Nonuniform deformation of niobium diffusion barriers in niobium-titanium wire

A major source of extrinsic limitation to the critical current density in Nb-46.5 wt.% Ti superconductors has been sausaging initiated by the formation of hard intermetallics of Cu-Ti-Nb at the Cu-superconductor interface during precipitation heat treatment. The main defense against this is the use of a diffusion barrier (normally Nb foil) between the Cu and the Nb-Ti, which has a tendency to deform in a nonuniform manner even prior to heat treatment. Initial examination of Superconducting Super Collider (SSC) R&D composites indicated that most of the nonuniform deformation of the barrier occurred at the interface between the Nb barrier and the Nb-Ti. The authors have examined the effect of varying the starting Nb-Ti grain size on the uniformity of barrier thickness reduction during wire drawing. For all the wires tested the nonuniformity of the barrier increased with drawing strains and with increasing initial Nb-Ti grain size. They also compared the barrier of cold drawn monofilament to that of warm extruded filaments. The extruded filaments had a significantly more nonuniform barrier.>