Morphology of (beta)-Al5FeSi Phase in Al-Si Cast Alloys

The (beta)-Al5FeSi platelets have long been thought to be brittle and responsible for the inferior mechanical properties of Al-Si cast alloys. Two typical cracks, transverse and longitudinal, are commonly observed in (beta)-Al5FeSi platelets. It is thought that the cracks are probably due to the presence of doubled-over oxide films (bifilm) though some transverse cracks may be due to the brittleness. The crack is actually the gap between the two dry sides of an entrained oxide film. Its wetted sides are the favoured substrates for the nucleation and growth of (beta)-Fe platelets. This discovery of the formation of (beta)-Fe platelets on folded films has answered many previously intractable questions encountered in (beta)-Fe phase. For the future, one may speculate that if oxide films could be eliminated from Al melts, it may prove possible to create alloys with unusually high toughness, despite high Fe contents.