Microstructure evolution and recrystallization mechanisms of a fine-grained nickel-based superalloy during thermal deformation process

Abstract

The thermal deformation behavior of fine-grained GH93 alloy was investigated through isothermal compression tests at deformation temperatures of 930–1020 °C, strain rates of 0.001–1 s−1 and deformation amount of 50 %. Based on the true stress-strain data, the power dissipation maps, instability maps and hot processing maps of this alloy were established at different strains. The precision of the hot processing maps was verified by deformation microstructures, and the optimal processing parameter was ascertained to be 975–1010 °C/0.001–1 s−1. The microstructure evolution, dynamic recrystallization (DRX) and twinning evolution were analyzed by means of EBSD and SEM, simultaneously the DRX mechanisms under different deformation parameters were elucidated. The results demonstrate that the degree of recrystallization increases with the increase of deformation temperatures and the decrease of strain rates. At lower deformation temperatures, γ′ phases at grain boundaries hinder grain boundaries to slip and encourage dislocation entanglement, thereby promoting recrystallization. As the deformation temperatures increase, γ′ phases gradually dissolve, and the pinning effect weakens, leading to grain coarsening. The proportion of ∑3 twin boundaries shows a continuous upward trend with increasing temperature, which is consistent with the trend of changes in recrystallization volume fraction. DRX mechanisms of the alloy under different process parameters include continuous dynamic recrystallization (CDRX) mechanism and discontinuous dynamic recrystallization (DDRX) mechanism. DDRX is the main recrystallization mechanism, and CDRX mechanism becomes more active with increasing temperatures and decreasing strain rates, while it is still only an auxiliary nucleation mechanism.