Comparative study of thermodynamic calculations with solidification microstructures of Al-Si-Fe and Al-Si-Mn ternary eutectic alloys

This study systematically investigated the solidification microstructures of Al-Si-Fe and Al-Si-Mn ternary alloys solidified at various cooling rates ranging from 0.3 to 145°C・s‒1. The thermodynamic calculations assessed three alloy compositions of Al-5%Si-1.5%Fe, Al-8%Si-1%Fe, and Al-5%Si-1.5%Mn exhibiting two-phase eutectic reactions of α-Al and intermetallics (β-Al5FeSi, αh-Al7Fe2Si or αc-Al15Mn3Si2) phases in solidification. The needle-shaped β phase was formed in the Al-Si-Fe ternary alloy. The microstructural characterizations revealed that the αc phase (rather than αh phase) and Si phase were dominantly formed in eutectic reactions. The αc phase with Chinese-script morphology was formed in the Al-5%Si-1.5%Mn ternary alloy. The dendritic α-Al phase was dominantly formed as the primary solidified phase, and the intermetallic phases were subsequently solidified at lower temperatures, which was independent of the cooling rate in solidification. The observed microstructures can provide insights into the solidification sequences, which were consistent with exothermal reactions detected by differential scanning calorimetry measurements at controlled cooling rates in solidification. The solidification sequence may not follow the calculated liquidus projections and its associated Scheil solidification simulations. The difference between experiments and calculations would be associated with the thermodynamic stability of the constitute phases below the solidus temperatures of Al-Si-Fe and Al-Si-Mn ternary systems.