**2. Materials and Methods**

Ti15Mo (wt%) alloy was supplied by Carpenter Co. (Richmond, VA, USA) in the form of a rod with the diameter of 10 mm. The as-received material was solution treated (ST) in an inert Ar atmosphere at the temperature of 810 ◦C for 4 h and subsequently water quenched. The cylindrical samples of the length of 5 mm were first cut from the rod and pressed with 6 GPa to achieve disk-shaped samples with the diameter of 20 mm and the thickness of approximately 1 mm. Disk samples were subsequently subjected to HPT deformation at room temperature at Ufa State Aviation Technical University (USATU), Russian Federation. Note that this pre-HPT deformation induced deformation of about 75% is significantly lower than the actual HPT deformation. The detailed description of the HPT method can be found elsewhere [5]. For this study, samples after *N* = 1 HPT rotation were prepared.

ST and HPT deformed samples (hereafter referred to as non-deformed and HPT-deformed, respectively) were aged at temperatures of 400 ◦C and 500 ◦C for 1, 4, and 16 h. The ageing was performed by immersing the samples to preheated salt-bath (i.e., with very high heating rate) without air access and subsequently water quenched. HPT samples for scanning electron microscopy (SEM) and microhardness study were prepared from the periphery part of the disks (>5 mm from the center) where the imposed strain is maximum. For XRD measurements, a quarter of a disk-shaped sample

was used. All samples were mechanically grinded and polished by standard methods followed by a three-step vibratory polishing.

The microstructure of the specimens was observed using SEM Zeiss Auriga Compact Cross Beam (Jena, Germany) equipped with the energy dispersive spectroscopy (EDS) detector operated at 4 kV. K-line and L-line for Ti and Mo, respectively, were used for quantification of EDS data.

XRD measurement of the non-deformed Ti15Mo alloy was performed employing a Bruker D8 Advance powder X-ray diffractometer using Cu K<sup>α</sup> radiation (Bruker AXS GmbH, Karlsruhe, Germany), with a variable divergent slit and a Sol X detector. The width of the beam was 6 mm and the sample was rotated, allowing it to probe the whole specimen surface. XRD measurements of the HPT-deformed Ti15Mo alloy were carried out on Bruker D8 Discover powder X-ray diffractometer. Vertical Bragg-Brentano geometry (2.5◦ Soller slits in both primary and secondary beam and 0.24◦ divergence slit) with filtered Cu K<sup>α</sup> radiation was used. Beam size of 20 mm × 5–15 mm (depending on the θ angle) was used. Note that the beam size is comparable to the size of the HPT disk. Diffraction patterns in both cases were collected at room temperature in the *2*θ range from 30◦ to 130◦ and were analyzed using LeBail approach in the program Jana2006 (Václav Petrícek, Michal Dušek and Luk ˇ áš Palatinus, Institute of Physics Academy of Sciences, Prague, Czech Republic).

The HEXRD measurement was carried out at the P07-HEMS beamline of PETRA III (Deutsches Elektronen-Synchrotron, Hamburg, Germany) [27] using the energy of 100 keV (λ = 0.124 Å) in a transmission mode perpendicular to the HPT surface. Patterns of entire Debye–Scherrer rings were acquired ex-situ from the bulk of Ti15Mo samples at room temperature. A PerkinElmer XRD 1621 image plate detector was used. The samples were kept fixed during the acquisition and measured 5 mm from the center of the sample with an incident beam of slit size of 1 <sup>×</sup> 1 mm2. The acquired diffraction patterns were processed by Rietveld structural refinement using FullProf software (Juan Rodriguez Carvajal, ILL Grenoble, France). Azimuthal averaging over 360◦ was performed first. HEXRD diffractograms after azimuthal averaging were treated by the March–Dollase approach in order to obtain at least rough estimates of volume fractions of individual phases. In fact, β phase peaks, which are the most intensive, could be fitted without the March–Dollase 'adjustment'. Nevertheless, diffractions of evolving α and ω phases had to be treated by the March–Dollase approach to achieve reasonable agreement with the measured data [28]. Even with the use of the March–Dollase approach the resulting R-factors of the fit accuracy range between 15–20 for α and ω phases, while R-factor for β phase is around 10.

The microhardness of the samples was measured using the Vickers method with the use of microhardness tester Qness Q10a (Golling, Austria). Note that all samples from HPT disks were cut from the area at least 5 mm distant from the center where microhardness of HPT *N* = 1 condition is saturated [8]. For each specimen 0.5 kg load and the dwell time of 10 s were applied. At least 20 indents were evaluated for each sample in order to get satisfactory statistical results.
