Age-hardening by miscibility limit of Au–Pt and Ag–Cu systems in an Au–Ag–Cu–Pt alloy

The age-hardening by miscibility limit of Au–Pt and Ag–Cu systems in an Au–Ag–Cu–Pt alloy was examined by characterizing the hardening behavior, phase transformations and changes in microstructure, and elemental distribution during aging. The hardness increased by the transformation of the parent α0 phase into the α1 and metastable AuCu I′ phases, but not by the further transformation of the metastable AuCu I′ phase into the stable AuCu I phase due to the simultaneously initiated lamellar-forming grain boundary reaction. The replacing ratio of matrix by lamellar structure was not directly proportional to the AuCu I phase formation. The relatively high Pt content caused the severe exclusion of Au from the Cu-rich layer of the lamellar structure due to the overlapped miscibility limit of both Au–Pt and Ag–Cu systems.