In-situ neutron powder diffraction study of annealing activated LaNi 5

Abstract

In-situ neutron powder diffraction measurements and the Rietveld refinement technique have been used to study a fully hydrogen activated LaNi 5 alloy during annealing in the temperature range 293 to 1123 K. Diffraction data were analysed by imposing a dislocation-induced anisotropic line broadening model, to reveal the variation of dislocation density and defect structure as a function of temperature. The activated sample contained predominantly edge dislocations with Burgers vector a/3〈−2110〉 on prismatic {0−110} slip planes associated with medium to long ranged strain fields ( M∼2–3). A small proportion of dislocations (∼10%) with the same Burger vectors on basal {0001} slip planes are also indicated by the refinements. This second type of dislocation appears to be more resistant to annealing than the dislocations on prismatic slip planes. A progressive decrease in dislocation density from ∼6×10 11/cm 2 to the limit of the resolution of our measurement (≲10 10/cm 2) was observed between 293 and 973 K. This annealing appears to cause the restoration of the large pressure hysteresis characteristic of unactivated LaNi 5. The most significant reduction in dislocation density occurs at ∼800 K. In contrast, TGA measurements show the release of trapped hydrogen at ∼500 K. The latter feature is associated with an anomaly in the c/ a ratio. The results suggest that, in addition to primary defects in the form of dislocations, a different class of defects such as vacancies and small dislocation loops may also exist in hydrogen activated LaNi 5. Further, whereas the former are undoubtedly associated with observed changes in the pressure hysteresis during activation, the latter are likely to be the favoured site for hydrogen trapping in activated LaNi 5.