Effects of chemical composition on the microstructure and mechanical properties of gravity cast Mg–xZn–yRE–Zr alloy

The microstructures and mechanical properties of Mg–xZn–1.25RE–Zr (x=3.5, 4.2, 5.0 wt%) and Mg–4.2Zn–yRE–Zr (y=1.0, 1.25, 1.5 wt%) alloys in as-cast and 325 °C peak-aged condition were investigated in this study. The as-cast Mg–xZn–yRE–Zr alloys consist of α-Mg matrix, T-phase and Mg51Zn20 phase. For Mg–xZn–1.25RE–Zr alloys, 4.2 wt% Zn addition led to smallest average grain size and better eutectics morphology of discontinuous network, short-bar and island shape. For Mg–4.2Zn–yRE–Zr alloy, increase in RE content gradually refined the microstructure and contributed to more triangular particles and networks. After peak-aged at 325 °C, the strengthening of Mg–xZn–yRE–Zr alloys occurred through the precipitations of rod-like β′1 phases. With Zn content from 3.5 to 5.0 wt%, the strengthening effect first increased and then decreased, with a turning point of 4.2 wt% Zn. The descend of strengthening effects at 5.0 wt% was mainly ascribed to the formation of plate-like β′2 phases and reduced number density of β′1 phases. With RE addition from 1.0 to 1.5 wt%, the strengthening effect increased because of the denser and finer β′1 phases. When x=4.2 and y=1.25–1.5 wt%, the Mg–xZn–yRE–Zr alloy reveals good combination of strength and elongation in 325 °C peak-aged condition, and was chosen as optimal chemical composition.