3.2.1. The Evolution of Microstructure during Ageing

SEM–BSE micrographs in Figure 4 show the evolution of the microstructures of the non-deformed material after ageing at 400 ◦C and 500 ◦C for 1–16 h. The microstructure consists of coarse-grained β matrix. At least one triple-junction is shown in each image. After ageing at 400 ◦C for 1 and 4 h, only β matrix is observed. In the specimen aged for a longer time (400 ◦C/16 h), nanometer-sized precipitates are seen in the SEM–BSE micrograph (note the higher magnification of this micrograph). These small ellipsoidal particles are particles of ω phase, which are visible due to chemical partitioning—ω phase particles are slightly Mo depleted [29]. Ageing at the higher temperature of 500 ◦C resulted in a precipitation of continuous and coarse α phase along grain boundaries (hereafter referred to as grain boundary α or GB α), which is also Mo depleted and appears as a long dark particle along the former β/β boundary (indicated by a yellow arrow). In the vicinity of the β grain boundaries, α phase particles with a typical lamellar morphology precipitated. Small ellipsoidal particles in the grain interior belong to ω phase. The contrast of these particles increases with the increasing ageing time due to ongoing chemical partitioning. After ageing at 500 ◦C for 16 h, tiny ellipsoidal ω particles are clearly seen in grain interiors, GB α is visible along the former β/β grain boundaries and α lamellae span from the GB α to the grain interiors. In conclusion, ω particles with ellipsoidal morphology can be observed by SEM in the grain interior after ageing at 500 ◦C and the fraction of α phase particles with lamellar morphology increases with increasing time of ageing at 500 ◦C. The coexistence of all three β, α, and ω phases is observed.

Figure 5 shows the microstructures of the HPT-deformed samples after ageing. Already after ageing at 400 ◦C/1 h, significant differences between the non-deformed and HPT-deformed specimens can be observed. In the non-deformed material, there is no evidence of α phase particles. On the other hand, small and equiaxed α particles already precipitated in HPT sample. In specimens aged at 400 ◦C for longer times of 4 h and 16 h, the volume fraction of the α phase increased and, simultaneously, α precipitates coarsened. Moreover, the precipitation is not homogeneous—some areas contain clearly more α phase particles. Ageing at 500 ◦C resulted in the formation of larger α particles, which are generally equiaxed, but not round—rather polygonal and sharp edged.

**Figure 4.** SEM–BSE micrographs of the non-deformed samples after ageing.

**Figure 5.** SEM–BSE micrographs of the samples deformed by high-pressure torsion (HPT-deformed samples) after ageing.

### 3.2.2. Evolution of Phase Composition during Ageing

The phase composition of the non-deformed and HPT-deformed samples before and after ageing is shown in laboratory XRD patterns in Figures 6 and 7, respectively. Both the measured (thin black curves) and fitted (colored curves) XRD patterns are shown. The interplanar distance is displayed on the horizontal axis for the comparison to the HEXRD data, while the y-axis shows the intensity in a logarithmic scale, allowing one to distinguish small peaks. The most important peaks, which best describe the evolution of emerging phases, are marked with arrows—full and open arrows for α and ω phase, respectively. A quantitative determination of phase content is not possible. The non-deformed specimen contains large grains with the size of hundreds of micrometers while HPT-deformed material is severely plastically deformed with high dislocation density and high internal stress resulted in the broadening of XRD peaks. Moreover, in both conditions, the grains have a preferred orientation as can be revealed from the relative intensity of the peaks. Therefore, laboratory XRD patterns could not be successfully fitted by any other method than the simple LeBail approach. However, several qualitative comparisons can be made.

**Figure 6.** X-ray diffraction (XRD) patterns (in log-scale) of aged conditions of the non-deformed Ti15Mo alloy. Black thin curves correspond to data, colored curves are numerical fits. Non-deformed without ageing (red curve) and aged conditions (other colored curves) are displayed. The patterns are vertically shifted for clarity. The most important peaks are marked by full and open arrows for α and ω phase, respectively. Two unfitted peaks around *dhkl* = 1.9 originated from the sample holder.

