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

Pure nickel and very dilute Ni-S systems (containing 50-340 at.ppm sulphur), irradiated by 5 MeV electrons at 270 K, 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 650 K 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 350 K, aggregate into nanovoids; (ii) above 550 K, 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 870 K.