| We report on hydrogen sorptive properties of Zr-Ti-Nb-Cu-Al-Ni icosahedral alloys under different mechanical activation conditions. Two compositions were investigated, Zr67Ti6.14Nb1.92Cu10.67Ni8.52Al5.75 and Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5 prepared respectively by partial crystallization from mechanically alloyed amorphous powders, and by copper mold casting. In the former, independently by the milling conditions, the gaseous absorption took place with loss of the icosahedral symmetry and precipitation of a δ-ZrH1.66-type nanocrystalline hydride which accompanied the formation of an amorphous hydride. Annealing treatments did not permit to restore initial structural conditions, so hindering the process reversibility. In the latter system, conversely, hydrogen absorption, activated by mechanical agitation in absence of milling ball, occurred by retaining the quasiperiodic features, just swelling the icosahedral structure. Thermally activated desorption preserved the QC symmetry and further successful hydrogenation indicated cycling possibility, at least partial, of the process. Hydriding kinetics, studied under isobaric conditions, followed sigmoidal trends, with specific characteristics depending on the alloy stoichiometry and the activation conditions. |
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