Observations on Titanium Diffusion in ITT Type Nb Sn Superconductors

Titanium doping has been successfully employed to increase high field J c in internal tin (IT) type conductors. Titanium doping is achieved by placing Nb-47Ti rods within the filament array. Doping occurs during reaction heat treatment as a result of titanium diffusing from the Nb-47Ti source into the niobium filaments. We have investigated doping of internal-tin-tube (ITT) type conductors using Nb-47Ti rods placed in holes drilled into the niobium annulus of each filament. The titanium distribution at various stages during reaction heat treatment was investigated by SEM/EDS and the A15 grain boundaries were examined with Scanning Auger Microanalysis. We observed that there is minimal titanium diffusion into the niobium until the Nb 3 Sn reaction front reaches the Nb-47Ti source, and that there is a loss of titanium to the residual filament core and the core/annulus interface. It was determined that the primary route for titanium diffusion is through the A15 grain boundaries.