Papers by Author: Igor V. Alexandrov

Abstract: The common feature of the technologies, such as the equal channel angular pressing (ECAP) is the use of a changeable deformation path, which changes the configuration (value and direction) of the acting forces that impose various directions of plastic flow. The changes destabilize a temporary dislocation structure as well as the spatial distribution of the areas with elevated elastic energy cumulated in deformed lattice. The application of that technique results in a potentially large deformation (up to several hundred percent) and plasticity of materials considered to be brittle at ambient temperature. The microstructure effects accompanied with intensive and large deformation are reflected in the space orientation of grains (crystallographic texture) as well as in the configuration of the residual stresses existing in deformed material. The presented results based on experimental data registered by X-ray diffraction and TEM techniques are helpful in interpreting deformation mechanisms operating in the bend-zone of the ECAP tool during processing Ti-ingot by the mentioned method of severe plastic deformation.

Abstract: The paper has viewed the manifestation of the paradox of severe plastic deformation (SPD), caused by the occurrence of preexisting deformation twins in ultrafine-grained Cu, which has been obtained by the combination of the SPD method, accomplished by an equal-channel angular pressing with the conventional methods of deformation-thermal treatment. The high strength of the obtained samples has proved to be conditioned by the occurrence of the high density of the coherent twin boundaries, serving as effective obstacles on the way of slipping dislocations. Moreover, the occurrence of the twins creates favorable conditions for the dislocation density increase both in the grains with the twins and in the grains without them. As a result the sample hardens, contributing additionally into its strength. Simultaneously it manifests high ductility. By doing so the deformation behavior of the sample is mainly conditioned by the grain boundaries of grains free from the twins. The results were obtained on the basis of the dislocation-based model which develops models of Y. Estrin and L. Tóth, M. Zehetbauer, and L. Remy.

Abstract: The results of a recent multiscale computer modeling are presented in this report. The conducted investigations are devoted to the processes, which take place in different metallic materials subjected to severe plastic deformation (SPD). It is presented that the developed models and approaches can be useful in the successful prediction and comprehensive analysis of the peculiarities of material flow and the ways of its homogenization, the understanding of principles of grain structure refinement, the achievement of given grain morphology, grain boundary misorientation spectrum and crystallographic texture, as well as in the evaluation of the active deformation mechanisms, estimating the level of structure-sensitive properties, etc. It is shown that multiscale modeling is a very promising approach which could supply the researchers with the possibility to take into account the complex influence of the different parameters, related to SPD processing and material in order to refine the grain size and obtain homogeneous bulk nanostructured materials.

Abstract: In the present work high pressure torsion (HPT) was imposed on commercial purity (CP) tungsten at different temperatures of 450 °С and 490 °С to achieve different microstructures and grain boundary misorientation spectra (GBMS). After HPT at 450°С, ultrafine grained microstructure with an average grain size of ~150 nm was developed in the metal. HPT at 490 °С results in an elongated structures with average width of ~500nm. EBSD investigations showed that over 92% are HAGB type in microstructure HPT-processed at 450°С, and in contrast, over 50% of grain boundaries are LAGB in sample processed at 490°С. Annealing at 900°С for 1h, of the sample with homogeneous UFG structure resulted from HPT at 450°C, leads to only limited decrease (~20%) in microhardness.

Abstract: In this article we present the results of the experimental research and those of the processes developing the crystallographic texture of computer modeling in CP Ti in the process of 1-4 equal channel angular pressing (ECAP) passes along the route ВС. The goal of the research was to determine the active deformation mechanisms, depending on the strain degree, accumulated in the ECAP process. The research was carried out by the method of X-ray analysis and by computer modeling. Computer modeling was carried out on the example of visco-plastic self-consistent model. Thereby, the basal, the prismatic and the pyramidal (of the 1st and of the 2nd order) slip systems were considered as possible active slip systems. Besides, the possibility of activating the tensile and the compressive twinning systems were taken into consideration. As the result of the carried out experimental research, the objective laws of forming preferred orientations were determined. For the first time, with the help of computer modeling, made up to the 4th ECAP pass, it was shown that the crystallographic texture development processes in CP Ti in ECAP, realized at temperature of 723 K, can be explained by activation of the basal, prismatic and pyramidal (of the 1st order) slip systems and compressive twinning systems. Therefore, the increase of the ECAP passes can lead to amplification of contribution of the basal and prismatic slip systems, as well as the insignificant weakening of the contribution of pyramidal slip systems (of the 1st order). Moreover, the compressive twinning can become obvious only in the 1st ECAP pass.

