Effect of structural changes on degradation of hydrogen absorbing capacity in cyclically hydrogenated TiMn₂ based alloys

Structural changes of cyclically hydrogenated TiMn₂ based alloys, in which the compositions of the TiMn₂ Laves phase are about 60 and 61at.% Mn, have been investigated to understand the mechanism of the degradation of their hydrogen absorbing capacity. There are no apparent changes in micro- and nano-scale metallurgical features in the Ti-60at.% Mn alloy, which exhibited superior endurance cyclic property. On the contrary, hydride formation, decrease in crystalline size and/or introduction of heterogeneous strain, and the deviation of Mn atom position have been observed in Ti-61at.% Mn alloy, which exhibited poor cycle property, arising from retained hydrogen in the TiMn₂ Laves phase. Therefore, it is considered that structural changes with cyclic hydrogenation sensitively depend on the composition of TiMn₂ Laves phase, governing the degradation behavior. The mechanism of the degradation of cyclic property is discussed in view of the substitution behavior in the hexagonal TiMn₂ Laves phase.