Omega-phase in Ti-Hf-Zr alloys produced by the hydride-cycle method

We report the presence of large proportions of ω-phase in Ti-Hf-Zr alloys, prepared using the hydride cycle technique. We show that the ω-phase extends across the concentration triangle of Ti-Zr-Hf and report the partitioning of the three metals across the two sites in this structure from neutron and X-ray data. We examine the symmetry of the order parameter governing the β-ω phase transition and show that a two-step model for the phase transition involving site ordering followed by displacement is not likely to be correct. We suggest that an interstitial solid solution of oxygen and octahedral vacancies exists, and that the observation of any ω-phase diffraction pattern of alloys of these metals at ambient temperature and pressure should be viewed as a potential sign of the presence of oxygen in the octahedral interstitial sites. (author)