Mechanism for Grain Refinement by Ruthenium in Palladium-Based Casting Alloys

Nielsen and Tuccillo proposed (J Dent Res 1966;45:964-969) that grain refinement of cast gold alloys, achieved by small amounts of Ir or Ru, occurred by homogeneous nucleation where atoms of these refractory elements reduced the surface free energy. From SEM observations, our group suggested that grain refinement of high-Pd alloys by Ru (~ 0.5 wt. %) might involve a peritectic mechanism (Brantley et al., Cells Mater 1993;3:103-114). Purpose: To use transmission electron microscopy (TEM) to gain insight into the mechanism of grain refinement by Ru in Pd-based casting alloys. Methods: Two high-Pd alloys (Liberty, Jelenko, 76Pd–10Cu–5.5Ga–6Sn–2Au; Protocol, Williams/Ivoclar, 75Pd–6Ga–6In–6Au–6.5 Ag), a Pd–Ag alloy (Super Star, Jelenko, 60Pd–28Ag–6In–5Sn), and an Au–Pd alloy (Olympia, Jelenko, 51.5Au–38.4Pd–8.5In–1.5Ga) were selected. Thin foil specimens were prepared for each alloy by previously described procedures (Cai et al., Dent Mater 1997;13:365-371) and examined with a Philips CM200 TEM. Phase compositions were determined by x-ray energy-dispersive spectrometric analysis, and crystal structures and lattice parameters were obtained by convergent-beam electron diffraction. Results: Pure Ru particles were found in the microstructures of the high-Pd alloys, but not in the microstructure of the Pd–Ag alloy, which had a lower concentration of Ru and evidence of residual dendritic structure. The Au–Pd alloy contained nearly pure Ru particles with ~ 4 wt. % Au (N=5). Pure Ru particles should not exist with a peritectic mechanism where grains are nucleated at Pd–Ru seed crystals. Conclusions: While the Nielsen-Tuccillo mechanism cannot be ruled out, unmelted Ru particles may be sites for grain nucleation in high-Pd alloys. The presence of these particles is plausible from the high melting temperature of Ru and the likelihood of rapid alloying by manufacturers. Supported by NIDCR Grant DE10147.