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 an d transmission electron microscopic observations. The hardening was attribu ted to the formation of the metastable AuCu 1' type ordered phase in the gr ain interior by the isothermal ageing at 225 and 450 degreesC at which two hardness peaks were observed by the isochronal ageing. By ageing at 450 deg reesC, the hardening did not begin immediately because the incubation perio d was required. The age hardening at 225 degreesC was characterized by a sl ow growth rate of the metastable AuCu 1' type ordered phase. The overageing with softening which occurred following prolonged ageing at 450 degreesC w as due to the formation of the lamellar structure composed of the Ag-rich a lpha (1) and AuCu 1 type ordered phases at grain boundaries. (C) 2001 Publi shed by Elsevier Science Ltd.