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