Electron Microscope Observation in an Age-Hardenable Dental Gold Alloy

Abstract

Transmission electron microscopy and electron diffraction studies have been carried out in order to make clear the crystallography and morphology of strengthening phases in a dental gold alloy, Au-35.4 at%Cu-17.8 at%Ag-9.7 at%Pd. Age-hardening observed in this alloy is caused by the formation of the AuCu I type tetragonal superlattice nuclei on the disordered matrix {100} planes. The interfacial coherency between the AuCu I type superlattice and the disordered matrix has been studied by referring to diffuse scattering in diffraction patterns and striated contrasts observed in the electron micrographs. The significant hardness increase is considered to be associated with the striated contrasts due to the tetragonal distortion. The structure of the final state has been identified as the AuCu I type ordered platelets which have the c-axis distributed along the three cube axes and have the [011] twin relation. Nodular precipitates formed at grain boundaries play no important role in the age-hardening.