Microstructural refinement of bulk Nb and Ta by severe plastic deformation for composite superconductor applications

Fine grained and homogeneous recrystallized microstructures in precursor Nb and Ta are necessary to avoid filament sausaging and layer fracturing during the production of Nb3Sn superconductor wires. In this study, the effectiveness of multi-pass equal channel angular extrusion (ECAE) to break up large as-cast microstructures (> 5min diameter columnar grains) in vacuum arc remelted Ta and electron beam remelted Nb is discussed. Extrusions were performed at room temperature in 90 degrees tooling via ECAE with routes C, B-c, and E. Results show significant refinement to average grain sizes of less than 30 mu m after four extrusions. Four passes via routes E and B-c showed the best potential for refinement due to deformation of the as-cast microstructure on orthogonal planes.