Dynamic deformation behavior and microstructural evolution of Sn-3.0Ag-0.5Cu lead-free solder

Abstract

The dynamic mechanical response of Sn-3.0Ag-0.5Cu lead-free solder at medium to high strain rates was investigated using a Split Hopkinson Pressure Bar (SHPB). The microstructural evolution of the material was characterized via Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD) techniques. The results indicate that the dynamic mechanical behavior of the solder is strain rate dependent. When the strain rate ranges from 250 s⁻¹ to 2300 s⁻¹, the primary deformation mechanism in the initial stage is dislocation slip, with the flow stress increasing as the strain rate increases. In the later stage, the deformation behavior is closely associated with work hardening and continuous dynamic recrystallization. When the strain rate ranges from 2300 s⁻¹ to 4000 s⁻¹, twinning becomes the dominant deformation mechanism in the early stage. The softening effect caused by twin formation competes with the hardening effect induced by dislocations, rendering the flow stress in the early stage insensitive to the strain rate. Subsequently, the extensive distribution of twin boundaries contributes to grain refinement, leading to a sudden increase in flow stress. The high (004) pole density oriented at approximately 28° to the loading direction is closely related to the extensive formation of twins. In the later stage of deformation, twinning-induced dynamic recrystallization causes material softening, resulting in a decrease in flow stress with increasing strain rate. Furthermore, the presence of numerous intermetallic compounds (IMCs) in the eutectic region causes local stress concentrations, promoting the nucleation of twins.