Types of Transformations

The local curvature of the Gibbs free energy of solution with respect to composition determines whether an unstable composition will achieve stability by small widely-spaced composition fluctuations or large isolated fluctuations in composition. The later is an important process called nucleation.

For the case of nucleation of very small nucleating particles, one needs to consider the extra energy associated with a surface of the particles. This extra energy has to be overcome by a decrease in the Gibbs free energy of the entire particle. This leads to the possibility of under-cooling and the prediction of minimum size of a particle (a nucleus) that can grow.