Microchemical and microstructural comparison of high performance Nb3Al composites

We have performed a comparison of the microstructures of state of the art Nb₃Al composites processed using both ordinary RHQT (rapid-heating, quenching and transformation) and TRUQ (transformation-heat-based up-quenching) routes. Cross-sections were examined in the as-quenched, untransformed, and final size strands including Cu-clad strand. Both grain size and microchemistry were examined using a high resolution FESEM in BEI mode, using low accelerating voltage for grain orientation contrast and high voltage for atomic number contrast. The grain size is relatively large in these composites with a Feret diameter of 1300 nm for the TRUQ processed strand (compared with 70 to 160 nm for Nb₃Sn composites). In the untransformed strand electron backscatter indicated residual chemical inhomogeneity associated with the jelly-roll precursor. In the final strands the variations were much less but longitudinal cross-sections revealed the residual chemical inhomogeneity extending along the strand length. In both the ordinary and TRUQ (Cu and Cu-clad) processed strands a 1 μm thick 2-phase reaction layer was revealed on the outside surface of the outer filaments that had an average composition of 10% Al and 90% Nb. D.C. Magnetization measurements at 12 K indicated a 1 T improvement in irreversibility field, H^*, for the TRUQ strand compared with ordinary RHQT strands.