Formation of banded structure in Pb–Bi peritectic alloys

Hypoperitectic Pb–25 at.%Bi alloys and hyperperitectic Pb–33 at.%Bi alloys were unidirectionally solidified under temperature gradient, G, upto 2.7×104 K m−1. For the Pb–25 at.%Bi alloys, the banded structure was formed in case of 250 mm initial melt length at the values of G/V higher than 2.6×1010 Ks m−2, although longitudinal separation of the two phases in growth direction was observed in case of 100 mm. For the Pb–33 at.%Bi alloys, the banded structure was formed at the value of G/V higher than 3.3×1010 Ks m−2 even in case of 100 mm melt length. Interface temperatures at steady growth suggested the banded structure was able to form only in the hypoperitectic region. The formation in the hyperperitectic region was explained by a model involving concentration change at nucleation of the secondary phase.