Rapid dendritic growth in undercooled Ag-Cu melts

The results of measurements of the reciprocal recalescence rise time (Δt)⁻¹ of undercooled melts of an Ag-Cu alloy with 65 at.% Cu are presented. It is shown that (Δt)⁻¹ is a semiquantitative measure of the growth rate. In agreement with observations already made on alloys with other compositions in this binary system, the growth rate as a function of the undercooling rises sharply at an undercooling corresponding to the T₀ temperature. This behaviour can be explained in the framework of the theories of rapid dendritic growth, when the kinetic displacements of the solidus and liquidus are taken into account. A simple empirical expression for this kinetic effect is proposed. For the Ag-65 at.% Cu alloy the predictions of the theory are expressed as a plot of growth rate against undercooling and also in the form of a kinetic phase diagram. The predictions are in good qualitative agreement with the experimental results.