A Low-gold Dental Alloy: Structure and Segregations

Abstract

The structure and concentration gradients were studied in the as-cast state for a low-gold alloy and a conventional Type III gold alloy. A much more lamellar eutectic phase was found at the grain boundaries in the low-gold alloy. TEM investigations showed that the interior of the grains consisted of fine lamellae, which probably were alternating Au-Ag and Au-Cu-rich bands due to the miscibility gap in the solid state. Microprobe analyses, where the beam overlapped several of the observed lamellae, displayed both the inter-dendritic and grain boundary segregations to be much larger for the low-gold alloy than for the Type III alloy. The lamellae observed in the as-cast state are quickly dissolved at 700°C into one phase, but the relaxation by diffusion of the concentration differences associated with grain boundary segregations required several hours because of the much larger distances involved. Aging at 350°C caused precipitation of ordered fct particles. On the basis of structure and alloy composition, they are most likely AuCu I and may contain some Pd.