Microstructures and defect structures in intermetallic compounds in the La-Ni alloy system

Microstructures and defect structures in intermetallic compounds in the La-Ni alloy system have been investigated by transmission electron microscopy in the composition range of La-77.8-83.2 at.%Ni, which corresponds to compositions between two intermetallic phases, La2Ni7 and LaNi5. The intermetallic phase, La5Ni19 of the Ce5Co19-type is found for the first time to exist as an equilibrium phase at a composition between La2Ni7 and LaNi5. This phase is stable at high temperatures around 1000 degrees C but decomposes into La2Ni7 and LaNi5 below 900 degrees C. Numerous stacking-faults are observed on basal planes in La5Ni19 but the density of these basal stacking faults is greatly reduced in La2Ni7. Most stacking faults in La5Ni19 are identified to be of the inter-block layer type while most of those in La2Ni7 are identified to be of the intra-block-layer type. (C) 1999 Elsevier Science S.A. All rights reserved.