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 x 10(4) 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 x 10(10) Ks m(-2), although longitudinal separation of the two phases in g rowth 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 x 10( 10) 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 h ypoperitectic region. The formation in the hyperperitectic region was expla ined by a model involving concentration change at nucleation of the seconda ry phase. (C) 1999 Elsevier Science S.A. All rights reserved.