Papers by Author: E.N. Popova

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Authors: E.N. Popova, Vladimir V. Popov, E.P. Romanov, S.V. Sudareva, L.V. Elohina, A.E. Vorobyova, A.K. Shikov, V.I. Pantsyrny, S.V. Sudiev
Abstract: Multifilamentary Nb3Sn-based superconducting composites manufactured by an internal-tin method have been studied by transmission (TEM) and scanning (SEM) electron microscopy. The main goal of this study is to reveal the effect of diffusion annealing regimes as well as the external diameter of the wires on the structure of nanocrystalline Nb3Sn layers (average grain size, grain size distribution, layer thickness, amount of Sn, etc.). It is demonstrated that multistep diffusion annealing results in quite a complete transformation of Nb filaments into Nb3Sn though some amount of the residual Nb remains in the filaments center. With an external diameter decrease the superconducting layers structure has been found to refine and get somewhat more uniform. An additional high-temperature annealing results in marked growth of Nb3Sn grain sizes and their scattering in sizes, which may negatively affect the current-carrying capacity of a wire.
Authors: E.N. Popova, Vladimir V. Popov, E.P. Romanov, N.E. Hlebova, V.I. Pantsyrny, A.K. Shikov
Abstract: The structure and properties of in situ Cu-Nb composites have been studied by the methods of TEM and SEM, X-ray analysis and mechanical testing. The evolution of Nb filaments structure at drawing and the effect of annealing and doping with Zr on the composites structure and texture have been investigated. Sharp fibre texture with <110>Nb develops in the ribbon-shaped Nb filaments, while the Cu matrix possesses a two-component fibre texture, <111>Cu and <100>Cu, appreciably affected by annealing. Annealing results in softening due to Nb filaments coagulation. In Zr doped composites two types of ZrO2 particles are observed, dispersed and coarse ones. The former result in the dispersion strengthening, while the latter may cause embrittlement.
Authors: E.N. Popova, Vladimir V. Popov, E.P. Romanov, S.V. Sudareva, E.A. Dergunova, A.E. Vorobyova, S.M. Balaev, A.K. Shikov
Abstract: Kinetics of formation of superconducting Nb3Sn layers and the structure of bronzeprocessed Nb/Cu-Sn composites with Zr, Zn or Mg-doped matrixes or Ti-doped Nb filaments of different geometry have been studied by the methods of TEM, SEM and electron-probe microanalysis. All the doping elements have been found to accelerate the rate of growth of the diffusion Nb3Sn layers and consequently their thickness. Correlation between the diffusion annealing schedules, the geometry of Nb filaments, the structure of the diffusion Nb3Sn layers and the current-carrying characteristics of multifilamentary Nb/Cu-Sn composites has been established.
Authors: Vladimir V. Popov, Gerrit Reglitz, Evgeniy V. Shorohov, E.N. Popova, A.V. Stolbovsky, Sergiy V. Divinski, Gerhard Wilde
Abstract: Formation of microstructure in Ni under equal-channel angular pressing (ECAP) and dynamic channel-angular pressing (DCAP), its thermal stability and diffusion properties of grain boundaries are investigated. Grain boundary diffusion in the ultrafine-grained Ni is found to be significantly faster than in the coarse-grained Ni, which indicates a 'non-equilibrium' (deformation-modified) state of grain boundaries in the former. The effect of non-equilibrium state of grain boundaries on the level of internal stresses is analyzed.
Authors: E.N. Popova, I.L. Deryagina, M.V. Polikarpova, D.C. Novosilova, A.E. Vorobyova
Abstract: The structure of high-purity copper and stabilizing copper of Cr-plated Nb3Sn-strands with different RRR (residual resistance ratio) values has been studied. Cr diffusion into peripheral layers of stabilizing Cu has been revealed. The variations of RRR and Cr content in Cu at annealing have been estimated. It is demonstrated that an additional drop of RRR may be caused by oxygen diffusion from Cr coating obtained by electroplating. Optimal regimes of the diffusion annealing of Cr-plated Nb3Sn-strands for ITER (International Thermonuclear Experimental Reactor) have been determined.
