Effects of Zn/Gd ratio on the microstructures and mechanical properties of Mg-Zn-Gd-Zr alloys

The microstructures and mechanical properties of three Mg-Zn-Gd-Zr alloys with specific Zn/Gd mass ratios to form different types of phases are systematically investigated by casting and subsequent indirect hot extrusion. With the Zn/Gd ratio increases from 0.27 to 1.4, second phases change from W (Mg3Gd2Zn3), X (Mg12GdZn) and Mg5(Gd,Zn) to W, I (Mg3Zn6Gd) and MgZn2. After extrusion, the needle-like and dispersively distributed I-phase provides effective strengthening effects to the experimental alloys. The lamellar-like X-phase in the as-extruded condition is hard and fragile to start cracks. W-phase owns superior ductility in these alloys. A high performance I-phase containing Mg-7Zn-5Gd-0.6Zr alloy is prepared by conventional casting and subsequent hot extrusion. It owns an ultimate and yield strength of 350 MPa and 285 MPa, respectively, and its elongation is 13.6%.