The Influence of Niobium and Niobium-Titanium Grain Size on the Drawing Instability of Niobium Diffusion Barriers in Niobium-Titanium Wire

A major source of extrinsic limitation to the critical current density in Nb--Ti superconductors has been sausaging initiated by the formation of hard Cu--Ti--Nb intermetallics during heat treatment. Traditionally, a Nb diffusion barrier is wrapped around each Nb--Ti filament as a means of protection. However, a Nb barrier is permeable to both Ti and Cu and local thinning, caused by non-uniform deformation during wire reduction, can decrease the amount of protection provided by a given barrier. We have examined the effect of varying the initial microstructural condition of both the Nb barrier and the Nb--Ti rod. Monofilaments were fabricated and their barrier deformation characteristics measured as a function of strain. Results show that an increasing initial Nb or Nb--Ti grain size escalates the rate at which the coefficient of barrier thickness variation increases as a function of strain. In addition, the two grain sizes appear to combine additively in promoting non-uniform deformat..