5. Influence of diffusion mechanisms on precipitation kinetics

Precipitation of a phase with a different composition from the matrix involves long-range diffusion. In the majority of cases, the mechanism that limits precipitation kinetics is substitutional diffusion by the vacancy mechanism, whose constant is directly proportional to the concentration of vacancies, and closely depends on their migration speed and interaction with solutes, which is reflected in the diffusion coefficient of the species in the matrix.

However, it should be borne in mind that, during precipitation, the concentration of vacancies is generally out of equilibrium, particularly as a result of sudden temperature changes, the most classic being quenching since the alloy was put into solution. The effective concentration of vacancies during precipitation can therefore be several orders of magnitude higher, with a corresponding increase in the diffusion...