Effect of grain boundaries on texture formation during dynamic recrystallization of magnesium alloys

Basal slip and dynamic recovery of dislocations have been long assumed as the main mechanisms accounting for the sharp texturing of magnesium alloys upon dynamic recrystallization at high temperature. Basal slip allows the basal plane to mitigate stress by reorienting the 〈c〉-axis normal to the loading direction, while dynamic recovery allows dislocations to rearrange into subgrains while maintaining the basal plane parallel to the main loading axis. These phenomena, though pertaining to unquestionable laws of plasticity and thermodynamics, do not fully explain the preferred selection of crystal orientation during nucleation and growth of dynamically recrystallized grains. For example, during extrusion, the basal plane reorients itself in one of two directions, where either the 〈112¯0〉 axis or the 〈101¯0〉 axis is parallel to the extrusion direction. Also, only a few (grain boundary) misorientation relationships are established between the recrystallized and the parent grains. In this paper, electron backscattered diffraction (EBSD) characterization on partially recrystallized microstructures, molecular dynamic simulations, and interfacial defect theory are used to uncover the mechanisms leading to the phenomena of texture formation during dynamic recrystallization. Simulations show that grain boundary energy and mobility emerge as the key governing effects in the experimentally-observed preferred orientation selection during nucleation and growth. Due to the low crystal symmetry inherent to the hexagonal close-packed structure of magnesium, certain grain boundaries possess both low interfacial energies and high mobilities relative to other boundaries, making them prime candidates for accommodating subgrain rotation. In particular, a new experimentally-observed {134¯0} twin boundary is associated with the texture formation; simulations and theory show that the twin boundary has a highly mobile b2/2 disconnection with a wide dislocation core, presumably causing the affected grains to dominate the final texture.