Superconducting and metallurgical properties of Nb 3Sn wires processed by internal tin route including hydrostatic extrusion

Abstract

Binary and Ti + Mg alloyed NB 3Sn wires both containing 19 × 132 and 55 × 588 filaments with diameters ranging between 1 and 5 μm have been produced by the internal Sn diffusion process, starting with cold hydrostatic extrusion. These wires exhibit a regular distribution of Sn cores and Nb filaments through the whole cross-section area. A heat treatment scheme including short preheating and a final reaction at 780°C yields J c (non-Cu) values of ≈1.5 × 10 5 A cm −2 at 10 T for binary Nb 3Sn wires. Ti + Mg alloyed Nb 3Sn wires reacted according to the same scheme show higher J c values above 12 T, i.e. 6.1 × 10 4 A cm −2 at 14 T, 1.8 × 10 4 A cm −2 at 18 T. The present Paper includes measurements of J c at applied fields up to 20 T and J c as a function of the applied uniaxial strain, ε, at 13.5 T as well as T c after different heat treatments. The analysis was completed by the SEM determination of the A15 grain size with different additives and the measurement of the Sn concentration profiles by means of Auger spectroscopy for filaments located at different matrix regions. The absolute Sn concentration values were determined by EDX. It is found that the simultaneous addition of Ti and Mg to the Sn facilitates Nb 3Sn grain growth and leads to finer grains.