Time-resolved X-ray diffraction studies of non-equilibrium solidification in deeply undercooled melts

by Olga Shuleshova (IFW Dresden)

109 (25b)



Solidification of virtually any substance is initiated by nucleation and growth of the solid phase at the temperature below the equilibrium liquids. In particular, for metallic alloys the difference between liquidus and nucleation temperatures, called undercooling, varies from incrementally small up to hundreds degrees of Kelvin. While conventional casting does not typically involve large undercooling of the melt prior to solidification, other production routs, e.g. gas atomization, additive manufacturing, etc., are inherently prone to it. Both retarded nucleation and rapid heat transport lead to acceleration of the solidification process such that no time remains to establish equilibrium conditions at the solid-liquid interface. Moreover, below the equilibrium, liquidus nucleation of the other solid phases can become thermodynamically and kinetically viable. It is this rapid non-equilibrium solidification that produces remarkably different microstructures and results in modified material properties. Due to fine length and short time scales of the event, its study requires non-trivial techniques. Here we discuss the possibilities to access the deep undercooling and in situ determination of subsequent solidification processes in industrially relevant systems by combination of the electromagnetic levitation technique with the time-resolved X-ray diffraction of synchrotron radiation. Experiments conducted at the high-energy materials science beamline P07 of Deutsches Elektronen-Synchrotron (DESY) allowed for the first time to determine the structure of the metastable phases, found in studied systems.