Atomistic State of Hydrogen Trapped by Vacancies and Formation of H-associated Vacancy Clusters in Nb as Observed by the Channelling Method

Abstract

In order to study hydrogen-defect interactions, the effects of ion irradiation and annealing on the lattice location of hydrogen dissolved in Nb, NbH0.023, have been investigated at room temperature by the channelling method utilizing a nuclear reaction 1H(11B,α)αα with a 2 MeV 11B+ beam. By irradiation at room temperature with a dose of about 1.4 × 1016/cm2 of about 2 MeV 11B ions, the lattice location changes from the original tetrahedral (T) site (T-1 state) to the Ttr site, which is displaced from a T site by 0.45–0.55 A towards its nearest neighbour lattice point. The Ttr-site occupancy remains the same even for an approximately three times higher irradiation dose. On subsequent annealing at 523 K for 1 h, the site occupancy changes to the occupancy of (55–70)% of H atoms at T sites (T-2 state) and (30–45)% of them at random (R) sites. By additional irradiation with a dose of about 1.4 × 1016/cm2 at room temperature subsequent to the annealing, the site occupancy changes to the occupancy of (35–50)% at T + (50–65)% at R or (30–40)% at T + (10–20)% at Ttr + (50–60)% at R. It is concluded that the irradiation-induced site change to the Ttr site is a result of the trapping of hydrogen by monovacancies, i.e., the formation of H-vac. complex-1. Most of the H atoms in the T-2 state are not free hydrogen, but associated with more vacancies, for which hydrogen located at a T site in a tetrahedron consisting of four vacancies (tetravacancy), i.e., H-4vac. complex-2, is proposed. The R-site occupancy corresponds to hydrogen in a H-associated larger vacancy cluster complex-3. It is demonstrated that the complex-1 does not act as a nucleus, whereas the complex-2 acts as a nucleus for the growth to the H-associated larger vacancy cluster complex-3 by trapping more irradiation-introduced vacancies at room temperature.