Solidification rate driven microstructural stability and its effect on the creep property of a polycrystalline nickel-based superalloy K465
Résumé
Different solidification rates may cause a significant difference in microstructure and creep property in different locations of turbine blades in aircraft engines. In this study, the effect of solidification rate on microstructural stability and creep property was revealed in a polycrystalline nickel-based superalloy K465. The K465 alloy was cast as a turbine blade, solid bars and hollow tubes. A larger cross section size caused a slower solidification rate in the blade shank and bar than that of the blade airfoil and tube. Microstructural characteristics and corresponding stress rupture properties under 975 °C/225 MPa were investigated after thermal exposure at 900 °C for 300–1000 h. Plate-like μ phase formed only in the interdendritic regions of the blade shank and the bar, but not in the blade airfoil and the tube after thermal exposure. The precipitation of μ phase was mainly responsible for the much worse stress rupture property of the bar in comparison with the tube. The microsegregation degree, chemical composition of γ matrix and precipitates including γ′ phase and various carbides, and dislocation configurations were examined. Compared to the tube, a slower solidification rate caused a higher degree of microsegregation, coarser γ′ precipitates and carbides, as well as a much higher dislocation density in the bar after standard solution treatment. The formation of μ phase was stress-induced and attributed to the remaining dislocations in the interdendritic regions. A longer solution treatment was suggested to effectively suppress the formation of μ phase in the bar and blade shank for practical applications. These results provide a guidance for the manufacturing and evaluation of microstructural degradation of turbine blades made from conventionally cast polycrystalline nickel-based superalloys.