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 (x = 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 Mg12Gd alloy. Small amount of Zn addition leads to the formation of basal precipitates of γ″ and γ′, accelerated-precipitation of the β1 phases and lower number density of the prismatic β′ phases, and modifies the morphology of the prismatic β′ 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 β1 precipitates, which do not precipitate in the Zn-free alloy. Under sand casting condition, GZ1208K alloy strengthened by the co-precipitates of the prismatic β′ and β1 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%.