**5. Conclusions**

Metastable β titanium Ti15Mo alloy was prepared by HPT and subsequently aged at 400 ◦C and 500 ◦C. Phase transformations were observed by XRD and SEM. The following conclusions can be drawn from this study:

Precipitation of the α phase in the HPT-deformed material is significantly enhanced by the high density of lattice defects such as dislocations or grain boundaries, which act as preferential nucleation sites for α phase precipitation.

*Metals* **2019**, *9*, 1194

α phase particles in the non-deformed material precipitate in the form of lamellae, while in the HPT-deformed material, α precipitates are small, equiaxed, and polygonal in shape suggesting that they all formed as grain boundary α at the β grains triple junctions.

Two-phase α + β microstructure continuously coarsens with increasing temperature and time of ageing.

Deformation by HPT significantly increases the microhardness due to microstructural refinement, but also due to the formation of ω phase.

During annealing, the microhardness of the non-deformed material is governed mainly by the evolution of ω phase. In the HPT-deformed material, the main strengthening mechanism is the precipitation of fine α phase particles.

**Author Contributions:** K.B. conducted SEM experiments, interpreted the results, and wrote the majority of the manuscript. J.S. and M.J. discussed achieved results and wrote parts of the manuscript. A.V. cooperated on SEM observations and microhardness measurements. P.S. interpreted achieved mechanical properties. P.B.-V. conducted HEXRD experiment. F.L. and P.D. conducted laboratory XRD experiments. V.P. and I.S. prepared material by HPT.

**Funding:** This work was financially supported by the Czech Science Foundation under the project 17-04871S and by ERDF, project No. CZ.02.1.01/0.0/0.0/15\_003/0000485. The Deutsches Elektronen-Synchrotron (DESY) is acknowledged for the provision of synchrotron radiation facilities in the framework of the proposal I-20150533 EC. Partial financial support by the Czech Ministry of Education, Youth and Sports under the project LTARF18010 is also gratefully acknowledged.

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