Precipitate and microstructural stability in alloys subjected to sustained irradiation

The sustained irradiation of a material by energetic particles leads to the continuous production of damage in the form of point defects, point-defect clusters, and forced atomic relocations, as reviewed in Chap. 1. These elementary processes lead to an acceleration of thermally activated diffusion owing to point-defect supersaturation, as well as a forced mixing of chemical species due to atomic replacements. In materials with precipitates or ordered phases, this forced mixing alone would lead to dissolution and chemical disordering, respectively. At high enough temperatures, however, these dynamical processes compete with thermally activated diffusion, which tends to restore an equilibrium state. The outcome of this competition depends of course on the relative intensity, or rates, of these processes, but also on their characteristic length scales. We review in some detail the evolution of pre-existing precipitates under irradiation to illustrate the complex material's response to these dynamical processes, including the potential self-organization of the microstructure. Similar effects are anticipated in materials undergoing order-disorder transformations. In addition, the kinetic coupling between point defects and chemical fluxes can lead to radiation-induced segregation and precipitation. Finally, we discuss the contribution of point-defect evolution to microstructural changes, which can produce dimensional changes and alter mechanical properties.