OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 2 2025 2 r u b s s s , (2) where σb are the back stresses (MPa); σr are the flow stresses under repeated loading (MPa); σu are the flow stresses under unloading (MPa). Flow stresses under repeated loading and unloading were determined from the hysteresis loops according to the scheme shown in Fig. 2. Fig. 2. Schematic showing the method for determining the yield stress at unloading (σu) and the stress at reloading (σr) Results and discussion The microstructure of the rod center (Fig. 3, a), subjected to 95 % CRF, consists of parallelogram-shaped domains (marked with a yellow dotted line), formed by mechanical twins of various systems. Dislocation cells can be observed inside such structural elements (marked with a green dotted line). In turn, the structure of the subsurface layer (Fig. 3, b) is ultrafine-grained (UFG). In this case, the size of the structural elements of the central part (700 ± 490 nm) (Fig. 3, a) significantly exceeds the sizes of the elements forming the structure of the subsurface layer (100 ± 50 nm) (Fig. 3, b). Following heat treatment at 600 °C, polygonization is activated over the entire cross section of the rod, which causes additional refinement of the structure due to the formation of dislocation walls (Fig. 4, a and 4, b). Heat treatment at 700 °C is accompanied by the formation of recrystallization nuclei in highly deformed subsurface layers (Fig. 4, d) and the continuation of dislocation redistribution processes in the rod center. As a result, areas with a reduced dislocation density ‑ “dislocation-free” regions ‑ are a b Fig. 3. Fine structure of the center (a) and subsurface layer (b) of a steel rod subjected to 95% CRF
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