Specificity of stage III in electron-irradiated Fe-Cr alloys

Abstract

It has been found that no typical features of stage III are traced in the resistivity recovery (RR) and positron lifetime (PL) data of electron-irradiated Fe-Cr alloys (4–10 at.%). None of the observed RR stages has shifted its temperature position with changing concentration of defects, which is characteristic of stage III. A new quantity was considered–the difference between RRs (DRR) of samples having different defect concentrations. The onset of free migration is indicated in the DRR plot by a peak, which corresponds in physical meaning to a partial (separated) RR peak of the free migration stage. Such a single peak was found in Fe-9Cr at 205–210 K and identified as the sign of stage III, however, the peak amplitude turned out inverted, i.e. it corresponded to a resistivity rise instead of the usual resistivity drop. In Fe-4Cr the peak amplitude is about zero. Such anomalous RR behaviour is related to a very high resistivity contribution of immobile di-vacancies formed in stage III. This contribution masks the resistivity reduction and makes stage III invisible in conventional RR spectra. Close PLs in mono- and di-vacancies make stage III poorly detectable by PL spectroscopy as well. The RR stage and PL increase at 220 K, considered earlier as the signs of stage III, are actually connected with the onset of long-range migration of interstitial atoms. Recombination and a release of mobile mono-vacancies resulted from this onset give rise, correspondingly, to a conventional RR stage and clearly detectable enlargement of the small vacancy aggregates formed in stage III.