Age-hardening mechanism by precipitation in a zinc- and indium-added silver-palladium alloy

The age-hardening mechanism of a copper-free silver-palladium alloy is unclear yet because the age-hardening mechanism is variable even with slight compositional changes. The aim of present study is to elucidate the age-hardening mechanism and relation of age-hardenability with the phase transformation and microstructural changes of a Zn- and In-added Ag-Pd alloy by means of the hardness test, X-ray diffraction study, field emission-scanning electron microscopic observation and energy dispersive spectrometer analysis. The matrix was composed of the Ag-rich α1 phase containing small amounts of solute elements, and the particle-like structures were composed of the PdZn-based β1 phase containing relatively large amounts of In. The lamellar-forming grain boundary reaction which is usually observed in Ag-Pd alloys containing Cu was not observed in the Zn- and In-added Ag-Pd alloy. The grain interior precipitate composed of the Pd-Zn-rich phase containing In resulted in apparent hardening from the early stage of aging process, and its coarsening by prolonged aging resulted in softening. In the specimen alloy, Zn and In formed extra phase preferentially with Pd, and the amount of Zn and In which took part in the precipitation hardening was very small.