Formation of Hydrogen-Associated Vacancy Clusters in Nb as Investigated from Site Occupancy Change of Hydrogen

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In order to study hydrogen-defect interactions, the effects of ion irradiation and annealing on the site occupancy of hydrogen dissolved in Nb, NbH0.023, have been investigated at room temperature by the channelling method utilizing a nuclear reaction 1H(11B,$\alpha$)$\alpha\alpha$ with a 2 MeV 11B+ beam. By irradiation at room temperature up to a dose of $1.4\times 10^{16}$/cm2 of 2 MeV 11B ions, the lattice location of hydrogen changes from the original tetrahedral ($T$) site ($T$-1 state) to the $T_{\text{tr}}$ site, which is displaced from a $T$ site by 0.45–0.55 Å towards its nearest-neighbour lattice point. This site occupancy remains the same even for an approximately three times higher irradiation dose. On the subsequent annealing at 523 K for 1 h, the state of hydrogen changes from the $T_{\text{tr}}$-site occupancy to the occupancies of (55–70)% of H atoms at $T$ sites ($T$-2 state) and (30–45)% at random ($R$) sites. The irradiation-induced site change from the $T$ site to the $T_{\text{tr}}$ site is due to the trapping of hydrogen by vacancies, i.e., the formation of H-vacancy complexes (complex-1). By additional irradiation with about $1.4\times 10^{16}$/cm2 at room temperature after the annealing, the site occupancy changes to (35–50)% at $T$ sites and (50–65)% at $R$ sites, or ${\sim}40$% at $T$ sites, ${\sim}10$% at $T_{\text{tr}}$ sites, and ${\sim}50$% at $R$ sites, or ${\sim}30$% at $T$ sites, (10–20)% at $T_{\text{tr}}$ sites, and (50–60)% at $R$ sites, indicating a difference in the state of hydrogen between the $T$-1 and $T$-2 states. In the $T$-1 state, hydrogen is in the isolated state, i.e., free hydrogen, while, in the $T$-2 state, most H atoms are associated with more vacancies. For this hydrogen–vacancy complex, a vacancy tetrahedron (tetravacancy) containing hydrogen inside at a $T$ site, H–4vac. (complex-2), is proposed. The $R$ site occupancy corresponds to hydrogen in a larger vacancy cluster (void). Therefore, on annealing, large H-associated vacancy clusters, such as H–4vac. complexes and H-associated voids, are formed. It is demonstrated that complex-1 does not act as a nucleus for the growth of a H-associated large vacancy cluster, whereas complex-2 acts as a nucleus and grows to a H-associated void by trapping still more irradiation-induced vacancies at room temperature.
2010-11-15

Formation of Hydrogen-Associated Vacancy Clusters in Nb as Investigated from Site Occupancy Change of Hydrogen

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