Solidification
Solidification requires undercooling below the melting point. Near equilibrium, stable solid phases are formed. However, nucleation from the container walls usually limits the degree of undercooling possible. Containerless processing techniques significantly widen the range of undercooling, maintaining a metastable liquid with large excess free enthalpy. A great variety of solidification pathways is possible. It emerges that the crystallographic phase is selected in the crystal-nucleation process. Subsequent crystal growth proceeds with a large variation 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. The 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 using electromagnetic levitation devices are carried out on ground as well as in weightlessness on parabolic flights, on sounding rockets, and aboard the ISS. In general, growth controls the microstructure and thus finally the macroscopic properties of a material. A major growth process in metals is that of dendritic growth: The crystals emerging during solidification follow a pattern that consists of a main stem with side arms growing along certain crystallographic directions.
Research at the institute on the solidification of metallic alloys is concerned primarily with advancing the understanding of alloy processing, so that structure and properties can be controlled as the materials are originally formed from the melt. Central processes involving crystal growth are influenced by buoyancy forces and thus gravity. Microgravity experiments allow to uncover the basic mechanisms behind structure formation under ideal, purely diffusive conditions.
The institute’s solidification research focuses on:
- Growth dynamics and microstructure evolution
- Metastable phase formation and multiphase formation
- Demixing and spinodal decomposition