Site Change Of Hydrogen Owing To Lattice Distortion In Niobium Alloys

The site occupancy of hydrogen in Nb alloyed with oversized Ta (atomic radius; r(Nb) = 1.43 angstrom and r(Ta) = 1.44 angstrom), Nb0.50Ta0.50H0.021 and Nb0.50Ta0.50H0.053, has been investigated at room temperature by the channelling method utilizing a nuclear reaction H-1(B-11,alpha)alpha alpha with a B-11 beam of an energy of 2.02 MeV. It was observed that, in both specimens, approximately 100% of H atoms are located at tetrahedral (T) sites. The half-widths of X-ray reflection lines have been measured for the non-hydrogen-doped Nb0.50Ta0.50 specimens. The half-widths for the Nb0.50Ta0.50 specimens are smaller than those for Nb alloyed with approximately the same concentration of undersized Mo atoms (r(Mo) 1.36 angstrom), Nb0.52Mo0.48, and close to the value for pure Nb. This result indicates that, in the Nb0.50Ta0.50 specimens, the lattice is undistorted or very weakly distorted as compared with the Nb0.52Mo0.48 specimens. From the present results on the half- widths and the site occupancy of hydrogen in the Nb0.50Ta0.50 alloys and those hitherto obtained for the Nb0.52Mo0.48 alloys, it is demonstrated that, in these two alloys, in the undistorted or very weakly distorted lattice, hydrogen occupies T sites, while, in the lattice more distorted at the intermediate level, hydrogen occupies d-T sites. The d-T sites are sites displaced from T sites by about 0.25 angstrom towards their nearest neighbour O sites. Thus, the solute concentration dependence of the site occupancy of hydrogen hitherto observed in the Nb-Mo and Nb-Ta alloys in the solute concentration range up to 60 at.% can be well explained on the basis of the lattice distortion induced by alloying.