High Temperature Creep Properties of Eutectic and Near Eutectic Silver-Copper Alloys: Application to Metal/Ceramic Joining

Calculations ofresidual stresses in eutectic Ag-Cu braze joints are often required to validate designs for a variety of metal/ceramic joining applications. In particular. Finite Element Analysis (FEA) codes have the capability of incorporating either elastic-plastic or minimum creep rate constitutive models for the braze material. Unfortunately, neither high temperature stress-strain or creep data are available in the literature for eutectic Ag-Cu alloy in the microstructural form encountered in braze joints. This paper presents the preliminary results of a study of the high temperature mechanical properties of eutectic 72Ag-28Cu and near-eutectic 67Ag-33Cu alloys at temperatures from 250°C to 750°C. For the case of the 67Ag-33Cu alloy, the stress exponent obtained from creep testing is observed to be a function of temperature. At 750°C, a stress exponent of 1.5 is observed, which suggests that grain boundary sliding controls deformation while at 350°C, the stress exponent of 4.6 suggests that matrix creep of the Ag-rich phase controls deformation at this temperature. Creep data for both alloys have been fit to a power law equation which incorporates a temperature-dependent stress exponent. Compressive stress-strain tests using an engineering strain rate of 5%/min., were made over a temperature range of 27° to 750°C. The linear work hardening slope is found to be fairly constant from room temperature up to 350°C, followed by a steep decrease in value above 450°C. FEA calculations are presented for a eutectic Ag-Cu braze joint between metallized alumina ceramic and Fe-29Ni-17Co alloy using two different types of constitutive relations for the brazement: (1) a high temperature minimum creep rate correlation (2) an elastic/plastic correlation using the temperature dependent yield stress and work hardening data. The former correlation, since it is time-dependent, permits a study of the effect of various cooldown cycles on the maximum residual stress in the alumina ceramic. Generally, lower residual stresses are predicted in the ceramic when the minimum creep rate correlation is used as the constitutive law for the eutectic Ag-Cu braze joint.