Prevention of Barrier Degradation in Fine Filament NbTi Conductor Produced from As-Cast NbTi Ingot

Abstract

A major factor limiting current density in fine filament NbTi conductors is sausaging of the filaments caused by Cu-Ti-Nb intermetallic formation. The intermetallic formation is primarily attributable to the mechanical degradation of the niobium diffusion barriers surrounding the filaments. Barrier breakdown is accelerated when large grains are present at the NbTi ingot surface in the monofilament billet. An innovative method has been developed by which to prevent barrier degradation when extremely large-grained, as cast NbTi ingot is utilized in conductor fabraction^1,2,3. Fine-grained NbTi sheets are interposed between the niobium barrier and the NbTi ingot. The NbTi sheets serve as a buffer, presenting a smooth profile to the niobium barrier. The method has been successfully applied to an as-cast plasma melted ingot. Barrier quality is dramatically improved as compared to un-buffered filaments. Current densities and n-values comparable to those obtainable conventionally are achieved. The approach makes it feasible to use as-cast ingot in conductor fabrication. Significant cost savings can be realized as compared with conventional NbTi conductor fabrication by virtue of fewer intermediate ingot processing operations and improved monofilament yields.