Formation and hydrogen adsorption properties of Ti-Hf-Ni quasicrystals and crystal approximants

The formation of a high-order rational approximant (RA) phase in rapidly quenched Ti-Hf-Ni alloys and the hydrogen absorption properties of that phase are reported. Electron diffraction patterns show systematic shifts of the diffraction spots from their expected positions for the icosahedral phase (i-phase); the diffraction patterns are consistent with those expected for a 3/2 RA phase RA-(Ti-Nf-Ni). Based on differential scanning calorimetry studies, RA-(Ti-Hf-Ni) is metastable, transforming between 350 and 500°C to a quasicrystal with strong phason disorder. This crystallizes at 620°C to a Ti₂Ni-type phase. RA-(Ti-Hf-Ni) readily absorbs hydrogen, up to 1.2 hydrogen atoms per metal atom ([H]/[M] = 1.2). Pressure-composition isotherm studies for gas-phase loading show that the pressure plateau for RA-(Ti-Hf-Ni) is similar to that observed for the i-(Ti-Zr-Ni) phase, although it occurs at a slightly higher pressure and extends from a smaller range of hydrogen concentrations. Unlike i-(Ti-Zr-Ni), no irreversible hydride phase forms with hydrogen loading at 250°C, suggesting that RA-(Ti-Hf-Ni) may have superior cycling properties, of interest for hydrogen storage applications.