Phase transformation by a step-growth mechanism in annealed La–Mg–Ni-based layered-stacking alloys

Abstract The present work contributes to the knowledge of a particular microstructure in which numerous stripes emerge in the annealed La–Mg–Ni-based hydrogen storage alloys. Stripe-shaped structures characterized by electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM) have been illustrated to be steps resulting from a step-growth mechanism of phase transformation during annealing. Nucleation of the La–Mg–Ni phases relies on the (0001) planes of the parent phase with a low mismatching configuration, leading to an orientation relationship of [ 10 1 ¯ 0 ] ∥ [ 10 1 ¯ 0 ] and (0001)//(0001) between the new and parent phases. Grain growth proceeds through a step-side face to avoid the stable phase boundaries on the step-terrace face, which is parallel to the (0001) planes of the new and parent phases. The present work also illustrates that nucleation of the new La–Mg–Ni structures can take place by introducing certain stacking faults under solid state, which provides different perspectives into regulating the microstructure by changing the annealing parameters. Moreover, the importance of the original microstructure, especially the effect of the grain size on the final phase constitution, is emphasized based on the present findings.