A scanning transmission electron microscopy study of two dental amalgams

Two fully aged amalgam alloys were examined using a scanning transmission electron microscope both in the transmission and scanning mode. The dispersed type amalgam containing a distribution of silver-copper spheres in addition to the Ag3Sn powder showed a markedly reduced gamma 1 grain size compared to a conventional Ag3Sn type amalgam. It is suggested that the increased compressive creep strength of the dispersed type material is a direct result of the reduced gamma 1 grain size and not due to a dispersion hardening effect from the cores of the remaining Ag-Cu spheres. Similarly, the formation of complex Cu-Sn intermediate phases at the Ag-Cu sphere surfaces are unlikely to lead to a dispersion strengthening effect. It is postulated that the reduced grain size in high copper amalgams is a consequence of the enhanced nucleating effect of a copper based phase on gamma 1.