Microstructure evolution characteristics of a powder metallurgy Ni-based superalloy during hot deformation

Microstructure evolution characteristics of a powder metallurgy (PM) Ni-based superalloy during hot deformation were investigated by isothermal compression tests at 1050 ˚C–1200 ˚C for strain rates of 0.001 s−1–1 s−1 in this work. And the relationships between the dynamic recrystallization (DRX) behavior, DRX mechanisms, twins, and γ´ were discussed. The results showed that discontinuous dynamic recrystallization (DDRX) which was nucleated by grain boundary bulging, occurred in all deformation parameters in this superalloy. For γ/γ´ poly-phase region deformation (1050 ˚C-1140 ˚C), the volume fraction of dynamically recrystallized (DRXed) grains increased with the increase in strain rates. The finding suggested that γ´ particles promoted the nucleation of subgrains in deformed grains, causing the continuous dynamic recrystallization (CDRX) mechanism, characterized by subgrain continuous rotation, to be another dominant DRX mechanism except for DDRX. Moreover, the formation of twin boundaries occurred under the comprehensive control of DRX and grain boundary migration. DRX promoted the formation of twin boundaries, but at high temperatures, the reduction of the free energy from grain growth was higher than twin formation, causing the possibility of twin formation to be reduced. For γ/γ´ poly-phase deformation, the area fraction and size of γ´ increased with decreasing compression temperatures and increasing strain rates, and high compression temperatures and low strain rates promoted the further dissolution of γ´. The optimum hot deformation processing conditions for PM Ni-based superalloy is 1110–1140 ˚C/0.01–0.1 s−1.