Recrystallization behavior and phase transformation in a hot-rolled pure cobalt during annealing at the elevated temperature

The evolution of microstructures and properties of the hot rolled cobalt (Co) during the isothermal annealing at different temperature was investigated. Recrystallization only occurred in hexagonal closed-packed (HCP) Co below martensitic transformation temperature (400 °C), the proportion of face-centered cubic (FCC) Co decreased dramatically and the Co sample became an HCP-Co single-phase with increasing annealing time due to the diffusive phase transformation promoted by high angle boundaries (HABs). For samples annealed at 400 °C, recrystallization occurred in both of HCP-Co and FCC-Co. The proportion of FCC-Co reached the highest value of 50% in sample annealed at 400 °C for 4 h, and the average grain size was less than 2.5 μm even after 12 h annealing due to the formation of special boundaries, annealing twins and the alternate permutation of nano FCC-Co lamellae and HCP-Co lamellae. The recrystallization mechanism was analyzed by the Johnson-Mehl-Avrami-Kolmogorov model and modified Arrhenius relationship. The nucleation and grain growth through the migration of HABs promoted the FCC→HCP diffusive phase transformation below martensitic transformation temperature. At temperature above martensitic transformation temperature, recrystallization kinetics of HCP-Co and migration of HABs was hindered by reverse martensitic transformation.