Hydrogen-induced defects in niobium studied by positron annihilation

Abstract

Changes of the defect structure of niobium induced by hydrogen loading are presented in this work. It was found that annealing of virgin bulk Nb (99.9%) at 1000degreesC for 1h leads to a complete recovery of defects. Subsequently, the defect-free samples were step-by-step electrochemically loaded with hydrogen up to x(H) - 0.06 [H/Nb atom ratio], i.e. in the alpha-phase region, where the Nb-H system represents a single-phase solid solution. The evolution of the microstructure with increasing hydrogen concentration was studied by X-ray diffraction and two complementary techniques of positron annihilation spectroscopy (PAS), namely positron lifetime spectroscopy and slow positron implantation spectroscopy with measurement of Doppler broadening. It was found that new defects were created due to hydrogen loading. The concentration of these hydrogen-induced defects increases with increasing hydrogen concentration. A comparison of PAS results with theoretical calculations revealed that complexes consisting of a vacancy, surrounded likely by four hydrogen atoms, were introduced into the samples due to hydrogen loading.