Diffusion in ion irradiated nickel: determination of atomic mixing, sink strength and fraction of freely migrating defects

Diffusion coefficients under heavy ion irradiation at 77 K, 800 K and 950 K were measured in nickel using specimens with several inserted thin layers of 63Ni or Co tracer in different depth. At 77 K the diffusion coefficient follows closely the depth dependence of the calculated displacement rate indicating that athermal atomic mixing is responsible for the observed transport at this temperature. Considerable self-diffusion is observed at 800 K and 950 K even far from the irradiated surface in a region were the displacement rate is negligible indicating long range diffusion of mobile radiation-induced point defects. The fit of a diffusion model for these defects to the observed depth dependence of the radiation-enhanced self-diffusion coefficients yielded effective values of the sink concentration and the fraction of freely migrating defects. In the non-damaged region of the specimens the sink concentration amounts to less then 4 x 10-7, while in the damaged region near the surface it is of the order of 10-6 to 10-5. The fraction of freely migrating defects is estimated to be between 0.7% and 7%, depending on the kind of irradiation. (orig.)