The center of crystallization is a solid phase nucleus in the melt from which crystallite grows. There are two mechanisms for the formation of crystallization centers: homogeneous and heterogeneous.
Homogeneous nucleation occurs in the zones:
- homogeneous melt state;
- with fluctuation of energy, where its level exceeds the average value of the melt energy;
- with a close arrangement of atoms corresponding to the ordering of the crystalline state;
- size larger than a certain critical size. Smaller embryos are thermodynamically unstable, since their growth leads to an increase in free energy, and therefore they dissolve in the melt.
Heterogeneous nucleation occurs in a heterogeneous melt on finished substrates, solid particles insoluble in the melt, mold walls, molds, provided that the surface energy of the boundary between the lining and the newly formed solid phase is less than the surface energy of the boundary between the nucleus and the liquid phase.
Literature
Ukrainian Soviet Encyclopedia: [12 p.] / Ch. ed. M.P. Bazhan; editorial: A.K. Antonov et al. - 2nd ed. - K. : Goals. ed. Ure, 1974-1985.
Popovich, A. Kondir, E. Pleshakov and others. Technology of structural materials and materials science: Workshop: Textbook. allowance / - Lviv: World, 2008. - 422 p.