Twin related silicon crystals in Al–Si alloys and their growth mechanism

The relationships between the morphologies and growth mechanisms of primary silicon crystals were studied using a hot etching technique to observe growth steps across polished cross-sections. The most frequent morphologies of primary silicon observed were octahedral and starlike. Octahedral crystals are most often bifurcated across a central twin plane to form spinel twins. Their faceted growth isfacilitated by dislocations emerging from the faceted surfaces which nucleate successive atomic planes that spread across the facet surface. Alternatively, accelerated corner growth (skeletal growth) can also nucleate the atomic planes. Starlike crystals usually exhibit five branches in a twin relationship, radiating from a centre. Evidence is presented that each branch can grow as afaceted octahedron, although initial growth may occur by the twin plane re-entrant edge (TPRE) mechanism. This mechanism is the accepted growth mode of a third morphology, plate like growth, which has been described previously. There is a great variety of other morphologies, mostly irregular, described in the literature. Most can be explained as either clusters of octahedral crystals or as the results of skeletal growth.Under relatively extreme conditions, skeletal growth can lead to hollow hopper crystals or pseudodendrites