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.