Phase equilibria have been used to construct a simplified model of 14 karat (14K) yellow jewelry alloys. In this model, silicon has a reduced solubility in solid 14K (Au-Cu-Ag-Zn) alloys compared to its solubility in pure copper. This reduced solubility results in low values of the partition coefficient for silicon in these alloys. The microsegregation of silicon in 14K yellow gold jewelry casting alloys was evaluated based on the principles of non-equilibrium solidification. The dynamics of non-equilibrium freezing described by the Gulliver-Scheil equation predict the build-up of silicon in the liquid phase to such an extent that unexpected, silicon-rich phases and compounds may appear in the solidification structure. These phases can have deleterious effects on mechanical properties and limit the use of certain types of grain refiners in (AuCu-Ag-Zn) families of alloys. The practical limits of silicon levels in 14K yellow gold alloys which do not allow the formation of Cu-Si intermetallics are estimated.There are mixed messages and opinions in the jewelry industry about the role of silicon as an alloying element in karat gold casting alloys. Dieter Ott (1) summarized the situation very well when he stated that "the use of silicon is almost a philosophical question (in karat gold alloys)."Silicon is known to perform useful deoxidation and 'brightening' functions. It is also known to increase the apparent fluidity of molten alloys and help in the production of investment castings which faithfully replicate intricate details in patterns. However, silicon promotes the development of large grains in cast structures and in high concentrations can cause the formation of low melting phases or precipitates which reduce mechanical properties. Figure 1 shows the brittle fractures associated with coarse grains in 14 karat (14K) yellow gold alloys containing silicon.Two recent studies of silicon-deoxidized 9K and 14K yellow golds indicate that the behaviour of silicon in these alloys is linked to the total precious metal content in a karat gold alloy. Normandeau (2) studied the effects of silicon in 10, 14, and 18K yellow gold jewelry alloys. In his study, the effect of silicon was correlated with the total concentrations of gold and silver in a casting alloy. The results indicate that as the total amount of gold and silver in a jewelry casting alloy increases, the alloy demonstrates loss of ductility and embrittlement when silicon exceeds a critical level. In other words, the alloys become less tolerant of silicon as caratage increases. Silicon-rich particles were observed on the grain boundaries of 14K fracture specimens when nominal silicon levels exceeded 0.175 wt% (3). Silicon concentration greater than 0.050 wt% in 18K alloys resulted in severely embrittled specimens for tensile testing.Grice (4) studied silicon-deoxidized low karat golds and reported the appearance of an alloy phase in the grain boundaries of alloys with silicon contents in the range of 0.10 -0.75 wt%. He concluded that 9K alloys containing high con...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.