Microstructure and electrochemical properties of melt-spun Nd_0.8Mg_0.2(Ni_0.8Co_0.2)_3.8 hydrogen storage alloy

The effects of rapid solidification on the microstructure and electrochemical properties of Nd_0.8Mg_0.2(Ni_0.8Co_0.2)_3.8 alloy were systematically investigated. The microstructure of alloys was characterized by scanning electron microscopy (SEM), X-ray diffractometer (XRD) and transmission electron microscopy (TEM). It was found that the melt-spun Nd_0.8Mg_0.2(Ni_0.8Co_0.2)_3.8 ribbons became thinner and the average grain size of the ribbons became smaller with increasing wheel speed. A fraction of amorphous phase was observed for the ribbons melt-spun at high wheel speed (≥20 m/s). Microstructural characterization revealed that two phases: (Nd,Mg)₂(Ni,Co)₇ main phase (Ce₂Ni₇ type structure) and NdNi₅ second phase (CaCu₅ type structure), existed in the samples in cast state and melt-spun. The cycle stability of the melt-spun alloys was significantly enhanced as compared with cast alloy, and the sample prepared at wheel speed of 20 m/s exhibited good comprehensive electrochemical properties.