Age-hardening and related phase transformation in an experimental Ag–Cu–Pd–Au alloy

The age-hardening behaviour, phase transformation and related microstructural changes of an experimental Ag–Cu–Pd–Au alloy were examined by means of hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations and electron probe microanalysis (EPMA). The specimen alloy showed apparent age-hardenability at the aging temperatures of 350 °C and 400 °C. By aging the solution-treated specimen at 400 °C, two phases of the Ag-rich α1 phase and the Pd-containing Cu-rich α2 phase were transformed into four phases of the Ag-rich α 1 ′ phase, the Cu-rich α 2 ′ phase, the CsCl-type CuPd phase and the AuCu(I) ordered phase. Microstructure of the solution-treated specimen consisted of the Ag-rich α1 matrix, Cu-rich α2 particle-like structures of various sizes and the lamellar structure of the α1 and α2 phases. When the peak hardness was obtained, the very fine lamellar structure consisting of the Ag-rich α 1 ′ and Cu-rich α 2 ′ phases was newly formed in the matrix. By further aging, the very fine lamellar structure grew and coarsened apparently, and the matrix was covered with the coarsened lamellar structure. The hardness increase was considered to be caused mainly by the diffusion and precipitation of Cu from the Ag-rich α1 matrix, and the hardness decrease in the latter stage of age-hardening process was caused by the coarsening of the very fine lamellar structure. The CsCl-type CuPd phase and the AuCu(I) ordered phase did not contribute to the hardness increase.