Influence of a novel SPD technique together with heat treatment on the microstructural characteristics and hardness of Mg-13Gd-4Y-2Zn-0.5Zr alloys

In this study, Mg-13Gd-4Y-2Zn-0.5Zr alloys were fabricated and subjected to 3 passes of cyclic expansion-extrusion with an asymmetrical cavity (CEE-AEC). The influence of the CEE-AEC together with the heat treatment on the microstructural characteristics and hardness were investigated systematically, through optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Vickers hardness testing machine. The results illuminated that the introduction of the strains through CEE-AEC has a significant effect on the heat treatment of the specimens. The precipitation of the second phases particles was remarkably accelerated, including the lamellar phases in solution treatment, the grain boundary precipitates and the equilibrium beta phases in ageing treatment. Likewise, the hardness of the investigated samples was obviously improved by the comprehensive effect of the CEE-AEC together with the heat treatment, and the peak aging time of the CEE-AEC samples was substantially advanced relative to the as-cast ones. The enhanced hardness owned relatively high thermal stability in the ambient temperature. The precipitation sequence of 3 CEE-AEC passes alloy aged at 225 degrees C was as follows: supersaturated solid solution Mg(S.S.S.S)->beta''(DO19)->beta'(bco)->beta(1) (fcc)->beta(fcc).