Effect of Trace Phosphorus Addition on the Solidification Behavior and Local Mechanical Response of β-Sn Alloys

To investigate the effects of trace phosphorus (P) addition on the solidification behavior, microstructure, and local mechanical response of β-Sn alloys, Sn–P alloys containing 0.02, 0.05, and 0.08 wt.% P were prepared. Phase constitution, solidification characteristics, and nanomechanical properties were characterized by X-ray diffraction, thermogravimetry–differential scanning calorimetry, and nanoindentation, respectively. Electron backscatter diffraction was additionally conducted on representative pure Sn and Sn–P0.08 samples to qualitatively compare the endpoint microstructures. All alloys were primarily composed of β-Sn, while localized P-rich secondary regions in Sn–P0.08 were indexed as Sn4P3. Trace P promoted heterogeneous nucleation and markedly modified solidification, increasing the solidification peak temperature by approximately 20–22 °C and reducing the undercooling from 39.83 °C to 17.50–20.33 °C. Relative to pure Sn, Sn–P0.08 exhibited more grain regions and grain boundaries, together with pronounced dendritic and locally refined features, indicating modified β-Sn nucleation and growth. Young’s modulus and hardness of Sn–P0.08 reached 29.86 GPa and 0.232 GPa, respectively. Inverse nanoindentation analysis revealed increased representative stress, strain-hardening exponent, and strength coefficient. Fracture-energy analysis showed superior crack-growth energy dissipation for Sn–P0.02, whereas Sn–P0.08 exhibited greater resistance to plastic deformation.