Microstructure and mechanical properties of Mg-xY-1.5MM-0.4Zr alloys

Age hardening, microstructure and mechanical properties of Mg-xY-1.5MM-0.4Zr (x=0, 2, 4, 6 wt.%) alloys (MM represents Ce-based misch-metal) were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the formed precipitates being responsible for age hardening changed from fine hexagonal-shaped equilibrium Mg₁₂MM phase to metastable β′ phase with bco crystal structure when Y was added into Mg-1.5MM-0.4Zr alloy, and the volume fraction of precipitate phases also increased. With the increase of Y content in Mg-Y-1.5MM- 0.4Zr alloys, it was found that the age hardening was enhanced, the grain sizes became finer and the tensile strength was improved. The cubic-shaped β-Mg₂₄Y₅ precipitate phases were observed at grain boundaries in Mg-6Y-1.5MM-0.4Zr alloy. It was suggested that the distribution of prismatic shaped β′ phases and cubic shaped β-Mg₂₄Y₅ precipitate phases in Mg matrix might account for the remarkable enhancement of tensile strength of Mg-Y-MM-Zr alloy. It was shown that the Mg-6Y-1.5MM-0.4Zr alloy was with maximum tensile strength at aged-peak hardness, UTS of 280 MPa at room temperature and 223 MPa at 250°C, respectively.