Hydrogenation of amorphous and nanocrystalline Mg-based alloys

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 nanocryrstalline 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) Mg87Ni12Y1 alloys show slower hydriding kinetics and lower hydrogen absorption capacity compared to the other Mg-based alloys studied. The amorphous Mg87Ni12Y1 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 Mg87Ni12Y1 alloys having different microstructure is practically the same. The crystallization of melt-spun Mg(75)Ni(20)Mm(5) and Mg87Ni12Y1 alloys is a two-step process. The primary crystallization of alpha-Mg (for Mg87Ni12Y1) takes place at about 160 degrees C, followed by transformation of the residual amorphous matrix into a metastable phase, assigned as fee Mg6Ni (isomorphic with fee Mg6Pd (alpha(o)=2.0108 nm)). This intermediate metastable phase decomposes during further annealing (at about 300 degrees C) into the equilibrium phases Mg2Ni and Mg. The product of the first crystallization reaction for the as-cast Mg(75)Ni(20)Mm(5) alloy is Mg2Ni, most probably realized by growth of the quenched-in Mg2Ni nanocrystals. The second reaction corresponds to transformation of the residual amorphous matrix into Mg(17)Mm(2). (C) 1999 Elsevier Science S.A. All rights reserved.