Age-hardenability and related microstructural changes during and after phase transformation in an Au-Ag-Cu-based dental alloy

the aim of this study was to clarify how the microstructural changes during and after phase transformation determine the age-hardenability of an au-ag-cu-based dental alloy. the rapid increase in hardness in the initial stage was the result of rapid atomic diffusion by spinodal decomposition into metastable ag-rich' and cu-rich' phases. the constant hardening after apparent initial hardening was the result of a subsequent transformation of the metastable ag-rich' and cu-rich' phases to the stable ag-rich α1phase and aucu i phase through the metastable aucu i' phase. during the increase in hardness, fine block-like structure with high coherency formed in the grain interior, which changed to a fine cross-hatched structure. a relatively coarse lamellar structure composed of ag-rich α1and aucu i phases grew from the grain boundaries, initiating softening before the grain interior reached its maximum hardness. as a result, the spinodal decomposition attributed to rapid hardening by forming the fine block-like structure, and the subsequent ordering into aucu i, which is a famous hardening mechanism, weakened its hardening effect by accelerating the lamellar-forming grain boundary reaction.