DENDRITIC SOLIDIFICATION OF UNDERCOOLED MELTS - MUSHY ZONE RECALESCENCE DYNAMICS

The nucleation and growth of crystals in deeply undercooled melts lead s to rapid dendritic growth accompanied by recalescence. The recalesce nce temperature provides an internal degree of freedom if the thermody namic system is closed to all energy and mass transfers over relevant observational time scales. Precision adiabatic recalescence experiment s on both pure and binary undercooled melts were conducted. The adiaba tic constraint limits the amount of net phase transformation from the metastable undercooled state, and also sets bounds on the microstructu ral parameters of the mushy zone, i.e. the volume fraction of the phas es, their length scales, and the interfacial compositions. As many as three distinct kinetic time scales are observed in these experiments: (1) a short time scale (ca. less than 1 s) of rapidly increasing tempe rature from the initial nucleation temperature, associated with propag ation of the dendrites and release of latent heat; (2) an intermediate time scale (ca. 10(3)-10(4) S) of slowly rising temperature, associat ed with coarsening of the mushy zone; (3) a long time scale (ca. days) of steadily falling temperature, associated with solid-state diffusio nal adjustments near the solid-liquid interface. Interpretation of the se experiments is provided in an attempt to understand the overall pro cess of non-equilibrium freezing of undercooled alloy melts.