Solidification

Solidification requires subcooling below the melting point. Stable solid phases form close to equilibrium. However, nucleation on the container walls usually limits the possible degree of undercooling. Containerless processing techniques considerably extend the undercooling range and produce a metastable liquid with a high excess of free enthalpy. A variety of solidification paths are possible. It can be seen that the crystallographic phase is selected in the crystallisation process. The subsequent crystal growth occurs with a wide variety of morphologies, such as dendritic or faceted. In many alloys, solidification is a multiphase process with coupled growth of two or more phases, such as eutectic or peritectic. Levitation of liquid droplets is combined with high-speed imaging to measure the velocity of the solid-liquid interface and the various effects of non-equilibrium solidification. To control the field-induced flow, experiments are performed with electromagnetic levitation devices both on the ground and in microgravity during parabolic flights, on research rockets and on board the ISS. In general, growth determines the microstructure and thus ultimately the macroscopic properties of a material. An important growth process in metals is dendritic growth: the crystals that form during solidification follow a pattern consisting of a main stem with side branches that grow along certain crystallographic directions.