Grain refinement, solid-solution hardening, and defect-induced strengthening in electroplated Ag–Bi alloy layer

Pure Ag has superior electrical conductivity than other metal elements. However, it has limitations in connector application due to insufficient mechanical robustness. In this study, Ag–Bi alloy layer demonstrated higher hardness than pure Ag coatings in both as-deposited and heat-treated (125°C) states. We elucidate the hardening mechanisms of electroplated Ag–Bi layer, which arise from the synergistic effects of solid-solution hardening, grain-boundary hardening, and defect (twins and stacking faults)-induced hardening. Specifically, we confirmed the solid solution of Bi within the Ag matrix, which effectively promoted Ag nucleation and suppressed grain growth up to 125°C via solute drag and Zener pinning. Furthermore, Bi incorporation significantly increased the stacking fault probability and generated dense nanotwins and stacking faults. This defect-induced hardening is the dominant strengthening mechanism, which contributed about 80% to the total strength increment. Our findings suggest the pathway for designing highly reliable, nonequilibrium noble metal alloys with superior thermo-mechanical stability for advanced contact applications. Impact Statement This study addresses the critical bottleneck issue (lack of mechanical robustness) of silver (Ag) electrodeposition films application in the connector industry using Bi alloying. Electroplated Ag–Bi layer was up to 3× harder than pure Ag layer. Conventional hard alloy research remains largely empirical and focuses on alloy composition optimization and the characterization of resultant hardness trends. However, we unveiled the strengthening mechanism in Ag–Bi alloy coating which is related to solid solution, grain refinement, and defects formation. Through this study, we found out that solute and defect engineering enables both mechanical superiority and thermal stability. Our findings suggest a rational framework or blueprint for the design of noble metal-based electronic components (connector, electric vehicle charger, and battery module), which requires high reliability and high performance.