K. Nogita et al., Eutectic solidification mode in sodium modified Al-7 mass %Si-3.5 mass %Cu-0.2 mass %Mg casting alloys, MATER TRANS, 42(9), 2001, pp. 1981-1986
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.