Abstract
If a liquid is undercooled below its equilibrium melting temperature an excess Gibbs free energy is created. This gives access to solidification of metastable solids under non-equilibrium conditions. In the present work, techniques of containerless processing are applied. Electromagnetic and electrostatic levitation enable to freely suspend a liquid drop of a few millimeters in diameter. Heterogeneous nucleation on container walls is completely avoided leading to large undercoolings. The freely suspended drop is accessible for direct observation of rapid solidification under conditions far away from equilibrium by applying proper diagnostic means. Nucleation of metastable crystalline phases is monitored by X-ray diffraction using synchrotron radiation during non-equilibrium solidification. While nucleation preselects the crystallographic phase, subsequent crystal growth controls the microstructure evolution. Metastable microstructures are obtained from deeply undercooled melts as supersaturated solid solutions, disordered superlattice structures of intermetallics. Nucleation and crystal growth take place by heat and mass transport. Comparative experiments in reduced gravity allow for investigations on how forced convection can be used to alter the transport processes and design materials by using undercooling and convection as process parameters.
Highlights
Metallic materials are prepared from the liquid state as their parent phase
A statistical analysis of the distribution function of maximum undercoolings in electrostatic levitation experiments hints to the onset of homogeneous nucleation in undercooled Zr-melts
This value indicates that results by density functional theory and molecular dynamics simulations may lead to an underestimation of the solid-liquid interfacial energies
Summary
Metallic materials are prepared from the liquid state as their parent phase. The conditions under which the liquid solidifies determine the physical and chemical properties of the as-solidified material. These are crystal nucleation and crystal growth Both of these processes are driven by an undercooling of the liquid below its equilibrium melting temperature to develop conditions where a driving force for the formation of supercritical nuclei and the advancement of a solidification front are created. This gives access to non-equilibrium solidification pathways, which can form metastable solids, which may differ in their physical and chemical properties from their stable counterparts. The non-equilibrium process of rapid solidification is accessible to direct analysis by investigating the recalescence profile Both nucleation and subsequent crystal growth are governed by heat and mass transport. The present article aims to give an overview on the current state of research on undercooled melts and their non-equilibrium solidification
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