Dynamic recrystallization and twinning mechanisms during the hot deformation of Haynes 188 cobalt-based superalloy

This study investigates the dynamic recrystallization (DRX) and twinning behaviors in Haynes 188 cobalt-based superalloy during hot deformation, aiming to reveal the correlation between forming conditions and microstructures. Three DRX mechanisms, including discontinuous (DDRX), continuous (CDRX) and twin-induced (TDRX), are identified, among which DDRX dominates under all conditions. The dominance of DDRX arises from the initial precipitate promoting DDRX nucleation, the fragmentation of twins hindering CDRX as well as the limited TDRX in deformed grains. Twins are extensively fragmented to accommodate deformation and occasionally induce TDRX grains, while new twins can also form within DRX grains and finally reach a dynamic equilibrium with increasing strain. Additionally, both temperature and strain rate have a non-monotonic effect on DRX fraction and twin density. DRX fraction increases with temperature due to enhanced grain boundary mobility, while twin density first decreases and then increases. This obvious decrease is due to initial twin fragmentation outpacing new twin formation when DRX is limited. Above 950 °C or at higher strain rates, twin density is positively correlated with DRX fraction, as twin formation is coupled with ongoing DRX. These findings provide fundamental guidance for designing thermomechanical processing routes to achieve finer microstructure and enhanced mechanical performance in Haynes 188 components.