Phase Stability in Irradiated Alloys

In nuclear materials, precipitation-hardening, characterized by the nucleation, growth and coarsening of second phase particles resulting from a metastable supersaturated solid solution, is commonly used for increasing the strength and hardness of alloys. However, with the harsh conditions encountered in nuclear reactors such as high temperatures, high pressure and high levels of radiation, the stability of the dispersed precipitation is altered and degrade the over-all mechanical properties of materials. Thus in nuclear industry, phase stability under irradiation play a key role in the integrity of materials. Under irradiation, collision cascades produced by energetic neutrons cause ballistic ejection of solute atoms from their original lattice sites and create point defects, such as vacancies and interstitials in excess concentration. These point defects are responsible for the enhancement of diffusion, a phenomenon also called radiation-enhanced diffusion, and for solute segregation, a phenomenon also called radiation-induced segregation. Ballistic jumps of atoms, enhanced diffusion and nonequilibrium segregation are synergetic processes able to significantly change the precipitation microstructure of materials. Among the changes induced by irradiation: radiation-induced dissolution and re-precipitation, radiation-enhanced Ostwald ripening, radiation-induced inverse Ostwald ripening, radiation-enhanced precipitation, radiation-induced precipitation and finally radiation-modified precipitation will be developed in this article and illustrated by examples from recent research work.