Microstructure Evolution in Cu-Pd-Ag Alloy Wires During Heat Treatment

Cu-Pd-Ag alloy wires with a relatively low Ag content are used widely in electronics. This alloy is usually subjected to age hardening to increase its strength. However, the alloy microstructure that results when the alloy is processed as a wire has not been clarified sufficiently to elucidate the hardening mechanism. A clarification of the microstructure is critical to control the mechanical properties of the alloy wire. In this study, we investigated the detailed microstructure variation of the alloy wire after heat treatment. Microscopy showed that the alloy wire contained many parallel rods with an Ag-rich α phase that extended along the axial direction of a Cu-rich α-phase matrix before heat treatment. After heat treatment at 623 K for 1 hour, the morphology of the rods in the matrix remained mostly unchanged. Detailed observations revealed that the matrix was transformed to a nanolamellar structure with β and α₂ phases. Many β′ phases with a thickness of a few atomic layers precipitated in the rods. Both nanostructures investigated in this study were expected to contribute to wire hardening through a short-fiber strengthening mechanism.