Powdering limit of hydrogen absorbing intermetallic compounds

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Mechanisms of the reversible hydrogen absorption and desorption in Ti_3Al-base alloys was studied by means of pressure-composition isotherm measurements and microscopy observations before and after hydrogen absorption on specimens subject to heat treatment and cold rolling. The alpha-hydride to beta-hydride phase transformation in the Ti_3Al-base alloys occurs at a very low pressure and is reversible under normal conditions. However, the transformation between beta and gamma hydrides can be made reversible if the specimen is heat treated so that it contains O-phase in its microstructure or is appropriately deformed by cold rolling. The beneficial effects of O-phase on hydrogen absorption/desorption were demonstrated in three Ti-AT-Nb alloys with compositions in the vicinity of Ti_2NbAl. The beneficial effects are found to be related to the structure and strain field of O-phase rather than the interface between the O-phase and matrix. Their hydrogen absorption and desorption properties are improved when they are mildly deformed. Microstructure observations show that the beta and gamma hydride transformation occurs in a way very similar to the martensitic transformation and can be analyzed within the frame work of the martensitic transformation. The lattice shear deformation is accomplished by a high density of screw dislocations in the transformed laths. Such screw dislocations can be introduced by cold rolling.The hydrogen absorption/desorption properties of other intermetallic materials may be expected to be improved similarly to Ti_3Al-base alloys. Thus, the hydrogen absorption and desorption of Mg-Ca-Ni alloys, which absorb hydrogen easily and form a very stable hydride, was investigated. However, we have not succeeded in enhancing their hydrogen absorption and desorption properties yet. Further work is needed to clarify the reason for this.