OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 2 2025 B2 austenite becomes the main phase, although martensitic peaks are present in small amounts (Fig. 3, b). This reverse MT is typical for the alloy in the martensitic state after large strain [40], and it is associated with an increase in dislocation density, which stabilizes the austenite [41]. The reverse transformation from martensite to austenite induced by strain was first observed in [42] and later confirmed in [43]. This transformation is observed after large plastic deformations and precedes the onset of amorphization. The authors attribute the phenomenon to a change from sliding and twinning mechanisms to rotational deformation modes. According to [40], the B2 phase is more resistant to large strain, while the B19’ phase is susceptible to disordering with the accumulation of crystalline structure defects. The reverse strain-induced MT, “martensite B19’ → austenite B2” is also observed after pulsed currentassisted rolling to a small strain (ε = 0.4) in the Ti50.0Ni50.0 alloy. The B2 phase becomes the dominant phase (Fig. 4). In addition to the reasons mentioned above, it should be noted that short-term, localized electrical heating is possible in this study. This localized heating may be sufficient to induce the “martensite B19’ → austenite B2” transformation, because the characteristic temperatures of MTs are sensitive to even small amounts of heat, and the Af temperature after rolling does not exceed 58 °C (Fig. 2, a). A further increase in strain up to ε = 0.8 in the Ti50.0Ni50.0 alloy during pulsed current-assisted rolling leads to a stress increase in the newly formed austenitic phase. The accumulation of these stresses provides a mechanism for the direct MT (austenite B2 → martensite B19’) to occur. Concurrently, there is a noticeable increase in the relative intensity of the martensitic diffraction pattern peaks, indicating an increasing proportion of the martensitic phase (Fig. 4). A subsequent increase in strain to ε = 1.4 again results in the main peak from the B2 phase becoming the most pronounced. This indicates a reverse straininduced MT from the previously formed martensite, themechanisms of which are dominated by the thermal action of the current. Thus, a cyclical martensitic transformation is observed in the Ti50.0Ni50.0 alloy during pulsed current-assisted rolling. The underlying reason is the alternating dominance of deformation mechanisms (stress increase in the austenitic phase due to strain) and heating from the pulsed current. In the Ti50.0Ni50.0 alloy, B2 austenite remained the dominant phase during pulsed current-assisted rolling, as it was in the initial quenched state (Fig. 5). There was no evidence of strain-induced martensite, which is typically observed in Ni-rich alloys during deformation [37]. Such strain-induced martensite usually results from a shift in the reverse MT temperatures to higher values (the effect of martensite stabilization by preliminary deformation [44]). A possible reason for its absence during rolling with current may be short-term, localized heating. Apparently, in this case, the thermal effect of the current dominates the mechanisms of strain-induced martensite formation, leading to high-temperature B2 austenite stabilization. A notable feature of the diffraction patterns from samples after rolling with current is the broadening of the main B2 (110) peak (Fig. 5, b, inset). This broadening is due to an increase in defects with increasing strain, as well as the presence of a pronounced peak corresponding to titanium oxides. In this case, the broadening of the main peak is logically associated with an increase in microstrains in the crystal lattice due to deformation. Oxide particles are frequently observed and studied by other authors in shape memory alloys [37, 45]. These particles enter the alloy during the smelting stage and are almost always present in Fig. 4. X-ray diffraction analysis results for Ti50.0Ni50.0 alloy after current-assisted rolling to various strain levels (ε)
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