Microstructural nature and stability of Co-rich TCP phases in Ru-containing single crystal superalloys

Abstract

Destruction of microstructural stability caused by topologically close-packed (TCP) phase precipitation restricted the industrial application of advanced single crystal superalloys. However, systematic investigations on microstructural nature and evolution of Co-rich TCP phases in Ru-containing single crystal superalloys with high level of Co addition have been rarely reported. In this study, experimental results indicated that the main TCP phase was μ phase after thermal exposure at 950 °C for 1000 h, while R phase was the main phase after thermal exposure at 1100 °C for 1000 h in the alloys containing high level of Co and different levels of Cr and Mo. The experimental and thermodynamic calculation results showed that R phase was more stable at 1100 °C than that of μ phase in the experimental alloys. Meanwhile, μ phase started to transform to R phase at 1100 °C for 50 h, and the stress accelerated the transformation of μ phase and R phase during creep test at 1100 °C/140 MPa in an alloy containing high levels of Co, Cr and Mo additions. Both R phase and μ phase were enriched in Re, W, Mo, Cr and Co, while R phase contained more Cr and μ phase was more enriched in Mo. This study contributes to better understand the microstructural evolution of TCP phases at different temperatures and to optimize the alloy design and thermodynamic database of Ru-containing single crystal superalloys with high level of Co addition.