Hydrogen diffusion in Ti-Nb45 at high hydrogen contents

The diffusion of hydrogen in the binary bcc-alloy Ti-45 wt.%Nb has been studied on the basis of anelastic relaxation due to the Gorsky effect. Diffusion coefficients in samples with hydrogen content between 0.03 and 0.37 H/M (ratio between hydrogen and metal atoms) have been measured at different temperatures. Other interstitial elements like oxygen also contribute to the anelastic behaviour of the alloy. To calculate the hydrogen diffusion coefficients, the influence of other elements on the anelastic behaviour was subtracted to get solely the hydrogen-induced anelastic relaxation. The diffusion coefficients of hydrogen in all samples show an Arrhenius-type temperature behaviour. With increasing hydrogen content the diffusion coefficients decrease at lower and increase strongly at higher content. This behaviour is attributed to the mutual blocking of the interstitial sites by hydrogen atoms and the distribution of energy levels for hydrogen storage at different interstitial sites in the alloy, depending on their local configuration. The tetrahedral sites that are surrounded by four titanium atoms (Ti4-sites) are preferentially occupied at low hydrogen contents. With increasing hydrogen content also sites with neighbouring niobium atoms will be occupied. The hydrogen storage at niobium containing sites results in a higher diffusion coefficient due to the lower activation energy for hydrogen diffusion on these sites.