Mechanisms for high strength and ultra-high electrical conductivity of Cu-3.5wt.%Ag alloy prepared by thermomechanical treatment

Abstract

Copper-based wire is the key conductor material in integrated circuit bonding wire, high-end audio and video transmission line, medical wire harness and other fields. It is of significant interests to improving the strength and retaining their good electrical conductivity of copper-based wires for broad applications. The combination of heat treatment and deformation to control the microstructure is a promising method for this objective. In this paper, the Cu-3.5 wt.%Ag alloy rod with diameter of 16 mm was prepared by the hot mold horizontal continuous casting. We discussed the effects of solution + aging (S.A), solution + aging + cold drawing (S.A.C), solution + cold drawing + aging (S.C.A) on the distribution of Ag phase, dislocation morphology, strength and electrical conductivity of Cu-3.5 wt.%Ag alloy. The results show that continuing cold deformation after aging makes the strength of Cu-3.5 wt.%Ag alloy is up to 417.2 MPa, which is 84.6% higher than that by S.A process. After S.A.C treatment, the dislocation density sharply increases, and the dislocation is arranged in a network structure. The dispersed nano-spherical Ag phase with an average size of 12 nm precipitation after aging strongly prevents dislocation slip. Moreover, the chain-like Ag phase effectively "intercepts" or "lock" the dislocations in the local area. The S.A.C process fully exerts the synergistic strengthening effect of aging precipitation strengthening and deformation strengthening induced by the interaction between nano-Ag phase and dislocation. This process is suitable for medical wiring harness, bonding wire and other application scenarios with higher strength requirements. On the other hand, if cold deformation is applied before aging, that is, S.C.A process. The electrical conductivity of Cu-3.5 wt.% Ag alloy reaches 95.5% IACS, which is close to 100% IACS of pure copper. The achievement of ultra-high conductivity is mainly attributed to the formation of a large number of band-shaped nano-Ag phase with an average length of 450 nm and width of 25 nm, and the arrangement direction remain consistent. In addition, the uniform distribution of dislocations is also beneficial to greatly improve the electrical conductivity. The S.C.A process is suitable for high-end audio and video transmission lines and other application scenarios that require higher electrical conductivity. • The strength of Cu-3.5 wt.% Ag alloy prepared by S.A.C process reaches 417.2 MPa, which is 84.6% higher than that by S.A process. • The electrical conductivity of Cu-3.5 wt.% Ag alloy prepared by S.C.A process reaches 95.5% IACS, which is close to 100% IACS of pure copper. • The strengthening mechanism of Cu-3.5 wt.%Ag alloy prepared by cold deformation after aging (S.A.C) is the synergistic effect of dispersed nano-spherical Ag phase precipitation and local chain-like Ag phase locking dislocation. • The achievement of ultra-high conductivity of Cu-3.5 wt.%Ag alloy prepared by cold deformation before aging (S.C.A) is mainly attributed to the formation of a large number of band-shaped nano-Ag phases with large aspect ratio and consistent alignment direction, and the uniform distribution of dislocations.