Stress effect on the swelling/shrinking behavior of an AB2 alloy during hydrogenation cycles

The stress influence on a Ti0.85Zr0.15Mn1.33V0.3 alloy during hydrogenation cycles has been studied in a special design test cell for mechanical studies during hydrogenation. The alloy was synthesized in an induction melting furnace with an inert Ar atmosphere. The full basic characterization of the material is presented, from a structural and morphological point of view by X-ray powder diffraction (XRPD), Scanning Electron Microscopy (SEM), Energy Dispersion X-ray spectroscopy (EDX) and laser grain size analysis, and, from a thermodynamic point of view by the measure of the Pressure composition Isotherms (P-c-I) at 23 and 80 °C. Besides, in order to measure the mechanical properties of the material, volume variations measurements during hydrogenation cycles were performed. The samples were milled up to sizes beneath 1.25 mm, and afterwards placed into a cylindrical die under three different levels of axial stress, 0.0325 MPa, 0.12 MPa and 0.84 MPa, for more than 100 cycles. For higher stress levels the powder bed shrinks as cycles continue, while for lower stresses the powder bed swells. A natural tendency for the powder to decrepitate and agglomerate was observed, as the size distribution of the particles reduces and the shape become uneven, generating an increase of the porosity and volume in the case of the lower stress sample. As the stress increases, the grains rearrange easily and a polydisperse distribution of particles facilitates the accommodation of smaller grains between bigger ones, producing a reduction in the porosity and volume of the hydride bed after several cycles.