Papers by Author: Sergey V. Dobatkin

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Authors: Jozef Zrník, Sergey V. Dobatkin, Ondrej Stejskal
Abstract: The article focuses on the results from recent experimental of severe plastic deformation of low carbon (LC) steel and medium carbon (MC) steel performed at increased temperatures. The grain refinement of ferrite respectively ferrite-pearlite structure is described. While LC steel was deformed by ECAP die (ε = 3) with a channel angle φ = 90° the ECAP severe deformation of MC steel was conducted with die channel angle of 120° (ε = 2.6 - 4). The high straining in LC steel resulted in extensively elongated ferrite grains with dense dislocation network and randomly recovered and polygonized structure was observed. The small period of work hardening appeared at tensile deformation. On the other side, the warm ECAP deformation of MC steel in dependence of increased effective strain resulted in more progressive recovery process. In interior of the elongated ferrite grains the subgrain structure prevails with dislocation network. As straining increases the dynamic polygonization and recrystallization became active to form mixture of polygonized subgrain and submicrocrystalline structure. The straining and moderate ECAP temperature caused the cementite lamellae fragmentation and spheroidzation as number of passes increased. The tensile behaviour of the both steels was characterized by strength increase however the absence of strain hardening was found at low carbon steel. The favourable effect of ferrite-pearlite structure modification due straining was reason for extended work hardening period observed at MC steel.
Authors: Liudmila M. Kaputkina, S.P. Efimenko, Sergey V. Dobatkin, E.M. Leschinskaya, V.M. Panovko, M.A. Kazakov
Authors: Jozef Zrník, Sergey V. Dobatkin, Martin Fujda, Jan Džugan
Abstract: By application of thermomechanical controlled rolling and accelerated cooling, the carbon steel grain refinement is limited to levels of ~ 5 μm in steels. The strain assisted or strain induced transformation could be considered for the refining process. The present work, likewise, deals with grain refinement of medium carbon steel containing 0.45 wt pct carbon having different initial microstructure modified by either thermal and/or thermomechanical treatment (TM) prior severe plastic deformation. In case of TM treated steel, structure refinement was conducted in two steps. Preliminary structure refinement has been achieved due to multistep open die forging process which provided total strain of 3. Uniform and fine recrystallized ferrite structure with grain size of the order of 2-5 μm and with nest-like pearlite colonies was obtained. The further grain refinement of steel samples having different initial structure was accomplished during warm Equal Channel Angular Pressing (ECAP) at 400°C. The steel samples of different initial structure were then subjected to six ECAP pressing passes through die channel angle of 120°. The microstructure development was analyzed in dependence of effective strain introduced (εef ~ 2.5 - 4). Employment of this processing route resulted in extensive deformation of ferrite grains where mixture of subgrains and ultrafine grain was found regardless the preliminary treatment of steel. As straining increases the dynamic polygonization and recrystallization became active to form mixture of polygonized subgrains and submicrocrystalline grains having high angle boundaries. The straining and moderate ECAP temperature caused the partial cementite lamellae fragmentation and spheroidization as straining increased. The lamellae cementite spheroidization was more extensive in TM treated steel samples. The tensile behavior was characterized by strength increase for both structural steel states; however the work hardening behavior was modified in steel where preliminary TM treatment was introduced to modified coarse ferrite-pearlite structure.
Authors: L.L. Rokhlin, Sergey V. Dobatkin, Tatiana V. Dobatkina, N.I. Nikitina, Mikhail V. Popov
Abstract: In this work severe plastic deformation (SPD) was applied to magnesium base alloys of the Mg-Sm system (2.8-5.5 mass %Sm). These alloys are characterized by high strength at elevated temperatures and high strengthening effect during aging. SPD was performed by torsion under pressure of 4 GPa at 20 and 200°C to ε ∼ 6. SPD results in significant strengthening of the Mg-Sm alloys due to the formation of submicrocrystalline structure. In all cases SPD accelerates the solid solution decomposition upon subsequent aging. The highest strengthening can be obtained if the solution treated alloy is aged at 200°C after SPD at room temperature. The state of high strength can be also reached if the following sequence of the operations is used: solution treatment + aging at 200 °C up to maximum hardness + SPD at 20°C + aging at 200°C accompanied by Sm –rich phase precipitation in the submicrocrystalline matrix.
Authors: Vladimir Serebryany, Mikhail V. Popov, A.S. Gordeev, V.N. Timofeev, L.L. Rokhlin, Yuri Estrin, Sergey V. Dobatkin
Abstract: Texture and microstructure formation during equal channel angular pressing (ECAP) of Mg-0.49%Al-0.47%Ca alloy were studied. The selected ECAP condition (route BC, N=6 passes, true strain ε ≈ 6.8, T=300°C) ensures an ultrafine-grained structure of the alloy and basal texture, inclined at an angle of 45-55º relative to the direction of extrusion. The expectation that such a change of the texture, together with the refinement of microstructure, should improve the low temperature ductility of this material was confirmed by tensile testing.
