**3. Results**

#### *3.1. Microstructure of the Oxygen Hardened Alloy*

LM and SEM images of the alloy treated at 700 ◦C, 850 ◦C and 1000 ◦C are shown in Figure 3. The alloy microstructure was dependent on the hardening temperature. The differences in the thickness of the oxygen-rich hardened zone and the grain size of the bulk of the material were observed. The highest thickness of the hardened zone, up to 470 μm, was found in the sample treated at 1000 ◦C (Figure 3e,f). In the case of alloy treated at 850 ◦C, the hardened zone thickness was about 160 μm (Figure 3c,d), while in the alloy treated at 700 ◦C it had the lowest thickness, up to 120 μm (Figure 3a,b). A coarsening of the α' laths in the oxygen-hardened zone with increasing treatment temperature was also observed. The voids in the hardened zone of the sample treated at 1000 ◦C were formed at a distance of ~20–30 μm from the surface. It should be noted that the presence of pores usually contributes to the formation of microcracks, which were also sporadically observed in this sample. A typical microcrack is marked with an arrow in Figure 3f. The observed microstructural defects exclude the sample hardened at 1000 ◦C from the intended biological applications.

**Figure 3.** Microstructure of the Ti–13Nb–13Zr alloy after oxygen hardening at 700 ◦C (**a**,**b**), 850 ◦C (**c**,**d**) and 1000 ◦C (**e**,**f**). LM (**a**) and SEM (**b**–**f**), cross-section samples. The zone with voids is marked with a dashed line in Figure 3f. An arrow in Figure 3f indicates a microcrack developed in the near-surface zone.

Investigation of the cross-section showed a significant grain growth resulting from the hardening process. The grain size estimated from LM and SEM images was in the range of 40–100 μm, 100–250 μm and up to 700 μm for the alloys treated at 700 ◦C, 850 ◦C and 1000 ◦C, respectively.

The sample treated at 700 ◦C was selected for a detailed microstructure characterization by TEM and STEM. The TEM and STEM images are shown in Figures 4 and 5. In the near-surface region (depth up to 10 μm from the surface), a high fraction of the Ti α'

(O) solid solution and low amount of fine laths of the Ti α" (O) solid solution in β phase were found (Figure 4). In the SAED pattern no. 2, the diffraction spots from crystal planes belonging to particular three [100] Tiα" zone axes corresponding to three sets of the Ti α" laths inclined by the angle of 60◦ are marked with black, green and red color, respectively.

**Figure 4.** TEM image of the near-surface region in the Ti–13Nb–13Zr alloy after oxygen hardening at 700 ◦C. SAED patterns of α' (hcp) and α" (orthorhombic, Cmcm) were taken from areas marked with 1 and 2, respectively. In the SAED pattern no. 2, the spots belonging to the three [100] α" zone axes are marked with black, green and red color. Indices of green spots are given.

**Figure 5.** STEM image of the microstructure of the near-surface region in the Ti–13Nb–13Zr alloy cross-section after oxygen hardening at a temperature of 700 ◦C (**a**) and concentration profile of oxygen obtained by TEM-EDS microanalysis performed in points 1–16 (**b**). The exemplary grains of the α' and β phases, as well as the areas 1 and 2 given in Figure 4, are marked in Figure 5a.

To examine the concentration profile of oxygen in the near-surface region, a TEM-EDS microanalysis was performed at 16 points located in the α' phase at a distance from 0.3 μm to 8 μm. The oxygen concentration profile is presented in Figure 5b. The results confirmed the increased content of oxygen in the zone close to the surface. At a point located 0.3 μm from the outer surface edge of the lamellae, the oxygen concentration was about 47 at.%, and with the increasing distance to 6 μm, it dropped gradually to about

6 at.%. At greater depths in the sample, the oxygen concentration in the α' phase remained constant at ~6 at.%. The obtained results of the oxygen distribution should be treated as approximate values since EDS microanalysis gives only a rough indication of light elements concentration. Nevertheless, the result is an indication that the Ti α' (O) solid solution is enriched in oxygen in the near-surface zone at a depth of up to 6 μm.

In our previous study [33], we showed that the near-surface region of the oxygen diffusion hardened two-phase (α + β) Ti–6Al–4V alloy consisted of Ti α (O) solid solution enriched with oxygen mainly. Oxygen is a strong interstitial solid solution strengthening element of titanium [36]. It is an α phase stabilizer and has a high solubility in the hcp α phase, up to 31.9 at.%. The solubility of oxygen in the β phase is much lower, maximum 8 at.% [41]. Therefore, the presence of the Ti α' (O) phase in the near-surface zone is preferred due to the diffusion of interstitial oxygen atoms.

According to [39], the plasma glow discharge strengthens the oxygen diffusion into the metallic substrate. It is likely due to an increase in the number of point defects formed during the first stage of the process. In addition, the plasma glow discharge inhibits the formation of the rutile layer on the titanium alloy surface. The oxides formed on the alloy surface act as limited reservoirs of oxygen atoms, which are then forced to diffuse into the alloy matrix and form a solid solution [32]. Therefore, the surface of the alloy investigated in this work was not covered by titanium oxide.
