Grain size dependence of active slip systems in an AZ31 magnesium alloy

Polycrystalline Alloys of Mg-3Al-1Zn (AZ31) were obtained by equal channel angular extrusion and subsequent annealing at 300°C for 24 h or at 500°C for 4 h. These samples had average grain sizes of 7 μm and 50 μm, respectively. Tensile tests were performed at room temperature at an initial strain rate of 1 × 10^-3 s^-1. The microstructure of the deformed samples at 2% strain was observed by transmission electron microscopy. Active slip systems were determined from the ratio of resolved shear stress (plastic anisotropy factor, PAF) for non-basal/basal slip systems. In the material with large grains (PAF = 15.4), non-basal a dislocations were active near grain boundaries, while basal a dislocations were dominant in the grain interior. In the material with fine grains (PAF = 0.88), non-basal a dislocations were active in all regions. The density of non-basal a dislocation segments was found to be 40% of the total dislocation density. The activation of non-basal a dislocations is attributed to induced stresses induced to maintain grain-boundary compatibility. In large grains this effect is limited near the grain boundary. In fine grains, this effect occurs within the entire grain.