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

Bulk samples of Gu-80% Pb hypermonotectic alloy were undercooled by up to 270 K (0.21 T_L) with glass fluxing technique. The undercooling behavior and the final microstructure were investigated experimentally. It was found that the macrosegregation decreased with the increase of undercooling exponentially. When undercooling reached 270 K, the volume fraction of macrosegregation was reduced by one order of magnitude. Meanwhile, high undercooling brought about significant changes to the microstructural morphology of S(Cu) phase. At small undercoolings, S(Cu) phase grew in dendritic manner. As undercooling increased, S(Cu) dendrite transformed gradually to spherical shell. This morphology transition was ascribed to the concurrent action of the phase separation within miscibility gap and the subsequent solidification process of L₂(Pb) matrix. As an essential step to model the final microstructure, theoretical calculations related to the nucleation of L₁(Cu) droplets were carried out.