Hardening and overaging mechanism of a commercial Au-Ag-Cu-Pd dental alloy

To reveal the aging behaviour and age-hardening mechanism of an Au-Ag-Cu-Pd alloy with small amounts of Pt, Zn and Ir, the hardness changes, phase transformation and changes in microstructure and element distribution during the aging process were observed. The specimen alloy showed apparent age-hardenability at the aging temperature of 400 °C. By aging, the hardness of the solution-treated specimen began to increase and reached a maximum value, and then, the hardness decreased continuously by further aging. The solution-treated specimen was composed of the Ag-Au-based α1 matrix containing Cu and the PdZn particle-like structures containing Cu and Pt. By aging the solution-treated specimen at 400 °C, the Ag-Au-based α1 matrix was replaced by the lamellar structures of the solute-depleted α′1 matrix and the AuCu I precipitates. The hardness increase in the early stage of the aging process seemed to be caused by the nucleation of the AuCu I phase from the matrix which induced the formation of lattice strains in the interface between the matrix and the AuCu I phase. The hardness decrease in the later stage of the aging process was caused by the coarsening and granulation of the AuCu I precipitates which released the strains that formed in the early stage of the AuCu I precipitation from the matrix. The PdZn particle-like structures did not contribute to the hardness increase.