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 degrees C and 1000 degrees C were constructed. Four and three three-phase regions were derived in the isothermal sections at 800 and 1000 degrees C, respectively. In addition, a new ternary compound named tau was discovered. The maximum solubilities of the three elements, Ti, Mo and Hf in tau were measured at 800 degrees C and 1000 degrees 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-MoHf 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 tau 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.