Gamma prime and TCP phases and mechanical properties of thermally exposed nickel-base superalloy

Abstract

The effect of long-term thermal exposure and casting superheat on microstructure, topologically close-packed (TCP) phases, γ ′ precipitation and mechanical properties of an experimental Ni-base superalloy were studied. The investigated alloys were produced by investment casting process under two levels of superheat. After solution heat treatment, at 1180°C for 2 h followed by air cooling; the two alloys under investigation were isothermally exposed at 845°C for 24, 200, 1000, and 1500 h. The long-term thermal exposure conditions have a significant impact on the precipitation and morphology of TCP and γ ′ phases. The σ phase precipitated as needle and platelet shapes, whereas the μ phase formed in plate and agglomerated shapes. The μ phase has high concentration of Cr, Mo, W, and Co, while the σ phase has high percentages of Ni and Ti. The μ phase was precipitated after thermal exposure of 200 h in the case of high superheat specimen and after 1000 h in low superheat specimen. The η phase found was also a thick plate-like shape in the microstructure of both alloys in the interdendritic zones. The optimum size and volume fraction of γ ′ precipitates were obtained after being thermally exposed for 200 and 1000 h for high and low superheat alloys, respectively. Consequently, the highest hardness level was achieved at the optimum conditions of γ ′ precipitates in high and low superheat alloys.