Heat treatment and mechanical properties of a high-strength cast Mg-Gd-Zn alloy

Different solution treatments at 480, 500 and 520 °C were carried out on the semi-continuously cast Mg96.34Gd2.5Zn1Zr0.16 alloy. It was found that different solution temperatures lead to formation of different phases. Besides α-Mg matrix and Zn-Zr compounds, the secondary phases along the grain boundaries change with solution temperature. At 480 °C and 500 °C, long period stacking ordered (LPSO) structured X phases and residual eutectic compounds (Mg, Zn)3Gd are observed, while at 520 °C, only residual eutectic compounds (Mg, Zn)3Gd exist. The different microstructures solution treated at different temperatures lead to different aging response and mechanical properties. Higher solution temperature results in higher aging response and better mechanical properties. The alloy solution treated at 520 °C for 8 h and aged at 200 °C for 64 h shows the best tensile properties at room temperature: ultimate tensile strength of 405 MPa, yield strength of 292 MPa and elongation of 5.3%. The influences of existing forms of Gd and Zn elements on tensile properties of Mg-Gd-Zn alloy were discussed, which indicates that the existing of Gd and Zn elements as precipitates leads to better strengthening effect than that in the form of LPSO structured X phase.