Electron diffraction analysis of an AB(2)-type Laves phase for hydrogen battery applications

A multicomponent AB(2) type nickel-metal hydride (Ni-MH) battery alloy prep ared by high-pressure gas atomization (HPGA) was investigated by transmissi on electron microscopy (TEM) in both the as-atomized and heat treated condi tion. TEM examination showed a heavily faulted dendritic growth structure i n as-atomized powder. Selected area diffraction (SAD) showed that this regi on consisted of both a cubic C15 structure with lattice constant a = 7.03 A ngstrom and an hexagonal C14 structure with lattice parameter a = 4.97 Angs trom, c = 8.11 Angstrom. The orientation relationship (OR) between the C14 and C15 structures was determined to be (111)[1 (1) over bar0](C15)//(0001) [11 (2) over bar0]C-14. An interdendritic phase possessing the C14 structur e was also seen. There was also a very fine grain region consisting of the C14 structure. Upon heat treatment, the faulted structure became more defin ed and appeared as intercalation layers within the grains. Spherical partic les rich in Zr and Ni appeared scattered at the grain boundries instead of the C14 interdendritic phase. The polycrystalline region also changed to a mixture of C14 and C15 structures. The phase stability of the C15 and C14 s tructures based on a consideration of atomic size factor and the average el ectron concentration is discussed. (C) 2000 Elsevier Science BN. All rights reserved.