonged aging both at 950°C and 1050°C, no topologically close-packed phase precipitation is found in the three alloys with Re content up to 4%. Re retards the  coarsening and is beneficial to improving the stress rupture life. The deformation mechanisms together with the dislocation configuration have been studied and discussed.

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.