Effect of Direct Current on Microstructure Evolution of Directionally Solidified Sn-70 wt pct Bi Alloy at Different Pulling Rates

The effect of direct current (DC) on the solidification behaviors of Sn-70 wt pct Bi alloy was studied using the directional solidification method. All the microstructures obtained at different pulling rates with DC treatment consisted of two typical structures, i.e., the transitional and steady ones. It was found that the transitional microstructures were two-phase structures under a positive DC, whose morphologies and the corresponding lengths of transitional zone were independent of the pulling rate. However, the transitional microstructures upon DC reversal, which were different from those under a positive DC, strongly depended on the pulling rate. The formation and difference of the transitional microstructures upon DC reversal were mainly attributed to Joule heating and concentration-dependent electromigration. The primary nubby Bi phase and dendritic β-Sn phase, caused by composition fluctuation due to the difference of Lorentz force subjected to Sn and Bi, precipitated in the steady zone regardless of the current polarity.