The effect of magnesium, copper and iron on eutectic solidification in hypoeutectic aluminum-silicon foundry alloys of the type A356 and 319 has been investigated. It is found that these elements affect the morphology of the interdendritic eutectic Growth interface. They are therefore also likely to have an effect on the final microstructure, the formation of microporosity and thereby the performance of the casting. The alloys investigated were: pure binary aluminum-silicon alloy, pure aluminum-silicon-magnesium alloy, pure aluminum-silicon-copper alloy, unmodified and strontium modified A356. The samples were directionally solidified with the same solidification conditions (growth rate, thermal gradient and cooling rate). A snapshot of the eutectic growth interface was obtained by quenching the samples during growth. The microstructure of the interface was characterized and the concentrations of different solute elements in the quenched liquid were measured by microprobe analysis. An irregular, flat eutectic growth interface was observed in the binary aluminum-silicon alloy, while the solute added a eutectic mushy zone. The extent of the eutectic mushy zone is dependent on the type and amount of solute added, i.e. a function of segregation tendency. A good correlation was found between the extent of the eutectic mushy zone, the microprobe measurements and phase diagram calculations. It is concluded that the instability of the eutectic growth front is caused by a constitutional undercooling mechanism. Different nucleation mechanisms can occur within the eutectic mushy zone depending on the amount of constitutional undercooling and the potency of nucleating particles.
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