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.

Acknowledgements

The author would like to thank his colleagues A.E. Davletshin and V.A. Pavlov for their assistance in irradiations, V.B. Vykhodets and T.E. Kurennykh for nuclear microanalysis, Yu.Yu. Tsiovkin for helpful discussions and explanations, V.V. Gapontsev for a figure design, and A.P. Druzhkov for valuable remarks when reading the manuscript. Special thanks should be given to Dr. C. Dimitrov for difficult questions in the course of many years discussion, many of which have been answered only in this paper. The author is also grateful to D. Terentyev and L. Malerba for their permanent interest in the work. This work was done within the RAS Program (project 13394) with partial support from the Russian Foundation for Basic Research (Grant 07.02.00020a).

Notes

†Liquid helium was blown out of Dewar, at the bottom of which some quantity of fine-dispersed snow dust was accumulated because of the moisture traces condensation from the helium collector system. In the course of flowing helium, this dust was dragged and thrown into a cryostat.

†Direct estimation of the SRO-induced contribution at the stage III peak is difficult because this contribution and the inverted partial RR compensate each other. Therefore the SRO-induced contribution to the RR rate was estimated at temperatures above the stage III peak position where the inverted RR rate is negligible.

†See for example the data on Fe-0.3Mo 33, where the suppressed IA migration (close pair, correlated and uncorrelated) gives rise only to one dissociation peak.

†Sample positioning in the sample holder and the RR percentage in Fe and Fe-Cr at 90 K were taken into account.

†In general, such overestimation may be a result of iron contamination with ∼0.1 at.% Cr 10. However, Cr is not a common contaminating impurity in iron so this assumption looks scarcely probable.