The nucleation of Fe-rich phases on oxide films in Al-11.5Si-0.4Mg cast alloys

The microstructures of Al-11.5Si-0.4Mg alloys with various Fe and Mn contents have been studied to investigate the potential influence of oxide films on the precipitation of Fe-rich phases from the liquid metal. Oxide films are incorporated into melts by an entrainment process. This is an enfolding mechanism of incorporation. Folded oxide films in melts have two sides: the dry unbonded inner surfaces and their wetted exterior surfaces. The wetted outer interfaces of oxide films appear to act as preferred substrates for the nucleation and growth of some Fe-rich phases. It is expected that there will be good atomic contact between the wetted side of the film and Fe-rich phases or aluminum matrix. However, the gap between the two dry sides of a double film constitutes a crack, thus explaining the commonly observed cracks in and around such intermetallics and aluminum matrix. The calculation of planar disregistries between some typical oxides and alpha-Fe phase (primary or script type) indicated that MgO, MgO . Al2O3, gamma-Al2O3, and alpha-Al2O3, might all be good substrates for the nucleation of alpha-Fe phase.