RAPID DENDRITIC GROWTH IN UNDERCOOLED AG-CU MELTS

The results of measurements of the reciprocal recalescence rise time (Delta t)(-1) of undercooled melts of an Ag-Cu alloy with 65 at.% Cu are presented. It is shown that (Delta t)(-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 T-0 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.