Ab initio investigation and thermodynamic modeling of the Mo–Ti–Zr system

Knowledge of the Mo–Ti–Zr phase diagram and its underlying thermodynamics is critically needed to guide the design of advanced U-Mo–Ti–Zr metallic fuel alloys for next-generation nuclear reactors as well as Ti–Mo–Zr–based biomaterials for medical applications. Here, we report on the development of a thermodynamic model for the Mo–Ti–Zr ternary system using the ab initio informed CALPHAD approach. Our ab initio evolutionary search discovers many experimentally unobserved stable compounds in the Mo–Ti system, but finds Ti–Zr to be a non-compound-forming system. In the Mo–Zr system, no stable compound other than the experimentally known C15 Mo₂Zr Laves phase has been found. Phonon calculations within the harmonic approximation have been performed to obtain the vibrational formation entropies of all ordered binary compounds in Mo–Ti and Mo–Zr systems. The mixing energies of the liquid phase and the mechanically unstable but dynamically stabilized bcc phase are further obtained using high-temperature ab initio molecular dynamics (AIMD) simulations. The predicted binary and ternary phase diagrams show overall good agreement with experimental measurements. Future experimental validation of the predicted stable compounds in Mo–Ti system and our model-predicted low-temperature hcp miscibility gap in Ti–Zr system will be of great interest.