Structure and morphology of an age-hardened goldcopper-silver dental alloy

Abstract

Transmission electron microscopy and electron diffraction studies have been carried out in order to make clear the correlation between the microstructure and age-hardening of a dental gold alloy Au-43.2 at.% Cu13.8 at.% Ag. At lower ageing temperatures the hardening is due to the development of a long period antiphase domain structure of the AuCu-II type. The strain field induced by the AuCu-I type superlattice coexisting with the AuCu-II type superlattice also contributes to the. age-hardening. An alternating coarse lamellar structure is formed by the mechanism of discontinuous decomposition at higher ageing temperatures. The phases composing this structure have been identified as the copper-rich α 1 phase and the silver-rich α 2 phase, which are both f.c.c. in structure. No age-hardening was observed in this temperature range. In the middle temperature range (350–400 °C) the α 1 phase becomes ordered, resulting in the AuCu-II structure type, but remains intact within the lamellae. The increase in hardness on ordering is related to the volume fraction of the ordered phase.