Solidification

Solidification is the phase change by which a liquid becomes a solid as its temperature falls below the melting point. It involves nucleation, the initial formation of small solid clusters, and crystal growth as atoms arrange into a lattice. Solidification can occur via homogeneous nucleation, where nuclei form spontaneously in the liquid, or heterogeneous nucleation, which occurs on impurities, container walls, or other interfaces. Undercooling, or supercooling, is the extent to which liquid is cooled below its melting point without solidifying; greater undercooling generally increases nucleation rate and can alter the resulting microstructure.

In metals and alloys, the rate of cooling and composition determine the microstructure: slow cooling tends to produce larger, well-formed grains; rapid cooling yields fine grains or amorphous structures in suitable systems; in alloys, phase transformations such as eutectic, peritectic, or invariant reactions create complex microstructures. The growth of solid phases proceeds through interfaces; diffusion of atoms and often the development of dendritic patterns or columnar grains.

Consequences and applications: Solidification determines mechanical properties, porosity, and residual stresses. In casting, controlling cooling rate minimizes shrinkage and segregation. In polymers, crystallization affects toughness and clarity. In geology, solidification of silicate melts forms igneous rocks. In cryology, water freezes to ice with crystalline structure, releasing latent heat.