Synergic effects of Gd and Y contents on the age-hardening response and elevated-temperature mechanical properties of extruded Mg–Gd(-Y)-Zn-Mn alloys

This paper investigated the effects of Gd and Y solutes on aging behaviour and corresponding mechanical properties of the extruded Mg–Gd(-Y)-Zn-Mn alloys at room and elevated temperatures. The results show that aging treatment provided significant improvement of ~100 MPa in strength by forming ellipsoidal β^′ nanophases in the as-extruded alloys. Partially substituting Y for Gd in the as-extruded Mg-Gd-Zn-Mn alloys can delay age-hardening response, but improve the strength increment after aging treatment. As the Y/Gd atomic ratio changed from 0 to 1, the Mg-1.75Gd-0.75Y-0.5Zn–Mn(at.%) alloy with a Y/Gd atomic ratio of 0.4 obtained the higher peak-hardness and mechanical properties. Enhanced age-hardening response and better mechanical properties were detected after separate additions of Y and Gd. The extruded-T5 Mg-2.5Gd-0.75Y-0.5Zn-0.3Mn alloy exhibited superior ultimate tensile strengths of 520 MPa at room temperature, 344 MPa at 250 °C, and 225 MPa at 300 °C. Fracture behaviours reveal that a change in predominant deformation mechanism from one based on dislocations to one mediated by grain boundary (GB) processes was found as the tensile temperatures arise from 250 °C to 300 °C. The activation of GB sliding of the fine grains partially resulted in the decrease of tensile strength at 300 °C.