PLANE FRONT SOLIDIFICATION OF 2 PHASE PERITECTIC TIN-CADMIUM ALLOYS -PART 2

Banding during plane front solidification of binary peritectic alloys consists of the deposition of alternating layers of the primary (alpha ) and peritectic (beta) phases. This phenomenon is considered in relat ion to the cycle of alternating metastable growth followed by nucleati on proposed by Trivedi to account for its formation. The study reveale d the following. Nucleation of at least one of the two phases must be caused by a heterogeneous nucleation catalyst foreign to the system, u nless large undercooling is generated. Fluctuations in growth conditio ns are very important in promoting nucleation. Nucleation will be foll owed by both a short and a long period of transient growth. Solid grow n under metastable conditions may be subject to partial remelting on r eheating to or above the peritectic temperature. For Cd-Sn peritectic alloys, the following points were found. The primary Sn phase (alpha) growing metastably below the peritectic temperature, under steady grow th conditions, will nucleate the peritectic phase (beta) when the Sn c ontent at the solid/liquid interface reaches similar to 12 wt-%, corre sponding to an interface temperature of similar to 206 degrees C. Stea dy state freezing of the alpha phase is possible for any Cd content le ss than similar to 3 wt%. The maximum undercooling that can be produce d for nucleation of the alpha phase during metastable growth of the be ta phase under steady growth conditions is similar to 0.5 K. (C) 1998 The Institute of Materials.