Formation of columnar grains during diffusional growth of Nb3Sn layer and its suppression

The manufacturing process of multifilamentary Nb ₃Sn superconducting wire includes annealing for diffusional growth of the Nb ₃ Sn intermetallic compound layer. During the process, the formation of columnar Nb ₃ Sn grains should be suppressed to maximize the flux pinning effect. However, the detailed mechanism underlying the formation of columnar Nb ₃ Sn grains has not been clarified yet, and is an obstacle to finding optimal process conditions. In the present work, we investigated the fundamental reason for the columnar grain formation, using a novel Monte Carlo Potts model which can simultaneously predict grain growth and interfacial reactions. We found that a decrease in thermodynamic driving force due to insufficient Sn at the interface causes the formation of columnar Nb ₃ Sn grains. Accordingly, columnar grain formation is expected to be suppressed when abundant Sn atoms are supplied rapidly to the reaction front. Based on the fundamental understanding of columnar grain formation, possible ways to suppress columnar grain formation are suggested. We especially focused on the difference in the columnar grain fraction depending on the wire strand type (bronze processed wire vs. internal-tin processed wire). Based on simulations and experimental analyses, we suggest a brief guideline for the design of wire strand geometry to avoid columnar grain formation.