Nucleation of Fe-intermetallic phases in the Al-Si-Fe alloys

Nucleation of Fe-intermetallic phases (i.e. binary Al-Fe, α-AlFeSi, β-AlFeSi, δ-AlFeSi, and q₁-AlFeSi phases) on the surface of different inclusions in six experimental Al-Si-Fe alloys was studied through a quantitative evaluation of the number of inclusion particles that have a direct physical contact with the nucleated phase as seen through the optical microscope. It was found that nucleation of each of the Fe-intermetallic phases was promoted on the surface of several inclusions under the same conditions of alloy composition and cooling rates. Some inclusions exhibited high potency for the nucleation of particular Fe-intermetallic phases under certain conditions and poor potency under other conditions. The potent nucleants for the primary α-Al phase such as γ-Al₂O₃ exhibited poor potency for the nucleation of the Fe-intermetallic particles that lie within the primary phase (intragranular particles). Reactive inclusions such as CaO and SiC are very potent nucleants for the intragranular Fe-intermetallic phase particles. The nucleation of the Fe-intermetallic phases in Al-Si-Fe alloys obeys the general features of nucleation, in particular, the effect of cooling rate and solute concentration on the potency of the nucleant particles: (1) it was observed that increasing the cooling rate enhances the heterogeneous nucleation of the Fe-intermetallic phases on the surface of different inclusions, and (2) the nucleation potency of inclusion particles in both α-Al and interdendritic regions improves with increasing solute concentration up to a certain level. Above this level, the solute concentration poisons the nucleation sites. Nucleation of the Fe-intermetallics in the alloys studied does not seem to be largely affected by the type of the nucleating surface.