Mechanism of ?1 formation in conventional dental amalgams

Abstract

This study reexamined the mechanism of β1 formation in conventional dental amalgams in the light of a previously unrecognized intergranular structure, the SnHg-δ2 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°C for 1 day, both amalgams were heat treated at 70°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 β1 formation. This mechanism involves a series of interrelated reactions. The primary one is a reaction between AgSn-γ and δ2 that leads to the nucleation of β1 at γ boundaries. Simultaneous operation of several secondary and tertiary reactions result in (1) further growth of β1 and its continuing change in composition, (2) loss of Sn from the δ1 matrix, (3) conversion of CuHg into η, and (4) a possible transformation of δ2 into Sn-rich β1 or α1.