Effect of solution treatment on the microstructural evolution and mechanical properties of an aged Mg-4Zn-0.3Ca alloy

The effect of solution treatment on the microstructure, mechanical properties, and creep resistance of cast Mg-4Zn-0.3Ca (wt%) alloy was investigated. Creep behavior was assessed by the impression creep testing at 175 and 225 °C, while shear yield stress (SYS) and ultimate shear strength (USS) values were measured by the shear punch testing (SPT) at room temperature and 175 °C. A semi-continuous network of Ca2Mg6Zn3 phase was found to form at grain boundaries after a single-step (SS) solution treatment (500 °C/4 h). This phase was, however, completely dissolved into the matrix after a double-step (DS) solution treatment (350 °C/20 h+510 °C/3 h). The SS treated samples exhibited better mechanical properties and creep resistances, due to the presence of the thermally stable Ca2Mg6Zn3 phase, mainly at grain boundaries. This phase, however, made the SS treated samples more susceptible to brittle fracture by acting as preferred sites for initiation of micro-cracks that could easily propagate through the intergranular phase network. On the other hand, DS treated samples had inferior mechanical properties and creep resistances, as compared to the SS treated condition, due to the elimination of the intergranular phases and activation of twinning during deformation.