Microstructure evolution and kinetics of recrystallization and grain growth of hot-rolled Mg-Gd alloy

The hot-rolled Mg-0.6Gd (wt%) alloy at 723 K for 85 % thickness reduction was subjected to annealing treatments at a temperature range of 573-723 K for durations ranging from 5 to 1440 min to study the microstructure evolution. In addition, the recrystallization and grain growth kinetics were evaluated using Vickers microhardness and mean grain size variation, respectively. The deformed microstructure characterized by the fragmentation of coarse initial grains and a high number of twins was retained after annealing at 573 and 623 K. In contrast, a microstructure with equiaxed recrystallized grains developed rapidly during annealing at 673 and 723 K. The mean grain size increased with both annealing temperature and time, reaching values of 11.8 ± 1.3, 21.1 ± 2.2, 36.1 ± 1.4, 48.9 ± 2.5 µm after 1440 min at 573, 623, 673 and 723 K, respectively. Hot-rolling and heat treatments altered the nature of the existing second-phase particles, which controlled the obtained microstructures via solute drag and the Zener pinning effect. Based on the Johnson-Mehl-Avrami Kolmogorov model, the recrystallization activation energy was 107.1 ± 14.1 kJ/mol, suggesting that the grain boundary diffusion controlled the static recrystallization. Grain growth behaviour was distinguished into two temperature regimes: from 573 to 623 K and from 623 to 723 K. At the 573-623 K range, the grain growth was restricted and the obtained activation energy of 200 kJ/mol evidenced the control of lattice self-diffusion. At the 623-723 K range, the activation energy decreased significantly to 50.1 ± 5.5 kJ/mol, indicating enhanced grain growth.