Optical microstructure and superconducting properties in jelly-roll Nb₃Al multifilamentary wire by rapid heating

To form the stoichiometric composition in the A15-phase, a rapid quench from the stable region at high temperatures is needed. Jelly-roll Nb₃Al wire with a Nb matrix is ohmically-heated up to 2000 °C during 0.1 sec in a vacuum. An optimization of rapid heating conditions is needed to an improve critical current density at high field because very short heating times and high temperature close to melting points of the Nb₃Al filaments and Nb matrix are used. The magnetic field dependence of critical current density, J_c, and the critical temperature, T_c, were studied as a function of the maximum temperature, T_m, using short samples that were heat-treated systematically. The cross-sections of these typical samples are analyzed by optical microscopy on various rapid heating conditions. In the short heating time of a range of 0.32-0.46 sec, the filament regions heated up to 2000 °C would be uniform and have relatively homogeneous superconducting properties. In the samples with long heating time, the filaments reacted with around matrix. The filament regions heated up to 2300 °C are inhomogeneous and some filaments have melted.