Phase separation and microstructural characteristics of undercooled Cu-Pb immiscible alloy

Bulk samples of Cu-80%Pb hypermonotectic alloy were undercooled by up to 27 0 K (0.21 T-L) With glass fluxing technique. The undercooling behavior and the final microstructure were investigated experimentally. It was found tha t the macrosegregation decreased with the increase of undercooling exponent ially. When undercooling reached 270 K, the volume fraction of macrosegrega tion was reduced by one order of magnitude. Meanwhile, high undercooling br ought about significant changes to the microstructural morphology of S(Cu) phase. At small undercoolings, S(Cu) phase grew in dendritic manner. As und ercooling increased, S(Cu) dendrite transformed gradually to spherical shel l. This morphology transition was ascribed to the concurrent action of the phase separation within miscibility gap and the subsequent solidification p rocess of L-2 (Pb) matrix. As an essential step to model the final microstr ucture, theoretical calculations related to the nucleation of L-1 (Cu) drop lets were carried out.