Synchrotron powder diffraction line broadening analysis of dislocations in LaNi5-H

Abstract

In situ synchrotron powder diffraction data for the LaNi5-H system were collected and the dislocation structures formed during the hydrogen activation cycle were studied by Rietveld refinement analysis in which anisotropic line broadening was modeled by dislocation scattering. Marginal line broadening was observed in the a phase formed by the very first hydrogen absorption by virgin metal. It is best modeled by dislocation a/3110 on basal {0001} slip planes, with density 1010-1011/cm2. In the ߠphase (LaNi5H趩 and the activated a phase (LaNi5H谮4, postdesorption), in contrast, the majority of dislocations is prismatic with a {010} slip plane and the same Burgers vector, with only a small proportion ( 15%) of dislocations having basal slip planes. High dislocation densities of the order of 1012/cm2 are associated with distributed strain fields of intermediate range. These results support the proposition that the dislocation structure is inheritable during the a-ߠphase transition, with the dislocations from the first-formed a phase on basal planes, and the majority created during nucleation and growth of the ߠphase on the prismatic planes, all being retained during subsequent phase transitions.