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

Abstract

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 α 1 phase 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 α 1 and 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.

Influence of thermo-mechanical history on the ordering kinetics in 18 carat Au alloys

“…2a) shows the same and even more pronounced shoulders on the fundamental L10 peaks. These shoulders most likely correspond to the α2 phase, which is an FCC phase with Au and Ag in solid solution [3,17,18,30].…”

“…At later stages, coarse twin lamellae are formed to accommodate the long-range strains arising from the growth of ordered regions with different c-axis orientations. The dental compatibility of the ternary Au-Cu-Ag system has steered the efforts into understanding the order-hardening mechanism [13] and age-hardening characteristics [13][14][15][15][16][17][18][19][20][21] of these alloys. The addition of small silver amounts to the binary equiatomic AuCu extends the early stages of ordering to longer ageing times by reducing the ordering rate [22], delaying the growth of inter-twin spacing as well as slowing down the growth of anti-phase domains [3].…”