Recrystallization mechanism and activation energies of severely-deformed magnesium during annealing process

Abstract

This study investigated the annealing process of severely-deformed magnesium through experimental testing and theoretical modeling to uncover the recrystallization mechanism and derive the corresponding nucleation and grain growth activation energies. The fine grain structure in magnesium was acquired through one type of severe plastic deformation techniques, i.e. equal channel angular pressing (ECAP). The recrystallization was achieved through annealing the ECAPed magnesium samples at various elevated temperatures (i.e., 300 °C, 350 °C, and 400 °C) and durations (i.e., 30, 40, and 60 min). The ECAP processing produced fully twinned samples that possessed heterogeneous microstructure with finer microstructure in a sample's periphery region and coarser microstructure in a sample's center region. The microstructure analysis disclosed that twin-free recrystallized grains primarily nucleated at twin boundaries through the novel integration of the bulging mechanism and the twinning-type local atomic rearrangement. The recrystallization nucleation activation energy (Qn) and Avrami exponent (n) for the ECAPed magnesium were determined to be 22.2 kJ/mol and 5, respectively. This Qn is significantly different from the literature-suggested self-diffusion activation energy and should be the twinning formation activation energy. This study also found that the grain growth activation energies varied with the annealing temperature and the corresponding values were deduced from the experimental data.