A study of irradiated FeCr alloys: deviations from Matthiessen's rule and interstitial migration

FeCr alloys with concentrations ranging from 50 p.p.m. to 3% Cr have been irradiated with electrons of 1.6 MeV at low temperature (30 K) and studied by resistivity measurements. Pre-irradiation measurements show important deviations from Matthiessen's rule in the annealed alloys: the resistivity increment per unit concentration of Cr in Fe depends strongly on the solute concentration. It increases from 180 mu Omega cm for a few per cent of chromium up to 400 or 600 mu Omega cm when the chromium concentration is lowered down to 50 p.p.m. This increase is explainable in the two-current model if one considers the dilute alloys with the residual impurities as ternary alloys. The radiation-induced resistivity, measured as 4 K, is also found to depend strongly on the Cr concentration. This can only be explained by assuming that the Frenkel pair specific resistivity, rho F , is an increasing function of the Cr concentration. This variation of rho F in the alloys can be interpreted, in the two-current model, as a deviation from Matthiessen's rule in the irradiated alloys considered as ternary alloys. Mixed Fe-Cr interstitials seem to be slightly more mobile than self-interstitials.