Compressive deformation and fracture behaviors of AZ31 magnesium alloys with equiaxed grains or bimodal grains

The compressive mechanical properties, microstructural evolution and fracture behaviors of AZ31 alloys with equiaxed grains (E-grain) or bimodal grains (B-grain) were investigated using optical microscopy, scanning electron microscopy and electron backscattered diffraction. The results indicate that the ratio of twinned grains to all grains increases for both E-grain and B-grain samples with increasing strain levels during compression deformation along the extruded direction. The average Schmid factor for {10-12} twins of B-grain samples is higher than that of E-grain samples, which lowers required activating stress for {10-12} twins in B-grain samples. This causes a higher quantity of twins in B-grain samples than that of E-grain samples at the same pre-compression level. The E-grain samples mainly exhibit parallel twins regardless of pre-compression levels, whereas the twins in coarse grains of B-grain samples are parallel to each other at low compressive strain level and then transform to crossed twins at high compressive strain level with parallel twins in fine grains. Additionally, the peak strength, strain hardening rate and fracture strain of E-grain samples are lower than those of B-grain samples, and they possess similar yield strength. The coarse grains in the B-grain samples effectively resist crack growth by deflecting and branching crack tips, bridging of the cracks and forming the secondary cracks. Meanwhile, the fine grains in B-grain samples accommodate plastic strain and mediate multiple grains' deformation. The concurrent effect of coarse grains and fine grains results in the improvement of plasticity.