The use of small angle neutron scattering in the study of hydrogen trapping at defects in metals

Abstract

Small angle neutron scattering (SANS) techniques have been used to investigate the trapping of hydrogen and deuterium on dislocations in palladium. Calculations of the expected form of this scattering are presented. It is shown that the different scattering lengths of H and D, respectively negative and positive, can be exploited to prove explicitly that the hydrogen is being trapped at the feature that is causing the SANS in the metal sample. In particular, at an edge dislocation, we can expect the H/D to be trapped in the region of lattice dilation below the edge. Thus, because the scattering length of palladium is positive, deuterium trapping will compensate for the reduction in the scattering density in the host lattice and hence will reduce the intensity of the SANS progressively as D is added, until the local scattering density deviation becomes positive. At this point, the overall SANS cross-section passes through a minimum. On the other hand, increasing H trapping will continuously increase the SANS intensity. The model for the expected scattering is described and measurements of the SANS intensities from dislocations in palladium are presented for a series of D concentrations. A minimum is observed in the SANS at about 1% atomic concentration, in reasonable agreement with the theory.