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

Abstract

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 <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn and Cu-V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> 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 <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> 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 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . At high fields, the performance of Cu-V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Ga in situ composites is significantly better than that of Cu-Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn conductors, with typical normalized values of J <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> of 1.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 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.