Age-hardening behaviour of a spinodally decomposed low-carat gold alloy

The age-hardening behaviour of a spinodally decomposed low-carat gold alloy was investigated by means of hardness test, X-ray diffraction (XRD), field emission scanning electron microscopic (FESEM) observations, and energy dispersive spectrometer (EDS). An apparent hardness increase occurred at the initial stage of the aging process without incubation periods. Then, after a plateau, the hardness increased to the maximum value, and finally, the softening by overaging occurred. The agehardening of the specimen is characterized by the fast increasing rate in hardness and the apparent delay of softening. By aging the solution-treated specimen, the fcc a0 phase was transformed into the Ag-rich a1, Cu-rich a2, and Zn–Pd-rich b phases through the spinodal decomposition process and the metastable phase formation. The first hardening stage which occurred during the early stage of spinodal decomposition without an apparent structural change was thought to be due to the interaction of dislocation with solute-rich fluctuations. The second hardening stage after the plateau was caused by the formation of the fine block-like structure with high coherency induced by the spinodal decomposition, which corresponded to the phase transformation of the metastable Ag-rich a01 phase into the stable Ag-rich a1 phase. The remarkably delayed softening was caused by the slow progress of coarsening and resultant chaining of the Ag-rich a1 precipitates in the Cu-rich a2 matrix due to the uniform fine scale of the structure.