Theoretical investigation of the role of alloying elements in the growth of A15 Nb3Sn

Abstract

Nb3Sn is one of the widely used low-temperature superconducting materials for various applications in wire form. The manufacturing process of the Nb3Sn wire is based on the diffusional growth reaction of Nb3Sn compound layers, where the diffusion kinetics in Nb3Sn compound determines the yield of Nb3Sn and eventually the superconducting properties of the wire. Alloying elements such as Ti are beneficial in improving the superconducting properties by enhancing the growth rate of the Nb3Sn layer, causing grain refinement, and increasing equiaxed grain fraction. To obtain a high-performance Nb3Sn wire, the fundamental details of diffusion kinetics and mechanisms underlying alloying effects should be well understood. However, even the governing diffusion mechanism in Nb3Sn matrix is not yet clarified, as well as mechanisms underlying the alloying effects. In the present work, the major diffusion mechanism of Sn and the effect of addition of Ti, a representative alloying element for the production of Nb3Sn wire, on the diffusional growth of the Nb3Sn layer were investigated using atomistic simulations. Further, microstructure evolution during the Nb3Sn reaction was investigated using Monte Carlo Potts model, and the alloying effect on columnar grain formation was discussed. First of all, the major diffusion mechanism in Nb3Sn was revealed as the grain boundary diffusion by using atomistic simulations. Volume diffusion of Sn is too slow to explain the experimentally observed growth rate of Nb3Sn layer, whereas grain boundary diffusion is fast enough. Grain boundary diffusion can be significant in the Nb3Sn layer since the grain size of Nb3Sn is generally small. Next, the most important alloying effect of Ti was revealed as the grain refinement by using atomistic simulations. Ti atoms tend to segregate at Nb3Sn grain boundaries but cannot enhance the diffusion of Sn enough to explain the experimentally observed enhancement in growth rate of Nb3Sn layer. Grain refinement due to the grain boundary segregation of Ti is thought to enhance the effective diffusion in the Nb3Sn layer since the major diffusion path is the grain boundary. Further, it was revealed that the appearance of coarse columnar Nb3Sn grains around the Nb-Nb3Sn interface is due to the lack of Sn supply to the Nb-Nb3Sn interface which reduces the driving force of nucleation, by using a Monte Carlo Potts model. The decrease in columnar grain fraction by adding Ti is mainly due to the promotion of effective diffusion of Sn through the Nb3Sn layer to the reaction front by grain refinement, while solute drag by grain boundary segregation may further help suppressing the coarsening of columnar grains. As a conclusion, the optimal alloying element is suggested as the one that can cause a significant grain refinement effect without hindering the grain boundary diffusion of Sn. Such an alloying element is also expected to naturally suppress the formation of columnar Nb3Sn grains, further improving the superconducting property of Nb3Sn wire.