Site Occupancies of Hydrogen in Nb Alloyed with Oversized Ta Atoms or Undersized Mo Atoms
作者:Hiroshi Matsuba、Eiichi Yagi、Chika Sugi、Yoshinori Okada、Kazuhiro Hirabayashi、Takashi Iida、Shigetoshi Koike、Tatuya Hayashi、Naota Higami、Yoichi Murakami、Akiko Takebayashi、Toru Yoshida、Takamasa Sugawara、Toetsu Shishido、Kiyoshi Ogiwara
DOI:10.1143/jpsj.80.104602
日期:2011.10.15
Site occupancies of hydrogen in Nb alloyed with 2 or 5 at. % of oversized Ta atoms have been investigated at room temperature for hydrogen concentrations of 0.01 and 0.029 at the hydrogen-to-metal-atom ratio (C-H = [H]/[M]), Nb0.98Ta0.02H0.01, Nb0.98Ta0.02H0.029, and Nb0.95Ta0.05H0.01, by the channelling method utilizing a nuclear reaction H-1(B-11,alpha)Be-8 with a B-11 beam of about 2 MeV. It has been clearly demonstrated that hydrogen occupies also tetrahedral (T) sites, in addition to the displaced T sites (d-T sites), which were observed at higher hydrogen concentrations in the Nb0.95Ta0.05 alloys. The d-T site is a site displaced from a T site by about 0.25 angstrom towards its nearest neighbour octahedral (O) site. The T site is more favourable, but the concentration of available T sites is limited, and excess H atoms occupy the d-T sites. This site occupancy is different from that in Nb alloyed with similar concentration of undersized Mo atoms, where hydrogen preferentially occupies trapped sites T-tr displaced from T sites attached to Mo atoms by about 0.6 angstrom towards the Mo atoms due to a strong attractive interaction between them, but the concentration of T-tr sites is limited and excess H atoms enter T sites. On the other hand, there exists no such a strong attractive interaction between hydrogen and an oversized Ta atom. With the help of measurements of the half-widths of X-ray reflection lines in Nb-Mo and Nb-Ta alloys, the site occupancy of hydrogen in the Nb-Ta alloys is explained in terms of an effect of lattice distortion, which was induced by alloying with solute atoms, as in the case of the site occupancy in the Nb-Mo alloys at higher Mo concentrations than about 40 at. %. The d-T site is a stable site for hydrogen in a slightly distorted bcc lattice.