INTERSTITIAL CLUSTER FORMATION IN METALS UNDER INTENSE IRRADIATION

A new mechanism of point defect clustering in irradiation environment is proposed. The process considered reflects a possibility for spontan eous (nondiffusional) clustering to exist in a highly enough supersatu rated solution of point defects under certain conditions. The influenc e of the production of di-interstitial clusters by this mechanism on t he kinetics of interstitial dislocation loop nucleation and growth is analyzed. It is shown that the intensity of irradiation must be high e nough and the temperature of the sample must be correspondingly low en ough for the effect of the spontaneous small interstitial clustering t o prevail over the usual mechanism of diffusion driven nucleation and growth of interstitial clusters. Both analytical and numerical solutio ns of an extended version of conventional rate equations demonstrate t he possibility of this phenomenon. This version takes into account the terms of higher order in an expansion of the chemical potential over the interstitial concentration. The new form of equations naturally fo llows from a canonical approach to chemical-type rate equations that i s known in nonlinear nonequilibrium thermodynamics. When density effec ts are negligible, one gets the conventional kinetics of diffusion dri ven interstitial loop formation. A simple model of interstitial loop n ucleation and growth through di-interstitials is used in the analysis of the clustering kinetics. A noticeable enhancement of interstitial l oop number density with smaller radii is expected for temperatures bel ow one third of the melting point, where the loop growth process due t o diffusional processes is weak enough. For this novel phenomenon to b e noticeable, a prolonged irradiation is necessary. Conditions and pos sibilities for experimental investigations of the predicted effect as well as the possible competitive processes are considered.