J. Bevk

Anomalous increase in strength of in situ formed Cu-Nb multifilamentary composites

Cu‐Nb wire composites with 0.105, 0.148, and 0.182 volume fraction of Nb filaments were produced in situ and their mechanical properties measured as a function of filament size and interfilament spacing. The yield stress and the ultimate tensile strength increased with both niobium volume fraction and overall composite reduction. At room temperature, the ultimate tensile strength of the Cu–18.2 vol% Nb composite reduced by 99.999% in cross‐sectional area (100–200 A filament thickness) reached the value of 2230 MN/m2 (323 ksi) and further increased to 2850 MN/m2 (413 ksi) when measured at 77 °K. These values are higher by a factor of 4 than the values predicted by the rule of mixtures based on the highest reported strength of both niobium and copper. The composite strength is as high as that of the best copper whiskers and is shown to closely approach the theoretical strength of the material. The anomalous increase in strength despite the low volume fraction of reinforcing filaments suggests that the filam...

Normal‐state resistivity of in situ–formed ultrafine filamentary Cu‐Nb composites

Normal‐state electrical resistivity has been studied in Cu‐Nb composites containing 7.5, 10.0, and 15.0 vol% niobium filaments formed by an in situ process. The resistivity of the as‐drawn and annealed composite wires was measured at temperatures between 300 and 825 K and just above the superconducting transition (∼9 K). The electron scattering from dislocations and interfaces was particularly pronounced in composites with submicron filaments and rapidly increased when the filament thickness decreased to a few hundred angstroms. The magnitude of the boundary scattering contribution in the smallest wires (25 mm in diameter) was found to be comparable to the predicted surface scattering in homogeneous copper wires of only 100 nm in diameter. In contrast to the bulk materials, dislocation scattering in highly reduced in situ composites can be the dominant scattering mechanism up to at least 300 K. These results imply a maximum dislocation density of about 1013 cm/cm3. The experimental findings are compared t...

Superconducting and mechanical properties of in situ formed multifilamentary Cu‐Nb3Sn composites

A systematic experimental study of the variations of superconducting transition temperature and critical‐current density is reported for in situ formed multifilamentary Cu‐Nb3Sn composites containing 10 at.% Nb and 2–3 at.% Sn annealed at either 650 or 700 °C. A particular emphasis was placed on the evaluation of uniformity, thermal stability, and mechanical strength. Critical‐current density was measured as a function of transverse magnetic field and was found to increase in samples measured in a bent position. The overall critical‐current performance is comparable to that of reinforced stabilized conventional composites. Large residual resistivity ratios are indicative of a clean high‐conductivity matrix surrounding each individual filament, an important requirement for thermal stability. High resistance to plastic flow in these composites is attributed to strong filament‐to‐matrix bonding and small interfilament spacing. The ultimate tensile strength at 77 °K reached a value of ∼100 ksi (690 MPa). The ...

Critical currents in liquid‐quenched Nb3Al

Critical current densities of over 1.8×106 A/cm2 at 4.2 °K and 150 kG have been achieved in Nb3Al tapes rapidly quenched from the liquid state at rates of 105–106 °C/sec. The superconducting transition temperature is about 16 °K, indicating a relatively high degree of order, and increases to 18.4 °K upon annealing at 750 °C. The observed microstructure depends sensitively on the quenching rate and consists of small submicron grains surrounded by a thin boundary layer of different composition. The grain size is roughly inversely proportional to the temperature gradient during quenching. The quoted critical current densities, the highest observed to date in any superconductor, should be regarded as a lower limit of the material critical value imposed by the heating problems due to the contact resistance.

Superconducting properties of in situ formed Cu‐V3Ga composites

Cu‐V3Ga composites were prepared from in situ formed multifilamentary Cu‐V wires containing 20 vol.% of vanadium filaments. The highest value of the upper critical field at 4.2 K of the reacted composites was found to be 22.4 T, with a corresponding midpoint superconducting transition of 15.5 K. The overall critical‐current density compares favorably with commercial V3Ga composites over the entire field range (2×105 A/cm2 at 4 T, 104 A/cm2 at 18 T).

