Influence of Si, Ni and Co additions on gold alloy for investment cast process

The aim of this paper is the study of the influence of addition of Si, Ni and Co on the microstructure and mechanical feature of Is-ct gold alloys, in order to reduce production rejects in the investment cast process. For this reason, a typical alloy containing Ag, Cu and Zn usually used for jewelry manufacturing, has been used to prepare four master alloys with variable concentrations of Si, Co and Ni; 18-ct gold alloys were also prepared. It has been demonstrated that the increasing of Si concentration induces embrittlement in gold alloys. XPS analysis on fractured samples revealed the presence of SiO2 at the grain boundaries of samples containing 0.25 wt.% of Si. The addition of Ni and Co permitted to obtain gold alloys with good mechanical properties and surface finishing. It has been also demonstrated that Ni and Co form insoluble intermetallic compounds with Si that decrease SiO2 content at the grain boundaries thus preventing embrittlement. XRD analysis permitted to identify the compounds as Ni3Si and Co2Si.