Low-temperature superconductors: Nb3Sn, Nb3Al, and NbTi

Low-temperature superconducting (LTS) wires are of significant importance in high-field magnet applications. Current developments of the LTS wires are attributed to many studies. Particularly, Nb3Sn is an attractive superconductor with substantial potential for performance improvement in view of an ideal microstructure that maximizes flux pinning properties. To date, various reviews have been reported on the physical properties of low-temperature superconductors. Therefore, this review focuses on understanding the fundamental phase formations and microstructural controls of low-temperature superconductors from the perspectives of growth kinetics, nucleation theory, and chemical potentials to facilitate the syntheses of these superconductors and advancement of wire production. Taking Nb3Sn as an example, the effect of Cu addition to Nb3Sn on Nb/Sn reactive diffusion is briefly described. Then, representative Nb3Sn formations are schematically summarized to broaden our understanding of the development behaviors of Nb3Sn. These behaviors are qualitatively reviewed in terms of Sn chemical potential. After mentioning the potential for performance improvement of Nb3Sn, the influences of element additions, specifically those of Zr and Hf additions, resulting in breakthrough microstructural refinements, on Nb/Sn diffusion are investigated. Subsequently, strengthening of the matrix via element additions is reviewed. Thereafter, taking Nb3Al as an example, the features of Nb3Al formation and basic development processes, including low-temperature processes, metastable phase transformations, and microstructural control, are described. Strain sensitivity, one of the most important properties of Nb3Al, is also briefly reviewed. Then, taking Nb alloy as an example, α-Ti precipitation in a binary Nb–Ti system is concisely summarized. Subsequently, recently reported new artificial pin incorporation based on a powder method is introduced, followed by a unique study of the application of high-temperature-tolerable Nb superconducting alloys in superconducting joints. This review makes a novel contribution to the literature as it provides a comprehensive understanding of phase formation in low-temperature superconductors.