A low-gold dental alloy: structure and segregations.

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 interdendritic 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 degrees 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 degrees C cause precipitation of ordered fct particles. On the basis of structure and alloy composition, they are most likely AuCu I and may contain some Pd.