Impact of precipitated phases on the microstructure and mechanical properties of eutectic Sn58Bi alloy

A new type of low-melting-point heterogeneous alloy (LMH), based on Sn58Bi alloy (SBE), was prepared using Cu, Zn, Ag, and Sb microalloying and rapid solidification. The addition of Cu and Zn elements in equal mass ratios resulted in the Cu₅Zn₈ phase being dispersed in the Sn55Bi3Cu3Zn (LMH3) alloy. Subsequently, the additional Cu reacted with Sn to form a nano-sized η′-Cu₆Sn₅ phase at the interface of the Sn-Bi eutectic phase. Sn53Bi3Cu3Zn2Ag2Sb (LMH32) was prepared by adding appropriate amounts of Ag and Sb based on LMH3, leading to the precipitation of Ag₃Sn and SbSn phases in the SBE matrix. The different precipitated phases in turn produced different types of fiber textures that existed in the matrix phase of SBE, LMH3, and LMH32. In addition, compared to LMH3, the presence of Ag₃Sn and SbSn phases in LMH32 caused the formation of Bi twins identified via electron backscattered diffraction. The twin planes (101̅1) and (11̅02) had corresponding rotation axes and rotation angles of 93.8°[7̅431] and 90.2°[5̅415̅], respectively. Nanoindentation results indicated that LMH32 exhibited higher creep resistance than Sn55Bi3Cu3Zn (LMH3) and SBE. The precipitated phase significantly refines the grain size and eutectic lath structure of the matrix phase (Sn and Bi). During the deformation process, the fine lath structure and the interfacial η′-Cu₆Sn₅ nanophase can hinder the slippage of the eutectic interface. Moreover, the presence of the precipitated phase forms a large strain gradient in the alloy matrix, which produces a significant back-stress hardening effect and induces substantial dislocation nucleation. The formation of more dislocations made the creep mechanism of LMHs depend on dislocation climbing.