Microstructures and hydrogen absorption-desorption behavior of an A2B7-based La-Mg-Ni alloy

The microstructural changes during hydrogen absorption-desorption cycles of an A2B7-based La-Mg-Ni alloy with a nominal composition of La1.5Mg0.5Ni7.0 were systematically investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The ternary La-Mg-Ni alloy was mostly composed of 2H-A2B7 phase with minor inclusions of 3R-A5B19, 2H-A5B19 and 3R-AB3 phases existing as parts of intergrowth structures with the major A2B7 phase. Most parts of the major 2H-A2B7 phase containing Mg exhibited an excellent crystal structure retention after the hydrogen absorption-desorption cycles at 80 °C. Two types of defected bands were found to develop after the first hydrogen absorption-desorption cycle. The first ones are amorphous bands developed inside the minor 3R-AB3 phase, while the second ones develop as heterogeneously strained regions inside the major 2H-A2B7 phase. Both the defected bands are considered to be responsible for the irreversible hydrogen capacity of the A2B7-based La1.5Mg0.5Ni7.0 alloy during the hydrogen absorption-desorption cycles at 80 °C.