Studies of off-stoichiometric AB2 metal hydride alloy: Part 1. Structural characteristics

In Part 1 of this two-part series of papers, phase abundances, lattice parameters, crystallite sizes, and microstructures of three series of AB(2)-based metal hydride alloys were studied. The base alloys with B/A stoichiometry of 2.0 in series 177, 190, and 193 are rich in C14, equal in C14/C15, and rich in C15 phases, respectively. In each series of alloys, the B/A stoichiometry varies from 1.8, 1.9, 2.0, 2.1, to 2.2. The effects of varying B/A stoichiometry to microstructures are the same for these three series of alloys. As the alloy formula changes from AB(1.8), AB(1.9), AB(2.0), AB(2.1), to AB(2.2), the following events occur: C14-to-C15 phase ratio decreases, both C14 and TiNi secondary phase lattice parameters and unit cell volume reduce; the a/c aspect ratio of C14 phase first decreases and then increases; abundances of non-Laves secondary phases decrease; and the Zr/Ti ratio in AB phase decreases. The C14/C15 ratio is closely related to the average electron density with a threshold that first decreases from 7.13 (AB(1.8)) to 7.08 (AB(1.9)) and to 7.06 (AB(2.0)) and then increases to 7.08 (AB(2.1)) and 7.09 (AB(2.2)) as the stoichiometry increases. The distributions of B-site elements are not uniform with most of the V, Cr, Mn, Co residing in AB(2) phase and Sn in Zr7Ni10 phase. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.