INFLUENCE OF STRESSES ON PRECIPITATION

Precipitation, which usually occurs by nucleation, growth and coarseni ng, may be affected by elastic stresses which influence the thermodyna mics of the system and thereby the driving force for precipitation. Su ch 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 k inetics of precipitation. The possible effects are exemplified by vari ous 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 decompositi on which is briefly discussed. Larger elastic stresses, which surpass the yield stress, result in plastic deformation with production and mo vement of dislocations. Dislocations may affect precipitation processe s by their stress fields, by being incorporated into interfaces to low er the interface energy and by providing easy diffusion paths. Again t he effects are discussed with respect to nucleation, growth and coarse ning. The effects are made use of for alloy processing by thermomechan ical treatments, and furthermore they may lead to structure changes - with corresponding changes in mechanical behaviour - during high-tempe rature service with creep.