Omega-phase in Ti-Hf-Zr alloys produced by the hydride-cycle method 1

We report the presence of large proportions of u-phase in Ti-Hf-Zr alloys, prepared using the hydride cycle technique. We show that the u-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 b-u phase transition and show that a two-step model for the phase transition involving site order- ing followed by displacement is not likely to be correct. We suggest that an interstitial solid solution of oxygen and octa- hedral vacancies exists, and that the observation of any u-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.