Experiment investigation and thermodynamic assessment of the ternary Ti-Al-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-Al-Hf at 900 °C and 1100 °C were established. A total of four three-phase regions and eight two-phase regions were found at the two temperatures, and a ternary compound τ was also found. The maximum solubility of Ti₂Al₅ for Hf at 900 °C was obtained, and the maximum solubility of binary compounds TiAl, TiAl₂, HfAl₂, Hf₂Al for the third element and Al, Hf, Ti in the τ phase was also measured. Based on the experimental data, the thermodynamic evaluation of the Ti-Al-Hf system was carried out using the CALPHAD(CALculation of PHAse Diagrams) method for two isothermal sections. The liquid phase, Bcc phase and Hcp phase were modeled using the substitutional solution model, and the ternary phase τ was treated as a stoichiometric compound. Its thermodynamic model was defined as (Hf)₂(Al)₂(Ti)₁. The final calculation results were in good agreement with the experimental results, including phase equilibrium and alloy composition.