Atomic migration and phase transformation processes in dental amalgams over 27 years, monitored by X-ray spectroscopy and X-ray powder diffraction

Abstract The identification of those intermetallic phases which constitute the complex multi-phase system of dental amalgams is rather contradictory in literature. This is intriguing, as the production processes of dental amalgams are standardized since a long time. The reason for the divergence on reported amalgam phases may be due to changes within the microstructure of dental amalgams occurring over time. With a combination of X-ray powder diffraction, Rietveld refinements and X-ray spectroscopy, we were able to show the differences in chemical phases and morphologies between a freshly prepared and a 27-year old dental amalgam sample. Atomic diffusion processes within the amalgam lead to transformation of initially formed intermetallic phases and to dissolution of phases present in the prealloyed metals which react with Hg to the dental amalgam. The main amalgam phases, Ag 1+ x Hg 1− x and Ag 2 Hg 3 , remain stable but change in their relative concentrations: the first increases over time, while the latter decreases. The content of Cu 6 Sn 5 as the third important constituent phase decreases over the years and concentrates in hollow spherical particles. The fourth phase is an fcc Ag–Cu mixed crystal which is only present in the freshly prepared amalgam and dissolves over the years. The silver amalgams form a matrix in which spherical Ag–Cu particles are embedded. Cu atoms migrate from within these particles towards their surface, react to η ′ bronze and are substituted by Hg atoms, whereas the Ag atoms show virtually no mobility.