Neutron diffraction study of the LaNi5-D system during activation

The microstructural changes occurring during the initial absorption of deuterium by virgin LaNi5 at 40 degrees C have been investigated using in-situ neutron powder diffraction. Rietveld profile refinement was used to determine the alpha and beta phase proportions, lattice parameters and microstrains. In absorption, we found that in the two-phase region (i) the lattice parameters of the alpha and beta phases were (within resolution) independent of the phase proportions; (ii) alpha-phase diffraction peaks remained essentially unbroadened relative to the virgin metal; (iii) the beta-phase peaks were relatively broad with the usual anisotropy of breadth. These findings imply that, as nuclei of beta phase form for the first time in a particle that is wholly alpha phase, the lattice expansion causes pure beta crystallites containing a very high density of lattice defects to fracture off the particle, i.e., decrepitate. Hence the nanoscale mixing and strong mechanical interaction between the alpha and beta phases noted in multiply cycled material are not observed during the initial absorption of D atoms, because the lattice parameter misfit cannot be accommodated. In desorption, and subsequently, there is sufficient accommodation of the lattice parameter mismatch between the alpha and beta phases for them to coexist in the same powder particle. (C) 1999 Elsevier Science S.A. All rights reserved.