Experiment investigation and thermodynamic assessment of the ternary Ti–Mo-Hf system

Based on experimental data measured by scanning electron microscope (SEM), X-ray diffraction (XRD) and electron probe microanalysis (EPMA), isothermal sections of Ti–Mo-Hf system at 800 °C and 1000 °C were constructed. Four and three three-phase regions were derived in the isothermal sections at 800 and 1000 °C, respectively. In addition, a new ternary compound named τ was discovered. The maximum solubilities of the three elements, Ti, Mo and Hf in τ were measured at 800 °C and 1000 °C. At the same time, the solid solubilities of Ti in HfMo₂_C15 and Mo in Hcp were also obtained. According to the measured experimental data, the Ti–Mo-Hf system was optimized using the CALPHAD (CALculation of PHAse Diagrams) method. The solution phases, liquid, Bcc and Hcp, were treated as substitutional solution, while the intermetallic compounds were modeled using sublattice models. HfMo₂_C15 was treated as (Hf, Mo, Ti)₁(Hf, Mo, Ti)₂. The ternary phase τ was considered as a stoichiometric compound and its thermodynamic modeling was defined as (Ti)₃(Mo)₃₍Hf)₁₄. The calculated results showed good agreement with the experimental phase equilibrium data, leading to the derivation of a set of self-consistent thermodynamic parameters for the Ti–Mo-Hf system.