Fluid-flow effects on phase selection and nucleation in undercooled liquid metals

Abstract

Fluid flow can have a profound effect on phase selection and nucleation in undercooled liquid metals.In phase selection experiments, the sample does not solidify directly to the thermodynamically stable phase, but instead first forms a metastable phase, then transforms to the stable phase after some delay. Previous experiments and modeling on phase selection in near-eutectic Fe-Cr-Ni stainless steels have shown that the lifetime of the metastable phase is limited by the incubation time for formation of a critical nucleus of the stable phase, and that for some conditions, fluid flow can change that incubation time by an order of magnitude or more. New microgravity experiments in this area will be performed on new materials, including peritectic alloys, in the Materials Science Laboratory Electromagnetic Levitator (MSL-EML) as a part of projects PARSEC and THERMOLAB. The models that predict the effect of fluid flow on phase selection will be extended to these new experiments.In another class of experiments, nucleation in quasicrystal- and glass-forming alloys, testing a new theory on nucleation kinetics requires control of the shear rate in fluid flow to prevent interference of the solute fields around the sub-critical nuclei. These theories will be tested in microgravity using the MSL-EML under the projects ICOPROSOL and THERMOLAB.