Trans-scale Analysis Reveals Design Principles for Mechanical Properties of Cu-Pd-Ag Alloy Wires with Nano Structures

Cu-Pd-Ag alloy is widely used in electronic device applications due to its relatively low electric resistance. For example, it is used to fabricate terminals or components of electronic devices as they require high strength coupled with low resistance. To obtain higher strength wire, age-hardening is usually conducted to this alloy wire. Consequently, the age hardening of these alloys by ordered-phase precipitation in the alpha phase has been studied far and wide. As reported in recent publications, aging of solution-treated specimens results in alloy transformation to the lamellar structure, while as coarsening of the fine lamellar structure results in alloy softening.

A review of the available vast literature has highlighted that various production processes yield different phases, and hence different hardening behavior. This observation indicates that, the alloy microstructure that results when the alloy is processed as a wire has not been clarified sufficiently to elucidate the hardening mechanism due to the lack of spatial resolution. Therefore, in order to control the mechanical properties and to ensure the reliability of the alloy wire, it is imperative that a clarification of the microstructure and hardening mechanism be established.

Recently, Japanese scientists at Ibaraki University, Prof. Chihiro Iwamoto, and N. Adachi in collaboration with F. Watanabe and R. Koitabashi at Yokowo Co., Ltd, investigated the detailed microstructure evolution of low-Ag-content Cu-Pd-Ag alloy wires after heat treatment using several microscopic techniques. Their main objective was to clarify on the micro and nano structure by trans-scale observation, which as been seen to be critical in controlling mechanical properties of the alloy wire. Their work is currently published in the research journal, Metallurgical and Materials Transactions A.

The authors obtained a commercial Cu-Pd-Ag alloy of predetermined Cu, Pd and Ag composition. The wire was then cut into small pieces of predetermined length and subjected to heat treatment in a vacuum chamber at preset temperature for a specified duration. Lastly, the microstructure and composition of the cooled wire alloy pieces was undertaken using various techniques, including: XRD analysis, scanning electron microscopy and field-emission scanning transmission electron microscopy.

Microscopy results showed that the alloy wire contained many parallel rods with a silver-rich alpha phase that extended along the axial direction of a copper-rich alpha-phase matrix before heat treatment. In addition, the researchers noted that despite being subjected to heat treatment, the morphology of the rods in the matrix remained mostly unchanged. Moreover, detailed observations revealed that the matrix was transformed to a nano-lamellar structure with beta and alpha₂ phases. Many beta prime phases with a thickness of a few atomic layers were also seen to precipitate in the rods. Both nanostructures which were expected to increase the hardness with different mechanism were considered to contribute to the mechanical properties of the wire through a short-fiber strengthening mechanism. The morphology of the rod-matrix which was determined by the wire-drawing process would control the final mechanical properties of the wire.

In summary, the study demonstrated the hardening mechanism of a Cu-Pd-Ag alloy wire by observing the micro and nano structure and proposed design principles for the mechanical properties of the alloy wire.