The residual resistivity ratio, R273/R20, is an important parameter for multifilament superconductors (strands) based on Nb3Sn that are used to manufature cables of magnetic systems. High values of RRR impart stability to the cable with regard to thermal excitations. Nb3Sn strands for magnetic system of the International Thermonuclear Experimental Reactor are manufactured from high purity oxygen-free copper with RRR > 250 units; however, after extended diffusion annealing intended to form superconducting phase Nb3Sn, the residual resistivity ratio values of the strands decrease. This work investigates the influence of diffusion annealing for 55–200 h during the final stage at 650°C on the residual resistivity ratio of chromium-plated Nb3Sn strands. The contents of chromium and oxygen have been analyzed using X-ray spectral microanalysis of the strand surface and peripheral copper layers. Mass spectrometry with inductively coupled plasma (ICP mass spectrometry) has been used to determine the total chromium content in the copper shell. The influence of chromium and oxygen diffusion from coating during annealing at 650°C on the residual resistivity ratio has been demonstrated. Based on the data of ICP mass spectrometry, the depth of the penetration of chromium in a copper shell has been assessed.

The effect of diffusion annealing on the structure of superconducting layers of the Nb3Sn phase and the extent of transformation of niobium filaments of multifilamentary Nb/Cu-Sn superconductors obtained using the bronze process has been investigated by scanning and transmission electron microscopy. The samples differed in the titanium concentration and method of alloying. It has been shown that the thickness of layers of the Nb3Sn phase grows with an enhancement in the titanium concentration in Nb filaments and that it is composites with the alloyed bronze matrix that exhibit the most favorable structure and morphology from the viewpoint of reaching the maximum current-carrying capacity of the conductor in all of the regimes of diffusion annealing under study.

The effect of internal doping of a NbTi composite wire by a large-heat-capacity substance (Gd 2 O 2 S ceramics) on the critical currents and stability against short thermal disturbances (with a typical time on the order of 1 ms) is studied experimentally and theoretically. The composite wire studied in this work is similar in design to conductors used in the international thermonuclear experimental reactor (ITER). The additive introduced into the wire in an amount of 5 vol % raises its specific heat ninefold at 4.2 K. It is found that the critical current of the (NbTi + Gd 2 O 2 S) wire increases by 8–11% in comparison with a reference NbTi wire depending on the external magnetic field varying between 5 and 7 T. Although the potential of high specific heat is not utilized completely, the critical thermal energies of the doped wires are three to four times higher than those of the undoped (reference) wires at near-critical currents.

The titanium-alloyed bronzes with enhanced (14.5 and 15.5 wt %) tin content obtained by the Osprey method have been studied by the methods of optical, scanning, and transmission electron microscopy with the use of quantitative microanalysis in the initial state and after hot extrusion. These alloys have been employed as bronze matrices of multifilamentary superconducting Nb/Cu-Sn composites. An increase in the tin content in the bronze matrix makes it possible to enhance the critical current density of the multifilamentary composite owing to the improvement of the structure and composition of a superconducting Nb3Sn compound that is formed upon diffusion annealing. It has been shown that the Osprey technology allows one to obtain bronzes with an enhanced Sn content and simultaneously to reduce the dendritic segregation characteristic of cast bronzes. The distribution of Sn and Ti in these alloys has been investigated. A more homogeneous structure in the initial state is characteristic of the alloy Cu-14.5Sn-0.24Ti (wt %). The increase in the tin content to 15.5 wt % leads to the formation of microregions of dendritic segregation with an (α + δ) eutectoid in the initial state, which slightly decreases plastic characteristics of this bronze.

Bronzes with an enhanced (14 wt %) tin content which were alloyed with titanium, zirconium, and boron have been studied in the as-cast, homogenized, and deformed states by scanning and transmission electron microscopy and X-ray diffraction analysis. These alloys are of interest as a matrix material for superconducting Nb/Cu-Sn composites in which the high tin content and the alloying of the bronze matrix make it possible to improve superconducting characteristics at the expense of optimization of the structure and properties of layers of the Nb3Sn compound formed at the niobium-bronze interface via reactive diffusion. The distribution of alloying elements in different states of the bronze has been investigated. It has been shown that Zr is uniformly distributed in the alloy and forms no coarse inclusions, whereas Ti forms in the as-cast state large platelike precipitates that can adversely affect technological characteristics of the bronze matrix and the composite as a whole.

