The effects of discontinuous precipitation and ordering on the age-hardening in Au-20Ag-30Cu (wt.%) alloy

Discontinuous precipitation (DP) and ordering have been typically considered to influence the mechanical properties of many alloy systems during the aging process. In the present study, the microstructural evolution and the effect of discontinuous precipitation and ordering on the age-hardening behavior in Au-20Ag-30Cu (wt.%) ternary alloy were investigated. Au-20Ag-30Cu (wt.%) alloy is found to be typically age-hardened. Thermodynamic calculation forecasts the discontinuous precipitation forming Cu-rich α1 phase and Ag-rich α2 phase, companied by the disorder-order transformation forming AuCu3 phase with L12 structure for this Au-Ag-Cu alloy. The DP is experimentally observed to form at grain boundaries consisting of alternative α1 and α2 two-phase lamellae. And the L12-type AuCu3 ordered phase, which presents nanometric spherical particle, forms at grain interior and the lamellar α1 phase of DP. The DP and L12-type AuCu3 nanoprecipitates both grow with the elevated aging temperature. The DP at grain boundaries is found to deleteriously affect the overall age-hardening with different deformation mechanisms between these two lamellar phases of DP. High-density dislocations are observed in the lamellar α1 phase of DP around the hardness indentation, which indicates that the interior L12-type AuCu3 nanoprecipitates in α1 phase can effectively impede the movement of dislocations to boost the age-hardening. The overall age-hardening is determined by the competitive effect of the DP and L12-type AuCu3 nanoprecipitates. This study offers theoretical foundation for controlling the microstructures and optimizing the mechanical properties of Au-Ag-Cu alloys.