Deformation mechanism of various Sn-xBi alloys under tensile tests

In this study, the thermal properties, mechanical properties, and microstructure evolution of Sn-xBi (x = 0, 10, 17, 20, 30, 40, 50, and 58 wt.%, respectively) alloys were investigated via differential scanning calorimetry, scanning electron microscope during and after tensile test. The results reveal that the shapes of precipitated Bi particles relate to the cooling rate, that is, sphere in water cooling condition and short rod like cooled in air. Beta tin phase strengthened by Bi particle precipitation offers superior performance than Sn-Bi eutectic structure. The result also clearly demonstrates the deformation and fracture modes of Sn-Bi alloys with different microstructure through in situ observations during tensile loading. The deformation of Sn-17Bi alloy deforms based on the grain boundary diffusion. For Sn-58Bi alloy, the deformation is mainly resulted by grain boundary sliding during even deformation stage and the phase sliding between Bi-rich and Sn-rich phase boundary after necking. And the latter accounts for the majority. Sn-50Bi alloy deforms through grain boundary sliding of eutectic phase and phase boundary sliding between beta tin phase strengthen by precipitated Bi and eutectic phase. Moreover, Sn-rich phase works as floating grains, does not participate in the deformation, but provides sliding interface with the eutectic part, which promotes the continuous plastic deformation.