Hardening and overaging Mechanisms in an Au-Ag-Cu-Pd alloy with In additions

Hardening and overaging mechanisms were examined in a semi-precious Au-Ag-Cu-Pd dental alloy with small amounts of In, Zn and Ir. The alloy showed maximum age-hardenability at the aging temperature of 400 degrees C. The hardness value increased to reach the maximum value, and then decreased continuously with aging time. In the early stage of aging process, the matrix of the single alpha(0) phase separated into the alpha(1) and AuCu I phases, and the fine InPd-based precipitates containing Zn and Cu formed at the grain boundaries. During further aging, the grain boundary precipitates grew toward the grain interior. In overaged specimens, the original matrix was replaced by the coarse lamellar structure composed of the AuCu I phase containing Pd and Zn and the Ag-Au-based alpha(1) phase of Cu-, Pd- and Zn-depleted. The hardness increase in the early stage of aging process was caused by the nucleation of the InPd-based phase and the AuCu I phase in the alpha(0) matrix; this introduced significant lattice strains into the interface with the matrix. The hardness decrease in the latter stage of aging process was caused by the formation and coarsening of the lamellar structure composed of the alpha(1) phase and the AuCu I phase. The minor constituent, In formed InPd-based grain boundary precipitates prior to the lamellar structure formation of alpha(1) and AuCu 1.