Non-Equilibrium Solidification of Undercooled Metallic Melts

Abstract

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.

Thermodynamic and electrochemical properties of holmium and HoxAly intermetallic compounds in the LiCl-KCl eutectic

“…For the formation of the metastable compound, two possible mechanisms can be proposed: first, during phase equilibrium, the liquid metallic solution might be overcooled below its equilibrium melting point temperature and an excess Gibbs free energy is therefore created. This gives an access to the solidification of metastable solids under nonequilibrium conditions [46]. However, this phenomenon cannot occur in our case because our electrolysis experiments were carried out at a specific temperature.…”

Microstructure, Solidification Texture, and Thermal Stability of 316 L Stainless Steel Manufactured by Laser Powder Bed Fusion

“…This can explain why the values of the constant k, obtained from the fitted lines in Figure 4, deviated from the~V L −0.5 dependence. This deviation, in turn, reflected the fact that crystallization can approach a rapid solidification regime at which diffusional processes are suppressed [40].…”

“…Consequently, it was suspected that at locations where the solidification rate was the highest, material could crystallize in the rapid solidification regime. Under these conditions, the diffusional path and the solidification rate are comparable, so solute atoms do not have enough time to segregate, and are trapped in the newly formed solid phase with a random distribution [40]. This regime does not imply a microsegregation phenomenon.…”

Melt undercooling and nucleation kinetics

“…Other examples also indicate that at low undercooling solidification is initiated at pre-existing crystal sites [7]. At the opposite extreme, metastable and amorphous phases can be synthesized at high undercooling by rapid quenching [8] or by special melt treatments such as fluxing or containerless processing [9,10].…”