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 degrees 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 degrees 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 degrees C for 4 h, and the average grain size was less than 2.5 mu m 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-MehlAvrami-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.

Automated summary

The evolution of microstructures and properties of hot rolled cobalt during isothermal annealing at different temperatures was studied. It was found that recrystallization only occurred in hexagonal closed-packed (HCP) cobalt below the martensitic transformation temperature, while the proportion of face-centered cubic (FCC) cobalt decreased significantly. With increasing annealing time, the cobalt sample transformed into a single-phase HCP-Co due to the diffusion phase transformation promoted by high angle boundaries (HABs). Recrystallization occurred in both HCP-Co and FCC-Co for samples annealed at 400 degrees C, with the proportion of FCC-Co reaching its highest value of 50% at 4 hours of annealing. Special boundaries, annealing twins, and the alternate permutation of nano FCC-Co lamellae and HCP-Co lamellae contributed to an average grain size of less than 2.5 μm even after 12 hours of annealing. The recrystallization mechanism was analyzed using 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 the martensitic transformation temperature. On the other hand, recrystallization kinetics of HCP-Co and migration of HABs were hindered by reverse martensitic transformation at temperatures above the martensitic transformation temperature.