*3.2. X-ray Analysis of the TiNi Alloy*

To determine the effect of multiple martensitic transformations on the structural characteristics of the Ti-50.8 at.% Ni alloy, an X-ray diffraction analysis was performed at room temperature. X-ray diffraction patterns of the alloy indicate that the main phase in the CG state is B2-austenite (Figure 10). After TC with the maximum number of cycles, the X-ray diffraction pattern indicates a change in the phase composition; instead of peaks on the B2 phase, a doublet peak of phase B19 is observed. A slight broadening of all peaks and a decrease in their intensity were observed (Figure 10). This can be explained by structural changes in the alloy—an increase in the density of dislocations accumulating during multiple cycles and an increase in internal microdistortions.

**Figure 10.** XRD patterns of the Ti-50.8 at.% Ni alloy in the coarse-grained state before thermal cycling and after maximum thermal cycling.

The X-ray diffraction pattern of the alloy in the UFG state also corresponds to the B2 phase of austenite (Figure 11). Moreover, there were more lines in this state than in the coarse-grained state. After TC, there was a broadening of the main peaks (Figure 11), which is caused by distortions of the crystal lattice and large values of microdistortions. However, there was no change in the phase composition of the alloy. The presence of only the B2 phase in the X-ray diffraction pattern in the ultrafine-grained state after the maximum number of cycles can be explained by the fact that, in this state, the structure is more homogeneous and the reverse transformation proceeds with the formation of an intermediate R phase with lower transformation energy. This all contributes to the reverse martensitic transformation, and in the UFG state it completely ends. In the case of the coarse-grained state, the structure is more heterogeneous, which affects the heterogeneity of the martensitic transformation; therefore, part of the material may contain a martensitic phase, the presence of which is recorded by X-ray diffraction analysis.

**Figure 11.** XRD patterns of the Ti-50.8 at.% Ni alloy in the ultrafine-grained state before thermal cycling and after maximum thermal cycling.

Based on the obtained X-ray diffraction data, the following structural parameters were calculated: coherent scattering regions (CSRs), lattice parameter (a), magnitude of the root-mean-square microdistortions of the crystal lattice (<ε2>1/2), and dislocation density (ρ) [26]. The results are

shown in Table 2. An analysis of the structural parameters showed that in both states there is a decrease in the CSR values, an increase in internal microdistortions, and an increase in the dislocation density related to them. The dislocation density increased more in the ultrafine-grained state than in the coarse-grained state, which suggests that a higher density of grain boundaries and a smaller grain size contribute to the intensity of defect accumulation.


**Table 2.** Structural parameters of the Ti-50.8 at.% Ni alloy.

Δ = Parameter difference between the initial state of the alloy and the state after TC.
