3.1.1. Structure in the Coarse-Grained State

In the initial state, after quenching, the alloy at room temperature has a predominantly equiaxial structure of the B2 phase (austenite) with an average grain size of about 20 ± 5 μm (Figure 2a). Moreover, globular inclusions 0.5–1 μm in size are observed in the structure inside and at the grain boundaries. Optical microscopy (OM) and scanning electron microscopy (SEM) (Figure 3) fail to accurately assess changes in the structure after repeated thermal cycling (Figure 2b); therefore, the TEM method was used (Figure 4).

**Figure 2.** Microstructure of the Ti-50.8 at.% Ni alloy obtained by optical microscopy (OM): (**a**) in the initial coarse-grained (CG) state, (**b**) after thermal cycling with a maximum number of cycles (*n* = 250).

**Figure 3.** SEM image of the microstructure of the Ti-50.8 at.% Ni alloy in the coarse-grained state: initial state (**a**), 250 thermal cycles (**b**).

**Figure 4.** TEM images of the microstructure of the Ti-50.8 at.% Ni alloy in a coarse-grained state with different numbers of thermal cycles: (**a**) *n* = 0, (**b**) *n* = 50, (**c**) *n* = 100, (**d**) *n* = 150, (**e**) *n* = 200, (**f**) *n* = 250.

According to the obtained TEM data in the CG state without thermal cycling in the microstructure of the alloy, grain boundaries and triple-grain junctions free of dislocations are observed (Figure 4a). After thermal cycling in the temperature range MT B2→B19 with a number of cycles equal to 50, developed dislocation clusters are present that form the so-called "dislocation forest". The average grain size decreased insignificantly and became about 18 ± 3 microns in size. With an increase in the number of thermal cycles in the structure, the formation of large clusters of dislocations and dislocation walls, which are formed during phase hardening, was observed. This was first noticeable near the grain boundaries (Figure 4b). In this case, there is a broadening of the extinction contours, which is also associated with an increase in the level of internal stresses and distortions of the crystal lattice. The average grain size at the maximum number of cycles compared with the initial value decreased by about 45% (the assessment was carried out according to OM and SEM). Thermal cycling with the maximum number of thermal cycles preserves the dislocation structure in the form of clusters and irregular walls and dislocation tangles (Figure 4f).

Figure 5 presents structures after *n* = 200 and *n* = 250 cycles with fields with extinction contours, the width of which reaches 150 ± 20 nm, which may indicate a high density of defects accumulated during multiple martensitic transformations.

**Figure 5.** Microstructures of the Ti-50.8 at.% Ni alloy in a coarse-grained state after thermal cycling with *n* = 200 (**a**) and *n* = 250 (**b**) with fields of the extinction contours.

Figure 6 shows a graph of the average grain size versus the number of cycles.

**Figure 6.** Graph of changes in the average grain size with an increase in thermal cycles in the CG state.

In addition, starting from 100 cycles of martensitic transformation, sections of the structure are observed in which packets of martensitic plates are visible (plate thickness in the range of 50–300 nm). Present in individual plates at high magnifications were composite (001) B19' nanotwins of type I with a width of up to 5 nm. The formation of nanotwins is probably associated with saturation of the structure after a certain number of cycles.
