Precipitation kinetics in metallic alloys: Experiments and modeling

Nanoscale precipitation is one of the most widely used microstructural tools to manipulate the properties of metallic alloys, and especially to reach high strength. Optimal microstructures are reached through complex solid state phase transformations involving non-isothermal heat treatments, metastable phases, complex chemistry, non-equilibrium vacancies, and interaction with structural defects. These phase transformations are controlled by an interplay between thermodynamics and kinetics, resulting through nucleation, growth and coarsening, in a large variety of precipitation trajectories that depend on both alloy chemistry and processing. Progress in both experimental characterization and modeling has tremendously improved the knowledge and description of these processes. The purpose of this overview is to describe the current level of understanding of precipitation kinetics, starting from the relatively simple situation of homogeneous precipitation of dilute coherent phases and including different levels of additional complexity regarding the diffusion mechanism, the effect of finite volume fraction, the effect of particle shape, the competitive multi-phase precipitation, the heterogeneous nucleation, and the non-isothermal effects.