UNIDIRECTIONAL SOLIDIFICATION OF A BINARY ALLOY AND THE EFFECTS OF INDUCED FLUID MOTION

Solidification of a binary substance in a bottom-chilled geometry is numerically simulated under conditions for which large-scale convection results from a compositionally induced, density inversion in the mushy region. Model results successfully predict characteristics of mushy region channel formation which have previously been observed but have heretofore eluded prediction. Channel growth is predicted to begin at the liquidus front and, due to localized freezing point depression (melting), to propagate downward toward the chill plate. The channels act as paths of least resistance for the transport of interdendritic fluid into the overlying bulk fluid. Fluid escaping from the mushy zone forms buoyant plumes at the mouth of each channel, and the plumes are sustained by penetration of bulk liquid across the liquidus front and subsequent advection toward the channels. Predictions for channel development and flow in the overlying melt agree qualitatively with previous experimental observations.