OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 2 2025 Meanwhile, the elongation to failure (δ) is in the range of 8‑12 %, while the uniform elongation (δu) does not exceed 1 %. After heat treatment at 600 °C, an increase in σu and σ0.2 of the E600 and H600 samples to 1,360‑1,370 MPa and 1,250‑1,330 MPa, respectively, is observed, but ductility does not change (Table). However, the strength characteristics of the C600 sample decrease after heat treatment at 600 °C. Namely, σu is 1,225 MPa, and σ0.2 is 1,060 MPa. Along with this, σ increases to 16 %, and σu increases to 9 % (Table). Heat treatment at 700 °C provokes a decrease in strength and an increase in ductility. In particular, the H700 sample possesses the attractive strength-ductility combination (σu = 1,145 MPa; σ0.2 = 1,054 MPa; δ = 16 %; δu = 8 %). The heterogeneous structure and texture in the cross section of the rod affect the mechanical behavior of the material under study. High strength characteristics and low ductility of the material after CRF are associated with a highly deformed structure. However, the material with the UFG structure (from the subsurface layer) exhibits somewhat greater strength and ductility (Fig. 5, a1). The increased strength of this region is attributed to the Hall-Petch strengthening [29, 30], while an increase in ductility is caused by the new grain nucleation [21]. It is worth noting that the pronounced effect of structural and textural heterogeneity on mechanical properties under tension is attained after heat treatment. After heat treatment at 600 °C, the strength of the heterogeneous material increases, while elongation to failure is of 10‑11%. On the one hand, an increase in strength characteristics is likely due to the formation of segregations of alloying elements on dislocations [31]. On the other hand, the preservation of ductility is associated with the partial polygonization (Fig. 4, a). Heat treatment at 700 °C, in turn, leads to further softening and an increase in ductility due to the development of polygonization in the center of the rod and the onset of static recrystallization in the subsurface layer. The latter is due to greater accumulated plastic deformation and thereby lower thermal stability [32]. It should be noted that the heterogeneous material after heat treatment at 600‑700 °C demonstrates the highest yield strength and good ductility. According to Ref. [33], a similar effect is ascribed to the increased value of back stresses. Structural heterogeneity is associated with strain partitioning during tensile testing [34]. So, in the strong subsurface Mechanical properties characteristics Region Heterogeneous (H) Centre (C) Edge (E) 95% CRF su (MPa) 1,242 ± 7 1,259 ± 28 1,303 ± 13 s0.2 (MPa) 1,210 ± 11 1,147 ± 100 1,197 ± 1 δ (%) 8.3 ± 1.2 8.4 ± 1.4 11.5 ± 0.9 δu (%) 1 ± 0.1 0.7 ± 0.2 1.1 ± 0.1 95% CRF + HT 600°C for 2 hours su (MPa) 1,355 ± 2 1,225 ± 35 1,374 ± 3 s0.2 (MPa) 1,330 ± 8 1,060 ± 30 1,252 ± 23 δ (%) 10.3 ± 0.5 16.1 ± 0.5 11 ± 0.3 δu (%) 0.8 ± 0.1 9 ± 0.3 1 ± 0.1 95% CRF + HT 700°C for 2 hours su (MPa) 1,145 ± 11 1,091 ± 4 1,102 ± 13 s0.2 (MPa) 1,054 ± 14 927 ± 15 877 ± 9.2 δ (%) 16 ± 2.7 18.5 ± 0.8 16 ± 1.3 δu (%) 8.1 ± 0.1 11.6 ± 0.5 8.8 ± 1.3
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