SMALL-ANGLE NEUTRON-SCATTERING STUDIES OF H(D) TRAPPING ON DISLOCATIONS IN METALS

Small Angle Neutron Scattering (SANS) from hydrogen and deuterium trapped on defects in metals potentially provides probably the only experimental technique for locating the trapped hydrogen. The point of the technique is that one gets interference between the scattering from the defect and from the hydrogen/deuterium rather than just the summation of intensities. In the present paper, we consider the case of trapping on edge dislocations. If the scattering was from a linear feature, we would expect the SANS to vary as Q-1. However, because the matrix density change varies as costheta/r, the SANS should, in fact, vary roughly as Q-3 (Atkinson and Hirsch 1957). Here H(D), will tend to trap in the stressed (expanded) regions. Hence, for the deuterium, the effects are of opposite sign, while for hydrogen they are additive. In all cases, the hydrogen will concentrate the scattering density effect closer to the dislocation and thus reduce the negative gradient towards Q-1. At high concentrations, however, the D scattering will dominate and it will give a higher SANS density than hydrogen. We report detailed numerical calculations of the SANS for this model using a Fermi-Dirac distribution for the trapped hydrogen (deuterium). Preliminary results are reported for samples of deformed iron and palladium. In both cases, there is evidence for a numerically smaller negative index for the Q dependence. In the former case, where the hydrogen concentration is necessarily small, interference is evident, as expected, in that the addition of deuterium decreases the scattering density while hydrogen increases it. In the latter case, which is complicated by strong SANS from the palladium, probably caused by major prior deformation of the sample, scattering from the larger quantities of trapped hydrogen (deuterium) appears to swamp the scattering from the bare dislocations because the scattering intensity increases in both cases.