An electron-microscopy study of neutron-induced defect clusters in niobium

Moderately pure niobium was irradiated to the fluence of 8 x 1019 neutrons (E > 0.1 MeV) per cm2 at 800C. Examination of the irradiated Nb by transmission electron microscopy revealed the formation of defect clusters of interstitials and vacancies in distinct configurations, size ranges, and distribution modes, post-irradiation annealing at temperatures Tsub(a) between 350 and 10000C produced three main events. At intermediate Tsub(a) up to 5000C, annealing was mainly to transform the complex interstitial clusters into simple loops. Between 500 and 7000C, the shrinkage and the eventual disappearance of the majority (approximately 80%) of the interstitial loops dominated the annealing phenomenon. Although the growth of the vacancy loops was detected even at low Tsub(a), it became a main event only when Tsub(a) exceeded 7000C. Despite the second main event, many interstitial loops persisted at Tsub(a) as high as 10000C. Whenever resolvable, the defect clusters of both interstitials and vacancies were found to lie on the brace111brace planes with b = (a/2)<111> normal to the loop planes. The three main events are explicable in terms of two major processes of annealing. One process involves a maximum of three mechanisms - a <001> shear, a rotation of the loop from brace110brace to brace111brace and the glide of prismatic loops - which enable multilayered interstitial clusters to become single-layered loops. The other major process is the migration of vacancies responsible for the second and third main events. (author)