Strain effects on critical current densities have been examined for conductors containing nearly stoichiometric Nb3Al filaments with fine grains. The Nb3Al phase in these multifilamentary conductors are prepared by phase transformation from supersaturated Nb(Al) bcc solid solution and show high-field critical current densities much larger than those for conventionally prepared Nb3Al conductors, where the Nb3Al phase is known to be off-stoichiometric. The degradation of critical current densities with −0.7% intrinsic strain is ca. 20% at 12 T, comparable with those for conventional Nb3Al conductors of high strain tolerance.

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Microstructures of rapidly-heated/quenched and transformed Nb& multifilamentary wires were studied by using transmission electron microscopy. Nb/AI composite wires are fabricated by a jelly-roll process. The Nb/AI composite filaments changed into Nb-AI supersaturated bcc solid solution with the rapidly-heating/quenching. The Nb-AI bcc phases consists of many crystal grains with diameters of 2-4 pm, surrounded with large-angle grain boundaries. Some spherical voids, about 0.1 micron in diameter, were also observed at the intra-and intergrains. All grain boundaries of the Nb-AI bcc phases are simple flat planes. Then, the Nb-AI bcc phases were transformed into A15 phases (grain size: 0.5-2.0 pm in diameter) with additional annealing. Secondary phases were not observed in the A15 filaments. Grain boundaries of the A15 phases show zigzag shape unlike those of the Nb-AI bcc phases, and every grain of A15 phases is an aggregation of sub-grains of 80-150 nm in diameter. Sub-grain boundaries are small-angle ones. Moreover, we found that many stacking faults formed in the A15 sub-grains in parallel with spaces of 10-20 nm. These numerous plane defects seem to be the main pinning centers in the rapidlyheated/quenched and transformed NbJAl wires. Index

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The advantages of the MgB2 compound are the high Tc value (39 K) and lower material cost than other superconducting metallic compounds. In addition, MgB2 wire has a shorter decay time for induced radioactive properties than Nb-based superconductors. However, the Jc value of MgB2 wire is poor compared with Nb-based superconductors, and further improvement of Jc is required if the compound is to be used in a future nuclear fusion reactor. In order to improve Jc, we investigated Cu addition using the Mg2Cu compound in MgB2/Fe-sheathed wires through the powder-in-tube (PIT) process. Tc values of Cu addition samples were higher than those of non-Cu addition samples and they were improved with increasing Cu addition. We obtained maximum core Jc values over 100 A mm−2 at 4.2 K under 8 T for 1 and 3 at.% Cu addition samples. Moreover, we found that Cu addition was more effective at improving the value of Jc in low and middle magnetic fields than the SiC addition process.

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