Papers by Author: Andrey Belyakov

Abstract: Some feature of discontinuous dynamic recrystallization (DRX) in an Fe-0.4%C-18%Mn austenitic steel during isothermal compression tests at temperatures of 973-1373 K and strain rates of 10^-3-10^-1 s^-1 were studied. The DRX microstructures consisted of various grains, i.e., DRX nuclei, growing DRX grains, and work-hardened DRX grains, which differentiated with the grain orientation spread (GOS). DRX was commonly promoted by a decrease in temperature-compensated strain rate, i.e., Zener-Hollomon parameter (Z), corresponding to an increase in deformation temperature and/or a decrease in strain rate. In contrast, the GOS distribution varied non-monotonously with Z. The large area fraction of DRX grains with small GOS below 1° appeared at definite temperature/strain rate conditions. The large fraction above 0.6 of DRX grains with small GOS was observed in DRX microstructures with a large ratio of CSL Σ3 boundary fraction to low-angle subboundary fraction. The GOS distribution in the DRX microstructures is discussed in terms of the DRX grain nucleation and growth rates.

Abstract: The microstructure evolution and mechanical properties of a copper alloy subjected to deformation at temperatures of 20 °C and 400 °C to total strains from 1 to 4 were examined. The formation of planar low-angle boundaries with moderate misorientations occurs within initial grains at relatively small strains regardless of deformation temperature. Upon further processing the misorientations of these boundaries progressively increase and the new ultrafine grains develop. Continuous dynamic recrystallization takes place during deformation at ambient and elevated temperatures. The kinetics of dynamic recrystallization is discussed in terms of a modified Johnson-Mehl-Avrami-Kolmogorov relationship. The large plastic straining results in significant strengthening, the ultimate tensile strength increases from 190 MPa in the initial state to 440 MPa and to 400 MPa after total strain of 4 at 20 °C and 400 °C, respectively. A modified Hall-Petch relationship is applied to evaluate the contribution of grain refinement and dislocation density to the overall strengthening.

Abstract: The mechanical properties of Fe-28%Mn-1.5%Al and Fe-0.6%C-18%Mn-1.5%Al-0.07%Nb (all in wt.%) steels subjected to hot plate rolling at a temperature of 1423 K with a total reduction of 60% were studied. The steels exhibited quite different mechanical properties in spite of almost the same original microstructures and similar stacking fault energies. The yield strength and total elongation of the Fe-28%Mn-1.5%Al steel are about 260 MPa and 45%, respectively, whereas those properties in the Fe-0.6%C-18%Mn-1.5%Al-0.07%Nb steel comprise 350 MPa and 53%, respectively. The tensile flow stress vs strain curves of the hot rolled steel samples can be described by Ludwigson-type relations with parameters being dependent on the strengthening mechanisms. Frequent deformation twinning in the Fe-0.6%C-18%Mn-1.5%Al-0.07%Nb steel promoted the strain hardening and improved the strength and ductility.

Abstract: The microstructure evolution during the annealing treatment of a recycled copper after cold rolling to total strain of 2.6 was investigated. The cold deformation resulted in the elongation of initial grains along rolling direction and the strain-induced formation of subboundaries. Annealing recovery occurred in the temperature range 100-250 °C. The recrystallized microstructures were observed after annealing at 300-400 °C. The hardness of partially recrystallized copper samples was interpreted in terms of dislocation strengthening. The recrystallization kinetics was estimated according to a Johnson–Mehl–Avrami–Kolmogorov equation using different methods for recrystallized fraction determination, i.e., the fractional softening, the grain orientation spread, and the Kernel average misorientation.

Abstract: The grain refinement is an effective approach to strengthen high-Mn TWIP/TRIP steels. The development of recrystallized microstructure with a grain size of about one micron increases the yield strength of high-Mn steels above 500 MPa. The fine grained microstructures can be easily developed by cold rolling followed by primary recrystallization. The recrystallized grain size can be expressed by a power law function of the strain hardening during the previous cold rolling with an exponent of -2. Taking the dislocation density as the main strengthener, the grain size is an inverse proportion to the dislocation density. Then, the number density of recrystallized grains can be expressed by a power law function of dislocation density evolved during cold rolling with an exponent of about 2.

