Abstract:

Annealing is one of the most efficient heat treatment to refine the grain microstructure in metal alloys. Prior to the annealing, an aging treatment can be implemented to precipitate the second phases for more nucleation sites in deformed mixed grains during subsequent recrystallization. However, the underlying mechanism behind the second phases and recrystallization has not yet been fully understood. Taking a typical deformed nickel (Ni)-based (GH4169) superalloy as an exapmle, this research aims to clairfy the connections between the recrystallization and delta (δ) phase (Ni₃Nb) during annealing. All deformed samples were also treated by aging and then annealing. The results show that the microstructure and component of δ phase dominated the recrystallization nucleation in the Ni-based alloy. Moreover, there was the presence of the δ phase interval limiting nucleation (PILN) and δ phase protecting nucleation (PPN), when a high content of close-set phase was precipitated from the matrix. The PILN determined the primary condition for the recrystallization nucleation that happended at the interval of δ phase larger than the critical nucleation size. The peak nucleation occurred in the PPN, where the recrystallization nucleus were free of the penetration by the adjacent grains, before the nucleation incubation time was reached. Thus, a new feasible J_N theory was proposed to determine the maximum domain of recrystallization nucleation under close-set precipitated phases. All the possible nucleation sites were assumed under PPN in the J_N theory. As such, the original microstructure can be expected to design for the annealing via the aging process. A quantitative evaluation of recrystallization nucleation can also achieved in the Ni-based alloys during aging.

Key words: Nickel-based superalloy, δ phase, Recrystallization, Nucleation, Metal alloys