Overview of processing of nanocrystalline hydrogen storage intermetallics by mechanical alloying/milling

The objective of this article is to overview processes of mechanical alloying/milling (MA/MM), and their modifications applied to produce nanostructured single- and multi-phase intermetallics, and their composites, for hydrogen storage. In the most typical processing, MA is used as a preliminary step in synthesizing a nanostructured intermetallic powder starting from elemental metal powders. In a subsequent step, the intermetallic powder is hydrogenised under high pressure of hydrogen to produce nanostructured intermetallic hydride. A modified processing variant combines the synthesis of nanostructured intermetallic and its subsequent hydrogenising in one step by MA of elemental metal powders directly under hydrogen atmosphere to form nanostructured intermetallic hydrides (so-called Reactive Mechanical Alloying—RMA). The MM can be applied to produce nanostructured intermetallic powders from pre-alloyed intermetallic cast ingots or to manufacture nanocomposites, by mixing with dissimilar material before milling, which could be hydrogenised in a separate process. In addition, pre-alloyed bulk intermetallics can be mechanically milled directly under hydrogen atmosphere (Reactive Mechanical Milling—RMM) in order to obtain nanostructured intermetallic hydrides as a final product. All the above processes are critically discussed in the present article. The effect of nanostructurization on the hydrogen sorption/desorption characteristics of intermetallics and/or their hydrides is also discussed.