Nanocrystalline powders of Mg-Ni-based light alloys for hydrogen storage processed by controlled mechanical milling

For vehicular and stationary power generation applications utilizing a hydrogen fuel cell technology, inexpensive lightweight metal/intermetallic alloy hydrides that absorb and desorb hydrogen offer a safe alternative to storage in compressed or liquid form. Light-weight and inexpensive elemental Mg metal and Mg2Ni intermetallic compound have the highest hydrogen storage capacity from all known metal/intermetallics, approaching 7.7wt% and 3.6wt%, respectively. It has been reported that nanocrystalline Mg-Mg2Ni alloys, processed by mechanical alloying (MA), offer very fast hydrogenation kinetics, which would be sufficient for practical applications. In the present work we report results of the systematic studies on the processing of nanocrystalline Mg-Ni alloys by mechanical (ball) milling of pre-alloyed ingots in a unique ball mill Uni-Ball-Mill 5, allowing milling under strictly controlled conditions under pure shearing (low-and high-energy), impact and mixed modes. Several ingots with the final composition of ∼28-33at%Ni (bal. Mg) were induction melted and solidified. The microstructure of all ingots was predominantly [eutectic (Mg+Mg2Ni)+Mg2Ni phase], in accord with the binary Mg-Ni phase diagram. However, due to some limited evaporation of Mg from the melt surface all the ingots additionally contained the residual MgNi2 phase. Subsequently, the pulverized ingots were mechanically (ball) milled in selected controlled modes up to 100h duration. The evolution of microstructure of the processed alloy powders and the concomitant changes of lattice constants of the constituent phases under milling up to 100 h duration are presented and discussed. (author)