In a complementary study, our measurements of Vickers hardness for an annealed Pd–Ag alloy (Rx 91, Jeneric/Pentron; 53.5Pd–37.5Ag–8.5Sn, balance: In and Ru) indicated that the hardness of the bulk alloy, the palladium solid solution matrix and the grain boundary regions varied with the annealing temperatures. Purpose: Employ transmission electron microscopy (TEM) to investigate the characteristic microstructures of the annealed Rx 91 alloy in more detail and gain insight into the hardening mechanism. Methods: Castings simulating a maxillary central incisor were prepared using standard dental laboratory techniques and sectioned with a slow-speed, water-cooled diamond saw. Specimens were isothermally annealed for 0.5 hour in a nitrogen atmosphere at temperatures from 400° to 950° C, using steps of 50° C. Foil samples prepared by mechanical grinding and polishing, ion milling, and plasma cleaning were annealed at three temperatures (409° C, 662° C, and 947° C) based upon the Vickers hardness measurements. The annealed foils were examined in the TEM (Philips CM200), using bright field, dark field, selected area electron diffraction, and convergent beam electron diffraction imaging to identify the phases and their structures. The compositions of the phases were determined by x-ray energy-dispersive spectrometric (EDS) analysis. Results: The Rx 91 alloy annealed at 409° C contained two phases: the Ag-enriched fcc matrix and an ordered grain boundary phase. Ordered fcc precipitates were present in the matrix phase after annealing at 662° C, where the alloy had its highest hardness, and growth of these precipitates at higher annealing temperatures lowered the hardness. Conclusion: The hardening mechanism of this Pd–Ag alloy is controlled by ordered precipitates (L1₂ type) that have special crystallographic orientations with the matrix. Supported by NIDCR Grant DE10147.

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