MECHANISM OF BETA(1) FORMATION IN CONVENTIONAL DENTAL AMALGAMS

This study reexamined the mechanism of beta(1) formation in conventional dental amalgams in the light of a previously unrecognized intergranular structure, the SnHg-delta(2) phase. Two amalgams, A (45.5% Hg) and B (65.0% Hg), were made from a Zn-free alloy of the following composition (wt%): Ag-70, Sn-26, Cu-4. After storage at 37 degrees C for 1 day, both amalgams were heat treated at 70 degrees C for up to 16 days. The structural changes in the two amalgams as a function of their heat treatment were characterized by differential scanning calorimetry and X-ray diffraction techniques. On the basis of the experimental data, we have proposed a new mechanism on pi formation. This mechanism involves a series of interrelated reactions. The primary one is a reaction between AgSn-gamma and delta(2) that leads to the nucleation of beta(1) at gamma boundaries. Simultaneous operation of several secondary and tertiary reactions result in (1) further growth of beta(1) and its continuing change in composition, (2) loss of Sn from the gamma(1) matrix, (3) conversion of CuHg into eta', and (4) a possible transformation of gamma(2) into Sn-rich beta(2) or alpha(2).