Erstarren

Erstarren is a term used in materials science to describe the transition of a liquid into a solid when temperature decreases below the melting point. The process begins with nucleation, where small stable solid particles form in the liquid under conditions of undercooling. Nucleation can be homogeneous, occurring in the bulk liquid, or heterogeneous, occurring at surfaces, interfaces, or impurities, which typically requires less undercooling.

The solidification front advances as atoms arrange into a crystal lattice, releasing latent heat of fusion. The rate of heat extraction and the cooling path strongly influence the resulting microstructure. Slow, uniform cooling tends to produce equiaxed grains, while directional cooling or high undercooling can yield columnar grains or dendritic structures. Alloy composition, presence of inclusions, and thermal gradients determine segregation patterns and porosity.

The microstructure established during Erstarren determines mechanical properties such as strength, hardness, ductility, and toughness. It also affects residual stresses and distortion in castings. Controlling Erstarren is a central concern in casting, welding, and additive manufacturing, where techniques include mold design, cooling rate management, grain refiners, inoculants, and heat treatment to tailor grain size and distribution.

Historically, studies of solidification helped develop casting technologies and alloy design. In practice, the term is often used interchangeably with solidification in English, while in German-language literature it is common as Erstarren to describe the entire freezing process.