Static recrystallization behavior of magnesium AZ31 alloy subjected to high speed rolling

Magnesium AZ31 alloy sheets were rolled at 100 °C at two speeds: (i) 1000 m/min and (ii) 15 m/min. The rolled specimens were annealed at temperatures from 200 °C to 500 °C for increasing intervals of time. The recrystallization kinetics were analyzed in terms of the Johnson-Mehl-Avrami-Kolmogorov model and found to involve two sequential annealing stages, characterized by two different Avrami exponents. Stage 1 is associated with recrystallization in the high stored energy regions, such as these containing shear bands and twins, while stage 2 concerns the recrystallization of some of the low stored energy regions. Nevertheless, in the specimens subjected to low reductions (<30%) and annealed at 200 °C for long time periods, some of the elongated grains remained unrecrystallized. This is attributed to the relative lack of formation of twins in these grains during rolling, i.e. to the very low stored energies in these regions. There was no significant difference in the microstructure evolution and recrystallization kinetics of the materials rolled at the two speeds. This is interpreted in terms of the Zener-Hollomon parameter, similar values of which applied to the two types of rolling and therefore to the microstructures produced.