Thermally- and irradiation-induced sulphur aggregation in Ni–S ageing system studied by positron annihilation

Abstract

Pure nickel and very dilute Ni–S systems (containing 50–340at.ppm sulphur), irradiated by 5MeV electrons at 270K, were studied by positron annihilation spectroscopy (PAS) and the electrical resistivity measurements. The ultrafine S-rich precipitates (clusters), which are formed by thermal ageing at about 650K in Ni–S, are observed clearly by PAS. The PAS data were interpreted by assuming that the positrons localize in the volume of the “defect-free” S-rich clusters due to affinity-induced confinement. Under irradiation, the accumulation of the vacancies in the Ni–S system is several times greater than that in pure Ni. This fact testifies the strong interaction between S atoms and self-interstitial atoms. During post-irradiation annealing, the formation and recovery process of the S-rich clusters are revealed: (i) the mobile vacancy–sulphur complexes, which are formed above 350K, aggregate into nanovoids; (ii) above 550K, the dissociation of vacancies from the nanovoids decorated with S-atoms leads to the formation of the S-rich clusters; and (iii) the S-rich clusters anneal out at about 870K.