Effect of Strain Rate on Deformation Mechanisms of a Mg Alloy

The effect of strain rate on dislocation slip and deformation twinning in a Mg-3wt.pctAl alloy was investigated by three-dimensional X-ray diffraction (3DXRD). In situ tensile tests were performed at strain rates of 10(-4), 10(-3), and 10(-2) s(-1). In each tested sample, more than 500 grains were indexed by 3DXRD, and their deformation was tracked. By measuring diffraction peak broadening, plastic deformation can be quantitatively analyzed in individual grains. Basal slip, prismatic slip, and pyramidal I < a > slips were identified in various grains during deformation, and their critical resolved shear stress (CRSS) values were systematically evaluated. At higher strain rates, non-basal slip systems exhibited increased activity, although basal slip remained dominant. Twin nucleation was observed in a limited number of grains, including some with orientations unfavorable for twinning. Interestingly, in certain cases, the resolved shear stress on the activated twinning systems was negative, indicating a deviation from the generalized Schmid law. This phenomenon is likely associated with prior dislocation activity in the matrix grains, which could create localized conditions conducive to twin formation.