Phase Evolution of Superconducting Sn–In–Bi Solder Alloys

Development of Pb-free superconducting solders is of great importance for the superconducting magnet manufacturers who often use Pb-Bi alloys as superconducting solders. Here, we study the phase evolution, microstructure, and superconducting properties of a number of alloys in the Pb-free Sn-In-Bi system using analytical scanning electron microscopy, differential thermal analysis, X-ray diffraction, and SQUID magnetometry. All the alloys exhibit melting points below 200°C and superconducting transition temperatures above 4.2 K. The choice of initial composition and cooling rate controls the microstructure (chemistry, volume fraction, morphology, and scale) of these alloys and strongly influences the final superconducting properties. The In-rich β-phase shows better superconducting properties compared with the other phases in the Sn-In-Bi system. As a result, by increasing the indium content in the alloy, volume fraction of the In-rich β-phase increases, leading to lower melting point, higher wettability, and better superconducting properties.