Neutron diffraction study of the LaNi5-D system during activation

The microstructural changes occurring during the initial absorption of deut erium by virgin LaNi5 at 40 degrees C have been investigated using in-situ neutron powder diffraction. Rietveld profile refinement was used to determi ne the alpha and beta phase proportions, lattice parameters and microstrain s. In absorption, we found that in the two-phase region (i) the lattice par ameters of the alpha and beta phases were (within resolution) independent o f the phase proportions; (ii) alpha-phase diffraction peaks remained essent ially 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 t hat is wholly alpha phase, the lattice expansion causes pure beta crystalli tes containing a very high density of lattice defects to fracture off the p article, i.e., decrepitate. Hence the nanoscale mixing and strong mechanica l interaction between the alpha and beta phases noted in multiply cycled ma terial are not observed during the initial absorption of D atoms, because t he lattice parameter misfit cannot be accommodated. In desorption, and subs equently, there is sufficient accommodation of the lattice parameter mismat ch between the alpha and beta phases for them to coexist in the same powder particle. (C) 1999 Elsevier Science S.A. All rights reserved.