The high-strength cold-deformed composites Cu-Nb and Cu-Nb-Zr obtained by the in situ method were studied by transmission electron microscopy, tensile tests, and microhardness measurements. The introduction of additional intermediate annealings in the process of fabrication of thin composite wire was found to lead to a decrease in the ultimate tensile strength and microhardness of the composites, which is caused by coalescence of the niobium fibers. Alloying with zirconium exerts a dual effect on the mechanical properties of the composites examined. On the one hand, Zr favors an appreciable increase in the ultimate tensile strength and microhardness. On the other hand, the alloying leads to a decrease in the energy capacity of fracture and a decrease in the plasticity. In the Cu-Nb-Zr composites, ZrO2 particles of two types (finely dispersed and coarse) were revealed. It was assumed that the coarse particles are formed in the process of solidification from the liquid state. These particles can exert a modifying effect on the structure of both niobium fibers and the bronze matrix but, on the other hand, can bring about embrittlement of the composite wire.

The relationship between the electrical conductivity anti microstructure of copper samples of 99.99 wt % purity obtained by vacuum electron-beam melting has been studied. The miclostructure was studied by transmission electron microscopy and optical metallography. The analysis of the content of impurity elements was performed by mass spectrography and laser mass spectrometry, A correlation was established between the parameter R-273 (K)/R-4.2 K the concentration of oxygen, and the amount of precipitates in copper. The difference in the values of the parameter R-273 K/R-4.2 K observed in the samples studied was shown to be due to differences in microstructure features such as the extension of grain boundaries and the concentration of precipitates of second phases.

The results of the study of the temperature-transition profile in magnetic fields and of the critical current Jc(T,B) in multifilamentary Nb3Sn conductors of different design are considered. We also analyze the contribution of the components with different critical parameters to the current-carrying capacity at high magnetic fields, and the possible causes of inhomogeneity of multifilamentary Nb3Sn conductors of different design formed by “bronze” technology and by “internal tin technique”.

Transmission electron microscopy, optical microscopy, X-ray diffraction, and microhardness measurements were used to study (1) the Cu-Nb(NbTi) microfilament composite, containing about two million niobium filaments filled with the NbTi alloy, after deformation of the composite assembly from 18 mm to 1.5, 0.8, and 0.4 mm in diameter and (2) in situ-produced Cu-Nh pseudocomposites after deformation of the conductor to a diameter of 0.67 mm and annealing at 300 and 600°C. The substructural and barrier strengthening mechanisms are discussed to explain the enhanced strength of these materials. Effects of deformation and annealing on the structure of the composites were studied. The strengthening is thought to be mainly caused by thin niobium filaments with a fine substructure rather than by a high dislocation density.

The kinetics of the formation of superconducting layers of Nb 3 Sn and the fine microstructure of single and multifilamentary Nb/Cu-Sn composites with a bronze matrix alloyed with Zr, Zn, and Mg have been studied by transmission and scanning electron microscopy, electron-microprobe analysis, and optical microscopy. All these alloying elements, magnesium especially, are found to increase the growth rate and thickness of the superconducting layer. The growth rate is shown to be determined by the grain-boundary diffusion of tin and to increase as the grain boundaries are refined from oxygen, which is bound in oxides of the alloying elements. The grain refinement found in the Nb 3 Sn layers of the zirconium-containing composites is suggested to be caused by zirconium segregation at grain boundaries.

Composites of Nb/Cu-Sn with a bronze matrix containing 2 wt % Ga were studied. The effect of this addition on the structure of the constituents of the composite before and after diffusion annealing is reported. Because of the decreased energy of stacking fault formation, the higher concentration of twins and e-phase precipitates is observed in the structure of the gallium-doped bronze matrix. During the diffusion annealing, gallium diffuses into the Nb 3 Sn layer, but a considerable amount of gallium remains in the bronze matrix. This diffusion is thought to be the reason for the enhanced grain growth in the superconducting layer, but it does not stimulate the growth of the layer, the thickness of which does not differ from that of the undoped composite

We studied the effect of small additions of magnesium, aluminum, zirconium, zinc and gallium on the structure and mechanical properties of tin bronzes and bronze matrices in superconducting Nb 3 Sn-based composites. Despite the beneficial effect of the alloying elements on superconducting properties of the composite as a whole, the doping was often found to lower plasticity of the bronze matrix, thus impeding the fabrication of multifilamentary wires. A decrease of the stacking fault energy on doping is thought to be one of the main reasons for the deterioration of plasticity through intensification of twinning and precipitation of the brittle E phase during cold working. Particles due to alloying elements were also found