Influence of heat treatment on microstructures and mechanical properties of gravity cast Mg–4.2Zn–1.5RE–0.7Zr magnesium alloy

Abstract

The microstructures and mechanical properties of Mg–4.2Zn–1.5RE–0.7Zr alloy were investigated under different heat treatment conditions. The as-cast alloy consisted of α-Mg phase, T-phase and Mg51Zn20 phase. After aging treatment (single-step (325 °C, 10 h) and two-step (325 °C, 4 h)+(175 °C, 14 h)), neither T-phase nor Mg51Zn20 phase dissolved into the matrix and the coarsening of α-Mg phase was not significant. When peak-aged at 325 °C for 10 h, dense short rod-like β′1 phase precipitated in the matrix. Further ageing at 325 °C led to coarsening of β′1 phase and a decrease in number density. Alloy aged at 325 °C for 10 h achieved the highest yield strength (YS) and ultimate tensile strength (UTS) of 153.9 MPa and 247.0 MPa, which were increased by 48 MPa and 23 MPa from as-cast condition, respectively. While the elongation slightly decreased to 15.6%. Comparatively, the YS and UTS of alloy two-step aged by (325 °C, 4 h)+(175 °C, 14 h) showed little difference from those of single-step aged alloy, but with a lower elongation of 13.4%. In addition, the fracture surfaces of Mg–4.2Zn–1.5RE–0.7Zr alloy under different thermal conditions were mainly characterized by quasi-cleavage feature, but with differences in the details.