Solidifications

Solidifications refer to the processes by which liquids become solids as temperatures decrease below their melting points. In metals and alloys, solidification begins with nucleation of solid phase inside the liquid, followed by growth of crystalline grains. Nucleation can be homogeneous, occurring spontaneously in deeply undercooled liquids, or heterogeneous, occurring at surfaces, impurities, or mold walls. Undercooling and alloy composition determine the number and distribution of nuclei.

During growth, atoms migrate to the solid–liquid interface, and the microstructure develops into grains. The solidification front can be equiaxed, with roughly equal dimensions in all directions, or columnar, elongated toward the heat flow. In many alloys a mushy zone forms, with solid and liquid phases coexisting and solute partitioning leading to microsegregation and, in some cases, macrosegregation.

Solidification affects material properties through the final microstructure. Rapid cooling yields fine grains and can create dendritic structures; slow cooling promotes coarser grains. In eutectic and peritectic systems, simultaneous solidification of multiple phases occurs at specific compositions and temperatures. Defects such as porosity, shrinkage, and hot tearing may arise from insufficient feeding, gas evolution, or thermal stresses.

Solidification is central to metal casting, alloy production, and crystal growth for semiconductors, as well as processes for polymers and ice formation. It is studied with phase diagrams, kinetics of nucleation and growth, and numerical models such as Scheil or lever-rule analyses to predict segregation and mushy-zone behavior. Controlling cooling rate, heat extraction, and alloy additives can tailor the resulting microstructure and properties.