Mössbauer effect study of interstitial atom trapping sites around⁵⁷Co impurities in tungsten

Abstract

The impurity interstitial interactions in diluteW⁵⁷ Co alloys during 3 MeV electron irradiation at 80 K and subsequent isochronal annealing have been studied using Mössbauer technique and electrical resistivity measurements. During 80 K irradiation, the population of one⁵⁷Co impurity single interstitial complex, two different trapping sites for di-interstitials and three⁵⁷Co impurity interstitial cluster complexes could be identified by the appearance of six additional absorption lines in the Mössbauer spectra with isomer shifts of +0.72 to +0.41 mm/s relative to the substitutional line and quadrupol splittings ranging from 1.89 to 0.17 mm/s. The trapping behaviour ofW⁵⁷ Co could be described by a nucleation trap model using a trapping radius ratior_o/r_v . = 0.25 (⁵⁷Co atom without captured interstitial/vacancy) and assuming an increase of the trapping radius per trapped interstitial ΔLr = r ₀. Comparing the Mössbauer data and the resistivity data, we found that on an average only about 23% of the interstitials are trapped in the direct vicinity of the⁵⁷Co impurities, while about 77% are bound farther on shallow traps.

The model is supported by the results of the isochronal annealing after irradiation. At annealing temperatures above 190 K, interstitials captured on shallow trap positions become mobile and reach the direct vicinity of the⁵⁷Co impurities.

Furthermore, the annealing behaviour shows that the only⁵⁷Co impurity single interstitial configuration populated during electron irradiation at 80 K is metastable, and rearranges at 140 K into several more stable configurations, consisting of a⁵⁷Co impurity with one interstitial trapped in the direct vicinity.