Transition of dynamic recrystallization mechanisms of as-cast AZ31 Mg alloys during hot compression

This study investigates the transition of dynamic recrystallization (DRX) mechanisms of as-cast AZ31 Mg alloys during hot compression. It is experimentally revealed that the transition between the twin-induced dynamic recrystallization (TDRX) and the grain boundary bulging dynamic recrystallization (GBBDRX) depends on the local strain accumulation and grain size. Based on the secondary twinning growth controlled by the grain size and stresses, a theoretical DRX transition criterion is built. Subsequently, two cooperating DRX schemes are implemented into the visco-plastic self-consistent crystal plasticity model to simulate the evolution of slip and twinning activity, textures and stress-strain relationships influenced by the double DRX mechanisms and the transition. The results show: (1) For as-cast Mg alloys with coarse grains, the TDRX is common at low-strain stage, and is the key to continuing plastic deformation. The smaller twin walled grains that inherit the c-axis orientation of the parent twins can weaken the deformation basal texture, enhancing basal slip activity and reducing the extension twinning activity; (2) The GBBDRX that can decrease local dislocation density replaces the TDRX if the post-saturation state of secondary twinning is achieved by the applied stress together with local stress within refined grains. The dominant GBBDRX mechanism strengthens the basal texture by limiting pyramidal slip. The activities of basal and prismatic slip are reduced and enhanced by the GBBDRX, respectively; (3) The TDRX leads to a dramatic decrease of strain-hardening rate, and the transition between the two successive DRX mechanisms switches the decrease of the hardening rate to flow softening. Furthermore, some control schemes of DRX transition are proposed according to the transition criterion.