OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 2 2025 (Fig. 6, d1, d3), whereas, in the subsurface layer, the shear texture B/ B ̅is detected (Fig. 6, d2, d4). In the direction from the center to the edge, the volume fraction of grains with the <001>-orientation decreases from 37 % to 4.4 %, and the fraction of grains with the <111>-orientation decreases from 48 % to 31 % (Fig. 6, c1–c4). Analysis of the obtained results showed that a pronounced heterogeneous structure is formed in the cross-section of the rod during the applied thermomechanical treatment. The phenomenon is associated with the non-uniform stress state of the rod during CRF [21]. Specifically, moderate tensile stress acts in the rod center and high compressive stress operates in the subsurface layers. The non-uniform stress state leads to the activation of various plastic deformation mechanisms in the center and subsurface layers of the rod. Mechanical twinning and dislocation slip are observed in the center of the rod, developing large structural elements in the form of regions with dislocation cells limited by packages of mechanical twins. In this case, according to Ref. [25], twinned microvolumes have the <111>-orientation, and microvolumes with a cellular structure have the <100>-orientation, which ultimately leads to the formation of the axial two-component texture <001>/<111> in the rod center. On the other hand, high compressive stresses in the subsurface layer contribute to the formation of shear bands, since the possibilities for deformation accommodation due to dislocation sliding and mechanical twinning are quickly exhausted during CRF. At the same time, shear bands were found after CRF with a degree of 60 % [21]. The formation of shear bands leads to the formation of a UFG structure via the rotational recrystallization mechanism, which was proposed by V.F. Nesterenko et al. [27]. In essence, during the deformation process, a shear band is enhanced, within which there are randomly distributed dislocations that form elongated dislocation cells. The latter transform into subgrains with an increase in accumulated plastic strain. During further deformation, these subgrains are fragmented with the subsequent formation of equiaxed micrograins. In addition, the change in the predominant mechanism of plastic deformation explains a decrease in the proportion of grains with <100>- and <111>-orientations in the direction from the edge to the center. It should be noted that the shear texture B/B ̅in the subsurface layer is caused by shear banding. Heat treatment at 600 °C does not have a significant effect on the microstructure and texture of the rod. However, partial polygonization occurs throughout the entire cross-section. After heat treatment at 700 °C, recrystallization nuclei were found in the subsurface layer, the fraction of which in the microstructure is less than 10 %. Tensile stress-strain curves (Fig. 7) were obtained for samples of different types (Fig. 1). After 95 % CRF for all types of samples (Samples C, E and H) the stress-strain curve is typical for cold-deformed steels with high strength and low ductility (Fig. 7, a1). When the yield strength is reached, a peak is observed on the curve, followed by a region of strain localization. Heat treatment at 600 °C does not have a significant effect on the shape of the stress-strain curves of the E600 and H600 samples, but an increase in strength characteristics is observed (Fig. 7, b1). In this case, the sample C600 shows an increase in the uniform deformation area and a decrease in strength characteristics. In turn, after heat treatment at 700°C, the curves in all cases demonstrate a lower level of strength characteristics and an increase in ductility (Fig. 7, c1). Based on the curves of strain hardening rate, it is evident that for all three types of samples after 95 % CRF, only one stage of strain hardening is observed, which is limited by the onset of strain localization (Fig. 7, a2-4). In turn, heat treatment at 600°C did not significantly affect the nature of the curve of samples E600 (Fig. 7, b3) and H600 (Fig. 7, b4). While on the curve obtained for the sample C600 (Fig. 7, b2), three stages of strain hardening can be distinguished. At the first stage, a sharp decrease in strain hardening occurs. The beginning of the second stage is characterized by a change in the slope of the curve, stabilization and an increase in strain hardening rate. Apparently, a decrease in the strain-hardening rate is related to the activation of dynamic recovery. The stage-by-stage nature of the strain-hardening behavior are described in detail in Ref. [28]. Namely, the first stage of strain hardening can be associated with the redistribution and annihilation of dislocations, which causes a decrease in the strain-hardening rate. With an increase in true strain, the second stage begins, which is associated with mechanical twinning. Therefore, the strain-hardening rate increases or sta-
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