The influence of partial substitutions of Fe in FeTi by transition metals (Cr, Mn, Co, Ni) and aluminum on the thermodynamic stability and hysteresis phenomena was systematically investigated. Intermetallic compounds examined were $Fe_0.9M_0.1Ti$ (M=Al, Cr, Mn, Co, Ni)and their quaternary alloys.
Pressure-composition isotherms of each intermetallics for both absorption and desorption of hydrogen were obtained using a modified sievert's apparatus. In all systems, equilibrium pressure for hydriding reaction at a given temperature was larger than for dehydriding reaction and this difference results in hysteresis energy loss during the hydriding and dehydriding cycles. Enthalpy data of the systems were also calculated from the P-C-T data.
Stability of the hydrides was increased and hysteresis energy loss during the hydriding and dehydriding cycles was reduced more or less significantly by small substitutions of Fe by other elements. It was considered that these effects of substitutions should be explained by intersitial hole size of the intermetallic compounds.
Hysteresis energy losses reduced significantly during the first few cycles of hydriding and dehydriding, and saturated gradually as the particle size becomes uniform. There was no evidence of the so-called aliquot size effect on hysteresis observed in other systems.
The origin of the hysteresis phenomena and true equilibrium pressure in the FeTi-type intermetallic compounds-hydrogen system were also discussed. The hysteresis energy losses are mainly due to plastic works in solid solution and hydride phase caused by a large volume change during the hydriding and dehydriding reaction.
