Critical Grain Size at Meso-Level after Deformation of Polycrystalline Metals and Alloys in Low-Stability State
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Russian Physics Journal, Vol. 63, No. 5, September, 2020 (Russian Original No. 5, May, 2020)
CRITICAL GRAIN SIZE AT MESO-LEVEL AFTER DEFORMATION OF POLYCRYSTALLINE METALS AND ALLOYS IN LOW-STABILITY STATE N. A. Koneva,1 A. I. Potekaev,2 L. I. Trishkina,1 T. V. Cherkasova,1,3 and A. A. Klopotov1,2
UDC 669.35:539.214
The paper deals with the dislocation and dislocation-disclination substructures of polycrystalline FCC alloys Cu–Al and Cu–Mn modified by tensile deformation. Observations are performed using the transmission electron microscopy. It is shown that the grain size of the alloy structure ranging from 10 to 240 μm, can serve as a critical parameter in the low-stability state during the deformation and transition from one stage of hardening to another. The dependences are obtained for the parameters describing the defect substructure and the mean grain size. These dependences are compared with the structure and phase composition of the alloys. The critical grain size of about 100 µm is found at a meso-level. When the grain size exceeds 100 µm, the main role in defect accumulation play intracrystalline processes. At stage II, the strain-hardening coefficient does not depend on the grain size of >100 µm and rapidly increases at a grain size of 0 at d > d1cr and k < 0 at d < d1cr. The sign change of the k coefficient means the replacement of the grain-boundary hardening by the grain-boundary softening. The grain-boundary processes include
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Tomsk State University of Architecture and Building, Tomsk, Russia, e-mail: [email protected]; [email protected]; [email protected]; 2National Research Tomsk State University, Tomsk, Russia, e-mail: [email protected]; 3National Research Tomsk Polytechnic University, Tomsk, Russia, e-mail: [email protected]. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 58–63, May, 2020. Original article submitted February 27, 2020. 1064-8887/20/6305-0773 2020 Springer Science+Business Media, LLC
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diffusion processes on the grain boundary, dislocation slip along the crystal lattice and grain boundaries, grain boundary migration, and have a strong effect on deformation. Another critical grain size d2cr at the micro-level is about 100 nm and associated with the dislocation-free grains or subgrains. Intracrystalline hardening disappears. Owing to their small size, the dislocation-free grains harden the micro polycrystal and change its deformation mechanisms. At the meso-level, the critical grain size is d3cr ≈ 10 µm. According to [8], stored dislocations dominate in the dislocation ensemble when d > d3cr, whereas at d < d3cr, the density of geometrically necessary dislocations is higher than that of statistically stored dislocations. Under these conditions, a gradient dislocation structure appeares due to the stress fields induced by disclinations locating on the grain boundaries and grain-boundary triple point intersections. In [9, 10] it was found that during the evolution of strain fields in metals and metal systems, each hardening stage on the
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