FINE GRAINED NB FOR INTERNAL TIN NB[sub 3]SN CONDUCTORS

Abstract

The push to drive superconductor strand technology to reach higher critical current density (J c ) 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.

Niobium rod quality and its impact on the production of Nb3Sn strand for the Divertor Tokamak Test Facility toroidal coils

“…Even thought the hardness ranges overlap, the seperation is complete by grain size for both critical current and AC loss. It has been observed in the past that the roughness of the niobium-copper interface in co-drawn materials is dependent on the initial grain size of the niobium 6 . As the niobium is body centered cubic and the copper is face centered cubic, the difference in slip systems causes some niobium grains to rotate prior to deforming to get into the easy direction.…”

Microstructure and mechanical properties of fine grain seamless Nb tube by a novel shear deformation process

“…The shear processed sample that underwent intermediate annealing, indicates a major rotated cube component (100)||RD. The tensile tests for this sample were performed along [4,3,0]||CD and [3,4,0]||LD. These rotations about the cube are not symmetry related and one does not expect the tensile properties to be similar.…”

“…A successful thermomechanical approach to decrease the severity of this problem is to grain refine bulk Nb by severe plastic deformation (SPD) steps and subsequent annealing [3,4] to obtain a uniform starting microstructure. The authors here present a thermomechanical processing approach by SPD involving simple shear and area reduction extrusion.…”