Microsegregation and its effects on quantity and morphology of secondary phases during solidification of Al-Fe-Sialloys

The effect of varying solidification conditions and evolution of solute element distribution on the microstructural evolution of Al-Fe-Si alloys has been investigated. An increase of cooling rate and/or a change of cooling schedule from a constant cooling rate to a slow-fast cooling schedule leads to an increase of the amount of eutectic and, consequently, the amount of secondary phases. This effect is attributed to a reduction of the amount of solutes in primary alpha-Al owing to faster cooling at lower temperatures. Microsegregation calculations show that solute distribution in alpha-Al will influence the Fe/Si ratio in the liquid at the onset of the eutectic reaction. It is concluded that changes in eutectic liquid composition exert an influence on secondary phase selection, and favour the formation of iron depleted phases with increasing cooling rate. Increasing cooling rate and/or change of cooling schedule from constant cooling rate to slow-fast cooling also leads to a transition from a faceted to a regular morphology of the secondary phases. This is accompanied by a refinement in size of the secondary phases. An increase in iron concentration and a decrease in silicon concentration in bulk alloy lead to an increase in the eutectic fraction.