Authors: E.N. Popova, E.P. Romanov, I.L. Deryagina, S.V. Sudareva, E.A. Dergunova, A.E. Vorobyova, S.M. Balaev
Abstract: Bronze-processed Nb3Sn-based multifilamentary composites with external diameter of 0.8 and 0.5 mm and coupled Nb filaments have been studied by transmission (TEM) and scanning (SEM) electron microscopy. After the two-staged annealing, 575°С, 150 h + 650°С, 200 h, commonly used for ITER conductors, a nanocrystalline layer of superconducting Nb3Sn compound is formed in every Nb filament as a result of solid-state reactive diffusion of Sn from the bronze matrix. It is demonstrated that in the wires of smaller external diameter the Nb filaments transformation into the Nb3Sn compound is more pronounced, that is the amount of the residual Nb is smaller. Besides, the nanocrystalline structure of the Nb3Sn diffusion layers is more perfect in 0.5 mm diameter wires, namely, the Nb3Sn grains are finer (their average size being 60 nm compared to 70 nm in 0.8 mm diameter wires) and are more uniform in sizes (the root mean square deviation of grain size distribution is correspondingly 15 and 17 nm).
Authors: V.V. Popov, E.N. Popova, D.D. Kuznetsov, A.V. Stolbovsky, G. Reglitz, S.V. Divinski, G. Wilde, E.V. Shorohov
Abstract: Evolution of structure and properties of Ni under severe plastic deformation by equal-channel angular pressing (ECAP) and dynamic channel-angular pressing (DCAP) has been studied by transmission and scanning electron microscopy and microhardness measurements, and the differences in formation of submicrocrystalline structure under different deformation techniques have been revealed. The thermal stability of structure obtained by these methods is compared. It is demonstrated that under the DCAP processing the material is strengthened faster, by lesser number of passes, and microstructure’s thermal stability is somewhat lower after ECAP compared to that after DCAP, although after equal number of passes ECAP results in a more homogeneous microstructure.
Authors: Jon Alkorta, C.J. Luis-Pérez, E.N. Popova, Martin Hafok, Reinhard Pippan, J. Gil Sevillano
Abstract: A commercially pure niobium has been subjected to SPD at room temperature ( ~0.11 TM) via ECAP (90º, route BC) up to 16 passes and via HPT up to shear strains γ =1000. ECAP-ed samples show an equiaxed structure after 8 and 16 passes with a decreasing average grain size. The results show that both the microstructure and mechanical properties of ECAP-ed samples do not reach a steady state up to at least 16 passes. HPT samples show at outer region a finer structural size but similar hardness values at similar equivalent strains. The nanoindentation results show an evident indentation size-effect even for the most deformed samples. The hardness values at the nano level converge for the recrystallized, the ECAP-ed and the HPT samples. This implies that, at the nano level, when the geometrically necessary dislocation density overcomes significantly the (initial) statistically stored dislocation density, hardness depends mainly on the physical intrinsic properties of the material (Burgers modulus, bulk modulus...) and the contribution of bulk mechanical properties (i.e., bulk yield strength) to hardness is smoothed down. Strain-rate sensitivity (SRS) of plastic strength has been also measured by means of rate-jump nanoindentation tests. The SRS is proportional to the inverse of hardness.
Authors: Vladimir V. Popov, E.N. Popova, D.D. Kuznetsov, A.V. Stolbovsky, E.V. Shorohov, Gerrit Reglitz, Sergiy V. Divinski, Gerhard Wilde
Abstract: Various modes of severe plastic deformation (SPD), such as high-pressure torsion (HPT) at cryogenic temperature, equal channel angular pressing (ECAP) and dynamic channel-angular pressing (DCAP), have been applied for nanostructuring of Ni, and the thermal stability of the structure obtained has been studied. The nanocrystalline structure with average grain sizes of 80 nm and the microhardness of 6200 MPa is produced by HPT in liquid nitrogen. DCAP and ECAP result in the submicrocrystalline structure of a mixed type, with ultra-fine grains separated by high-angle boundaries along with deformation bands and coarse cells with low-angle dislocation boundaries. The thermal stability of the structures obtained by ECAP and DCAP is approximately the same, and it is higher than after the HPT at cryogenic temperature.
Authors: Vladimir V. Popov, V.N. Kaigorodov, E.N. Popova, A.V. Stolbovsky
Abstract: The properties of grain boundary cores and adjacent areas in polycrystalline Nb manufactured by rolling with subsequent recrystallization annealing and nanocrystalline Nb obtained by high pressure torsion have been investigated by the Mössbauer emission spectroscopy on 119mSn nuclei.
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