Nucleation During Phase Transitions

Abstract

Experience has shown that, during cooling of a liquid, crystallization does not start immediately when the solidification temperature Tg is reached. The new phase is formed only after a more or less severe undercooling ΔT below the equilibrium temperature. This phenomenon is especially noticeable with transformation or precipitation reactions in solids. It is based on the fact, that in a situation, void of energy influences, casual fluctuations of the atomic structure of the starting phase cause the appearance of a very small volume fraction of an atomic arrangement, which is equal, or at least similar to that in the end phase. These new phases, called nuclei, can grow into macroscopic dimensions. This kind of new phase formation is called a nucleation-growth process. Only when all phases belonging to a heterogeneous equilibrium are formed, can the laws of phase equilibria be valid.