Thermodynamic re-modelling of the Cu–Nb–Sn system: Integrating the nausite phase

Journal Article · 22 February 2022 · Calphad

DOI: https://doi.org/10.1016/j.calphad.2022.102409 · OSTI ID:1981550

Lachmann, Jonas  ^[1]; Kriegel, Mario J.  ^[1]; Leineweber, Andreas ^[1]; Shang, Shun-Li ^[2]; Liu, Zi-Kui  ^[2]

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1. Technische Universität Bergakademie Freiberg (Germany)
2. Pennsylvania State Univ., University Park, PA (United States)

Currently available Cu–Nb–Sn phase diagrams lack the recently discovered nausite phase (Cu,Nb)Sn₂, which is an important intermediate in the course of thermal processing of superconducting Nb₃Sn wires. Processing decisively determines the resulting microstructure of Nb₃Sn and, thus, its superconducting properties. Lack of suitable and complete phase diagrams, however, obstructs rational design of such thermal processing procedures. To close this gap and to obtain valid knowledge of homogeneity and stability range of nausite, various Cu–Nb–Sn samples, which are heat-treated between 300 °C and 500 °C, are investigated. By means of energy-dispersive X-ray spectroscopy (EDX), a temperature-dependent homogeneity range of nausite is observed, which covers average mole fractions of Cu between 0.09 and 0.15. This is correlated with a change in the mean atomic volume and can be seen in the lattice parameters determined by X-ray diffraction (XRD). Additionally performed first-principles calculations on different CuSn₂ and NbSn₂ model structures confirm this trend. Furthermore, the peritectic decomposition of nausite to NbSn₂ and liquid at 586 °C is determined by means of in situ XRD and differential scanning calorimetry (DSC). By using the CALPHAD (CALculation of PHase Diagrams) approach, all these findings are used to extend a previous thermodynamic description of the Cu–Nb–Sn system by including the nausite as an additional phase. Finally, with this noteworthy integration, the updated modelling of the Cu–Nb–Sn system can be used for optimizing the multistage heat-treatment steps during processing superconducting Nb₃Sn wires.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF)
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 1981550
• Journal Information: Calphad, Journal Name: Calphad Journal Issue: C Vol. 77; ISSN 0364-5916
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English