Thermodynamic re-modelling of the Cu–Nb–Sn system: Integrating the nausite phase. (June 2022)

• Record Type: Journal Article
• Authors: Lachmann, Jonas; Kriegel, Mario J.; Leineweber, Andreas; Shang, Shun-Li; Liu, Zi-Kui
• Is Part Of: Calphad. Volume 77(2022)
• Publication Date: 2022-06
• Subjects: CALPHAD -- nausite -- Nb3Sn -- Cu–Nb–Sn -- First-principles calculations; Phase diagrams -- Data processing -- Periodicals; Thermochemistry -- Data processing -- Periodicals; Diagrammes de phases -- Informatique -- Périodiques; Thermochimie -- Informatique -- Périodiques; Thermodynamica; Electronic journals; 541.363
• DOI: 10.1016/j.calphad.2022.102409
• Languages: English
• ISSNs: 0364-5916

Abstract

Abstract: 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 Nb3 Sn wires. Processing decisively determines the resulting microstructure of Nb3 Sn 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. 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 Nb3 Sn wires.