Precipitation of β′ phase and hardening in dental-casting Ag–20Pd–12Au–14.5Cu alloys subjected to aging treatments

The age-hardening behavior of the dental-casting Ag–20Pd–12Au–14.5Cu alloy subjected to aging treatment at around 673 K is well known, and this hardening has been widely employed in various applications. To date, the age-hardening of this alloy has been explained to attribute to the precipitation of a β phase, which is a B2-type ordered CuPd phase or PdCuxZn1 − x phase. In this study, results obtained from microstructural observations using a transmission electron microscopy and a scanning transmission electron microscopy revealed that a fine L10-type ordered β′ phase precipitated in the matrix and a coarse-structure region (consisting of Ag- and Cu-rich regions) appeared after aging treatment at 673 K and contributed to increase in hardness. The microstructure of the coarse β phase, which existed before aging treatment, did not change by aging treatment. Thus, it is concluded that the fine β′ phase precipitated by aging treatment contributed more to increase in hardness than the coarse-structure region and coarse β phase.