Microstructural evolution of NiCoCrAlY coated directionally solidified superalloy

Abstract

The microstructure evolution of the interdiffusion zone (IDZ), secondary reaction zone (SRZ), and substrate diffusion zone (SDZ) formed beneath the coating/substrate interface in a directionally solidified (DS) nickel-based superalloy during thermal exposure has been investigated. Recrystallization occurred in the substrate below the coating at both 900 °C and 1000 °C. However, significant differences in the evolution of the SRZ and topologically close-packed (TCP) phases were observed at 900 °C and 1000 °C. Numerous rode-like σ phases precipitated in the γ'-based cellular recrystallization (CR) zone at 900 °C for 100 h. In contrast, after 100 h at 1000 °C, a continuous γ layer was formed in the IDZ and moderate amounts of needle-like σ precipitates were found in the SDZ. Furthermore, after thermal exposure for 1000 h at 900 °C, the rod-like σ-phases within the SRZ gradually dissolved with substantial precipitation of M23C6, and the number of needle-like σ-phases increased and grew in size. Nevertheless, after exposure at 1000 °C for 1000 h, the SRZ was completely consumed by the IDZ and the needle-like σ phases in the SDZ were incorporated with the γ layer and dissolved progressively.