Direct measurement of hydrogen dislocation pipe diffusion in deformed polycrystalline Pd using quasielastic neutron scattering

The temperature-dependent diffusivity D(T) of hydrogen solute atoms trapped at dislocations - dislocation pipe diffusion of hydrogen - in deformed polycrystalline PdHx (x∼10-3[H]/[Pd]) has been quantified with quasielastic neutron scattering between 150 and 400 K. We observe diffusion coefficients for trapped hydrogen elevated by one to two orders of magnitude above bulk diffusion. Arrhenius diffusion behavior has been observed for dislocation pipe diffusion and regular bulk diffusion, the latter in well-annealed polycrystalline Pd. For regular bulk diffusion of hydrogen in Pd we find D(T)=D0exp(-Ea/kT)=0.005exp(-0.23eV/kT)cm2/s, in agreement with the known diffusivity of hydrogen in Pd. For hydrogen dislocation pipe diffusion we find D(T)≃10-5exp(-Ea/kT)cm2/s, where Ea=0.042 and 0.083 eV for concentrations of 0.52×10-3 and 1.13×10-3[H]/[Pd], respectively. Ab initio computations provide a physical basis for the pipe diffusion pathway and confirm the reduced barrier height.