Microstructural development and mechanical properties of hypereutectic Sn–Cu solderalloys

In this study a transient directional solidification system was used, which permitted the assessment of microstructures along castings lengths of hypereutectic Sn–2.0 wt% and 2.8 wt% Cu alloys, for a wide spectrum of experimental tip growth rates (VL) and tip cooling rates (T
• ). The primary Cu6Sn5 IMC developed typical M-shaped or H-shaped morphologies. However, rod-like particles prevailed along the Sn-rich β matrix of both alloys evaluated. Smaller inter-branch spacings (λ) are shown to be associated with the alloy having higher Cu content, and the presence of Cu3Sn (ε) intermetallic compound (IMC) was detected by X-ray diffraction (XRD). Such IMC developed probably due to the combination of high cooling rates during solidification and incomplete peritectic reaction along the Cu-enriched regions of the casting, which provided less η than predicted under equilibrium conditions. Hall–Petch type relationships are proposed interrelating Vickers hardness (HV), ultimate tensile strength (σu) and elongation to fracture against λ. The two alloys examined showed roughly similar trends considering HV and σu, despite having a very different ductility behavior. However, the mechanical strength of each alloy is shown to be controlled by different microstructure features. The higher fraction of eutectic rod-like Cu6Sn5 intermetallics observed for the Sn–2.0 wt% Cu alloy was shown to be associated with the corresponding higher experimental mechanical strength.