Study on solution and aging heat treatment of a super high strength cast Mg-7.8Gd-2.7Y-2.0Ag-0.4Zr alloy

Mg-7.8Gd-2.7Y-2.0Ag-0.4Zr (wt.%) alloy was prepared by traditional metal mold gravity casting, and then was solutionized and aged. The solution and aging processing were optimized, the microstructure evolution during solution and aging of the alloys was investigated, the isothermal aging hardening process was studied, and the room temperature tensile properties and strengthening mechanism of the alloys were studied. The results show that alloys exhibit obvious double peak aging characteristics during the isothermal aging process due to the precipitation process difference of β′ and γ″, and a higher aging temperature is beneficial to shorten the time to peak hardness. The optimal solution and aging parameters were 510 °C for 6 h and 200 °C for 32 h, respectively. The lattice distortion of α-Mg caused by Ag is much greater than that of Gd and Y, so the strengthening effect of Ag is the most significant. The yield strength, ultimate tensile strength, and elongation of the alloy after the optimized solution and aging process reach 273.1 MPa, 410.7 MPa, and 4.85%, respectively. The contribution of precipitation strengthening to the yield strength of alloy after aging at 200 °C and 225 °C is about 51%.