Martensitic transformations in TiNi-based alloys during rolling with pulsed current

Introduction. Shape memory alloys based on TiNi possess a set of properties, including biocompatibility, corrosion resistance, low density, high specific strength, thermal stability, shape memory effect, and superelasticity. A significant number of studies are currently dedicated to various deformation methods of processing such materials, aiming to enhance their mechanical properties and shape memory characteristics. One such method is plastic deformation with the simultaneous application of pulsed current. Since the shape memory properties in TiNi-based alloys are due to the presence of thermoelastic martensitic transformations, the combined effect of deformation and current on these transformations is of particular interest. The purpose of this work is to investigate the characteristics of thermal and deformation-induced martensitic transformations in Ti50.0Ni50.0 and Ti49.2Ni50.8 alloys during rolling with simultaneous application of pulsed current. Research methods. The paper analyzes samples of Ti50.0Ni50.0 and Ti49.2Ni50.8 alloys after rolling with pulsed current at a density of 100 A/mm², a pulse duration of 100 μs, a pulse ratio of 10 to various strain levels (ε=0; 0.4; 0.8; 1.2). The study of the staging of martensitic transformations was carried out using differential scanning calorimetry at a heating/cooling rate of 10 °C/min in the temperature range of −150 to +150 °C. The phase composition was studied by X-ray diffraction analysis using Cu-Kα radiation at U=40 kV and I=40 mA in the angular range of 2θ=15 to 100 ° with a step size of Δθ = 0.05° and an exposure time of 5 s. Results and discussion. The results show that current-assisted rolling leads to the manifestation of a two-stage direct martensitic transformation during cooling in both alloys. Furthermore, increasing the strain level broadens the temperature range of the R-phase existence. The possibility of stabilizing the high-temperature austenitic B2 phase in the Ti49.2Ni50.8 alloy, as well as the emergence of a cyclically occurring deformation-induced “martensite-austenite-martensite” transformation in the Ti50.0Ni50.0 alloy, are demonstrated. Possible mechanisms for these features are discussed.