Formation and Thermal Stability of ω-Ti(Fe) in α-Phase-Based Ti(Fe) Alloys

In this work, the formation and thermal stability of the omega-Ti(Fe) phase that were produced by the high-pressure torsion (HPT) were studied in two-phase alpha-Ti + TiFe alloys containing 2 wt.%, 4 wt.% and 10 wt.% iron. The two-phase microstructure was achieved by annealing the alloys at 470 degrees C for 4000 h and then quenching them in water. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were utilized to characterize the samples. The thermal stability of the omega-Ti(Fe) phase was investigated using differential scanning calorimetry (DSC) and in situ high-temperature XRD. In the HPT process, the high-pressure omega-Ti(Fe) phase mainly formed from alpha-Ti. It started to decompose by a cascade of exothermic reactions already at temperatures of 130 degrees C. The decomposition was finished above similar to 320 degrees C. Upon further heating, the phase transformation proceeded via the formation of a supersaturated alpha-Ti(Fe) phase. Finally, the equilibrium phase assemblage was established at high temperatures. The eutectoid temperature and the phase transition temperatures measured in deformed and heat-treated samples are compared for the samples with different iron concentrations and for samples with different phase compositions prior to the HPT process. Thermodynamic calculations were carried out to predict stable and metastable phase assemblages after heat-treatments at low (alpha-Ti + TiFe) and high temperatures (alpha-Ti + beta-(Ti,Fe), beta-(Ti,Fe)).