Abstract: The effect of cold working followed by annealing on the development of ultrafine grained microstructure and mechanical properties of an Fe-12%Mn-0.6%C-1.5%Al medium-manganese steel was studied. The steel was cold rolled with intermediate annealings and then annealed at 873 K or 923 K for 30 min. The yield strength and total elongation of the Fe-12Mn-0.6C-1.5Al steel after cold rolling were 1200 MPa and 14%, respectively. The heat treatments resulted in the formation of two phase (austenite-ferrite) ultrafine grained microstructures with average grain sizes of 0.9 to 1.2 μm, depending on the annealing temperature. The annealed ultrafine grained steel samples exhibit the yield strength in the range of 800-950 MPa, the ultimate tensile strength in the range of 1150-1200 MPa, and total elongation of 12% to 19%.

Abstract: The microstructure and creep properties of a P911-type steel normalized at 1060°C and then subjected to one-step tempering at 760°C for 3 h or two-step tempering at 300°C for 3 h + 760°C for 3 h were examined. The transmission electron microscope (TEM) observations showed that the tempered martensite lath structure (TMLS) with a lath thickness of 340 nm evolved after both tempering regimes. High dislocation densities of 3×10¹⁴ or 5×10¹⁴ m^-2 retained after one-and two-step tempering respectively. M₂₃C₆ carbides with a mean size of 120 nm and V-rich MX carbonitrides having a “wing” shape with an average length of about 40 nm precipitated on high-and low-angle boundaries and within ferritic matrix, respectively. A number of Nb-rich M(C,N) carbonitrides with a mean size of 20 nm precipitated on dislocations during low temperature tempering. The creep tests were carried out under constant load condition at 650°С at applied stresses of 100 and 118 MPa. Analysis of creep rate versus time curves showed that the use of two-step tempering decreases the minimum creep rate providing an increase in the creep strength in long-term conditions.

Abstract: The formation of nanocrystalline structures and mechanical properties were studied in a nitrogen-bearing 304-type stainless steel subjected to severe plastic deformation (SPD). The steel samples were processed at ambient temperature using three different methods, i.e., caliber rolling, multidirectional forging and high pressure torsion. All these techniques resulted in pronounced grain refinement. The microstructures consisting of austenite/ferrite crystallites with transverse dimensions of 50 and 30 nm evolved in the rolled and forged samples, respectively. The austenite fractions comprised approximately 0.4. In contrast, the microstructure consisted mainly of austenite with an average grain size of about 25 nm evolved after high pressure torsion. All samples of the stainless steel subjected to severe plastic deformation demonstrated significant strengthening. The ultimate tensile strengths of 2065 MPa and 1950 MPa, were obtained after rolling and high pressure torsion, respectively. The ultimate tensile strength of samples subjected to multidirectional forging was 1540 MPa.

Abstract: The grain refinement and kinetics of submicrocrystalline structure formation in a Cu-0.3%Cr - 0.5%Zr alloy during large plastic deformation were investigated. The fraction of high-angle boundaries and the fraction of ultrafine grains were used to estimate the kinetics of grain refinement and submicrocrystalline structure evolution during large plastic deformation. The multidirectional forging (MDF), equal channel angular pressing (ECAP), and high pressure torsion (HPT) were used as methods of large plastic deformation. Comparative analysis showed that the grain refinement process occurred faster during HPT and MDF in comparison with ECAP. The fraction of ultrafine grains achieved almost 1 after 3 HPT turns and after MDF to the total strain of 4; while the one reached only 0.29 after 4 ECAP passes. The modified Johnson-Mehl-Avrami-Kolmogorov equation could be applied to determine the kinetics of grain refinement in copper alloy during large plastic deformation as a function of true strain.

Abstract: The deformation microstructures and mechanical properties of an austenitic stainless steel subjected to warm plate rolling were studied. The warm rolling was carried out at 300°C to different total true strains of 0.5, 1, 2 or 3. The structural changes during warm rolling were characterized by the elongation of original grains towards the rolling direction and the development of spatial network of strain-induced high-angle boundaries leading to the evolution of ultrafine-grained microstructure at sufficiently large strains. The grain refinement was assisted by the development of deformation twinning. After straining to 3, the transverse grain size decreased down to 220 nm in the warm rolled samples. The warm plate rolling resulted in significant strengthening. The microhardness increased from 2910 MPa to 4192 MPa with increase in the total strain from 0.5 to 3. Correspondingly, the yield strength approached 1005 MPa after warm rolling to a total strain of 3.