Investigation of the Effects of Low Temperature Heat Treatments on the Microstructure and Properties of Multifilamentary, Tube-Type Nb₃ Sn Strands

Multifilamentary, tube-type composites consisting of subelements of Nb with a simple Cu/Sn binary metal inserts were studied in an attempt to enhance the performance boundaries of these conductors. TransportJcvalues for these multifilamentary strands were found to be as high as 2440 A/mm2at 12 T, 4.2 K for 217 strand conductors. We focused on correlating the composition and morphology of the intermetallic A15 to the transport and magnetic properties for varying heat treatments. In particular, lower temperature HTs were studied, specifically 625degC and 635degC as a function of time. The extent of A15, the ratio of the coarse/fine grain areas, and amount of the untransformed 6:5 were then observed as a function of time-temperature at these lower temperatures where the ternary phase diagram three-phase region boundaries have shifted from their 675degC positions. Compositional analysis was carried out on the A15 phase to study the variation in stoichiometry as a function of temperature. Fractography was also performed to investigate the effect of temperature and alloying on the morphology and grain size of the fine grain A15.