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

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.