Small-angle neutron scattering measurement of deuterium trapping at dislocations and grain boundaries in palladium.

Abstract

Small angle neutron scattering measurements have been performed on deformed single and polycrystalline palladium with and without deuterium dissolved in the solution phase at room temperature. The purpose of these experiments was to directly measure the spatial distribution of trapped deuterium at dislocations in the deformed metal. The net scattering cross section for the same sample with and without deuterium shows a behavior expected from deuterium correlation with dislocations forming rod-like scattering structures. The measured cross sections indicate the trapped deuterium is within 2 to 3 Burgers vectors of the dislocation core. On average 1 to 3 deuterons per A are trapped at the dislocations in the deformed samples. The measurements also indicate the straight, rod-like correlation geometry extends on average 50 to 100 A along the dislocations. Accurate volume averaged dislocation densities are obtained from an absolute calibration of the net scattering data. Dislocation densities on the order of 5 $\times$ 10$\sp{11}$ cm/cm$\sp3$ were found for all samples investigated. Net scattering from a well annealed polycrystalline palladium sample exhibiting a behavior expected from spherical shells has been observed. This net scattering is attributed to deuterium trapping at grain boundaries in the polycrystalline sample. Net scattering in excess of that expected from deuterium correlated at dislocations was also observed in a deformed polycrystalline measurement. This too is attributed to deuterium trapping at grain boundaries. A value of 0.4 $\pm$ 0.2 deuterons per A$\sp2$ of grain boundary is calculated from both of these measurements. A 1/Q$\sp4$ scattering behavior was observed in all measurements on polycrystalline samples. This behavior is typical of scattering for isolated particles and is due, at least in part, to MnO particles present in the polycrystalline material. The dislocation substructure of the deformed palladium samples was characterized by transmission electron microscopy (TEM). This analysis illustrated the cellular arrangement that evolved in palladium during cold working. The presence of MnO particles also was confirmed by TEM analysis.