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 α and β 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 β-Cu on α-Ag is larger than that of α-Ag on β-Cu, but the critical nucleation work for β-Cu to heterogeneously nucleate on α-Ag in hypoeutectic alloys is obviously smaller than that for α-Ag on β-Cu in hypereutectic Ag–Cu alloys. Thus, α-Ag phase is a better nucleant for β-Cu phase than vice versa and the Ag–Cu alloys solidify with non-reciprocal nucleation, reflected by only a halo of α-Ag around primary β-Cu phase in the hypereutectic Ag–Cu alloy.