Molybdenum-titanium phase diagram evaluated from ab initio calculations

The design of next generation β-type titanium implants requires detailed knowledge of the relevant stable and metastable phases at temperatures where metallurgical heat treatments can be performed. Recently, a standard specification for surgical implant applications was established for Mo-Ti alloys. However, the thermodynamic properties of this binary system are not well known and two conflicting descriptions of the β-phase stability have been presented in the literature. In this study, we use ab initio calculations to investigate the Mo-Ti phase diagram. These calculations predict that the β phase is stable over a wide concentration range, in qualitative agreement with one of the reported phase diagrams. In addition, they predict stoichiometric compounds, stable at temperatures below 300°C, which have not yet been detected by experiments. The resulting solvus, which defines the transition to the β-phase solid solution, therefore occurs at lower temperatures and is more complex than previously anticipated.