Polytypism of La-Ni phases in multicomponent AB5 type hydride electrode alloys

The microstructure of multi-component AB(5) type alloy Ml(NiCoAl)(5) (Ml, Lanthanum riched misch rare earth metal) has been characterized using transmission electron microscopy, high resolution electron microscopy and energy dispersive spectrum analysis. The intermetallic phases Ml(5)(NiCoAl)(19) of Ce5Ni19 (hexagonal 2H) type and Ml(2)(NiCoAl)(7) phase of La2Ni7 type with two kinds of polytypes (modifications) are found for the first time to exist as equilibrium phases in stoichiometric MI-Ni-Co-Al alloys after arc melting. All of the polytypes existing in the multicomponent alloys keep well-defined orientation relationship with the AB(5) type matrix expressed as the [0-10](CaCu5)\ \ [0-10](polytype) and (001)(CaCu5)\ \ (001)(polytype). The misfit dislocations at the interfaces between polytypes structure and parent alloy matrix were clearly observed in high resolution electron microscopy (HREM) image. The possible reasons, which cause the formation of the polytypes and the effect of the polytypes on the hydrogen absorbing/desorbing properties for the multicomponent alloys are discussed. (C) 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.