**5. Conclusions**

The high-pressure torsion (HPT) treatment of two-phase Ti–Fe alloys consisting of a mixture of α-Ti and TiFe led to the formation of the high-pressure ω-Ti(Fe) phase mainly at the expense of α-Ti. The concentration of iron in ω-Ti(Fe), which was ~1 wt.%, was alternatively estimated from the difference in the phase fractions before and after the HPT process and from the measured lattice parameters of ω-Ti(Fe). The overall iron concentration in the samples (2 wt.%, 4 wt.%, and 10 wt.% Fe) primarily influenced the amount of TiFe, which inhibited the phase transition α-Ti → ω-Ti(Fe) to some extent, in particular in sample containing 10 wt.% Fe. The comparison with previous investigations, which were carried out on samples that were annealed above the eutectoid reaction, i.e., on samples mainly containing β-(Ti,Fe), has shown that the high-pressure ω-Ti(Fe) phase can be much more easy produced from β-(Ti,Fe) than from α-Ti. Thermodynamic calculations confirmed this experimental finding. The combination of *in situ* high-temperature X-ray diffraction and differential scanning calorimetry revealed that ω-Ti(Fe) starts to decompose exothermically already at 130 ◦C. Still, it survives up to ~320 ◦C, where its amount tremendously decreases. The thermal stability of ω-Ti(Fe) in the samples under study was lower than in the alloys, which were annealed above the eutectoid reaction. Nevertheless, the decomposition pathway via a supersaturated α-Ti(Fe) phase was also observed in the present work.

**Author Contributions:** Conceptualization, M.J.K., D.R. and B.B.S.; methodology, A.K., M.J.K. and B.B.S.; software, M.J.K. and M.R..; validation, M.J.K. and M.R.; formal analysis, M.J.K. and M.R.; investigation, M.J.K. and M.R.; resources, D.R. and H.H.; writing—original draft preparation, M.J.K. and M.R.; writing—review and editing, O.F., D.R., B.B.S., J.I.; supervision, O.F. and D.R.; funding acquisition, J.I., O.F., H.H., B.B.S. and D.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the German Research Foundation (grant numbers RA 1050/20-1, IV 98/5-1, HA 1344/32-1, FA 999/1-1), Russian Foundation for Basic Research (grant number 18-03-00067) and State tasks of the Russian federal ministry of education and science (grant number n/a), and by the Karlsruhe Nano Micro Facility.

**Acknowledgments:** The authors would like to thank Alena Gornakova from the Institute of Solid State Physics (Russian Academy of Sciences) for her assistance in the preparation of the alloys, Th. Kreschel from the Institute of Iron and Steel Technology (TU Bergakademie Freiberg) for the determination of the oxygen content in the samples by means of carrier hot gas extraction method, and Ch. Schimpf and A. Walnsch from the Institute of Materials Science (TU Bergakademie Freiberg) for conventional X-ray measurements and SEM/EBSD investigations.

**Conflicts of Interest:** The authors declare no conflict of interest.
