Mi-Hyang Cho

Age-hardening by grain interior and grain boundary precipitation in an Au-Ag-Pt-Zn-In alloy for multipurpose dental use

The complex precipitation mechanisms related to the age-hardening of Cu-free Au-Ag-Pt-Zn-In alloy for multipurpose dental use was studied by means of hardness test, X-ray diffraction (XRD) studies, field emission scanning electron microscopic (FE-SEM) observations, energy dispersive spectrometer (EDS) analysis, and electron probe microanalysis (EPMA). The early diffusion and then clustering of the In-concentrated phase in the grain interior, together with the early diffusion and then ordering of the PtZn phase in the grain boundary, introduced the internal strains in the Au-Ag-rich α1 matrix, resulting in the hardening process. As the Au-Ag-rich α1’ and PtZn β lamellarforming grain boundary reaction progressed, the phase boundaries between the solute-depleted face-centered cubic (FCC) α1’ matrix and the face-centered tetragonal (FCT) PtZn β precipitate reduced, resulting in softening. In the particlelike structures composed of the major Pt-Au-rich α2 phase and the minor Pt-Zn-rich α3 phase, the separation of In and Zn progressed producing the In-increased Pt-Au-rich α2’ phase and the Zn-increased PtZn α3′ phase with aging time without restraining the softening. The miscibility limit by complex systems of Au-Pt, Ag-Pt, Au-In and In-Zn resulted in the phase transformation and related microstructural changes.

Age-hardenability related to precipitation and lamellar-forming grain boundary reaction in dental low-carat gold alloy

Abstract A dental Ag–Au–Cu–Pd alloy with small amounts of Zn and Ir was examined to determine the association of the age-hardenability with the phase transformation process and microstructural changes by means of hardness testing, X-ray diffraction study, field emission scanning electron microscopic observation and energy dispersive spectrometry. The fast and apparent age-hardenability of the present alloy was possibly attributed to phase separation by spinodal decomposition and the formation of a fine and coherent cuboidal structure composed of the Ag–Au-rich α1 phase and the metastable Cu–Au-rich α2′ phase. The lamellar-forming grain boundary reaction was induced by internal strain before the maximum hardness value was obtained. Moreover, growth of the grain boundary lamellar structure toward the grain interior and its coarsening acted as a strain-releasing softening mechanism, which caused a decrease in age-hardenability. A change in Ag and Pd content might alter the start point of lamellar-formation and the age-hardenability of Ag–Au–Cu–Pd alloys.