THE NATURE OF THE C-DEFECTS IN NICKEL AND THEIR ROLE IN THE INTERPRETATION OF RADIATION-DAMAGE IN METALS

In previous Perturbed-Angular-Correlation (PAC) studies of the gamma-g amma emission of In-111 probe nuclei in cold-worked or particle-irradi ated nickel, it has been found that thermal annealing in the temperatu re regime of recovery stage III leads to the formation of so-called C- defects (Cubic defects). This is indicated by the occurrence of a new frequency of about 80 Mrad/s, in addition to the frequency (almost-equ al-to 200 Mrad/s) that is due to In-111 on substitutional sites. Obvio usly, the C-defects are complexes consisting of In-111 and the intrins ic point-defect species that migrates freely in recovery stage III. Th erefore, they have played an important role in the long-standing contr oversy on whether the recovery-stage-III defects are vacancies (one-in terstitial model) or self-interstitials (two-interstitial model). The present paper reports on a novel experimental effort to reveal the nat ure of the C-defects by combining PAC studies on nickel samples differ ently pretreated in a systematic way, investigations of the Extended X -ray Absorption Fine Structure (EXAFS) on In-doped nickel, and measure ments of the decay rate of In-111 nuclei in the Electron-Capture-Induc ed Decay (ECID). On the basis of the results of these experiments it i s concluded that the defects trapped by substitutional In-111 atoms (I n(s)) in recovery stage III are self-interstitials (I), as expected ac cording to the two-interstitial model. Moreover, there is evidence tha t the C-defects are In interstitials on tetrahedral sites (In(i)) that form exclusively in the vicinity of the specimen surface from In(s) - I pairs via the reaction In(s) + I --> In(i).