The Influence of Calcium on the Primary Mg2Si Phase in the Hypereutectic Mg-Si Alloys

The microstructure of Mg-5Si alloy consists of the primary coarse Mg2Si phase, α-Mg solid solution and eutectic α-Mg + Mg2Si, in which eutectic Mg2Si phase solidifies in the form of Chinese script particles. When 0.2 wt.% of Ca was added to the Mg-5Si alloy the size of primary Mg2Si phase remained unchanged. The modification effect of calcium on the primary Mg2Si phase was effective only in the Mg-5Si-0.5Ca alloy. The morphology of the primary Mg2Si phase is changed from the coarse dendrite shape to polyhedral shape and the size of primary crystals is significantly reduced. The addition of 0.6 wt.% Ca to Mg-7Si alloy did not cause the modification of primary Mg2Si phase.