Phase equilibria and experimental observations were used to contract a model of Au-Cu-Ag-Zn-Si alloys. In this model, silicon has a reduced solubility in solid Cu-Ag-Au-Zn alloys compared to its solubility in pure copper. This reduced solubility results in low values of the partition coefficient for silicon during freezing in these alloys. The dynamics of non-equilibrium freezing described by the Scheil equation predict the build-up of silicon in 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 alloying elements in Au-Cu-Ag-Zn families of alloys. Electron microscopy was used to measure microsegregation of gold, silver, copper, zinc and silicon in cast samples of 14K yellow gold. Mechanical properties were measured in 14K yellow gold alloys that contained different concentrations of silicon and iridium grain refiner. Silicon-free, grain-refined alloys had tensile strengths of 56,142 psi and elongations of 49.9% in 1 inch. A conventional silicon-deoxidized 14K alloy had a tensile strength of 43,505 psi and an elongation of 34.7% in 1 inch.

Author: John C. McCloskey, Dr. Shankar Aithal, Paul Randy Welch