Stress effects in superconducting Cu‐V3Ga in situ composites

The effect of the uniaxial tensile stress on the critical‐current density Jc, and upper critical field Hc2, of in situ formed Cu‐V3Ga composites has been investigated. Jc was found to gradually decrease at stresses greater than ∼200 MPa. The effect is small, amounting to a decrease of only 20% at 16 T and 1 GPa. However, no permanent degradation was observed even at stress levels in excess of 1 GPa. Similarly, Hc2 was found to be rather insensitive to applied stress. The observed decrease of about 0.5 T at 1 GPa was fully recovered after releasing the stress.

In situ formed multifilamentary composites part I: Coupling mechanisms, stress effects and flux pinning mechanisms

Recent developments on in situ formed multifilamentary composites are reviewed and their superconducting and mechanical properties discussed in terms of the underlying physical mechanisms. The evidence is presented for a strong size dependence of the strengthening, flux-pinning and coupling mechanisms and, in turn, the composite normal-state and superconducting transport properties. The importance of the composite microstructure and micro-geometry is illustrated with data on Cu-Nb, Cu-Nb 3 Sn and Cu-V 3 Ga conductors. In particular densely spaced interfaces are shown to interact effectively with both matrix crystal dislocations and flux-line lattice, resulting in strongly anisotropic material properties. The importance of the proximity-effect coupling is discussed for Nb 3 Sn-based composites below the microstructural percolation threshold where the self-field critical current densities (normalized to the filament volume fraction) reached values of 1.4 × 107A/cm2. At high fields, the performance of Cu-V 3 Ga in situ composites is significantly better than that of Cu-Nb 3 Sn conductors, with typical normalized values of J c of 1.4 × 107A/cm2at 18 Tesla and 4.2 K. Possible use of Cu-Nb in situ composites in high-field magnet design is also discussed in view of their remarkable strength (up to 2.9 GPa at 77 K) and high normal-state conductivity.

Critical currents of in situ formed multifilamentary Cu-Nb3Sn composites

Critical current densities of in situ formed Cu-Nb3Sn composites with discontinuous filaments were measured as a function of superconducting volume fraction, matrix resistivity, area reduction ratio, and applied magnetic field. In agreement with recent modelling by Tinkham and co-workers, the effective superconducting volume fraction in a given composite was found to be field-dependent, necessitating the distinction between microstructural and electrical percolation. In composites with a low filament volume fraction, proximity effect coupling, controlled by matrix resistivity, was found to be the dominant factor determining both the composite remnant resistivity and the critical current density. For sufficiently high filament volume fractions and area reduction ratios, the remnant resistivities fall below the level of detection, as predicted by theory, and critical current densities become comparable to those of continuous filament composites. SEM, TEM, and STEM analysis reveal a dense distribution of submicron, ribbon-like Nb3Sn filaments in relatively pure Cu matrix. The microstructure of the filaments is equi-axed with an average grain size of ∼ 400Å, ensuring effective flux pinning.

Superconducting critical properties and AC losses in a large sample of in situ formed Cu‐Nb3Sn composite

Critical‐current density, upper critical field, and hysteresis ac losses have been evaluated for a large (100‐m long) sample of in situ formed Cu‐Nb3Sn composite with superconducting volume fraction λ≃0.23. The magnetic field dependence of the critical current, measured in wire and tape samples of different reductions, cannot be explained by a model based on a single pinning mechanism. The enhancement of the pinning force in highly reduced tape composites is attributed to surface flux pinning at the filament‐matrix interfaces. Hysteretic ac losses obtained by calorimetric and electronic measuring methods confirmed the results reported previously for small in situ samples.

Enhancement of the Young’s modulus in the ultrafine Cu‐Nb filamentary composites

Measurements of the Young’s modulus are reported for in‐situ formed Cu‐Nb wire composites containing 7.5 and 15.0 vol.% niobium filaments. Comparison of the experimental values for the as‐drawn and annealed wires shows rapidly diverging trends with decreasing wire diameter. In composites with the smallest filaments (50–200 A) the modulus increases by almost 100% upon annealing and exceeds the maximum values calculated from bulk elastic constants. However, plastic strain (in elongation) of as little as 0.02% results in a substantial (∼25%) reduction in modulus, suggesting that most of the enhancment in the annealed composites originates from the thermally induced elastic strains at the matrix‐filament interfaces.