Powdering limit of hydrogen absorbing intermetallic compounds

通过对热处理和冷轧后的Ti_3Al基合金吸氢前后的压力-成分等温线测量和显微观察，研究了Ti_3Al基合金可逆吸氢和放氢的机理。Ti_3Al基合金中α-氢化物向β-氢化物的相变发生在很低的压力下，并且在正常条件下是可逆的。然而，如果试样经过热处理，使其微观结构中含有O相或通过冷轧适当变形，则β和γ射线之间的转变可以是可逆的。在Ti_2NbAl附近的三种Ti-AT-Nb合金中，O相对吸放氢有明显的促进作用。研究发现，这种有益效果与O相的结构和应变场有关，而与O相与基体的界面无关。当它们轻微变形时，它们的吸氢和放氢性能得到改善。微观结构观察表明，β和γ氢化物转变发生的方式非常类似的马氏体相变，可以分析的框架内的马氏体相变。晶格剪切变形是由转变板条中的高密度螺位错完成的。冷轧可以引入这种螺旋位错，其它金属间化合物材料的吸放氢性能也有望得到类似于Ti_3Al基合金的改善。因此，研究了易吸氢并形成稳定氢化物的Mg-Ca-Ni合金的吸放氢行为。然而，我们还没有成功地提高它们的吸氢和解吸性能。需要进一步的工作来澄清这一原因。

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.