Hydrogen as a local probe: Diffusion and short-range order inTi1−yVyalloys

Diffusion coefficients of hydrogen have been measured in disordered ${\mathrm{Ti}}_{1\mathrm{-}\mathrm{y}}$ ${\mathrm{V}}_{\mathrm{y}}$ alloys, as a function of temperature, hydrogen concentration, and alloy composition. Diffusion and electron microprobe analysis data indicate the existence of short-range order (clustering) in ${\mathrm{Ti}}_{1\mathrm{-}\mathrm{y}}$ ${\mathrm{V}}_{\mathrm{y}}$ alloys, as observed before in the related ${\mathrm{Ti}}_{1\mathrm{-}\mathrm{y}}$ ${\mathrm{Nb}}_{\mathrm{y}}$ alloy system by diffusive x-ray scattering. The diffusion data are analyzed by means of a model, incorporating site-dependent activation energies, effective hydrogen-hydrogen interactions, and selective-blocking factors, generalized for alloys with short-range order. Site energies are calculated by means of the embedded-cluster model, assuming the energy of the hydrogen atoms to be predominantly determined by the nearest-neighbor metal atoms. An approximate relation between the short-range-order parameter σ and the site distribution is established. For σ<0 a ‘‘chain’’ model and for σ>0 a ‘‘cluster’’ model are in good agreement with Monte Carlo calculations. Using the short-range-order parameter σ as the only fit parameter, the short-range order in ${\mathrm{Ti}}_{1\mathrm{-}\mathrm{y}}$ ${\mathrm{V}}_{\mathrm{y}}$ alloys is determined from diffusion and solubility data. The quantitative determination of the short-range order in ${\mathrm{Ti}}_{1\mathrm{-}\mathrm{y}}$ ${\mathrm{V}}_{\mathrm{y}}$ alloys and ${\mathrm{Mo}}_{1\mathrm{-}\mathrm{y}}$ ${\mathrm{Ti}}_{\mathrm{y}}$ alloys from diffusion and solubility data, which are in excellent agreement with diffusive x-ray scattering data, shows that hydrogen can be a valuable tool to investigate the microscopic structure of concentrated disordered alloys.