**2. Materials and Methods**

Ti-15Mo powder was prepared by gas atomization and supplied by TLS Technik GmbH and Co. Spezialpulver KG, Bitterfeld-Wolfen, Germany.

The milling was performed for 4 h at 700 revolutions per minute (RPM) milling speed in Union Process 01-HD (Union Process, Akron, OH, USA) attritor (1400 cm3) device in liquid argon. Stainless steel balls of a diameter 6.35 mm and ball to powder ratio (BPR) 16:1 were used. 1.8 g of stearic acid was added into 180 g of Ti-15Mo powder as a process control agent to prevent cold welding.

Both gas atomized (initial) and cryo-milled (referred to as milled) powders were compacted by a spark plasma sintering (SPS) device by FCT Systeme GmbH (Rauenstein, Germany). The sintering was performed for 3 min in the temperature range from 750 ◦C to 850 ◦C. The temperature was measured by a thermocouple inserted into the graphic die 4 mm from the sintered sample. The powder was compressed by a pressure of 80 GPa and heated 50 ◦C below the desired temperature always in one minute (heating rates were from 700 ◦C/min to 800 ◦C/min depending on the sintering temperature). The sample was subsequently heated up to the desired sintering temperature with the heating rate of 100 ◦C/min. These two steps were designed to avoid temperature overshooting. The example of the temperature and pressure evolution during sintering at the temperature of 750 ◦C is shown in Figure 1. The cooling was not actively controlled.

**Figure 1.** An example of the temperature and pressure evolution during the sintering process.

Scanning electron microscope (SEM) observations were performed with the scanning electron microscope FEI Quanta 200F (FEI, Hillsboro, OR, USA). For this purpose, both initial and milled powders were simply stuck on a conductive foil. Bulk samples for SEM microstructural observations were prepared by standard mechanical grinding and polishing followed by the three step vibratory polishing. Fracture surface after tensile tests were also observed.

The fraction of the α-phase and porosity of samples were determined by image analysis in ImageJ (version 1.52r, Wayne Rasband, Research Services Branch, National Institute of Mental Health, Bethesda, MD, USA). For this purpose, 10 micrographs of size of about 1000 μm2 from each sample were made by a scanning electron microscope FEI Quanta 200F operated at 10 kV and at 5 kV in case of sintered initial powder and sintered milled powder, respectively. The fraction of the α-phase was determined from electron back-scatter (BSE) micrographs. Porosity of samples sintered from initial powder was determined by image analysis from secondary electron (SE) micrographs. The very low overall porosity of sintered milled powder disallowed its determination by image analysis. The Archimedes method was used instead. The error of this measurement is given only by the precision of the sample weighing, which is 0.5 mg. For comparison, the porosity of the initial powder sintered at 800 ◦C was measured by both methods. It was found that image analysis underestimates the overall porosity and Archimedes method proved to be more reliable. Porosity of sintered initial powder was therefore calibrated according to the porosity of the initial powder sintered at 800 ◦C.

Contamination of material by oxygen was determined in powders as well as in selected sintered samples by carrier gas hot extraction (CGHE). The microhardness was measured by the Vickers method (0.5 kgf load, 30 indents per sample) using Qness Q10a (Qness, Golling, Austria) instrument with automatic evaluation of the measurement.

X-ray diffraction (XRD) measurements were performed on a Bruker D8 Advanced diffractometer (Bruker AXS, Karlsruhe, Germany) in Bragg–Brentano geometry using Cu K<sup>α</sup> radiation, λ = 1.54051 Å, variable divergence slits and a Sol-X detector. Bragg–Brentano geometry is an arrangement in which the incident and diffracted beams are focused on a circle with the measured sample in the middle. Diffraction patterns were collected at room temperature in the 2 range from 30◦ to 130◦ with a step size of 0.02◦ and an exposure time of 5 s/step. The patterns were fitted and refined employing le Bail algorithm with a pseudo-Voigt profile using program Jana2006 (V. Petrícek, M. Dušek and L. Palatinus, ˇ Institute of Physics Academy of Sciences, Prague, Czech Republic).

The position of the flat dog-bone shaped tensile sample in the sintered tablet is shown in Figure 2. The sample was 20 mm long and its gauge length was 5 mm. The total width was 7 mm while the gauge length and thickness were 1.2 mm and 1 mm, respectively. The diameter of the hole for the pin was 2.3 mm. The "bricks" in Figure 2 symbolize arrangement of flat milled powder particles in the tensile sample caused by their stacking in the sintering die [24]. Tensile tests were performed at Instron 5882 machine. The constant crosshead velocity was 0.03 mm/s, resulting in the initial strain rate of . ε = 10<sup>−</sup>4s−1.

**Figure 2.** A position of tensile sample in a sintered tablet/cylinder (the green line is described in a discussion of tensile tests below).
