Ti _12.5Zr ₂₁V ₁₀Cr _8.5Mn _xCo _1.5Ni _46.5-x AB ₂-type metal hydride alloys for electrochemical storage application: Part 1. Structural characteristics

The microstructures of a series of AB ₂-based metal hydride alloys (Ti _12.5Zr ₂₁V ₁₀Cr _8.5Mn _xCo _1.5Ni _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 Zr ₇Ni ₁₀-type, Zr ₉Ni ₁₁-type and TiNi-type phases. As the Mn content in the alloy increases, the fraction of the C14 phase increases, whereas the fraction of C15 decreases. In the IDRs when the alloy's Mn content increases the Zr ₉Ni ₁₁ phases and Zr ₇Ni ₁₀ phase fraction first increases and then decreases, while the TiNi-based phase fraction first increases and then stabilized at 0.02. IDR compositions can be generally expressed as (Ti,Zr,V,Cr,Mn,Co) ₅₀Ni ₅₀, which accounted for 7-10% of the overall alloy volume fraction.