Binary Ti-Fe system. Part II: Modelling of pressure-dependent phase stabilities

Based on the experimental and calculated data from literature, the thermodynamic assessment of the binary Ti-Fe system under hydrostatic pressure was performed. The pressure-independent part of the Gibbs energy was modelled using the heat capacity data. The description of the pressure-dependent part of the Gibbs energy was based on the molar volume, thermal expansion and bulk modulus data, and on their pressure derivatives. Using the Murnaghan equation of state, the binary phase stabilities in the Ti-Fe system were assessed to reproduce the experimental results in the pressure range between 2.3 and 2.7 GPa. The thermodynamic functions were converted into the parameters of a phenomenological pressure model, which is implemented in the Thermo-Calc software. The reliability of the assessment is illustrated on the unary systems Ti and Fe containing high-temperature and high-pressure phases, and on the binary Ti-Fe system containing intermetallic phases TiFe and TiFe2 in addition to the terminal solid solutions. It is shown that the calculation of the T-0 lines can help in understanding the formation of alpha'-Ti(Fe) martensite and the high-pressure phase omega-Ti and its metastability at atmospheric pressure.