Hydrogenation of amorphous and nanocrystalline Mg-based alloys

Abstract

Abstract Amorphous and nanocrystalline Mg-based alloys were produced by rapid quenching (melt-spinning) and their hydrogenation properties were studied. The thermal stability and crystallization behaviour of the as-quenched and hydrogenated alloys were investigated as well. It was found that the as-cast nanocrystalline/amorphous Mg 75 Ni 20 Mm 5 (Mm=Ce, La-rich mischmetal) alloy possesses the best hydriding properties (hydrogenation kinetics and hydrogen absorption capacity) with maximum hydrogen capacity of 4.0 wt.% H. The difference in the hydriding properties of the as-quenched nanocrystalline and completely crystallized (with grain size in the range of 100–150 nm) Mg 75 Ni 20 Mm 5 alloys was found to be insignificant. The amorphous and crystallized (after heat treatment) Mg 87 Ni 12 Y 1 alloys show slower hydriding kinetics and lower hydrogen absorption capacity compared to the other Mg-based alloys studied. The amorphous Mg 87 Ni 12 Y 1 exhibits faster initial hydrogenation kinetics than the partially and fully crystallized alloys with the same composition, due to faster hydrogen diffusion in the amorphous phase, but the hydrogen absorption capacity of all Mg 87 Ni 12 Y 1 alloys having different microstructure is practically the same. The crystallization of melt-spun Mg 75 Ni 20 Mm 5 and Mg 87 Ni 12 Y 1 alloys is a two-step process. The primary crystallization of α-Mg (for Mg 87 Ni 12 Y 1 ) takes place at about 160°C, followed by transformation of the residual amorphous matrix into a metastable phase, assigned as fcc Mg 6 Ni (isomorphic with fcc Mg 6 Pd ( a o =2.0108 nm)). This intermediate metastable phase decomposes during further annealing (at about 300°C) into the equilibrium phases Mg 2 Ni and Mg. The product of the first crystallization reaction for the as-cast Mg 75 Ni 20 Mm 5 alloy is Mg 2 Ni, most probably realized by growth of the quenched-in Mg 2 Ni nanocrystals. The second reaction corresponds to transformation of the residual amorphous matrix into Mg 17 Mm 2 .