Grain refinement through fragmentation of dendrites in undercooled melts

The widely observed microstructural transitions from coarse dendritic to fine equiaxed grains in solidification of undercooled melts have been puzzling for many years. Here we describe recent progress made in solving this puzzle. We review the basic elements of a simple model that assumes that these transitions result from the fragmentation of dendrites by remelting during the period following recalescence when the interdendritic melt solidifies. Furthermore, we review the results of detailed experiments on Ni–Cu alloys specifically designed to test the predictions of this model. A good overall quantitative agreement is obtained between the model predictions and the experimental observations. In particular, the model predicts the existence of four grain refinement transitions, three of which are observed experimentally. It also describes the dependence of the microstructural transition undercoolings on the cooling rate that follows recalescence and on the alloy composition, in agreement with the experimental findings.