Development of structural heterogeneities in a magnesium alloy processed by high-pressure torsion

An investigation was conducted on a magnesium AZ31 alloy to determine whether the deformation process is homogeneous or heterogeneous during high-pressure torsion (HPT). Disks were processed by HPT at a temperature of 463 K for up to 5 turns and then sectioned vertically across the diameters in order to examine the grain size distributions and the microhardness values on the cross-sectional planes. The results show that deformation is not homogeneous in the through-thickness direction during processing by HPT. The variations in grain size, and therefore in hardness, are due in part to the different grain refinement mechanism that occurs in hcp magnesium alloy by comparison with fcc aluminum. Shear bands were also observed in these samples and they confirm the heterogeneity of the deformation. The heterogeneous nature of flow in HPT processing is probably associated with local variations in the flow stress of the samples during processing.