The non-deformed Ti15Mo alloy contains a mixture of β and ω phases. However, the identification of the ω phase content is difficult due to overlapping peaks of β and ω peaks. Nevertheless, ω peaks (11–22)<sup>ω</sup> and (11–21)<sup>ω</sup> can be observed at the inter planar distances dhkl ≈ 1.2 Å and dhkl ≈ 1.8 Å, respectively, as shown in Figure 6. Ageing of the non-deformed specimen at 400 ◦C/1 h (red curve in Figure 8) resulted in an increase of the intensity and narrowing of ω peaks (open arrows). In addition, small α phase peaks are also visible (full black arrows). XRD patterns of the non-deformed material aged at 400 ◦C for 1, 4, and 16 h are very similar. In the specimen aged at 500 ◦C for 1 h, the α phase is clearly present (full arrows in Figure 6). Moreover, its volume fraction increases with increasing ageing time (4–16 h), as also confirmed by SEM observations (cf. Figure 5); ω phase is still present in the specimen aged 500 ◦C even for the longest time of 16 h reported in [29].

**Figure 7.** XRD patterns of aged conditions of the HPT-deformed Ti15Mo alloy: HPT-deformed without ageing (black curve) and aged under different conditions (colored curves) are displayed. The patterns are vertically shifted for clarity. The most important peaks are marked by full and open arrows for α and ω phase, respectively.

**Figure 8.** High-energy synchrotron X-ray diffraction (HEXRD) pattern of the HPT-deformed sample.

The XRD pattern of the HPT deformed specimen exhibits significantly broadened peaks due to enhanced dislocation density and reduced crystallite size in this specimen. Moreover, the peaks are slightly shifted to different values of interplanar distances due to residual stresses in the deformed material—the direction of the shift depends on the type of the residual stress [30].

The ω phase content in the HPT sample seems to be inferior to that in the non-deformed sample; only a tiny peak can be resolved at the interplanar distance *dhkl* ≈ 1.2 Å. However, the most intensive peaks of the ω phase coincide with the peaks of the β phase.

In order to obtain more precise information about volume fraction of individual phases, HEXRD measurement was carried out on the HPT-deformed sample. In contrast to the laboratory XRD, HEXRD provides a better signal-to-noise ratio and the simultaneous measurement of the scattering signal in various directions due to the use of a 2D detector and subsequent azimuthal averaging. Consequently, a better resolution of small peaks, namely those of the ω phase, is achieved. Figure 8 shows the HEXRD pattern of the HPT specimen. Both the measured and fitted intensities as well as the difference curve between the fitted and measured intensity are shown in Figure 8. The results indicate that the HPT-deformed alloy is a two-phase material with volume fractions of the β and ω phase of 72% and 28%, respectively (the error of the volume fractions estimation is approximately ±5%).

In order to get more accurate results, the selected HPT-deformed specimen after ageing at 400 ◦C/1 h was examined using HEXRD. Both the measured and fitted intensity and the difference between the fitted and measured data are displayed in Figure 9.

**Figure 9.** HEXRD pattern of the HPT-deformed sample after aging 400 ◦C/1 h.

The volume fractions of individual phases in HPT-deformed sample before and after ageing at 400 ◦C/1 h are summarized in Table 2. The non-aged HPT-deformed material contains a high-volume fraction of the ω phase (28%). After ageing, the volume fraction decreases to approximately 9%. This is caused by enhanced volume fraction of the α phase, which reaches 23% in the aged condition.

**Table 2.** Volume fraction of individual phases in HPT-deformed Ti15Mo alloy as determined from high energy synchrotron X-ray diffraction (HEXRD). The experimental errors are also shown.

