Morphological evolution and growth mechanism of primary Mg2Si phase in Al–Mg2Si alloys

Three-dimensional morphologies of primary Mg2Si in Al–Mg2Si alloys were studied in detail using field emission scanning electron microscopy, and its growth mechanism was also discussed. Primary Mg2Si crystals in the alloys display various geometric shapes, such as octahedron, hopper, truncated octahedron, cube and enormous dendrite, although they tend to form faceted octahedron (equilibrium shape) with minimized total surface free energy. The higher growth rate along 〈1 0 0〉 induces the degradation of the six {1 0 0} facets to corners, while eight {1 1 1} facets remain to form octahedron Mg2Si because of their lower growth rates. However, the growth velocities of the 〈1 0 0〉 and 〈1 1 1〉 directions can be manipulated by means of external growth conditions, which are responsible for the evolution of Mg2Si crystals into other morphologies.