FINE GRAINED Nb for INTERNAL TIN Nb₃Sn CONDUCTORS

S.Balachandran¹, R.E.Barber², Y. Huang³,
H.Miao³, J.A. Parrell³, S. Hong³, R.B. Griffin⁴, K.T.Hartwig¹

¹ Department of Mechanical Engineering,
Texas A&M University, College Station TX 77843-3123, USA.
² ShearForm Inc, Bryan TX 77801,USA
³ Oxford Superconducting Technology, Carteret, NJ 07008,USA. .
⁴ Department of Mechanical Engineering, Texas A&M University, Qatar

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 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 and higher J_c.

KEYWORDS: Cu-Nb, fine grain Nb, high J_c, RRP, ECAE, SPD, interface roughness.

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IEEE/CSC & ESAS EUROPEAN SUPERCONDUCTIVITY NEWS FORUM (ESNF), No. 10, October 2009.
On June 29, 2009, this manuscript was submitted for possible publication in “Advances in Cryogenic Engineering-Materials"