Solidification sequence and as-cast microstructure of a model Pb-Bi-Sn alloy with quasi-peritectic reaction

Abstract

The Pb39Bi25Sn36 alloy was directionally solidified at varied withdrawal rates followed by quenching. From metallographic observation, composition analysis and differential scanning calorimetry (DSC) analysis, the solidification sequence of Pb39Bi25Sn36 alloy was characterized. The Sn-dendrite is observed as the primary phase during solidification, followed by the β(Pb7 Bi3)/Sn divorced eutectic. At lower temperatures, the β(Pb7Bi3)/Sn binary eutectic is formed in the interdendritic region. However, the quasi-peritectic reaction [L+α-Pb→β(Pb7 Bi3)+Sn] does not occur as the prediction based on ternary phase diagram analysis, which can be attributed to the unequilibrium solidfication condition. Moreover, the as-cast microstructure is significantly refined with the increasing withdrawal rates. The quenched sample only exhibits the lamellar structure of Sn, β(Pb7Bi3) and Bi phases. The results can provide instructions to optimize the as-cast microstructure of ternary alloys with quasi-peritectic reaction.