, , , , K. Young, B. Chao,
The microstructures of a series of AB_(2)-based metal hydride alloys (Ti_(12.5)Zr_(21)V_(10)Cr_(8.5)Mn_(x)Co_(1.5)Ni_(46.5-x)) designed to have different fractions of non-Laves secondary phases were studied by x-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectrometry, and electron backscatter diffraction. The results indicate that the alloys contain a majority of hydrogen storage Laves phases and a minority of fine-structured non-Laves phases. Formation of the phases is accomplished by dendritic growth of a hexagonal C14 Laves phase. The C14 phase is followed by either a peritectic solidification of a cubic C15 Laves phase (low Mn containing alloys) or a C14 phase of different composition (high Mn containing alloys), and finally a B2 phase formed in the interdendritic regions (IDR). The interdendritic regions may then undergo further solid-state transformation into Zr7Ni10-type, Zr9Ni11-type and TiNi-type phases. With the Mn content in the alloy increases, the following trends were found: the abundance of the C14 phase increases, C15 and Zr_(9)Ni_(11)type phase abundances decrease, Zr_(7)Ni_(10)type phase abundance first increases and then decreases, TiNi-based phase abundance first increases from 0 % to 4 % and then stabilized at 2 % . The IDR compositions can be generally expressed as (Ti,Zr,V,Cr,Mn,Co)_(50)Ni_(50), which accounted for 7 % to 10 % of the overall alloy volume fraction.
Journal of Power Sources
Hydrogen absorbing materials, hydrogenation, metal hydride electrode, transition metal alloys