Interaction of hydrogen and deuterium with dislocations in palladium as observed by small angle neutron scattering

Small angle neutron scattering (SANS) measurements on Pd samples containing dislocations with a density of a few 10(11) cm(-2) reveal an additional in tensity for a scattering vector of 0.02 Angstrom (1) to 0.2 Angstrom (1) af ter loading with hydrogen (H) or deuterium (D). The corresponding net cross section is inversely proportional to the scattering vector as expected for line type scattering objects with a superimposed exponential decrease stem ming from scattering within the Guinier-regime. This experimental finding i s in accordance with a model where extended segregation of H or D within th e dilated regions of edge dislocations occurs. In a first order approximati on this corresponds to a precipitation of cylindrically shaped hydrides alo ng the dislocation line and can be treated quantitatively yielding radii in agreement with SANS data. Whereas gas volumetric measurements at the same total concentration reveal no difference for the amount of H- and D-segrega tion, there is a pronounced effective difference in SANS intensities which cannot be explained by the different scattering lengths alone. However, the different sign of the latter quantity in combinations with an expected vol ume expansion within the hydride/deuteride region provides a reasonable exp lanation of the intensity difference observed. Knowing the amount of segreg ated H or D from gas volumetry and the dislocation density from electron mi croscopy the SANS results can be explained in a self consistent way. (C) 20 01 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reser ved.