OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 2 2025 a b Fig. 5. X-ray diffraction analysis results for Ti49.2Ni50.8 alloy after quenching from 800 °C in water (a) and currentassisted rolling to ε=0.4 (b) with an overlay of tabular data corresponding to the B2 phase (red lines) and Ti4Ni2O particles (green lines); inset: comparison of the broadening of the main B2 (110) peak after quenching (blue) and current-assisted rolling, ε=0.4 (grey) its composition. The broadening of the corresponding X-ray peak may indicate their crushing by pulsed current-assisted rolling, but the thermal effect of the current is insufficient to dissolve them. Microstructural studies using TEM reveal significant fragmentation of the initial grains after pulsed current-assisted rolling up to ε = 0.4 in the Ti49.2Ni50.8 alloy (Fig. 6, a). Deformation shear bands are observed within the structure, primarily oriented along the rolling direction. The thickness of these bands ranges from approximately 500 nm (most commonly observed) (Fig. 6, b) to 30 nm (Fig. 6, c). After straining to ε = 1.4, the microstructure exhibits a similar grain morphology but is more homogeneous. In this condition, thin shear bands (approximately 20-30 nm thick) are observed within wider (400-500 nm) deformation bands (Fig. 6, d). Fig. 7 presents the TEM results for the Ti50.0Ni50.0 alloy during pulsed current-assisted rolling up to ε = 0.8 and 1.4. These results indicate that deformation in this alloy occurs through twinning of the initial martensitic plates. The bright-field structure image after straining to ε = 0.8 is characterized by the presence of thin (20-30 nm) deformation bands. The electron diffraction pattern exhibits double reflections (Fig. 7, a). As strain increases to ε = 1.4, the deformation bands become even thinner, reaching thicknesses of less than 10 nm. The electron diffraction pattern corresponding to this state is characterized by reflections that are elongated along a circle, indicating strong lattice distortions after pulsed current-assisted rolling. The arrangement of the rings is typical for the B2 austenite phase; however, some areas exhibit martensitic reflections (reflections with similar interplanar distances near (110)) (Fig. 7, b). A comparison of the deformation process during pulsed current-assisted rolling for the TiNi-based alloys, considering their initial austenitic and martensitic structures, reveals that the Ti50.0Ni50.0 alloy undergoes more intense deformation. This observation is supported by the results of X-ray diffraction analysis. Thus, a characteristic feature of structure formation in the Ti50.0Ni50.0 alloy is the cyclical nature of the direct and reverse “martensite → austenite → martensite” transformations during pulsed currentassisted rolling. A possible explanation for this phenomenon is the alternating dominance of strain-induced martensitic transformation mechanisms and the localized influence of the thermal action of the current on the characteristic martensitic transformation temperatures. In contrast, a special characteristic of the effect of pulsed current during rolling on martensitic transformations in the Ti49.2Ni50.8 alloy is the absence of strain-induced martensite B19’ and the stabilization of the high-temperature austenitic B2 phase. These distinct features of MT manifestation can be utilized to control the structural and phase state of shape memory alloys in order to maximize functional properties such as reversible strain, recovery stress, and superelasticity.
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