OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 3 2022 Fig. 1. Shape and dimensions of TiNiHf alloy samples: a – ingot; b – strip for rolling a b The current density j varied from 200 A/mm2 at the beginning of the process to 580 A/mm2 at the fi nal passes with a pulse duration of 200 μs and a frequency of 500 Hz. At smaller values of current density, the deformation behavior of the alloy did not differ from that during rolling without current, and fracture occurred already after the fi rst passes. The true strain е was calculated by the equation е = ln S0 / Sf (where S0, Sf are the cross-sectional area of the strip before and after rolling). Themicrostructurewas studied using a Versamet-2 Union optical microscope with a magnifi cation of 50 to 100. Samples for light microscopy were ground on abrasive paper with a grain size of P120 to P2,500, followed by polishing. After mechanical grinding and polishing, the samples were etched in following solutions: 1HF:3HNO3:6H2O2. The deformation hardening of the alloy was determined via Vickers hardness tests. The tests were carried out at room temperature on a LECOM 400-A hardness tester under a load of 1 N with an exposure of the indenter for 10 s. Results and discussion In the present paper a method for processing hard-to-deform brittle TiNiHf alloys with a reduced Ni content by cold rolling with a pulsed current was applied and studied for the fi rst time. Phase Composition and Microstructure The X-ray diffraction pattern of the alloy in the initial state at room temperature is shown in Fig. 3. The lines of martensite and (Ti, Hf)2 Ni phase are confi dently indicated on the X-ray diffraction pattern. The absence of visible lines of the high-temperature phase – austenite – confi rms that the temperature of the beginning of the reverse martensitic transformation exceeds 25 °C. A weak broadening of the X-ray lines indicates a low crystal lattice defi ciency, which is typical for of a recrystallized structure. Thus, based on the results of X-ray phase analysis, it can be concluded that the embrittling (Ti, Hf)2 Ni phase is contained in a signifi cant amount in the initial sample. The microstructure of the TiNiHf alloy in the initial state and after rolling with a current up to a strip with a thickness of 0.6 mm is shown in Fig. 4. In the initial state, TiNiHf alloy has a recrystallized structure Fig. 2. Scheme of current supply and strain direction: 1 – work materials; 2 – cylindrical rolls; 3 – force direction; 4 – current direction
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