Influence of Stresses on Precipitation

Abstract
Precipitation, which usually occurs by nucleation, growth and coarsening, may be affected by elastic stresses which influence the thermodynamics of the system and thereby the driving force for precipitation. Such stresses may be internal, e.g. coherency stresses, or external, i.e. externally applied stresses. The consequences for nucleation, growth and coarsening are discussed in detail with special emphasis on the kinetics of precipitation. The possible effects are exemplified by various cases of the orienting of precipitate particles by external stress. The effect of elastic stress on phase solubility may not only affect precipitation by nucleation and growth, but also spinodal decomposition which is briefly discussed. Larger elastic stresses, which surpass the yield stress, result in plastic deformation with production and movement of dislocations. Dislocations may affect precipitation processes by their stress fields, by being incorporated into interfaces to lower the interface energy and by providing easy diffusion paths. Again the effects are discussed with respect to nucleation, growth and coarsening. The effects are made use of for alloy processing by thermomechanical treatments, and furthermore they may lead to structure changes - with corresponding changes in mechanical behaviour - during high-temperature service with creep.