Fine grained Nb for internal tin Nb3Sn conductors

The push to drive superconductor strand technology to reach higher critical current density (Jc) values and reduce production costs has led to innovative approaches in manufacturing technology. The Restacked Rod Process (RRP®) by Oxford Instruments is one such process which involves Nb bar extrusions in a Cu sheath. Commercially available Nb used in the initial RRP extrusion leads to nonuniform deformations of the Nb bar which in turn leads to a jagged Cu-Nb interface. This report presents a feasible methodology to remedy the problem of nonuniform deformation of Nb through severe plastic deformation (SPD) of precursor Nb to obtain smaller grains in starting Nb. Cu-Nb monocore extrusion and drawing experiments were accomplished at Oxford Instruments using Nb bars of nominal dimensions 45mm diameter by and 78mm long and with grain sizes in the range of μm to mm. Results of Cu-Nb interface roughness measurements show that a finer starting grain size gives a significantly lower roughness and better Nb core conformance to initial shape. Our experiments indicate that refinement of the initial Nb grain size to below ∼50μm could enable fabrication of RRP conductor with improved wire yield. © 2010 American Insititute of Physics