Effect of Ca addition on the microstructure and mechanical properties of heat-treated Mg-6.0Zn-1.2Y-0.7Zr alloy

In this study, the effects of Ca addition on the microstructure, thermal properties, and mechanical properties of a Mg-6.0Zn-1.2Y-0.7Zr (ZWK611) alloy at room temperature and 150 °C were investigated. With an increase in the Ca content, the ignition resistance of the ZWK611 alloy improved and the grains became finer. The as-cast ZWK611 alloy consisted mainly of the dendritic α-Mg matrix and I-phase (Mg₃Zn₆Y) at the grain boundaries. On the other hand, the τ- (Ca₂Mg₆Zn₃) and I-phases were formed in the Ca-added ZWK611 alloy. The fraction of the τ-phase increased with an increase in the Ca content. After the solid-solution treatment, these phases remained partially at the grain boundaries of the Ca-added ZWK611 alloys and an additional W-phase (Mg₃Zn₃Y) was observed. The phases remaining at the grain boundaries restricted the grain growth through the grain boundary pinning effect. The aging treatment resulted in the formation of MgZn' precipitates in the α-Mg matrix of the alloys. These precipitates were more uniformly distributed in the Ca-added alloys than in the alloys without Ca. Thus, the heat treatment-induced precipitation improved the tensile and creep properties of the T6-treated alloys. The T6-treated ZWK611+0.7Ca alloy exhibited the best mechanical properties at room temperature and 150 °C among all the tested alloys.