Diffusion processes in irradiated materials are considerably promoted by the nonequilibrium mobile vacancies and interstitials introduced by irradiation. To quantify these effects as is necessary for an understanding of, for example, void swelling or phase stability, the production rate of freely migrating defects must be known for the specific irradiation conditions. This poses a considerable problem for neutron and ion irradiations, where defects are produced in cascades. Because of spontaneous recombination and clustering of defects, only a small number of freely migrating defects is expected in comparison to the calculated displacement rate.
Diffusion coefficients of nickel in nickel and in Fe-20Cr-20Ni and Fe-20Cr-60Ni were directly measured with secondary ion mass spectroscopy (SIMS) under 300-keV Ni+ irradiation in the temperature range between 650 and 1000 K. Dose rates between 10-4 and 3 × 10-2 dpa/s and doses larger than 0.2 dpa were applied. The results were analyzed by means of a rate equation model. For the self-ion irradiated nickel a value of 1.5% of the calculated displacement rate was deduced for the rate of freely migrating defects independent of temperature. Similar values were estimated for the alloys.