Growth of octahedral primary silicon in cast hypereutectic Al–Si alloys

From the present experimental results on the crystal morphologies of primary silicon, different sequences of crystal growth of octahedral primary silicon can be expected. At the initial stage, primary silicon crystal along [1 0 0] direction will advantage to rapidly grow and become stable first branchings. As a result, growth is suppressed in all but the high-mobility [1 0 0] direction due to the strong faceting tendency of the growing primary silicon crystal. If V[1 0 0]/V[1 1 1]=1.5, the primary silicon crystal will grow as a perfect octahedron. If V[1 0 0]/V[1 1 1]>1.5, the primary silicon crystal will grow as an imperfect octahedron. According to the present experimental results, it can be concluded that the internal surface of the hollow within imperfect octahedral primary silicon should be relatively round. Macrosteps, generated at the corners and the edges of imperfect octahedral primary silicon, will grow toward the centers of the {1 1 1} facets and join there due to the presence of the relatively low impurity concentrations. On the contrary, macrosteps will not join at the centers.