Dendritic solidification of undercooled melts: mushy zone recalescence dynamics

Abstract

The nucleation and growth of crystals in deeply undercooled melts leads to rapid dendritic growth accompanied by recalescence. The recalescence temperature provides an internal degree of freedom if the thermodynamic system is closed to all energy and mass transfers over relevant observational time scales. Precision adiabatic recalescence experiments on both pure and binary undercooled melts were conducted. The adiabatic constraint limits the amount of net phase transformation from the metastable undercooled state, and also sets bounds on the microstructural parameters of the mushy zone, i.e. 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: (1) a short time scale (ca. less than 1 s) of rapidly increasing temperature from the initial nucleation temperature, associated with propagation of the dendrites and release of latent heat; (2) an intermediate time scale (ca. 10³–10⁴ s) of slowly rising temperature, associated with coarsening of the mushy zone; (3) a long time scale (ca. days) of steadily falling temperature, associated with solid-state diffusional adjustments near the solid-liquid interface. Interpretation of these experiments is provided in an attempt to understand the overall process of non-equilibrium freezing of undercooled alloy melts.