Microstructural Change Due to Aging and Its Effect on Fatigue Properties in Sn-Sb-Ag-Ni-Ge Alloy

In this study, the microstructural changes and coarsening behavior of Ag₃Sn in Sn-6.4Sb-3.9Ag-0.25Ni-0.003Ge (mass%) during high-temperature aging were investigated. Additionally, low-cycle fatigue tests were conducted to compare the fatigue behavior of Sn-6.4Sb-3.9Ag-0.25Ni-0.003Ge with that of Sn-3.0Ag-0.5Cu. At room temperature, SbSn phases are dispersed in the β-Sn matrix. As the temperature rises, Sb atoms dissolve in the β-Sn phase; thus, the SbSn phases disappear, and some of the atoms aggregate. The activation energy was 45 kJ/mol for the coarsening of Ag₃Sn in Sn-6.4Sb-3.9Ag-0.25Ni-0.003Ge due to aging. Ag₃Sn coarsening was estimated to be controlled by the lattice diffusion of Ag atoms in the β-Sn phase. Furthermore, it was confirmed that the solid solution of Sb atoms in the β-Sn phase reduces the solubility limit of Ag atoms in the β-Sn phase, which delays the coarsening of Ag₃Sn. Regarding fatigue properties, while both alloys exhibited comparable low-cycle fatigue behavior at room temperature, the fatigue ductility exponent's increase was confirmed to be suppressed for the Sn-6.4Sb-3.9Ag-0.25Ni-0.003Ge alloy at 175 °C. This trend suggests that the delayed coarsening of Ag₃Sn maintains the cyclic strain-hardening exponent, thereby influencing high-temperature fatigue behavior.