SEM Microstructural Observations and Vickers Hardness Measurements for Pd-Ag Alloys

The clinically popular palladium-silver alloys appear to have relatively wide variation in mechanical properties. Purpose: Characterize the microstructures of two new Pd-Ag alloys (IS 64 and d.Sign 59; both Williams/Ivoclar Vivadent) and their Vickers hardness in the as-cast and heat-treated conditions. Vickers hardness should correlate with yield strength for Pd-Ag alloys, since their work hardening mechanisms should be similar. The nominal alloy compositions are: d.Sign 59 (59.2Pd-7.9Ag-.2Sn-.7In-.3Zn and <1.0 Pt, Ru, Re and Li); IS 64 (59.9Pd-6.0Ag-.0Sn-.8Au-.8Ga-.5In-.0Pt and <1.0 Ru and Re). Methods: Specimens simulating the coping for a maxillary central incisor were cast and subjected to heat treatment simulating the porcelain firing cycles. The specimens were resin-mounted, polished, and etched in aqua regia solutions, and coated with a thin layer of gold-palladium alloy for SEM observation. This coating was then removed, and specimens were polished and etched again for Vickers hardness measurements (1 kg and 300 g). Results: Both Pd-Ag alloys had dendritic as-cast microstructures and experienced substantial microstructural homogenization during the simulated porcelain-firing heat treatment. After this heat treatment, both microstructure had well-defined equiaxed grains and precipitates with dimensions not exceeding a few microns. The Vickers hardness after heat treatment agreed well with the reported value of 230 for both alloys. The presence of grain boundary porosity is indicative of the importance of care in melting and casting these alloys. Conclusions: Fundamental understanding about the origins of yield strength and ductility of these alloys requires transmission electron microscopy because of the small size of the precipitate particles. Both alloys fit our metallurgical model where the ratios of the component elements have critical importance in determining the amount and morphology of the secondary phases in the palladium solid solution matrix. Supported by NIDCR Grant DE10147.