Authors: Vladimir Serebryany, T.M. Ivanova, T.I. Savyolova, Sergey V. Dobatkin
Abstract: Various equal channel angular pressing (ECAP) regimes by routes A and Bc were applied to a commercial MA2-1 (Mg-5wt.%Al-1wt.%Zn-0.4wt.%Mn) alloy for the development of texture which is different from the one of conventionally extruded and annealed alloy. In order to avoid the grain-size effect, the ECAP-processed alloy was annealed to coarsen the grains. The alloy texture before and after the ECAP was determined by the approximation of the X-ray measured pole figures with the canonical normal distributions of central type. The ECAP implementation results in the formation of ultra-fine grained structure of the alloy with an average grain size of 2.0-2.4 µm. The ECAP also drastically changes the initial axial texture characterized by a sharp basal component by splitting it into several more scattered orientations. The degree of the orientation scattering depends on the ECAP regime and route. The annealing of alloy after ECAP results in the grain size growth to the initial state of the extruded and annealed alloy. In addition to that the texture changes of the ECAP-processed alloy after annealing, unlike the structure changes, don’t result in texture of the initial state. The mechanical tensile properties of the annealed alloy substantially depend on the preceding ECAP routes. The yield strength of the annealed alloy decreases after all routes of ECAP. On the contrary, the uniform elongation compared with the one of the initial state of the alloy decreases after 4A route and increases after 4Bc route of ECAP. The effect of the texture and structure on the yield strength and tensile elongation of the alloy after ECAP and annealing was estimated using calculation of the generalized Schmid factors for specific preferred orientations of the active deformation systems and Hall-Petch relationship.
Authors: Alex M. Glezer, Margarita R. Plotnikova, Sergey V. Dobatkin, Nikolai S. Perov, Anna V. Shalimova
Abstract: The effect of severe plastic deformation in Bridgman’s chamber on magnetic properties of the amorphous alloys of metal-metalloid type Ni44Fe29Co15Si2B10, Fe50Ni33B17, Fe70Cr15B15 obtained by the melt quenching was studied. The substantial alteration of saturation magnetization depending on the number of ferromagnetic and antiferromagnetic components in the alloy was revealed. It was supposed that the internal separation into nanoscaled areas enriched and depleted by ferromagnetic components took place in the amorphous matrix under the action of severe plastic deformation.
Authors: Sergey V. Dobatkin, V.I. Kopylov, Reinhard Pippan, O.V. Vasil'eva
Abstract: At present, the possibility of the formation of high-angle grain boundaries upon severe cold deformation, in particular, equal-channel angular (ECA) pressing is reliably proved. The structure formation upon multi-cycle ECA pressing substantially depends on the route determining the shear plane in the sample upon repeated passes. The route is defined by the rotation of the sample around its axis upon the multi-cycle ECA pressing. There are four main routes: route A, in which the sample is deformed by many passes without any rotations; route Ba, in which the sample is rotated by ± 90°; route Bc, in which the sample is sequentially rotated in the same direction by 90°and route C, in which the sample is rotated by 180° about its axis before each subsequent pass. By the methods of SEM, TEM and EBSD analysis it was shown that the fraction of high-angle boundaries in a-Fe upon cold ECA pressing with an angle of 90° between the channels and N=4 depends on the deformation route and increases according to the route sequence: Ba-C-Bc.
Authors: I. Khmelevskaya, Sergey Prokoshkin, Vladimir Brailovski, K.E. Inaekyan, Vincent Demers, Irina Gurtovaya, Andrey Korotitskiy, Sergey V. Dobatkin
Abstract: The main functional properties (FP) of Ti-Ni Shape Memory Alloys (SMA) are their critical temperatures of martensitic transformations, their maximum completely recoverable strain (er,1 max) and maximum recovery stress (sr max). Control of the Ti-Ni-based SMA FP develops by forming well-developed dislocation substructures or ultrafine-grained structures using various modes of thermomechanical treatment (TMT), including severe plastic deformation (SPD). The present work shows that TMT, including SPD, under conditions of high pressure torsion (HPT), equal-channel angular pressing (ECAP) or severe cold rolling followed by post-deformation annealing (PDA), which creates nanocrystalline or submicrocrystalline structures, is more beneficial from SMA FP point of view than does traditional TMT creating well-developed dislocation substructure. ECAP and low-temperature TMT by cold rolling followed by PDA allows formation of submicrocrystalline or nanocrystalline structures with grain size from 20 to 300 nm in bulk, and long-size samples of Ti-50.0; 50.6; 50.7%Ni and Ti-47%Ni-3%Fe alloys. The best combination of FP: sr max =1400 MPa and er,1 max=8%, is reached in Ti-Ni SMA after LTMT with e=1.9 followed by annealing at 400°C which results in nanocrystalline (grain size of 50 to 80 nm) structure formation. Application of ultrafine-grained SMA results in decrease in metal consumption for various medical implants and devices based on shape memory and superelastiсity effects.
Authors: Irina S. Tereshina, Evgeniya A. Tereshina, Gennady S. Burkhanov, Sergey V. Dobatkin
Abstract: In the present work a study of the influence of structural state on magnetic hysteresis properties of (Nd,Ho)2(Fe0.8Co0.2)14B compound was carried out. Starting alloy was prepared by induction melting in an Ar atmosphere. Nd-Ho-Fe-Co-B alloys with a nanograin structure were obtained by severe plastic deformation (SPD). Electron microscopy and X-ray analysis were used for the structural investigation. The magnetization measurements were performed using a SQUID magnetometer. It is shown that the relatively high values of coercive force are observed in case of achieved nanograin structure in (Nd,Ho)2(Fe0.8Co0.2)14B. The effect of structural state on hysteresis properties of Y2(Fe0.8Co0.2)14B is also investigated.
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