Correlation of substitutional solid solution with hydrogenation properties of TiFe1-xMx (M=Ni, Co, Al) alloys

The hydrogenation properties of TiFe partially substituted with a small amount of Ni, Co, or Al for Fe were studied. Complete solid solutions were observed for all compositions, but different characteristics of the pressure-composition-temperature (P-C-T) curves were observed. For Ni- and Go-substituted alloys, the shape of the P-C-T curves and the hydrogen storage capacities were very similar to those of pure TiFe. The plateau pressures in the two-phase (alpha-beta) region always remained flat, but decreased drastically as the amount of nickel or cobalt was increased. The different hydrogenation behaviors are explained on the basis of different values of hydride formation enthalpies of TiFe, TiNi, and TiCo. When Fe was partially replaced by Al, a solid solution was also formed, but the sizes of the octahedral sites were changed. The hydrogen storage capacity was decreased and the slope of the plateau pressure increased as the amount of aluminum increased. The change of slope of the plateau pressure is explained and modeled by the various sizes of the octahedral sites. (C) 1999 Elsevier Science S.A. All rights reserved.