OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 3 2022 Deformability and Hardness Experimental results have revealed that after rolling of fl at samples without current (Fig. 5, a) or with current density j < 200 A/mm2 (Fig. 3b), the TiNiHf alloy fractures brittle already after the fi rst 3-4 passes (e ≤ 0.07) and without the formation of edge defects (Fig. 5 a, b). In most cases, the sample is divided into several parts. It should be noted that the thickness reduction does not exceed 5 %. The deformability increases with the increase of the current density j ≥ 200 A/mm2 allowing to preserve the entity of the sample (Fig. 5, c, d, e). The microfracture always starts from the side edges of the sample, which increases but does not lead to macrofracture (Fig. 6). The edge microcracks formed during the rolling process due to the concentration of predominantly tensile stresses during the transition from the bulk state in the original sample to the plane-stressed state in a thin sample. Obviously, the application of a pulsed current during the rolling process inhibits the formation and propagation of cracks. а b c d Fig. 5. Appearance of samples during rolling without current (a) and with current (b, c, d, e) at true deformation: a–е = 0; b–е =0.07; c–е = 0.39; d–е = 0.85; f–е = 1.47 e The change in the dimensions of the sample cross section, as well as the hardness value, engineering and true strain during the rolling process of the sample with a pulsed current are presented in Table 2. The obtained results and analysis of the hardness value after rolling showed that an increase in the accumulated strain after rolling with current leads to almost linearly increase in the hardness value (Table 2). It can be assumed that the strengthening is a consequence of several factors: an increase in the volume fraction of martensite due to the transformation of residual austenite during deformation; changes in the starting temperature of the forward martensitic transformation Ms as compared to the measurement temperature (20 °C); an increase in the dislocation density and substructural refi nement and an increase in the number of intermetallic particles at the grain boundaries. The nature of deformation hardening and the absence of the signs of recrystallization also indicate the minimum thermal effects in the process of rolling with current. Fig. 6. Stereomicroscopic image of the sample rolled with current, j = 580 A/mm2, e = 1.47
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