Mechanical milling and subsequent annealing effects on the microstructural and hydrogenation properties of multisubstituted LaNi5 alloy Original Research Article

The influence of milling and subsequent annealing on the microstructural properties of commercial-type Mm(Ni, Mn, Al, Co)5.2 alloy have been investigated. On milling, a two stage process is observed from X-ray diffraction (XRD) profile analysis. At short milling times (t < 200 min), ball collisions lead to crystallite-size decrease by dislocation formation together with a related increase of lattice strain. Later (t > 200 min), further milling causes partial amorphous phase formation, while crystallite size as well as lattice strain of the crystalline phase remain constant (50 Å and 0.6%, respectively). At this stage, a volume cell expansion occurs which has been attributed to atomic site-interchange between A and B-type elements. The same two milling stages are observed by scanning electron microscopy regarding particle morphology: a fracture stage governed by plastic deformation (short milling times) and an agglomeration stage governed by cold-welding (long milling times). Microstructural properties of alloy take initial values on annealing at moderate temperatures, i.e., improvement of crystallization and recovery of cell volume. The relationship between alloy microstructure and hydrogenation properties for multisubstituted LaNi5 alloy is outlined. The occurrence of amorphous phase produces detrimental effects in both reversible H-capacity as well as H-kinetics as compared to the nanocrystalline phase.