Nanocrystalline Mg-Ni-based hydrogen storage alloys produced by nanocrystallization

Nanocrystalline Mg-Ni-Y alloys were produced by crystallization of amorphous precursors and by direct quenching of the melt, using a melt-spinning technique. The crystallization behaviour of Mg(2)Nj-based and Mg-based amorphous as well as nanocrystalline alloys, containing large amount of amorphous phase, were studied by transmission electron microscopy, differential scanning calorimetry, x-ray and electron diffraction. During heating the as-quenched Mg-2(Ni,Y) alloys crystallize by three dimensional growth of quenched-in nanocrystals (with average size of 2-3 nm), embedded into the amorphous matrix, with an activation energy of 140+/-7 kJ/mol, which value coincides with the activation energy of Mg self-diffusion. The crystallization of the melt-spun Mg-based alloy (Mg87Ni12Y1) was found to be a two-stage process, which leads to a nanocrystalline microstructure with an average grain size less than 100 nm. The influence of hydrogen on the thermal stability and crystallization of the melt-spun alloys was investigated. It was found that hydrogenation of the as-quenched amorphous and nanocrystalline alloys leads to a change in the crystallization mechanism during subsequent annealing, as the microstructure remains nanocrystalline even after heating up to 350 degrees C. The hydriding properties of the as-quenched alloys have been studied as well. The maximum hydrogen absorption capacity and hydrogenation kinetics of the melt-spun Mg-2(Ni,Y) alloys were found to be better than those of the conventional polycrystalline magnesium alloys and comparable to the H-absorption characteristics of nanocrystalline ball-milled Mg2Ni.