Influence of heat treatment on the evolution of microstructure and mechanical properties of Mg-7Gd-5Y-0.6Zn-0.8Zr magnesium alloy

In this study, the microstructure, mechanical properties and fracture behavior of semi-continuous cast Mg-7Gd-5Y-0.6Zn-0.8Zr alloy were investigated in different heat treatment conditions. After solution treatment at 500 °C for 10 h, amounts of SFs precipitated inside grains and almost all the eutectic phase Mg3(Gd, Y, Zn) phase dissolved into the α-Mg matrix. The precipitation sequence was Mg(S.S.S.S)→β'' (DO19)→β' (bco)→β1 (fcc) and SF→LPSO structure during ageing treatment at 210 °C. In the Mg alloy, the solution strengthening response was relatively weak, while the strong age strengthening response took place. Owing to the dissolution of eutectic phase and precipitation of SFs, both the strength and elongation of the Mg alloy were improved during solution treatment. The easier dislocation slip and cross-slip and the activation of deformation twins contributed to higher elongation in the as-cast and solution-treated alloy, which evidently decreased in the peak-aged alloy. The fracture model was quasi-cleavage fracture in the as-cast and solution-treated alloy, which transformed to cleavage fracture in the peak-aged alloy.