THERMAL RECALESCENCE AND MUSHY ZONE COARSENING IN UNDERCOOLED MELTS

Precision adiabatic recalescence experiments on both pure and binary undercooled melts have been conducted. The adiabatic constraint limits the amount of net phase transformation from the metastable undercooled state (creating a mushy zone) and also sets bounds on the microstructural parameters of the mushy zone, namely the volume fraction of the phases, their length scales, and the interfacial compositions. As many as three distinct kinetic time scales are observed in these experiments: a short time scale (less than approximately 1 s) of rapidly increasing temperature from the initial nucleation temperature, associated with the propagation of the dendrites and release of latent heat; an intermediate time scale (approximately 10(3) - 10(4) s) of slowly rising temperature, associated with coarsening of the mushy zone; and a long time scale (days) of steadily falling temperature, associated with solid-state diffusional adjustments near the solid-liquid interface.