Microstructure Investigation on Metal Hydride Alloys by Electron Backscatter Diffraction Technique

The microstructures of two metal hydride (MH) alloys, a Zr7Ni10 based Ti15Zr26Ni59 and a C14 Laves phase based Ti12Zr21.5V10Ni36.2Cr4.5Mn13.6Sn0.3Co2.0Al0.4, were studied using the electron backscatter diffraction (EBSD) technique. The first alloy was found to be composed of completely aligned Zr7Ni10 grains with a ZrO2 secondary phase randomly scattered throughout and a C15 secondary phase precipitated along the grain boundary. Two sets of orientation alignments were found between the Zr7Ni10 grains and the C15 phase: (001)Zr7Ni10A//(110)C15 and [100]Zr7Ni10A//[011]C15, and (011)Zr7Ni10B//(100)C15 and [100]Zr7Ni10B//[313]C15. The grain growth direction is close to [313]Zr7Ni10B//[111]C15. The second alloy is predominated by a C14 phase, as observed from X-ray diffraction analysis. Both the matrix and dendrite seen through a scanning electron microscope arise from the same C14 structure with a similar chemical composition, but different orientations, as the matrix with the secondary phases in the form of intervening Zr7Ni10/Zr9Ni11/(Zr,Ni)Ti needle-like phase coated with a thin layer of C15 phase. The crystallographic orientation of the C15 phase is in alignment with the neighboring C14 phase, with the following relationships: (111)C15//(0001)C14 and [110]C15//[1120]C14. The alignments in crystallographic orientations among the phases in these two multi-phase MH alloys confirm the cleanliness of the interface (free of amorphous region), which is necessary for the hydrogen-storage synergetic effects in both gaseous phase reaction and electrochemistry.