The majority of phase transformations, which take place in alloys, are diffusional and proceed by thermally activated movements of atoms across a concentration gradient. Of significant importance in relation to thermal industrial processing, are phase transformations involving precipitation reactions resulting to a second phase in the form of a particle population. During these transformations three basic physical mechanisms are taking place, the nucleation of new particles, the growth of the nucleated particles and the coarsening of the particles resulting to a particle population which can be described by a particle size distribution (PSD). The material final mechanical properties depend on the particles volume fraction and size and consequently on the shape of the PSD. The PSD is readily calculated with the KWN model  which treats nucleation, growth and coarsening concurrently.

Reference: Kampmann, R., Eckerlebe, H., Wagner, R. (1987). Precipitation kinetics in metastable solid solutions, Mater. Res. Soc. Symp. Proc., 57, pp. 525-542.