Site Occupancies of Hydrogen in Nb Alloyed with Oversized Ta Atoms or Undersized Mo Atoms

Abstract

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]), Nb 0.98 Ta 0.02 H 0.01 , Nb 0.98 Ta 0.02 H 0.029 , and Nb 0.95 Ta 0.05 H 0.01 , by the channelling method utilizing a nuclear reaction 1 H( 11 B,α) 8 Be with a 11 B 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 Nb 0.95 Ta 0.05 alloys. The d - T site is a site displaced from a T site by about 0.25 A 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 ...