Thermodynamic description of the Ti-Mo-Nb-Ta-Zr system and its implications for phase stability of Ti bio-implant materials

Abstract Titanium alloys are great candidates for applications such as biomedical implants that require biocompatibility, a low Young's modulus and a high strength. However, the properties of Ti alloys are highly dependent on phase stability. In the present work, a database for the Ti-Mo-Nb-Ta-Zr system has been evaluated using the CALPHAD (CALculation of PHAse Diagram) approach. The database was completed by evaluating the accuracy of previously modeled systems from literature and modeling systems that, to the best of the authors’ knowledge, had no modeling available in literature. All of the binary systems that make up the Ti-Mo-Nb-Ta-Zr system had previously modeled thermodynamic descriptions available in the literature and in most cases had multiple different descriptions available, which meant determining which previous thermodynamic description most accurately modeled the binary system with a direct focus on the bcc phase. In order to determine the accuracy of the multiple available thermodynamic descriptions from literature a combination of experimental data (also obtained from the literature) and computed thermochemical properties of the bcc phase from DFT (Density Functional Theory)-based first-principles calculations (present work) were used. Once the thermodynamic descriptions for the binary systems were chosen, focus shifted to the Ti-containing ternary systems. The Ti-Mo-Zr, Ti-Nb-Zr and Ti-Ta-Zr systems had previous thermodynamic description available in literature, which were incorporated without changes into the working database. The Ti-Mo-Ta, Ti-Nb-Ta and Ti-Mo-Nb systems had, to the authors’ knowledge, no descriptions available in the literature. Interaction parameters were determined for the Ti-Mo-Ta and Ti-Nb-Ta systems, and no interaction parameters were introduced for the Ti-Mo-Nb system. The database introduced by this work satisfactorily predicts the thermodynamics of the Ti-Mo-Nb-Ta-Zr system.