Papers by Author: I.L. Deryagina

Abstract: In the present study, the Nb₃Sn-based multifilamentary wires with coupled Nb filaments have been investigated by SEM and TEM after various regimes of intermediate annealing including short high-temperature heat treatments and after two-staged diffusion annealing. The formation of some amount of pre-reacted Nb₃Sn layers has been revealed in all the wires studied, and their amount depends on the wire diameter, temperature and duration of the intermediate heat treatment. The structure of final diffusion layers is also affected by the regimes of these preliminary treatments. This research enables the revealing of the optimal heat treatment schedules for the formation of most perfect nanocrystalline structure of superconducting layers ensuring the highest critical current densities.

Abstract: The structure and thermal stability of Cu-18Nb multicore composite fabricated by repeated cold-drawing of in situ melted mixture of Cu and Nb and subjected to high-pressure torsion (HPT) have been studied by SEM, TEM, X-ray analysis and microhardness measurements. In the cold-drawn state ribbon-like Nb filaments the thickness of 30-70 nm are located in Cu-matrix with sharp texture <110>_Nb║<111>_Cu║drawing axis. The Nb lattice is distorted, the interplanar spacing (110)_Nb being extended along the drawing axis and compressed perpendicular to it, which testifies a semi-coherent character of Cu/Nb interfaces. At annealing these distorsions gradually vanish, and coagulation of Nb ribbons starts at 400С, actively develops at 600С and finishes at 800С with the formation of sausage-like filaments with round transverse sections, which is accompanied with about two-fold decreasing of microhardness. Under the HPT the composite structure is considerably refined, and almost equiaxed grains the sizes of 20-30 nm are formed, which gives rise to a dramatic increase of microhardness. The thermal stability of Cu-Nb composite after cold drawing and HPT is appreciably higher than that of pure Nb and Cu nanostructured by severe plastic deformation.

Abstract: The evolution of structure and morphology of superconducting Nb₃Sn layers forming under various regimes of the diffusion annealing in multifilamentary Nb/Cu–Sn superconductors of different design is briefly reviewed based on the authors’ (with their coauthors) original studies and the available publications. Different modes of the solid-state diffusion formation of the superconducting phase are compared. Possible mechanisms of the Nb₃Sn nucleation upon the diffusion reaction of the Nb filaments and bronze matrixes are discussed. The effects of different doping elements, especially Ti, on the formation of the superconducting phase, its structure and morphology, and, consequently, on the current-carrying capacities of multifilamentary Nb₃Sn-based wires are considered. Special attention is paid to the effect of the mode of Ti doping, the content of this alloying element and the regimes of the diffusion annealing on the structure, morphology and resulting performance of the bronze-processed wires with coupled Nb filaments worked out at Bochvar Institute of Inorganic Materials for the creation of International Thermonuclear Experimental Reactor.

Abstract: The structure and morphology of Nb₃Sn layers in superconducting Nb/Cu-Sn composites with ring (tubular) Nb filaments have been studied by transmission (TEM) and scanning (SEM) electron microscopy after various regimes of diffusion annealing. It is demonstrated that the tubular geometry of Nb filaments, in which Sn diffuses from the bronze matrix both from inside and outside, ensures practically complete transformation of Nb into the superconducting Nb₃Sn phase. Besides, at certain regimes of the diffusion annealing this geometry enables the improvement of the superconducting layers morphology compared to that of wires with continuous filaments, namely, to obtain wide Nb₃Sn layers with fine equiaxed grains and to avoid the formation of columnar grains, which promotes enhanced current-carrying capacities of the wires.

Abstract: The structure and properties of multi-rod Cu-Nb composites with the true strain of 10.2 and 12.5 have been studied by TEM, SEM and microhardness measurements. The non-uniform distribution of Nb ribbons throughout the composite cross sections was revealed, at higher strain their structure being more dispersed. In both wires the Cu/Nb interfaces are partly coherent, and the Nb lattice is more distorted at interfaces than in the bulk. The behavior at heating was studied in the temperature range of 300-800^оС. In the range of 600-800^oC complete coagulation of Nb filaments accompanied with drastic microhardness drop is observed. The thermal stability of Cu-Nb nanocomposites is higher than that of Nb and Cu nanostructured by SPD.

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.

Abstract: The structure of Nb3Sn-based, bronze-processed Ti-doped multifilamentary superconducting wires has been studied by TEM and SEM after the first (5750C,100 h) and the second (6500C,100 h) stage of the diffusion annealing. The Nb3Sn layers formation in all the composites proceeds by one and the same mechanism and starts with nucleation of particles and very fine grains of this phase in Nb filaments where Sn diffuses from the bronze matrix. Ti, inserted both in the bronze matrix, or Nb filaments, diffuses into the growing superconducting layer and promotes its more active formation. At the first stage of annealing (5750C, 100 h) Nb3Sn grains have an average size of 40 nm, and at the second stage (6500C, 100 h) they increase by a factor of 1.5 and the grain size distribution gets wider. After the two-stage annealing the amount of the residual niobium is small, and some Nb filaments, especially in doped composites, almost completely transform into Nb3Sn. In the Nb3Sn layers of a zone of columnar grains is adjacent to the residual Nb.

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).