Microstructure and mechanical behaviour of a Mg94Zn2Y4 alloy processed by equal channel angular pressing

A Mg94Zn2Y4 (at%) alloy containing the long-period stacking ordered (LPSO) phase and the Mg24Y5 phase was processed by equal channel angular pressing (ECAP). The ECAP processing develops a bimodal microstructure consisting of large deformed grains (Mg and LPSO) and sub-micron sized dynamically recrystallised (DRXed) grains. The DRXed grain boundaries are decorated with a large numbers of nano-sized Mg24Y5 precipitates. The presence of LPSO lamellae refined the deformed grains by kinking and promoted dynamic recrystallisation during the ECAP process. The ECAP processed alloy was then subjected to the small punch test (SPT). SPT result shows that the ECAP processing increased significantly the strength of the alloy. Under the biaxial tensile stress induced by SPT, the sample started to crack along the ECAP shear direction shortly after the linear elastic region on the load-displacement curve, and the DRXed grains are potential crack sources. These phenomena may be explained by different deformation behaviours of the fibre textured coarse grains and the random oriented DRXed grains, and the distribution of the DRXed grains.