*Article* **Nb/Sn Liquid-Solid Reactive Diffusion Couples and Their Application to Determination of Phase Equilibria and Interdiffusion Coefficients of Nb-Sn Binary System**

**Jiali Zhang, Jing Zhong, Qin Li \* and Lijun Zhang \***

State Key Lab of Powder Metallurgy, Central South University, Changsha 410083, China; jializhang@csu.edu.cn (J.Z.); zhongjingjogy@gmail.com (J.Z.)

**\*** Correspondence: qinli333@csu.edu.cn (Q.L.); lijun.zhang@csu.edu.cn (L.Z.); Fax: +86-731-8871-0855 (L.Z.)

**Abstract:** Nb3Sn plays an irreplaceable role in superconducting parts due to its stable performance under high field conditions. Accurate phase equilibria and interdiffusion coefficients are of great significance for designing novel Nb3Sn superconductors. However, the related experimental information is still in a state of scarcity because of the difficulty in fabrication of Nb-Sn alloys caused by the large difference in melting points of Nb and Sn. In this paper, a simple but pragmatic approach was first proposed to prepare the Nb/Sn liquid-solid reactive diffusion couples (LSDCs) at 1100 ◦C and 1200 ◦C, of which the phase identification of the formed layer and the measurement of compositiondistance profiles were conducted. The formed layer in Nb/Sn LSDCs was confirmed to be Nb3Sn compound. While the measured composition profiles were employed to determine the phase equilibria according to the local equilibrium hypothesis and the interdiffusion coefficients with an aid of the latest version of HitDIC software. The determined phase equilibria of Nb3Sn, (Nb) and liquid show good agreement with the assessed phase diagram. While the calculated interdiffusion coefficients and activation energy for diffusion in Nb3Sn are consistent with both experimental and theoretical data in the literature. Moreover, the growth of the formed Nb3Sn layer in Nb/Sn LSDCs was also found to be diffusion controlled. All the obtained phase equilibria and interdiffusion coefficients are of great value for further thermodynamic and kinetic modeling of the Nb-Sn system. Furthermore, it is anticipated that the presently proposed approach of fabricating liquid-solid reactive diffusion couple should serve as a general one for various alloy systems with large differences in melting points.

**Keywords:** liquid-solid diffusion couple; Nb-Sn system; Nb3Sn; phase equilibria; diffusion; HitDIC
