Ti/Zr/Hf-based quasicrystals

The formation, stability and structures of the icosahedral phase (i-phase) and related complex crystal and amorphous phases in Ti/Zr/Hf alloys are reviewed. Most attention is focused on the TiZrNi quasicrystal, demonstrating that this i-phase is a low-temperature stable quasicrystal, forming almost 300 °C below the liquidus temperature. The persistence of the i-phase at low temperatures after very long duration anneals suggests it as the first realistic candidate for a ground-state quasicrystal. A structural model for i(TiZrNi) is presented, the first for any Ti/Zr/Hf-based quasicrystal; the energy is lower than for competing crystal phases, in agreement with the results of the annealing studies. With sufficient undercooling, metastable i(TiZrNi) nucleates in preference to a polytetrahedral C14 Laves phase, suggesting a local icosahedral order in the liquid. X-ray data are presented to support that conclusion. Hydrogen storage properties of these alloys are discussed briefly and pressure composition isotherm measurements are presented and analyzed. A new Ti-Hf-Ni crystal approximant that has superior hydrogen adsorption properties is discussed. Origins of quasicrystal nanostructures, formed by crystallizing Zr-based metallic glasses are proposed.