Grain size and void formation in Mg alloy AZ31

The present study examines void formation during tensile loading in coarse and fine-grained AZ31 magnesium alloy using tensile testing, scanning electron microscopy and micro X-ray tomography techniques. Reducing the grain size from 30 to 4.5 μm doubles the total tensile elongation. At failure, the fine-grained material displays a higher volumetric number density of voids compared to the coarse-grained counterpart. Large voids with lower sphericity are considerably more prevalent in the coarse-grained material. Depending on the grain size, two different void nucleation mechanisms could be distinguished; failure at twin or grain boundaries or at second-phase particles. The dominant mechanism in the fine-grained material is failure at second-phase particles. The void volume fractions at failure were comparable in both materials. We propose that the present results can be understood in terms of the effect of grain size on the rate at which the void fraction grows with strain. The larger voids formed in the large grain sized samples lead to a more rapid increase in void fraction with strain and thus failure ensues at lower strains.