Effect of heat treatment on LPSO morphology and mechanical properties of Mg–Zn–Y–Gd alloys

The mechanical properties and microstructure were investigated under different Zn content and heat treatment conditions in a Mg–Zn–Y–Gd cast alloy. A part of the long period stacking order (LPSO) phases transformed to W-Mg3Zn3RE2 phases with an increase in Zn content from 0.9 at.% to 1.8 at.%, and the ultimate tensile strength (UTS) increased from 229 MPa to 248 MPa. With solution treatment at 480°C, the content of the LPSO phase and strength sharply decreased in the Mg-1.8Zn-0.8Y-0.8Gd alloy, whereas this change was not significantly observed in the Mg–0.9Zn–0.8Y–0.8Gd alloy. After solution treatment, the elongation significantly improved and the UTS sharply decreased in both alloys. The lamellar and filminess LPSO phases were observed with aging treatment at 200°C. Moreover, the strengthening efficiency of lamellar and filminess LPSO phases was lower than that of the block LPSO phases. Therefore, the UTS of the T6 state was lower than that of the as-cast alloy.