Abstract: Bulk nanostructured Cu samples with 20 mm in diameter and 1 mm in height were processed by HPT at a pressure of 6 GPa by 10 rotations. Measurements of texture by means of state-of-the-art XRD technique have been achieved in directions parallel and normal to the torsion axis. Local texture measurements were performed by a spot size of 500 -m in steps of 3.5 mm in order to examine the homogeneity of deformation. The texture data were resolved to shear plane and analyzed in terms of ideal shear orientations. At both inner and outer areas of the disc plane typical shear textures are observed. However, the intensity of components of textures at inner areas is higher than that of outer one. These results can be interpreted in terms of dynamic lattice relaxations rather than by heterogeneities in deformation.

Abstract: In the present investigation, a bimodal structured alloy with ultrafine-grained (UFG) eutectoid matrix embedded with micrometer-grained pre-eutectoid phase was introduced into the hypo-eutectoid Cu-10.8wt.%Al and Cu-11.3wt.%Al alloys by means of pre-pressing heat-treatment, equal-channel-angular pressing (ECAP) and subsequent annealing. Different size of micrometer grained pre-eutectoid phase was obtained by controlling the cooling rate during pre-pressing heat-treatment of the hypo-eutectoid alloy. The tensile deformation behavior of the developed microstructures is characterized by a maximum tensile yield strength up to 800MPa, which is three times higher than that of the un-treated alloy. It is found that the size of the micrometer grained pre-eutectoid phase is critical to the improvement of the bimodal structured alloy. With larger micrometer grained pre-eutectoid phase, no obvious improvement in plastic elongation was observed with the increase of volume fraction of the pre-eutectoid phase from 20% to 40%, but a decrease in the yield tensile strength was observed. An optimal combination of strength and ductility was obtained particularly in those samples embedded with small-sized micrometer-grained pre-eutectoid phase, which provide extra strain gradient hardening effect.

Abstract: X-ray investigations revealed that the increase in the applied pressure during high pressure torsion (HPT) of commercially pure Ti leads not only to substructure refinement with an increase of the dislocation density and microstrain level but also to an α→ ω phase transition at room temperature. The coexistence of both α and ω phases, the latter known as a high pressure phase, in the ratio approximately of 1:3 has been obtained after removal of thehigh pressure. Texture analysis of electodeposited Ni after HPT discovered a new form of crystallite orientation distribution in the nanocrystalline state. A nearly random orientation crystallite distribution has been observed unlike the “traditional” case of a shear texture forming in cubic symmetry metals. The crystallographic texture data obtained were considered as experimental evidence of the changed plastic deformation mechanisms in nanocrystalline Ni produced by HPT.

Abstract: In the investigation the 3D version of the Estrin-Tóth dislocation model was used to analyze deformation behaviour of pure Cu, subjected to high pressure torsion (HPT) under pressures equal to 0.8, 2, 5, 8 GPa. As a result of the computer simulation, the nature and reasons for strain hardening are analyzed, the dislocation density evolution versus degree of SPD and graincell size versus degree of SPD curves were plotted. It is shown that the model adequately reflects the acting deformation mechanisms and structural changes during HPT at different applied pressures. It has been stated that an increase of the applied pressure at HPT leads to an increase in the activity of dislocation sources and sinks in the grain-cell walls. Misorientations between boundaries are estimated. It is revealed that an increase of the applied pressure contributes to a growth of the misorientation angles between neighbouring grain-cells.

Abstract: New data on structure and texture features of Cu and Ti rods, subjected to ECAP at 20oC and 400oC respectively, were obtained by means of advanced X-ray diffractometric methods. A deformation inhomogeneity through rod’s cross-section was studied by reduced cubic samples 3x3x3 mm, cut out from different regions of rods. The inhomogeneity of ECAP rods is characterized by distributions of texture and substructure parameters. Main tendencies in structure formation by ECAP are revealed.