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

The hydrogenation properties of TiFe partially substituted with a small amo unt of Ni, Co, or Al for Fe were studied. Complete solid solutions were obs erved for all compositions, but different characteristics of the pressure-c omposition-temperature (P-C-T) curves were observed. For Ni- and Go-substit uted alloys, the shape of the P-C-T curves and the hydrogen storage capacit ies were very similar to those of pure TiFe. The plateau pressures in the t wo-phase (alpha-beta) region always remained flat, but decreased drasticall y as the amount of nickel or cobalt was increased. The different hydrogenat ion behaviors are explained on the basis of different values of hydride for mation enthalpies of TiFe, TiNi, and TiCo. When Fe was partially replaced b y Al, a solid solution was also formed, but the sizes of the octahedral sit es 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 var ious sizes of the octahedral sites. (C) 1999 Elsevier Science S.A. All righ ts reserved.