Age-hardening reactions in a type III dental gold alloy

The age-hardening reactions in a commercial type III dental gold alloy were studied by means of hardness test, X-ray diffraction study and scanning and transmission electron microscopic observations. The hardening was attributed to the formation of the metastable AuCu 1′ type ordered phase in the grain interior by the isothermal ageing at 225 and 450°C at which two hardness peaks were observed by the isochronal ageing. By ageing at 450°C, the hardening did not begin immediately because the incubation period was required. The age hardening at 225°C was characterized by a slow growth rate of the metastable AuCu 1′ type ordered phase. The overageing with softening which occurred following prolonged ageing at 450°C was due to the formation of the lamellar structure composed of the Ag-rich α1 and AuCu 1 type ordered phases at grain boundaries.