Embedded droplet measurements and an adsorption model of the heterogeneous nucleation of solidification

Abstract

An adsorption model has been developed to describe the heterogeneous nucleation of solidification in an A-B eutectic or monotectic alloy system. The interface between A-rich α solid and B-rich liquid is treated as a mixture of A solid, B solid, A liquid and B liquid atoms, randomly distributed as a monolayer between the two phases. The interfacial energy is calculated by summing pairwise bonding energies, and is then minimized to determine the equilibrium interface solid fraction and composition. With decreasing temperature, the interface monolayer changes sharply from liquid to solid, with a composition close to pure B. This sharp onset of interface adsorption of solid B atoms corresponds to α acting as a catalyst for the heterogeneous nucleation of B-rich β solid. Adsorption close to the eutectic temperature and therefore efficient nucleation catalysis is promoted by a large difference between the melting points of A and B, and a small difference between the solid and liquid immiscibilities of A and B. Predicted undercoolings for the onset of adsorption and nucleation catalysis can be obtained directly from simple phase diagram data, and give good agreement with previous measurements in the AgPb and AlSn alloy systems.