Age-hardening by the formation of metastable phases in an In-added Au–Ag–Cu–Pd alloy

The age-hardening of an In-added Au–Ag–Cu–Pd alloy was examined by characterizing the hardening behavior, phase transformations and changes in microstructure, and elemental distribution during aging over the temperature range 200 to 500 °C. The major product phases, Ag-rich α1 and AuCu I, were formed homogeneously from the parent α0 phase by the transformation of precursor metastable phases in the grain interior. This led to a significant age-hardening response. A minor product phase, InPd, containing Cu and Zn, contributed to the overall hardness increase by initiating a grain boundary precipitation reaction. Continued long-time aging led to a hardness decrease due to overaging, which involved the formation of a lamellar microstructure composed of the stable Ag-rich α1 and AuCu I phases. At the longest aging time at 400 °C, the lamellar structure consumed the matrix α1 and AuCu I phases.