We previously reported fatigue limits (AADR Abst. No. 1754, 2001) and characteristic microstructures observed with the transmission electron microscope (TEM) for fatigued high-palladium dental alloys (AADR Abst. No. 133, 2001). Purpose: Employ TEM to investigate the fatigued microstructure of a representative Pd–Ag alloy (Super Star, Jelenko; 60Pd–28Ag–6In–5Sn, with < 1 wt. % Ga, Ru/Re), in order to gain insight into the mechanism for fatigue failure. Methods: Fractured bars of the Super Star alloy, which had been subjected to simulated porcelain-firing heat-treatment and fatigued under uniaxial tension-compression stress at 10Hz and an R-ratio of –1, were selected for TEM (Philips CM200) examination. Regions near the fracture surface were sectioned into slices, using a slow-speed water-cooled diamond saw. Foil specimens suitable for TEM observation were obtained by mechanical grinding and polishing these slices, following by ion milling and plasma cleaning. Bright field, dark field, convergent beam electron diffraction and selected area electron diffraction imaging techniques were used to obtain the microstructural information. Phase compositions were measured by x-ray energy-dispersive spectrometric (EDS) analysis. Results: Dislocations were present in the fcc palladium solid solution matrix phase, and interaction between dislocations and precipitates in the matrix phase was evident. Twins were found in the previously reported (AADR Abst. No. 132, 2001) face-centered tetragonal (fct) phase; this phase did not contain twins prior to the fatigue testing. Conclusions: Fatigue deformation in this Pd–Ag dental alloy was controlled by slip (movement of dislocations) in the fcc matrix phase and twinning in the fct phase. The fct phase appeared to have a beneficial effect on the fatigue performance of this alloy. Supported by NIDCR Grant DE10147.

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