Development of high strength sand cast Mg-Gd-Zn alloy by co-precipitation of the prismatic β' and β1 phases

In order to design a high strength sand cast Mg-Gd-Zn alloy, the microstructures and mechanical properties of cast Mg-12Gd-xZn-0.4Zr (chi = 0, 0.4, 0.8) (wt%, signed as GZ1200K, GZ1204K and GZ1208K) alloys were studied. Zn addition changes the constitution of the strengthening phases in Mg-12Gd alloy. Small amount of Zn addition leads to the formation of basal precipitates of gamma and gamma', accelerated-precipitation of the beta(1) phases and lower number density of the prismatic beta' phases, and modifies the morphology of the prismatic beta' precipitates. Compared with the Zn-free alloy, the Zn-containing alloys (GZ1204K & GZ1208K) having higher yield strength in peak-aged and over-aged conditions are mainly due to the extra prismatic beta(1), precipitates, which do not precipitate in the Zn-free alloy. Under sand casting condition, GZ1208K alloy strengthened by the co-precipitates of the prismatic beta' and beta(1) phase indicates a good combination of strength and ductility at room temperature: yield strength of 270 MPa, ultimate tensile strength of 348 MPa and elongation of 2.6%.