Electrical resistance studies of hydrogen diffusion and of hydride precipitation in niobium

Diffusion of hydrogen in niobium was studied by using thermotransport to establish a concentration gradient in a periodic width modulated sample. A sensitive Kelvin double bridge allowed the kinetics of return to uniform concentration to be monitored. In undeformed Nb, the Arhennius plot exhibited a curvature, in agreement with previous results. Deformation indiced by hydride precipitation and by cold work removed the curvature. These effects are considered to be due to dislocation trapping of H. The undeformed data were fit to the Flynn-Stoneham quantum tunneling model of diffusion. The value of the lattice Debye temperature found was unphysically high (900K); the validity of the model is therefore in question. The precipitation of hydride in Nb was studied by balancing the Kelvin double bridge such that the bridge output was proportional to the concentration of unprecipitated hydrogen, and following the concentration of free hydrogen with temperature. Repeated cycles of precipitation and reversion showed at first a decreasing, and then constant hysteresis (difference between precipitation and reversion temperatures)