Insight into the effect of Ti-addition on diffusion-controlled growth and texture of Nb3Sn intermetallic superconductor phase

Abstract

The alloying of Ti with Cu(Sn) and Nb significantly increases the grain boundary diffusion-controlled growth kinetics of Nb3Sn accompanied with a decrease in the activation energy in the Cu(5.5 at.% Sn, Ti)/Nb and Cu(5.5 at.% Sn)/Nb(Ti) diffusion couples. In either case, the β-(Ti,Nb) precipitates form at the grain boundaries of Nb3Sn. On the other hand, the ternary intermetallic phase, Nb3Sn2Ti3, is present in the interior of the Nb3Sn phase matrix only when Ti is added to Nb. The pinning forces on the grain boundaries of Nb3Sn exhorted by the β-(Ti,Nb) precipitates and related microstructure refinement results in an enhanced growth kinetics of the product phase, Nb3Sn. The addition of 0.5 at.% Ti to Cu(Sn) has a stronger influence on the growth kinetics and the activation energy for the growth of Nb3Sn compared to 3 at.% Ti to Nb owing to a higher fraction of smaller and equiaxed grains with high angle grain boundaries of Nb3Sn. The Ti-free Nb3Sn phase layer grows with a weak texture, a commonly observed behavior in other material systems for the product phases grown by diffusion-controlled mechanism in the interdiffusion zone. On the contrary, a very strong crystallographic texture of the Ti-containing product phase, Nb3Sn, is reported that has a unique pattern depending on the orientation of the adjacent Nb or Nb(Ti) grains. The Cu atoms segregate to the grain boundaries of Nb3Sn over a distance of ∼2–5 nm with a depletion of Nb.