Evidence for highly localized damage in internal tin and powder-in-tubeNb3Sn strands rolled before reaction obtained from coupled magneto-optical imaging and confocallaser scanning microscopy

Abstract

Nb3Sn strands for high-current, high-field magnets must be cabled before reaction while theconductor is still composed of ductile components. Even though still in the ductile,deformable state, significant damage can occur in this step, which expresses itself byinhomogeneous A15 formation, Sn leakage or even worse effects during later reaction. Inthis study, we simulate cabling damage by rolling recent high performance powder-in-tube(PIT) and internal tin (IT) strands in controlled increments, applying standardNb3Sn reaction heat treatments, and then examining the local changes using magneto-opticalimaging (MOI), scanning electron microscopy (SEM) and confocal laser scanningmicroscopy (CLSM). These combined characterizations allow any local damage to thefilament architecture to be made clear. MOI directly reveals the local variation ofsuperconductivity while CLSM is extremely sensitive in revealing Sn leakagebeyond the diffusion barrier into the stabilizing Cu. These techniques reveal amarkedly different response to deformation by the PIT and IT strands. The studydemonstrates that these tools can provide a local, thorough, and detailed view of howstrands degrade and thus complement more complex extracted strand studies.