Effect of trace Sn content on solidification microstructure of Al-Bi monotectic alloys

Abstract

Solidification experiments were performed to explore the effect of trace amount of micro-alloying element Sn on the solidification microstructure of Al-Bi monotectic alloy. Bi-rich particles are refined by the addition of micro-alloying element Sn. The refining effect increases first and then almost keeps unchanged with the Sn addition level. An analytical model is proposed to determine the interfacial energy γ_L1/L2 between the Al-rich melt (L₁ phase) and Bi-rich liquid droplets (L₂ phase) in an Al-Bi monotectic alloy melt with the addition of micro-alloying element Sn based on Gibbs absorption isotherm. Calculation results demonstrate that the micro-alloying element Sn shows a strong segregation potency to the interface between the two liquid phases and decreases γ_L1/L2. The variation in γ_L1/L2 with the Sn addition level shows a similar tendency as the average size of Bi-rich particles. The critical Sn amount to refine the Bi-rich liquid droplets formed during the liquid–liquid phase transformation is determined.