Low-temperature hydrogenation of Mg-Ni-Nb₂O₅ alloy processed by high-pressure torsion

Powder mixtures with a nominal composition Mg-5 wt%Ni-2 wt%Nb₂O₅ are used to produce ultrafine-grained bulk material by high-pressure torsion (HPT) processing. Samples subjected to 10 and 20 revolutions in torsion under a pressure of 3 GPa developed an ultrafine grain size, mainly in the 100−200 nm range. The hydrogenation properties of these HPT-processed materials have been characterized at temperatures in the interval of 373–723 K. The hydrogenation experiments clearly show faster kinetics of hydrogenation and higher storage capacities in the HPT materials compared to the initial mild-milled powder mixture. The storage capacities have been limited to about 5.5 wt% H₂ by the formation of a substantial amount of MgO in the materials after several hydrogenation cycles. However, as a matter of practical relevance, the production of bulk pieces of the Mg-Ni-Nb₂O₅ alloy having an ultrafine grain structure allows handling of the material without special precautions against ignition, as well as its storage in contact with air for several months with no evident deterioration of its fast activation in posterior hydrogenation treatments.