Abstract
Undercooled melts are metastable with respect to the lower Gibb's free energy of the corresponding solid state and due to the activation barrier against nucleation. This gives access to direct solidification into metastable solid phases. Nucleation of metastable bcc phase in FeNi alloys has been demonstrated if undercoolings below its melting temperature are achieved and if the nucleation barrier is decreased by offering nucleation seeds of proper catalytic potency. Moreover, measurements of rapid dendrite growth velocities on containerlessly undercooled Ni-base alloys reveal deviations from local equilibrium at the solid-liquid interface. These are generated by a kinetic undercooling of the solidification front and solute trapping effects at high crystallisation velocities. The microgravity environment of space gives access to an extended degree of undercooling and thus to an enhanced range of metastable materials.
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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