Achieving simultaneous strength-ductility enhancement of SnBi alloys via ultrasonic hierarchical refinement

SnBi alloys, a promising candidate widely used in thermal interface materials, oil well plugs, and semiconductor packaging, suffer from low ductility due to the long and coarse primary dendrites. In this paper, an ultrasonic treatment strategy was implemented on the Sn30Bi (wt%) melt to fragment the dendrites, refine the microstructures, and further enhance the mechanical properties of solidified alloys. The effects of ultrasonic parameters on dendrite size, grain structures, and mechanical properties of the alloy were systematically investigated. It is found that the microstructures of Sn30Bi alloys were reorganized from coarse β-Sn dendrites to equiaxed cellular structures with an average size of 55 μm, and the grain size was further refined to an average size of 25 μm under the ultrasonic treatment temperature of 165 °C, ultrasonic amplitude of 10 μm, and ultrasonic vibration duration of 30 s. This hierarchical refinement led to a 20 % increase in strength (from 59 MPa to 71 MPa) and a 54 % improvement in elongation (from 12 % to 19 %) for the Sn30Bi alloy. The primary refinement from dendrites to cells extended crack propagation paths, while the intracellular secondary refinement strengthened β-Sn cells, further leading to crack deflection and arrest. The correlation between ultrasonic processing parameters and alloy microstructures provides practical guidance for SnBi alloy melt treatment to improve mechanical performance, while the hierarchical refinement induced by ultrasonic treatment deepens mechanistic insights of ultrasonic effects on alloy microstructures.