Microstructure, resistivity, and hardness of aged Ag-7wt.%Cu alloy

Abstract

Ag-Cu alloys are used as both decorative materials because of beautiful appearance, and conductors due to excellent combinations of strength and electrical conductivity. The strength and electrical conductivity of Ag-Cu alloy are closely related to precipitation behavior of Cu-rich phase in Ag matrix. The morphology, size and volume fraction of Cu-rich phase have been highly concerned. In this work, a series of aging temperatures is used in both supersaturated solid-solution and cold-rolled Ag-7wt.%Cu samples to investigate the relationship between the precipitation behavior of Cu-rich phase and property by using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, and properties measurements (hardness and resistivity). The DSC results of as-solid-solution Ag-7wt.%Cu alloy show a distinct exothermic precipitation reaction of Cu out of Ag matrix ranging from 300 C to 350 C, and the activation energy is estimated to be (1111.6) kJ/mol according to Kissinger equation. Because of the existence of deformation energy, the DSC results of cold-rolled Ag-7wt.%Cu sample show a distinct exothermic precipitation reaction of Cu from Ag matrix between 290 C and 330 C, and the activation energy is (12812) kJ/mol. XRD analysis indicates that the dissolved Cu in Ag is dependent on ageing temperature, and the change of solubility of Cu in Ag is calculated by XRD curve. Microstructural analysis demonstrates that spherical Cu-rich phases are precipitated from Ag-matrix at 450 C in both solid-solution and cold-rolled Ag-7wt.%Cu alloys. Moreover, the banded structure of Cu-rich phase is found in the solid-solution sample after being aged at 450 C. The deformation twinning Ag is found in the cold-rolled sample. The precipitation and dissolution of Cu-rich phase in Ag matrix play important roles in the resistivity and microhardness. With ageing temperature increasing (ageing temperatures range from 200 to 450 C), the electrical resistivity of as-solid-solution aged sample decreases and the microhardness increases, however, both electrical resistivity and microhardness of as-cold-rolled aged sample decrease. With ageing temperature increasing further (over 450 C), the electrical resistivity increases and the microhardness decreases in both aged samples. Because of the formations of dislocation and deformation twinning Ag, the microhardness of cold-rolled sample reaches to 217 HV, which is higher than that of solid-solution sample. Strengthening and electrical resistivity models are built based on the microstructural characterization and concentration contributions. These theoretical predictions are in good agreement with experimental values. Our model demonstrates that the precipitation and dissloution of Cu in Ag significantly affect the electrical conductivity, and dislocation and deformation twinning play important roles in microhardess in Ag-Cu alloy. This work clarifies the influencing mechanism of different microstructures on the microhardness and resistivity of Ag-Cu alloy.