Microstructural changes by heat treatment for single crystal superalloy exposed at high temperature

The effects of heat treatment for recovering microstructure of Ni-based single crystal superalloy with added C and Hf after thermal exposure have been evaluated. The optimum heat treatment conditions were drawn from the microstructure analysis, after thermal exposure at 982 °C for 3000 h and rejuvenation treatment. The solvus and solidus temperatures of this alloy were 1305 and 1315 °C respectively, and most of Ta and Ti-rich carbide dissolved after performing γ′ solid solution treatment at 1305–1310 °C. The fraction of microporosity and eutectic phase showed a tendency of increase at solvus temperature. Also, an uphill diffusion of Al, Cr, Ta and Re was observed in eutectic phase at the solvus temperature, and the change of the composition of Ta was relatively higher than other elements. The gradient of heating curves in DSC analysis showed its sharp change after γ′ solid solution treatment at the temperature below solvus temperature of 1305 °C because of the existence of un-dissolved coarse γ′ in the interdendritic region.

► Most of carbides in the single crystal superalloy dissolved at the solvus temperature.
► The fraction of eutectic and microporosity increased only at the solvus temperature.
► The gradient of DSC heating curve changed sharply according to solution treatment.
► The up-hill diffusion in the eutectic phase was observed at the solvus temperature.