Microstructure evolution of directionally solidified Sn-Bi alloy under different medium-density direct current

The effect of medium-density direct current (DC) on directional solidification behaviors of Sn-70 wt% Bi alloy is investigated. The results show that the microstructures subjected to DC exhibit two types: the unstable and stable ones, respectively. In the case of DC of +50 and +100 A cm^-2, the primary Bi phase precipitates in the unstable zone, which is obviously different from that with negative DC. Combined with the cooling curves, it is proved that medium-density DC can induce significant electromigration in Sn-Bi alloy. In the stable zone, the eutectic spacing increases when DC of +50 and ±100 A cm^-2 are applied during solidification, which mainly attributes to the weak convection caused by Lorentz force, the enhancement of interface energy and the decrease of temperature gradient. The difference of the eutectic spacing upon DC reversal is ascribed to electromigration. As DC increases to 150 A cm^-2, the unstable microstructures become the divorced eutectic, and the primary Bi phase and dendritic β-Sn precipitate in the stable zone.