TRANSMISSION ELECTRON-MICROSCOPY STUDY OF UNHYDRIDED, DEHYDRIDED AND ANNEALED LANI5

The influence of hydrogen absorption on the microstructure of LaNi5 powders has been investigated by transmission electron microscopy. At the surface of the unhydrided and dehydrided LaNi5 grains a reaction layer is observed. By means of selected area electron diffraction this layer is found to consist of metallic nickel and a second constituent, most probably La2O3. This observation suggests that this surface layer is not induced by the hydrogen absorption reaction. Within dehydrided LaNi5 grains planar-like defects, which are formed during previous hydrogen absorption, were observed. The habit plane, strain field and average spacing of these planar-like defects have been determined. From the correspondence between the above characteristics and the reduced coherence length perpendicular to the c-axis, as derived from anisotropic peak broadening in X-ray diffraction analyses, it was concluded that a correlation exists between the presence of planar-like defects and peak broadening. This correlation is further sustained by the fact that they both disappear upon annealing at 600-degrees-C.