Microprobe analysis of a Au-containing alloy and its amalgam

Under near-equilibrium conditions, an alloy ingot containing approximately 64% Ag, 26% Sn, and 10% Au was found by X-ray diffraction to consist of large grains demonstrating the gamma (Ag-Sn) structure. Microprobe analysis of these grains showed a 64.2% Ag, 25.3% Sn, and 10.5% Au composition with very small amounts of AuSn2 and AuSn4 segregated at the grain boundaries. When this alloy is manufactured in the form of rapidly colled spherical particles, X-ray diffraction shows both beta (Ag-Sn) and gamma (Ag-Sn) structures with Au substituting for Ag, and microprobe analysis demonstrates a segregation of small high Sn, high Au areas having a AuSn2 composition. When Hg is mixed with these spherical particles, gamma1, some gamma2, and Sn-Au phases (AuSn2 and AuSn4) form. The Au that combines with Sn seems to come from the beta or gamma phases rather than from the Sn-Au segregations in the alloy. The small amount of the gamma2 phase that is initially formed apparently disappears after a short period of time although there is some question as to whether a very small amount may still remain. The Ag-Hg hase (gamma1) is essentially the same as found in conventional amalgams as is the small amount of gamma2 initially formed. Furthermore, a nonequilibrium state seems to be necessary for the early and possibly complete elimination of gamma2. In the as-cast condition, after a period of time at 37 C, A Ag-Hg-Sn phase (beta1) is formed that is similar to that found in conventional amalgams that serve as restorations in the mouth for a period of time. However, in the 135-annealed amalgam, no beta1 appears.