In situ analysis of phase constituents evolution upon hydrogen cycling of cold-forged Mg-Ni powders

Abstract

Development of hydrogen storage in metal hydrides requests efficient, simple and safe processes such as room temperature forging. Samples elaborated in this way from Mg and Ni powders result in bulk pellets whose only identified phases are Mg and Ni. Deuterium absorption occurs from 250 °C in these samples due to the co-existence of basal fiber texture and structural defects, both induced by forging. In situ neutrons diffraction analysis shows the progressive formation of the Mg2NiD0.3 / Mg2NiD4 phases upon deuterium cycling and allows to identify the multi-step absorption and desorption mechanisms at temperatures as low as 270 °C. The growth of the Mg2NiD0.3 / Mg2NiD4 phases occurs in a short time during the second step of the deuterium absorption – as Mg2NiD0.3 transformed into Mg2NiD4 – and progresses from the periphery of the initial Ni particles towards their center. The mechanism is controlled simultaneously by deuterium diffusion and migration of Mg atoms through the already formed Mg2NiD4 deuteride. The reaction between Mg and Ni is not observed in the absence of deuterium, even after annealing at 285 °C for 48 h, thus we evidence a deuterium-induced mobility of the metal atoms.