Banding structure formation during directional solidification of Pb-Bi peritectic alloys

Directional solidification experiments on Pb-Bi peritectic alloys were carried out at very low growth rate (v=0.5 μm/s) and high temperature gradient (G=35 K/mm) in an improved Bridgman furnace. The banding structures were observed in both hypoperitectic and hyperperitectic compositions (Pb-xBi, x=26, 28, 30 and 34). Tree-like primary α phase in the center of the sample surrounded by the peritectic β phase matrix was also observed, resulting from the melt convection. The banding microstructure, however, is found to be transient after the tree-like structure and only the peritectic phase forms after a few bands. Composition variations in the banding structure are measured to determine the nucleation undercooling for both α and β phases. In a finite length sample, convection is shown to lead only to the transient formation of bands. In this transient banding regime, only a few bands with a variable width are formed, and this transient banding process can occur over a wide range of compositions inside the two-phase peritectic region.