Eutectic solidification mode in sodium modified Al-7 mass %Si-3.5 mass %Cu-0.2 mass %Mg casting alloys

The influence of sodium (Na) on nucleation and growth of the Al-Si eutectic in a commercial hypoeutectic Al-Si-Cu-Mg foundry alloy has been investigat ed. The microstructural evolution during eutectic solidification was studie d by a quenching technique. By comparing the orientation of the aluminium i n the eutectic to that of the surrounding primary aluminium dendrites by EB SD, the eutectic solidification mode could be determined. The results show that the eutectic solidification starts near the mould wall and evolves wit h front growth opposite the thermal gradient on a macro-scale, and on a mic ro-scale with independent heterogeneous nucleation of eutectic grains in in terdendritic spaces. Na-modified alloys therefore behave significantly diff erently from those modified by other elemental additions.