Papers by Author: Alexander Korshunov

Abstract: The influence of multipass processing by Twist Extrusion (TE) on distribution of mechanical properties by volume in commercially pure (CP) titanium billets is investigated. Experiments show that the mechanical properties are almost homogeneous in the billet cross-section already after the second pass of TE. This can be explained by mixing effect and saturation of properties as well. Warm TE leads to the formation of high strength properties in combination with high plasticity. Ultimate and yield stresses of the billet processed by two cycles of TE increased, in comparison with initial state, by 30% and 60% respectively. The value of the reduction in area remained at the initial value. This fact is indicating a high technological plasticity of the material, i.e. its ability for further shaping by metal forming methods.

Abstract: A two-dimensional Ising-like model for the triangular spin-chain lattice, where each spin chain is treated as a rigid superspin, is proposed to investigate the dynamics of magnetization in frustrated triangular spin-chain systems. The superspins are assumed to interact with the nearest neighbours and external agency (heat reservoir and external magnetic field) that causes them to change their states randomly with time. A probability of a single spin-flip process is assumed in a Glauber-like form. This technique allows describing the steps in the magnetization curves observed in Ca3Co2O6 and their dependence on the magnetic field sweep rate and temperature.

Abstract: Annealed oxygen-free and tough-pitch copper samples have been processed by equalchannel angular pressing (ECAP) by route BC. The samples included 8 x 8 mm section pieces and a 40 mm diameter bar. Thermal stability was assessed based on the changes in the standard mechanical properties (conventional yield strength, tensile strength, elongation, proportional elongation and contraction) after annealing at different temperatures for 1 hour. Thermal stability of the same grade of material has been found to be different for different batches and to depend on the structural conditions of deformed material. The zone of thermal stability for copper of the two grades of interest does not depend on the material’s chemical composition.

Abstract: Annealed commercially pure copper and titanium were processed by equal-channel angular pressing (ECAP) using routes BC and C. Pressed samples had a square section with a side length of 8 mm. Anisotropy was analyzed based on the conventional yield strength at compression in three mutually perpendicular directions. The tests were conducted with the as-received materials and after different number of ECAP passes. For copper, which is isotropic in the as-received state, considerable anisotropy manifests itself after the first pass and holds out after the subsequent passes. Anisotropy of as-received titanium holds out at further pressing and makes about 25 percent. Anisotropy after all the passes is similar both for room and elevated temperature. Anisotropy manifests itself during pressing in both routes, BC and C, although its behavior differs for the different routes.

Abstract: During the last decade it has been shown that severe plastic deformation (SPD) is a very effective for obtaining ultra-fine grained (UFG) and nanostructured materials. The basic SPD methods are High Pressure Torsion (HPT) and Equal Channel Angular Extrusion (ECAE). Recently several new methods have been developed: 3D deformation, Accumulative Roll Bonding, Constrained Groove Pressing, Repetitive Corrugation and Straightening, Twist Extrusion (TE), etc. In this paper the twist extrusion method is analyzed in terms of SPD processing and the essential features from the “scientific” and “technological” viewpoint are compared with other SPD techniques. Results for commercial, 99.9 wt.% purity, copper processed by TE are reported to show the effectiveness of the method. UFG structure with an average grain size of ~0.3 μm was produced in Cu billets by TE processing. The mechanical properties in copper billets are near their saturation after two TE passes through a 60º die. Subsequent processing improves homogeneity and eliminates anisotropy. The homogeneity of strength for Cu after TE is lower than after ECAE by route BC, but higher than after ECAE by route C. The homogeneity in ductility characteristics was of almost of inverse character. The comparison of mechanical properties inhomogeneity in Cu after TE and ECAE suggests that alternate processing by ECAE and TE should give the most uniform properties.

Abstract: This paper investigates microstructures and mechanical properties of the TI-6AL-4V ELI alloy processed by ECAP and extrusion with various morphology of α and β-phase. Preliminary thermal treatment consisted of quenching and further high-temperature ageing. The present work reveals that the decrease of volume fraction of α-phase globular component in the initial billet results in a more homogeneous structure refinement during SPD, lower internal stress, enhancement of microstructure stability and mechanical properties. An ultimate strength of UTS ≥1350 MPa was obtained in the Ti-6Al-4V ELI alloy while maintaining a ductility of δ≥11%.

Abstract: Titanium VT1-0 was processed by four passes of equal-channel angular pressing (ECAP) using two routes, BC and C. Pressed samples had a square section with a side length of 8 mm. Mechanical properties at tension (tensile strength, conventional yield strength and elongation) were determined at 9 points across the sample using small-size specimens, 1.5 mm in diameter, cut out along the pressing direction. Heterogeneity in the mechanical properties across the sample was determined based on the value of the variation coefficient. One can observe that heterogeneity in the mechanical properties generally tends to decrease with the number of ECAP passes.

Abstract: Equal-channel angular (ECA) pressing is the main technique of the severe plastic deformation (SPD) method, applied for fabrication of bulk nanostructured metal materials. At the same time the practical realization of this technique is a rather challenging task. This is connected with the fact that the material during the ECA pressing is subjected to large strains under high imposed pressure at relatively low temperatures. Simulation with the help of the finite element method (FEM) or the variation-difference (VDM) method is widely applied to analyze the process of ECA pressing. A variety of as commercial as well as in-house developed programs are used by researches, when conducting this analysis. As a result the correlation between the modeling results, obtained at different laboratories as well as their adequacy, i.e. possibilities of their application for the analysis of the experimental data become topical issues. In order to find answers to the questions put by there has been performed computer simulation of 1st pass of ECA pressing by an example of pure copper at 4 different laboratories, engaged in SPD problems. Meanwhile, the investigators used different software packages, however, initial simulation conditions were set equal. This refers in particular to geometry sizes and the form of the die-set possessing square transverse section of the channels, as well as to the inner and outer curvature radii of the channels in the point where they intersect, and to the form and dimensions of the billet, strain rate, strain curve, isotropic model of the material. The modeling temperature was ambient. The die-set and the punch were assigned as absolutely solid non-deformable bodies. Taking into account the symmetry of the solving task, the modeling was conducted for a half of the billet, cut along the vertical plane, coming through its geometrical center. The friction coefficient was assigned equal to zero, in order to avoid influence of friction on the character of the material flow as well as not to complicate the problem at the given stage of comparison. Other modeling parameters were chosen by each researcher on his own, basing on his experience and conventional approaches to modeling. Comparison of the obtained modeling results was made by means of matching of the calculated values of the level of the accumulated strain along the bulk of the billet, pressing efforts, and the geometrical form of the billet after ECA pressing. Modeling results were compared with the results of the experimental researches.

Abstract: The paper represents results of computer modeling of texture formation in pure copper subjected to severe plastic deformation (SPD) realized by equal channel angular pressing (ECAP). Several polycrystalline models, namely the Taylor model, Sachs model, and self-consistent viscous plastic (VPSC) model, were applied and their predictions of texture evolution for different routes and number of ECAP passes were compared. For these calculations, simple shear deformation was used for the deformation realized by ECAP. Using the VPSC model, a single ECAP pass was revisited, but employing a 3D model of material flow, conducted by the variation-difference method, for the ECAP deformation. The influence of the inner and outer radii at the channel intersection and the friction coefficient on the homogeneity of the texture development in the cross section of the bulk ingot was investigated.