Beneficial effects of O-phase on the hydrogen absorption of Ti-Al-Nb alloys

The beneficial effects of O-phase on hydrogen absorption/desorption were de monstrated in three Ti-Al-Nb alloys with compositions in the vicinity of Ti 2NbAl. The alloys were first quenched from high temperature and then aged f or a certain period of time at a lower temperature to get O-phase precipita tion in the matrix before pressure-composition (P-C) isotherm measurements. All three alloys absorb hydrogen to beta hydride at a very low equilibrium pressure. Further absorption to gamma hydride is difficult for the quenche d O-phase free specimens. beta-> gamma hydride transformation occurs in all the aged specimens and the reversible absorption/desorption between beta a nd gamma hydrides is observed in some aged specimens with relatively high v olume fraction of O-phase in the matrix. The hydrogen absorption/desorption abilities of these alloys become poor when the Nb content is decreased. Th e determined formation heat for beta- > gamma hydride transformation is in the range of -30-45 kJ/mol H-2, which is larger than that of binary Ti3Al. It is found that the value of formation heat becomes more negative with the increasing amount of O-phase and is relatively independent of the interfac e between O-phase and matrix. This indicates the beneficial effect is relat ed to the O-phase structure itself. (C) 2001 Elsevier Science Ltd. All righ ts reserved.