The effect of solidification rate on the microstructure and electrochemical properties of Co-free Ml(NiMnAlFe)5 alloys

In this work, Fe-containing Co-free Ml(NiMnAlFe)(5) alloy samples were prepared by both arc-melting and melt-spinning processes at different solidification rates, and tested for electrochemical performance. The electrochemical tests indicated that higher solidification rates led to a better cycling stability, but lower discharge capacity and high-rate dischargeability. The X-ray diffraction results indicated that the as-cast alloy and the melt-spun alloys were both of the CaCu5-type structure single phase, but the lattice constants of the alloys were varying. The scanning electron microscope and EPMA results indicated that the microstructure of melt-spun alloy formed at the relative lower solidification rate (5 m/s) was of very fine dendritic structure and that for the alloy formed at a relative higher solidification rate ( 15 m/s) was of cellular structure, and both had a more homogeneous composition than the as-cast alloy, which had a typical dendrite structure with noticeable composition segregation. The Co-free MlN(4.1)Mn(0.35)Al(0.3)Fe(0.25) prepared by melt-spinning at 15 m/s rate is a promising candidate for low-cost electrode alloy. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.