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

Abstract

The morphology evolution of primary Mg₂Si in the Al–20Mg₂Si–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 °C and then to 200 °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 Al–20Mg₂Si 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.