Development of Indium-Based Low Temperature Solder Alloys for Martian Conditions

Indium solder or indium-based solder alloys, known for their exceptional ductility and ultra-low melting points, have huge potential for specific applications like space exploration, without the concern of tin-pest. However, the strength of indium-rich solder alloys is generally weaker than that of conventional solder alloys. To address the limitation and develop an indium-based solder alloy that is suitable for space exploration, a specific hardening phase <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-\text{AgIn}_{2}$</tex> was selected to strengthen the microstructure dominated by the soft <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\beta-\text{In}_{3}\text{Sn}$</tex> phase. Additionally, 7wt<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">%</sup> Bi was added to leverage the solid solution strengthening within the microstructure. The microstructure of the designed alloy, In-25Sn-7Bi-1.2Ag-0.25Cu (ISBA1.2) (in wt.%) was characterised by scanning electron microscopy (SEM). The solidification behaviour was predicted by Thermo-Calc and experimentally verified through differential scanning calorimetry (DSC). Mechanical properties were tested at room temperature and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-20^{\circ}\mathrm{C}$</tex> and results were compared with commercial solder alloys and In-Sn-Bi alloys with different microstructures. The designed alloy with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{AgIn}_{2}$</tex> particles and Bi solute atoms exhibited good strengthening effect and more stable mechanical properties at variable temperatures compared to In-Sn-Bi alloys containing <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\gamma-\text{InSn}_{4}$</tex> phase.