Indications of the formation of an oversaturated solid solution during hydrogenation of Mg–Ni based nanocomposite produced by mechanical alloying

An oversaturated solid solution of H in a nanocomposite material formed mainly by nanocrystalline Mg₂Ni, some residual nanocrystalline Ni and an Mg rich amorphous phase has been found for the first time. The nanocomposite was produced by mechanical alloying starting from Mg and Ni elemental powders, using a SPEX 8000D mill. The hydriding characterization of the nanocomposite was carried out by solid–gas reaction method in a Sievert's type apparatus. The maximum hydrogen content reached in a period of 21 Ks without prior activation was 2.00 wt.% H under hydrogen pressure of 2 MPa at 363 K. The X-ray diffraction analysis showed the presence of an oversaturated solid solution between nanocrystalline Mg₂Ni and H without any sign of Mg₂NiH₄ hydride formation. The dehydriding behaviour was studied by differential scanning calorimetry and thermogravimetry. The results showed the existence of two desorption peaks, the first one associated with the transformation of the oversaturated solid solution into Mg₂NiH₄, and the second one with the Mg₂NiH₄ desorption.