Non-reciprocal Heterogeneous Nucleation in Solidification of Ag-Cu Alloys

Taking Ag-27 at% Cu hypoeutectic and Ag-67 at% Cu hypereutectic alloys as examples, a theoretical criterion was established based on the classical nucleation theory to predict the non-reciprocal nucleation. For a eutectic alloy composed of alpha and beta solid phases, non-reciprocal nucleation will not occur if the critical nucleation work of one phase on the other phase is approximately equal to that of the latter on the former. Otherwise, one phase can act as the more effective nucleation substrate for the other phase and non-reciprocal nucleation takes place. For Ag-Cu alloys, the wetting angle of beta-Cu on alpha-Ag is larger than that of alpha-Ag on beta-Cu, but the critical nucleation work for beta-Cu to heterogeneously nucleate on alpha-Ag in hypoeutectic alloys is obviously smaller than that for alpha-Ag on beta-Cu in hypereutectic Ag-Cu alloys. Thus, alpha-Ag phase is a better nucleant for beta-Cu phase than vice versa and the Ag-Cu alloys solidify with non-reciprocal nucleation, reflected by only a halo of alpha-Ag around primary beta-Cu phase in the hypereutectic Ag-Cu alloy.

Automated summary

This study establishes a theoretical criterion based on the classical nucleation theory to predict non-reciprocal nucleation using Ag-Cu alloys as examples. It is found that in hypoeutectic alloys, the beta-Cu phase acts as a more effective nucleation substrate for the alpha-Ag phase, while in hypereutectic alloys, the opposite is observed. This non-reciprocal nucleation phenomenon is reflected in the solidification process of the alloys.