A novel annealing method to uniformly refine deformed mixed grain microstructure of a solution-treated Ni-based superalloy

Abstract

Increasing static recrystallization (SRX) nucleation rate and decreasing grain growth rate are the main ways to obtain uniform and fine grain microstructure by annealing treatment. However, it is contradictory to raise the SRX nucleation rate and reduce grain growth rate during isothermal annealing treatment. This is because the increase of the SRX nucleation rate needs to raise annealing temperature, while the decline of grain growth rate needs to reduce the annealing temperature. To solve the contradiction, a novel method named cooling recrystallization annealing treatment (CRT) is designed and verified. For the CRT, the relatively high annealing temperature in the primary stage can promote SRX nucleation, and the gradually decreased annealing temperature can reduce the growth rate of SRX grains. Besides, an aged treatment is first carried out to precipitate high content of the δ phase before the CRT. Enough δ phases not only provide large numbers of sites for SRX nucleation but also limit the grain growth due to the pinning effect, especially in the primary high-temperature stage. The results show that the novel method can refine deformed mixed grain microstructure well when the suitable cooling rate, start and final recrystallization annealing temperatures are employed. The deformed mixed grain can be uniformly refined to an average size of 8.26 µm with the route of 900°C×12 h+1020°C→970°C×20 min.