OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 2 2025 Characteristic temperatures of martensitic transformations Alloy Initial processing Direct martensitic transformation C2→B19ˊ Reverse martensitic transformation B19ˊ →C2 Phase composition at room temperature Ms, °С Mf, °С As, °С Af, °С Ti49.2Ni50.8 Quenching (water) 800 ºС (1 hour) −5 ̊ −37 −5 17 austenite C2 Ti50.0Ni50.0 45 25 58 77 martensite C19ˊ Ms, °С – the direct martensitic transformation (B2→B19’) start temperature; Mf, °С – the direct martensitic transformation finish temperature; As, °С – the reverse martensitic transformation (B2→B19’) start temperature; Af, °С – the reverse martensitic transformation finish temperature. samples were cooled in water to avoid additional current heating. Post-deformation annealing, when necessary, was carried out at 450 °C for 1 hour after rolling. Thermal martensitic transformation temperatures were studied by differential scanning calorimetry (DSC) using a Mettler Toledo 822e apparatus. Calorimetric curves were obtained in the temperature range from −150 to 150 °C with a heating/cooling rate of 10 °C/min. Strain-induced martensitic transformations were studied by performing phase analysis on the samples after rolling with current to different strains. X-ray diffraction phase analysis was performed using an ARL X’TRA X-ray diffractometer (Switzerland) with Cu Kα radiation in the angle range 2θ = 15-100° with a step size of Δθ = 0.05° and an exposure time of 5 s at a voltage U = 40 kV and a current I = 40 mA. Qualitative phase assessment was carried out using the WinXRD computer software package (ARL X’tra software) by comparing it with the database of the International Centre for Diffraction Data (ICDD) PDF-2 [36]. Structural studies after rolling were carried out by transmission electron microscopy (TEM) using a high-resolution JEM 2100 microscope from JEOL (Japan) at a maximum accelerating voltage of 200 kV. Results and discussion The calorimetric studies of the Ti50.0Ni50.0 alloy after pulsed current-assisted rolling and annealing at 450 ℃ showed the presence of a two-stage MT with an intermediate R-phase (Fig. 2, a). This phase is common in nickel- enriched alloys [37], but some authors observe it in Ti50.0Ni50.0 alloys and attribute its presence to high internal stresses, for example, after thermal cycling [38] or plastic deformation [39]. In the Ti49.2Ni50.8 alloy, the R-phase is observed immediately after annealing in the undeformed state, and it is associated with the presence of Ti3Ni4 particles [37]. While there is no shift in the B2→R MT start temperatures, a shift in the R→B19’ start temperature is noticeable (Fig. 2, b). This effect, where the pulsed current expands the R-phase temperature range, is also observed when comparing MTs with the initial undeformed state. Acharacteristic feature of TiNi-based alloys is that MTs manifest not only during cooling and heating but also during deformation [37]. According to X-ray diffraction analysis, all peaks in the diffraction pattern of the Ti50.0Ni50.0 alloy in the initial quenched state correspond to the B19’ martensitic phase with a monoclinic lattice (Fig. 3, a). Cold rolling without current leads to a reverse martensitic transformation. Consequently, Fig. 1. Schematic of current supply circuit: 1 – mill rolls; 2 – cylindrical sample; 3 – feed table (sliding contact); 4 – pulsed current source; 5 – current lines
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