Papers by Author: Rustam Kaibyshev

Abstract: Microstructure, precipitation behaviour and mechanical properties of an Al-5.64Cu-0.33Mn-0.23Mg-0.14Zr-0.11Ti (in wt. %) alloy subjected to thermomechanical processing (TMP) involving equal-channel angular pressing (ECAP) at ambient temperature to total strains (ε) of ~1 and ~2 followed by aging at 180°C for 0-28 h have been investigated and compared with conventional aging at the same temperature (T6 state). TMP led to significant increase in yield stress (YS) and ultimate tensile strength (UTS) and decrease in elongation-to-fracture as compared to the peak-aged T6 state. The YS, UTS and δ values attained after ECAP to ε ~ 2 followed by peak ageing were ~510 MPa, ~540 MPa and ~7.6%, respectively. The changes in mechanical properties were related to microstructure evolution and precipitation behaviour. TMP conditions obtaining a high-strength state of the Al-Cu-Mg alloy are discussed.

Abstract: The commercial Zr-modified 5083 aluminum alloy was homogenized to precipitate nanoscale Al₆Mn particles and then undergone to equal-channel angular pressing (ECAP) at 300 °C to a true strain of ~12 via B_C route. The obtained ultrafine-grained material was subjected to friction-stir welding (FSW). The welding variables were selected to provide reasonable homogeneous microstructure distribution across the weld zone and thus to ensure a highly uniform elongation during subsequent superplastic tests of the joints. Superplastic behavior of the obtained welds is discussed.

Abstract: Stress serration patterns and kinematics of deformation bands associated with the Portevin-Le Chatelier (PLC) effect were examined for an Al–6%Mg–0.35%Mn–0.2%Sc–0.08%Zr–0.07%Cr (in wt.%) alloy with two grain sizes: 22 μm and 0.7 μm. The fine-grained structure of the alloy was obtained using equal-channel angular pressing (ECAP) at 320°C up to a total strain of ~12. Tensile tests were carried out at room temperature and strain rate ranging from 10^-5 s^-1 to 10^-2 s^-1. In addition, high-frequency local extensometry technique was applied to monitor the evolution of the axial strain distribution during deformation. Depending on the strain rate, conventional A, B, C, or mixed types of serrations were observed on the stress-strain curves. These types of behavior usually correspond to different kinematics of the PLC bands, including band propagation and localization. However, the propagation regime was found to dominate in the investigated alloy in the entire strain-rate range. This unusual behavior of deformation bands and their features depending on the grain size are discussed.

Abstract: Influence of friction stir welding (FSW) on microstructure of an Al-4.57Mg-0.35Mn-0.2Sc-0.09Zr (wt. pct.) alloy was studied. Following parameters of FSW were used: the rotation speeds of 500, 650 and 800 rpm, the traverse speed of 75 mm/min and the tilt angle of 2.5°. Defect-free welds were obtained using all these parameters. FSW leads to the formation of fully recrystallized microstructures with average grain sizes less 2 μm and a moderate dislocation density of ~10¹³ m^–2 in the stir zone. No evidence for abnormal grain growth was found in the heat affected zone of the weld. The nanoscale Al₃(Sc,Zr) dispersoids coarsened to 21 nm but retained coherent interfaces and cube-cube orientation relationship with the matrix.

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.

Abstract: The microstructure evolution and strength properties of a Cu-0.096%Cr-0.057%Zr alloy subjected to equal channel angular pressing (ECAP) at a temperature of 673 K via route B_C to total strains of 1 to 4 were examined. The planar low-angle boundaries with moderate misorientations form within initial grains during the first ECAP passes. Upon further processing the misorientations of these boundaries progressively increase and the formation of new ultrafine grains occurs as a result of continuous dynamic recrystallization. Partially recrystallized ultrafine grained structure evolves at strains above 4. After straining to 4 the (sub) grain size attains 0.65 μm. The large plastic straining provides significant strengthening. The ultimate tensile strength increases from 190 MPa in the initial state to 420 MPa after 4 ECAP passes. A modified Hall-Petch analysis is applied to investigate the contribution of grain refinement and dislocation density to the overall strengthening.

Abstract: The paper reports on the effect of severe plastic deformation on mechanical properties of an Al-4.57Mg-0.35Mn-0.2Sc-0.09Zr (in wt. pct.) alloy processed by equal channel angular pressing followed by cold rolling (CR). The sheets of the 5024 alloy with coarse grained (CG) structure exhibited a yield stress (YS) near 410 MPa and an ultimate tensile strength (UTS) of 480 MPa, while the YS and UTS of this material with ultrafine-grained (UFG) structure increased to 530 and 560 MPa, respectively. On the other hand, the elongation to failure decreased by a factor of 2 and 4 after CR and CR following ECAP, respectively. It was shown that dislocation strengthening attributed to extensive CR plays a major role in achieving high strength of this alloy. Besides these macroscopic characteristics, jerky flow caused by the Portevin-Le Chatelier (PLC) instability of plastic deformation was examined. The formation of UFG structure results in a transition from mixed type A+B to pure type B PLC serrations. No such effect on the serrations type was observed after CR.