NOVELTY - A nanocrystalline powder comprises polycrystalline particles containing crystals of aluminum alloy and extracrystalline catalyst present on and in the particles. The aluminum alloy is sodium-aluminum, lithium-aluminum, magnesium-aluminum, or zirconium-aluminum. The catalyst is carbon, and titanium, platinum, palladium, vanadium, zirconium, cobalt, nickel, iron, and/or lanthanum.    USE - For use in storing hydrogen.    ADVANTAGE - The inventive nanocrystalline includes aluminum alloy that takes the form of nanocrystalline powder comprising polycrystalline particles. The particles are small enough to enhance the surface area of the powder and minimize the distance that hydrogen has to diffuse into the particle to achieve full hydrogen storage capacity.    DETAILED DESCRIPTION - A nanocrystalline powder comprises polycrystalline particles containing crystals of aluminum alloy and extracrystalline catalyst present on and in the particles. The aluminum alloy is NaAlx, LiAlx, MgAl2x, or ZrAl2x, where x is 0.9-1.1. The catalyst is carbon, titanium, platinum, palladium, vanadium, zirconium, cobalt, nickel, iron, lanthanum, and combination of titanium, platinum, palladium, vanadium, zirconium, cobalt, nickel, iron, and/or lanthanum. INDEPENDENT CLAIMS are also included for:    (a) a method of forming a hydrogen storage medium comprising providing a charge comprising powdered aluminum and powder of second metal in predetermined atomic ratio, impact milling the charge to form a nanocrystalline alloy powder comprising crystals of MAlx and intercalating a catalyst into particles of alloy powder; and    (b) a metal hydride for reversible hydrogen storage of formula MAlx-yM'yH-4.    M = sodium, lithium, magnesium or zirconium;    M' = silicon, potassium, calcium, barium, titanium, lithium, sodium, magnesium, or zirconium;y = 0.1-0.5.    Provided that x is 0.9-1.1 (preferably 1) for each M being lithium or sodium, and x is 1.8-2.2 (preferably 2) for each M being magnesium or zirconium.