The microstructure and critical current density of Nb-48 wt.%Ti superconductor with very high alpha-Ti precipitate volume and very high critical current

Superconducting strand has been fabricated with the highest yet recorded 5 T, 4.2 K critical current density for conventionally processed Nb-Ti. Starting from an ingot of high homogeneity Nb-48 wt.%Ti alloy, both mono- and multifilamentary superconductors were fabricated using multiple heat treatment and drawing schedules with very long heat treatments. Using high-resolution FESEM and TEM, the influence of treatment conditions on the microstructure was investigated. With 1000 to 8000 h of total heat treatment time, the volume of precipitate approaches the decomposition limit of the solid solution and the critical current density increases, reaching a maximum > 4(.)10(5) A/cm(2) at 5 T, 4.2 K. The amount of the precipitated alpha-phase reaches 25-33 vol. %, and the precipitate thickness is 1-4 nm. In order to create a more ordered multilayer structure in the high precipitate-content strand, the samples were rolled as a final stage of processing. The highest level of current characteristics is reached with this combined processing (drawing - rolling). The pinning mechanism, responsible for the high current characteristics in these strands is analyzed.