7. Effect of non-stoichiometry of precipitates

The simplest situation in a precipitation process is the formation of a phase of fixed composition, in a matrix whose solute composition is progressively depleted until equilibrium is reached. However, this situation is rarely encountered in practice, and the composition of precipitates generally varies during the different stages of precipitation, with very significant consequences for kinetics.

During the early stages of germination, tomographic atom probe measurements have shown that precipitates are much richer in solvent than the equilibrium concentration. Furthermore, in multi-constituent systems, when a precipitated phase may contain several chemical species, the ratio between their concentrations generally evolves over time, and the distribution of these chemical species may be spatially heterogeneous within each precipitate. These deviations from stoichiometry...