Thermodynamic calculations explaining the stability of high-pressure ω phase in the Ti-rich part of the binary Ti–Fe system
Titanium-rich Ti–Fe alloys are promising materials for numerous engineering applications, as they possess good corrosion resistance, high wear resistance, biocompatibility and appropriate mechanical properties. The adjustment of the microstructure, e.g., through the mechanical and thermal treatment, improves further the properties of the alloys. The key parameters are the amount and distribution of the low-temperature α-Ti phase, the high-temperature β-(Ti,Fe) phase and the high-pressure ω-Ti(Fe) phase. For a better understanding of the phase formation and stability of ω-Ti(Fe), different initial states were produced by heat treatments at 470 and 800°C, and subjected to high-pressure torsion (HPT). The phase fractions, the distribution of individual phases and their orientation relationships in the initial and in the HPT-processed alloys containing between 2 and 10 wt.% Fe were analyzed using X-ray diffraction (XRD), electron microscopy (SEM, (HR)TEM) and electron diffraction (EBSD, SAED). The thermal stability of the ω phase was investigated by means of thermal analysis and high-temperature in situ XRD. The alloys quenched from the bcc β-(Ti,Fe) solid solution (from 800°C) show a martensitic microstructure for Fe contents below 2 wt.% and the formation of the athermal ω phase at approximately 4 wt.% Fe. Alloys annealed at 470°C exhibit a two-phase α-Ti + TiFe microstructure after quenching. During the HPT process, the phases were partially or completely transformed to ω-Ti(Fe), depending on the Fe content and the initial phase composition. Differences were also detected for the thermal stability of the HPT-induced ω-Ti phase. The phase transformations were interpreted by means of the pressure-dependent thermodynamic calculations that were performed with the Thermo-Calc software. The thermodynamic modelling were started from the unary titanium and iron systems. For that purpose, experimental and ab initio calculated heat capacities, molar volumes, thermal expansions and bulk moduli were utilized. The modelling of the binary Ti–Fe system was based on the assessment of Ref. .