STRAIN EFFECTS ON THE SUPERCONDUCTING PROPERTIES IN NB3AL ULTRA-THIN FILAMENTARY WIRES

Because of large upper critical fields above 30 T at 4.2 K, the Nb3Al superconductor may be regarded as promising for high field superconducting magnet use. Nb3Al multifilamentary wires which have recently been fabricated by a modified composite diffusion process show the high critical current densities at high fields, comparable to those for practical Nb3Sn wires. In this study effects of tensile strain on superconducting properties in the Nb3Al multifilamentary wires have been investigated to see if these wires are useful for applications where superconductors are subjected to high mechanical loads. Critical temperatures and currents have been studied for multifilamentary wires containing more than 1 million of Nb3Al filaments as a function of applied tensile strain. Relations of critical currents under strain to composite configurations and microstructures of wires are examined through tensile tests and scanning electron microscopic observations of fractured surfaces. While critical currents for samples with Al-Mg or Al-Cu-Ge alloy cores in the Nb matrix decrease monotonically with applied strain, samples with a Cu-Ni alloy sheath around the Nb matrix show peaks in the critical currents vs. strain curves. The strains for the occurrence of permanent degradation in critical current have been more than 1% and the strain dependences of critical currents in these Nb3Al wires have been much smaller than that for Nb3Sn wires.