Mechanisms of grain refinement and improved kinetic property of nanocrystalline Mg-Ni-La hydrogen storage alloys prepared by nanocrystallization of amorphous

Abstract

Mg_x(Ni_0.8La_0.2)_100-x, where x = 60, 70, 80, exhibiting a nanocrystalline microstructure, were prepared through the crystallization of amorphous alloys. The investigation encompassed the phase constitution, grain size, microstructural stability, and hydrogen storage properties. Crystallization kinetics, along with in-situ high-energy XRD characterization, revealed a concentrated and synchronous crystallization of Mg₂Ni and RE-Mg-Ni ternary phases with the increase in La and Ni content. The attributed synchronous crystallization process was found to be a result of the close local affinity of Mg₂Ni and RE-Mg-Ni ternary phases, as assessed by the thermodynamic Miedema model. Significant secondary phase pinning effect, arising from the high likelihood of well-matching phase structures between Mg₂Ni, LaMg₂Ni, and LaMgNi₄, was validated through both the edge-to-edge matching model prediction and experimental observation. The fine and homogeneous microstructure was shown to be a consequence of fast crystallization kinetics and the secondary phase pinning effect. Improved activation performance and cycling stability were observed, stemming from grain refinement and excellent microstructural stability. Our study provides insights into mechanism of grain refinement of nanocrystalline microstructure tailored by phase constitution and crystallization kinetics in the amorphous-crystallization route. We also demonstrate the potential of material design guided by phase equilibria and crystallographic predictions to improve nanocrystalline with excellent microstructural stability.