Effect of Ca additions on the microstructure, thermal stability and mechanical properties of a cast AM60 magnesium alloy

The effect of 0.5, 1.2, and 2.0 wt.% Ca additions on the microstructure, thermal stability, and mechanical properties of the cast Mg–6Al–0.3Mn alloy (AM60) was investigated. Addition of 0.5 wt.% Ca did not form any new phase but suppressed the discontinuous precipitation of the β-Mg17Al12 phase by being dissolved in both the second phase and magnesium matrix. In the materials containing higher amounts of Ca, however, the Al2Ca phase with a lamellar morphology appeared in the microstructure and the volume fraction of β-Mg17Al12 diminished. It was found that Ca generally refines the microstructure and enhances the thermal stability of the alloy. This was documented by the retention of the initial fine dendritic microstructure, hardness, and ultimate shear strength (USS) of the Ca-containing materials after long-term annealing at 673 K. The observed improvements are believed to be caused by the solid solution strengthening of Ca in the Mg matrix, and the particle strengthening effects of the thermally stable Al2Ca phase which are formed as an interconnected network. This behavior is in contrast to that of the base material which developed a coarse grain structure with decreased strength and hardness after the same thermal treatment.