Microstructural evolution and improved hydrogenation–dehydrogenation kinetics of nanostructured melt-spun Mg–Ni–Mm alloys

The microstructural evolution of as-quenched ribbons and ball-milled hydrides of the Mg–10Ni–2Mm alloy was studied by TEM. These studies showed a refinement of the microstructures during the applied processing and a nucleation of MmMg12 intermetallic at the grain boundaries of Mg and Mg2Ni. The interface between MmMg12 and Mg2Ni is semi-coherent, with an ordered repetition of the consistent atomic arrangements. The kinetics of H-absorption/desorption is improved due to the fast hydrogen diffusion in the nanograins, thus, providing paths for H-exchange. TEM studies showed (a) stability of the nano-sized grains in the ball-milled Cu-1000 (the surface velocity of the copper wheel: 1000 rpm) sample that underwent cycling of hydrogen desorption and absorption during heating to 350 °C; (b) formation of MmH3−x hydride from MmMg12 and its preferential location at grain boundaries of MgH2. Clearly, MmH3−x and Mg2NiH4 act as nucleation centres to initiate the formation of MgH2, thus, promoting hydrogen absorption by the Mg alloys. Pressure–composition–temperature diagrams show the presence of two plateaux, Mg–MgH2 and Mg2Ni–Mg2NiH4. The MgH2 plateau showed no hysteresis and practically no slope, while the plateau for Mg2NiH4 exhibited both a pronounced hysteresis and a slope, particularly for the nanocrystalline sample. The maximum hydrogen storage capacity of the nanocrystalline sample was higher than that of the microcrystalline one.

Research highlights▶ Nanocrystalline microstructures synthesised by the melt-spinning technique. ▶ The hydrides obtained by Reactive Ball Milling process. ▶ The microstructural evolution of the melt-spun ribbons and hydrides studied by TEM. ▶ The effect of nanostructuring on the hydrogen storage properties.