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 spontaneous (nondiffusional) clustering to exist in a highly enough supersaturated solution of point defects under certain conditions. The influence of the production of di-interstitial clusters by this mechanism on the kinetics of interstitial dislocation loop nucleation and growth is analyzed. It is shown that the intensity of irradiation must be high enough and the temperature of the sample must be correspondingly low enough for the effect of the spontaneous small interstitial clustering to prevail over the usual mechanism of diffusion driven nucleation and growth of interstitial clusters. Both analytical and numerical solutions of an extended version of conventional rate equations demonstrate the 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 follows from a canonical approach to chemical-type rate equations that is known in nonlinear nonequilibrium thermodynamics. When density effects are negligible, one gets the conventional kinetics of diffusion driven interstitial loop formation. A simple model of interstitial loop nucleation and growth through di-interstitials is used in the analysis of the clustering kinetics. A noticeable enhancement of interstitial loop number density with smaller radii is expected for temperatures below one third of the melting point, where the loop growth process due to diffusional processes is weak enough. For this novel phenomenon to be noticeable, a prolonged irradiation is necessary. Conditions and possibilities for experimental investigations of the predicted effect as well as the possible competitive processes are considered. (orig.)