Microstructural evolution of a Ni-Co based superalloy during hot compression at γ′ sub-/super-solvus temperatures

The effects of strain rate on the microstructural evolution and deformation mechanism of a Ni-Co based superalloy were investigated by isothermal compression tests performed at γ′ sub-solvus (1090 °C) and γ′ super-solvus temperatures (1150 °C) with a wide strain rate range from 0.001 to 10 s-1 under a true strain of 0.693. Electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and transmission electron microscope (TEM) techniques were used to characterize the microstructures. The results revealed that the dynamic recrystallization (DRX) volume fraction increased and stored energy of the γ matrix grains decreased with increasing the strain rate during γ′ sub-solvus temperature deformation, while the opposite phenomena were observed during γ′ super-solvus temperature deformation. The comprehensive effect of initial grain size, primary γ′ phase, twins and adiabatic temperature rise led to these results. The primary γ′ particles undergone the deformation behavior within itself and obviously accelerated the DRX of the matrix. The microstructural evolution proved that discontinuous dynamic recrystallization (DDRX) was the dominant mechanism during the hot deformation carried out at both γ′ sub-solvus and γ′ super-solvus temperatures. Primary γ′ particles obviously accelerated the nucleation step and retarded the growth step of DDRX during γ′ sub-solvus temperature deformation. Besides, the acceleration effect of primary γ′ particles on DDRX increased with the increase of strain rate. Continuous dynamic recrystallization (CDRX) was confirmed to be an assistant mechanism during γ′ super-solvus temperature deformation and was promoted with the increase of strain rate.