TRANSMISSION ELECTRON-MICROSCOPIC INVESTIGATION OF HIGH-PALLADIUM DENTAL CASTING ALLOYS

Objective. The purpose of this study was to use transmission electron microscopy to examine four representative high-palladium alloys and ga in insight into possible strengthening mechanisms. Methods. Castings o f two Pd-Cu-Ga alloys and two Pd-Ga alloys were thinned by jet polishi ng and ion milling, followed by plasma cleaning, to yield foil specime ns. Multiple specimens were prepared for each alloy. Bright-field imag es, dark-field images and selected-area electron diffraction patterns for the alloys in the as-cast condition, after simulated porcelain-fir ing heat treatment, and after annealing at 980 degrees C were analyzed by standard transmission electron microscope (TEM) techniques. The ov erall compositions of the ultrastructures for the specimen foils were determined by conventional standardless energy-dispersive spectroscopi c analyses with the TEM, and mean values of the elemental compositions were compared to the nominal alloy compositions provided by the manuf acturers. Results. There was generally good agreement (differences les s than 2 wt%) between the overall ultrastructure composition and each nominal alloy composition, except for Protocol from which In may have been lost during casting or formed intermetallic compounds that were n ot detected by TEM. The same fine-scale tweed structure within paralle l bands of similar to 100-200 nm width was observed for all four alloy s in the as-cast condition and after simulated porcelain-firing heat t reatment. The persistence of the ultrastructure in the specimens of th e two Pd-Cu-Ga alloys annealed at 980 degrees C and quenched in ice wa ter indicated very rapid formation from the palladium solid solution. The presence of {100} and {110} forbidden reflections for the [001] zo ne suggested that the tweed structure is ordered, although further res earch is necessary to establish this conclusion. Significance. The pre sence of a similar tweed structure in both the Pd-Cu-Ga alloys and the Pd-Ga alloys of substantially lower hardness shows that some other st rengthening mechanism accounts for the high hardness and strength gene rally observed for Pd-Cu-Ga alloys.