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

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. - Abstract: 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 deg. 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.