Morphology evolution of primary Mg2Si in Al-20Mg2Si-0.1Ca alloys prepared with various solidification cooling rates

The morphology evolution of primary Mg2Si in the Al-20Mg(2)Si-0.1Ca alloy was investigated by a mould temperature-controlled solidification technique at various solidification cooling rates. It is found that by lowering the pre-set mould temperature, i. e. increasing the cooling rates, the influence of external factors, i. e. the adsorption of Ca atoms on {100} facets, on the morphology evolution of primary Mg2Si can be strengthened, leading to primary Mg2Si with varying morphologies. With the decrease of mould temperature from 600 to 400 degrees C and then to 200 degrees C and finally to room temperature, primary Mg2Si transformed from a mixture of an equiaxed-dendrite and an octahedron to a truncated octahedron and then to a truncated cube and finally to a cube; in contrast, primary Mg2Si still remained dendritic in the Ca-free Al20Mg(2)Si alloy. Moreover, the skeleton-type growth process of the truncated cubic primary Mg2Si in the modified alloy was identified, where growth steps with some hillocks were observed for the first time. Our study offers a simple and low-cost method to control the morphology and size of primary Mg2Si crystals in the Al-Mg-Si alloy, which is beneficial to the design of lightweight Al-Mg-Si alloys with high strength and toughness.