3.1.2. Microstructure in the Ultrafine-Grained State

After applying the ECAP method, the transformation of the initial coarse-grained structure into an inhomogeneous grain–subgrain ultrafine-grained structure with an increased dislocation density was observed (Figure 7). The microdiffraction pattern corresponds to an ultrafine-grained structure with the presence of diffuse cords.

**Figure 7.** Microstructure of the Ti-50.8 at.% Ni alloy in the ultrafine-grained (UFG) state: (**a**) optical microscopy (OM), (**b**) scanning electron microscopy (SEM), (**c**) bright-field image, (**d**) dark-field image.

As the number of martensitic transformation cycles increases, grains are observed in the structure with predominantly nonequilibrium boundaries, which may indicate a highly defective structure (Figure 8). Nevertheless, complex dislocation structures formed in the grains—various accumulations and tangles—and the density of dislocations increased. The average size of structural elements in the UFG state without thermal cycling was 320 ± 15 nm, which decreased to 260 ± 20 nm after maximum thermal cycling (Figure 9).

**Figure 8.** *Cont*.

**Figure 8.** TEM images of the microstructure of the Ti-50.8 at.% Ni alloy in the UFG state: (**a**) *n* = 100, (**b**) *n* = 100 with nanotwins, (**c**) *n* = 150, (**d**) *n* = 200, (**e**) *n* = 250, (**f**) *n* = 250 with large magnification.

**Figure 9.** Changes in the average grain size with an increase in thermal cycles in the UFG state.
