Ti and Sn diffusion and its influence on phase formation in internal-tin Nb3Sn superconductor strands

Nb₃Sn superconductor wires were investigated in terms of Nb₃Sn phase formation and Ti diffusion. Two series of subelement strands (HP and EG) with Ta, Ti addition were fabricated. For comparison, a special model sample consisting of an annular Nb region containing Sn and surrounded by Cu was fabricated. All samples were given a standard initial up-ramp-and-hold schedule (185°C/24 h (ramp rate 10° C/hour) + 340°C/ 48 h (ramp rate 25°C/hour) + 700°C/up to 150 h (ramp rate 10° C/hour)). Reaction heat treatments at 700°C were performed for various times on these subelements. The diffusion of Sn and Ti was studied by SEM, EDS, EPMA and STEM analysis. The Nb₃Sn reaction rate was determined for strands with and without Ti, and Ti was seen to double the rate of Nb₃Sn formation for these strands. EDS analysis showed that within the filament annular shell, an even distribution of Ti existed with a concentration of 0.4-1.0 at. %. However, for the highest Ti bearing sample, it had a Ti Nausite at the filament boundaries which may reduce the strands superconductor property.