In-Situ Powder Diffraction Studies of Metal-Hydrogen Microstructures

Abstract

In-situ powder diffraction has been used to elucidate the microstructures of several metal-hydrogen systems. Information derived fiom diffraction is correlated with the defect structures that can be observed with TEM. The in-situ technique, in which diffraction data are collected from a sample loaded to a known hydrogen concentration on the beam line, is invaluable in the determination of metal-hydrogen crystal structures because it allows a rigorous test of space group feasibility through comparison with an accurately known hydrogen content. Research on the LaNi5 system focused on activation and degradation properties. In particular, the y phase has been studied under both extrinsic and intrinsic degradation conditions up to 300°C and 1.1 kbar hydrogen pressure to address the question of thermodynamic instability with extended cycling. En route to unravelling the complicated inter-phase relationship that develops between the a, y and P phases at elevated temperatures, new structure solutions for the y and P phases were derived. A multi-staged mechanical relaxation occurs that is sensitive to the occupation of the 6m position in the p and y phases. Under these conditions, the y phase can morph fiom a P-like structure to an a-like structure. The question of 'trapped' hydrogen was resolved with all D accounted for in interstitial positions in the nphase. The strain relationships between coexisting a and P phases have been investigated in the LaNi5-,Sn,H, system for x = 0.0, 0.1 and 0.2. The substituted alloys display a peculiar prismatic compression of the a phase that can be directly correlated with the removal of a/3 less than 21 10 more than {0i 10) dislocation cores, revealed as a reduction in the severity of anisotropic line broadening. lt is apparent that characteristics of the a-p interface in the parent LaNi5 system are retained in substituted materials, and that variation in the ratio of the densities of a/3 less than 21 10 more than (01 10) and a13 less than 21 10 more than10001) dislocations causes compression of the a phase between layers of phase. The first ever in-situ neutron diffiaction study of the PdDx system above the critical point was executed. D was found to occupy both octahedral and tetrahedral positions. This result is controversial, however the evidence is compelling. Tetrahedral occupation was retained at room temperature and through the two-phase region.