Heterogeneous nucleation of Mg2Si on CaSb2 nucleus in Al–Mg–Si alloys

Abstract

Primary Mg₂Si with a truncated octahedral morphology and size over a range of 8–15 μm were prepared by simultaneously adding Ca and Sb into the Al–Mg–Si melt. It is revealed for the first time that CaSb₂ can act as the nucleus of primary Mg₂Si during solidification, which changes the morphology and refines the size of primary Mg₂Si significantly. It is useful for improving the mechanical properties of the Al–Mg–Si alloy. The skeleton-type growth processes of truncated octahedral primary Mg₂Si crystals were identified by observation of the three-dimensional (3-D) morphologies of the crystals extracted from the Al–Mg–Si alloy. The content of Ca and Sb plays an important role in determining the morphologies of primary Mg₂Si, which can be transformed from coarse dendrites to defective truncated octahedra and finally to the coexistence of perfect truncated octahedra and flat truncated octahedra with increasing the content from 0 to 0.2 and then to 0.5 wt%. Our study provided a one-step less-cost method to change the morphologies and sizes of primary Mg₂Si, which is important to tailor new light-weight alloys with high strength and toughness.