Enhanced diffusion of Nb in Nb-H alloys by hydrogen-induced vacancies

The diffusion coefficient of 95Nb in pure Nb and Nb-H alloys whose hydrogen concentration ranges between H/Nb = 0.05 and 0.34 in atomic ratio has been determined in the temperature range 823-1598 K using a serial sputter-microsectioning technique. The diffusion coefficient of Nb in the Nb-H alloys was found to increase significantly with increasing hydrogen concentration. The dependence of the diffusion enhancement on temperature and hydrogen concentration was examined in some detail, and explained tentatively in terms of average occupation number of hydrogen atoms per vacancy, r. The diffusion enhancement comes primarily from the decrease of the activation energy Q, resulting from the increase of r with increase of hydrogen concentration. Some remaining problems with this interpretation are pointed out for future investigations.