Relationships between the morphologies and growth mechanisms of primary Si crystals in Al-Si alloys were studied by using a hot etching technique to observe growth steps across polished cross-sections. The most frequent morphologies which were observed were octahedral and star-like. The octahedral crystals were most often bifurcated, across a central twin plane, to form spinel twins. Their faceted growth was facilitated by dislocations which emerged from the faceted surfaces and nucleated successive atomic planes that spread across the facet surface. Accelerated corner (skeletal) growth could also nucleate atomic planes. The star-like crystals usually exhibited 5 branches, in a twin relationship, which radiated from a center. It was shown that each branch could grow as a faceted octahedron, although initial growth could occur via the twin-plane re-entrant edge mechanism. The latter was the accepted growth mode of a third, plate-like, morphology. There was a wide variety of other morphologies, most of which were irregular. The majority of these could be explained as being either clusters of octahedral crystals, or the result of skeletal growth.

R.Y.Wang, W.H.Lu, L.M.Hogan: Materials Science and Technology, 1995, 11[5], 441-9