Surface tension (γ) on the surface of crystal and liquid influence on the critical size of the nucleus. The bigger is energy of interphase boundary, the more energy fluctuations are necessary and nucleuses origination is found more difficult. Melt capability for overcooling depends on similarity of packing of atoms short-range order in liquid and crystal state. If the similarity is small, the surface tension between crystals and liquid is big and bigger fluctuations energies will be necessary for interphase surface origination. Spontaneous origination of nucleuses in pure state is found only in thoroughly refined liquids. This refinement is made by repeated filtration, vacuum distillation, crystallizing out from the solution and by other ways. In technical metals there are always nonmetallic inclusions, on the surface of which the heterogeneous origination of crystallization centers happens even under conditions of small overcooling. Crystals origination catalyzes and by surface of vessel walls (mold), in which the liquid is crystallized.
During heterogeneous origination small crystal occurs on the surface of extraneous solid body, as on the substrate. It is usually assumed that it has form of hemispherical dome. The small crystal becomes stable when reaching certain size. During this not quantity of atoms in the crystal is significant, but curvature of surface of its boundary with liquid.
Inclusions have positive effect on crystal origination, which grate is similar by type (isomorphic) with the grate of crystallizing material and parameters of matching grates of inclusion and crystallizing metal are close (the difference is no more than 9%). In this case in resultant on the adsorbed layer of inclusion surface atomic arrangement will be almost the same, as in the case of the crystal of this material or near to it. As a result, isomorphic inclusions represent good substrate and liquid consolidation begins under conditions of lower overcooling level, than in the case of homogeneous origination.