Coupling of precipitation and recrystallization phenomena in a γ-γ' Ni-base superalloy
Résumé
Recrystallization and second phase precipitate evolution during hot forging sequences were investigated in a Ni base superalloy. The alloy under study consists of a ã matrix and ã’ particles (Ni3(Al,Ti)): large primary particles aiming at grain boundary pinning, and fine hardening ones which are dissolved at the forging temperatures but form again during cooling. The applied forging conditions allow (dynamic and post-dynamic) recrystallization to occur and precipitation to evolve simultaneously. While many studies are dedicated either to recrystallization in ã matrix or ã’ precipitation phenomena, the focus is placed here on the coupling of both phenomena: recrystallization influencing ã’ precipitation, and ã’ particles influencing recrystallization mechanisms. Nucleation and growth of ã’ into ã and possibly of ã into ã’ occur as diffusional transformations. Scenarios are proposed to account for the observed evolutions, based on consideration of stored energy, chemical composition of grains and precipitates, boundary energies and migration.