Aged Bi-supersaturated Sn–5Bi–3In alloy for strengthening, corrosion resistance, and corrosion protection for FeCoNiMnCu substrate

Low-melting Sn–Bi alloys are attractive for metallic joining but suffer from Bi precipitation–induced micro-galvanic corrosion. This study investigates a Sn–5Bi–3In alloy (SBI53) to clarify the role of minor In addition in controlling microstructural evolution, mechanical properties, corrosion behavior, and soldering performance, using Sn–5Bi (SB5) as a reference. After aging at 150 °C, SB5 develops a fine Bi-dispersion microstructure, whereas SBI53 transforms into a stable Bi-supersaturated single-phase β-Sn solid solution, as confirmed by EBSD. This structure remains stable after long-term storage (5000 h), resulting in enhanced hardness (∼28 HV). Aged SBI53 shows localized grain-boundary corrosion in 3.5 wt% NaCl solution, unlike the widespread Sn/Bi micro-galvanic corrosion in aged SB5, owing to suppressed Bi-rich precipitation and reduced Sn activity. XPS analysis confirms corrosion products consistent with the proposed mechanism. SBI53 also demonstrates excellent wettability and forms robust joints with FeCoNiMnCu (FCNMC), achieving shear strengths of ∼31 MPa. Furthermore, SBI53 acts as a sacrificial anode, effectively protecting FCNMC from corrosion. These results highlight SBI53 as a promising low-temperature solder and corrosion-protection material for HEA systems.