Mechanisms of DRX nucleation with grain boundary bulging and subgrain rotation during the hot working of nickel-based superalloys with columnar grains

Nickel-based superalloys were thermomechanically processed at temperatures of 1050–1170 °C and strain rates of 0.001–10 s⁻¹ to study the hot-deformation behavior and dynamic recrystallization (DRX) of the columnar structures. Electron backscatter diffraction (EBSD) was employed to characterize the DRX microstructure of the columnar grains during the thermomechanical processing of the alloys. A linear relationship between the critical strain-hardening rate and critical stress was derived and validated. The critical conditions for DRX were directly determined from the corresponding peak values, which could be used to effectively predict the critical conditions for DRX to occur. It was determined that discontinuous-DRX (DDRX) and continuous-DRX (CDRX) both have an important role in the microstructural evolution. During deformation of the columnar grains, the DRX nuclei first developed at the serrated grain boundaries (GBs) via grain boundary bulging. Subsequently, the continuous misorientation accumulation led to subgrain rotation, which accelerated nucleation within the deformed grains. Owing to DDRX, the DRX nuclei tended to develop at the GBs parallel to the loading direction, leading to the heterogeneous distribution of the newly formed, fine DRX grains. The findings of this study provide a basis for the precise control of the microstructures of nickel-based superalloys with columnar grains during the hot-working process, and therefore, could be used to tailor their properties.