Effects of Casting Conditions and Annealing on Microstructures and Vickers Hardness of Dendritic Pd-Cu-Ga Dental Alloys

Three Pd-Cu-Ga alloys with as-cast dendritic microstructures and very similar compositions, two containing less than 1 wt% boron and the third boron-free, were cast with normal bench-cooling or rapid-quenching into water. Quenched specimens were also heat treated at temperatures of 1000°, 1200°, 1500° and 1800°F that span the firing cycles for dental porcelain. Similar values of Vickers hardness were observed for all three alloys, suggesting little effect from boron on yield strength. The hardness was relatively insensitive to the experimental conditions, except for heat treatment at 1500° and 1800°F where significant softening occurred with transformation of the microstructure to Pd2Ga and the palladium solid solution

The Viscometry of Two Dental Cements

Master of ScienceProsthodonticshttps://deepblue.lib.umich.edu/bitstream/2027.42/148016/1/ot_190.pd

Mechanism for Formation of Lamellar Constituents in Grain-Refined Pd-Cu-Ga Dental Alloys

Grain-refined Pd-Cu-Ga dental alloys solidify with a lamellar microstructural constituent that affects a variety of clinically relevant properties. While formation of this constituent has been attributed to eutectic solidification, an alternative mechanism of discontinuous precipitation has been proposed. Using a representative grain-refined Pd-Cu-Ga dental alloy, casting procedures involving two different rates of solidification were used: (a) A standard thin-walled coping configuration for a metal-ceramic restoration was cast into a room temperature mold, followed by rapid quenching into an ice-water mixture. (b) A thin plate specimen was cast into a standard elevated-temperature mold, with the same subsequent rapid quenching procedure. Neither casting was subjected to the standard air-abrasion procedure following devesting that is used in dental laboratories. An outer surface layer, approximately one grain thick, containing only the palladium solid solution, was observed in the microstructures of the two different castings, and the eutectic constituent was present at greater depths. This observation is consistent with physical metallurgy principles for the freezing of an alloy containing a eutectic constituent, and the alternate hypothesis that the lamellar constituents might represent discontinuous precipitates has been discarded

Heat-Treatment Behavior of High-Palladium Dental Alloys

Four high-palladium dental alloys were cast, quenched following solidification, and heat treated at temperatures ranging from 200° to 1,800°F. The Vickers hardness of each alloy was measured, and microstructural changes were studied by scanning electron microscopy (SEM). Phase transformations were investigated by thermomechanical analysis (TMA). Heat treatment at 1,600° and 1,800°F significantly decreased the hardness of one Pd-Cu-Ga alloy, compared to the as-cast condition, as did heat treatment at 1,800°F for the other Pd-Cu-Ga alloy and one Pd-Ga alloy, and yielded fine-grained microstructures of the palladium solid solution. There were generally no significant changes in the hardness of the other Pd-Ga alloy for heat treatments over the entire temperature range, which was attributed to persistence of the as-cast dendritic structure. For all alloys, heat treatment at 1,200°F resulted in formation of discontinuous precipitates, which had very similar hardness to that of the palladium solid solution. Variations in the amount of a previously unidentified hard phase with temperature were assumed to account for the hardness variations in the harder Pd-Cu-Ga alloy. The TMA results suggested that a second-order phase transformation occurred around 1,050° to 1,400°F in all alloys, which may be formation of an ordered tweed structure observed by transmission electron microscopy in another study

Effects of Solidification Conditions and Heat Treatment on the Microstructure and Vickers Hardness of Pd-Cu-Ga Dental Alloys

Two representative Pd-Cu-Ga dental alloys, one with a dendritic as-cast microstructure containing a eutectic interdendritic constituent and the other with an equiaxed fine-grained as-cast microstructure containing a near-surface eutectic constituent, have been subjected to rapid quenching following casting, in addition to the conventional bench cooling recommended by the manufacturers. The quenched alloys were subsequently heat treated at temperatures of 1200°, 1500° and 1800 op that span the range of the firing cycles for dental porcelain. Scanning electron microscopic examination showed that the lamellar eutectic constituents normally present in the microstructures of the as-cast and bench-cooled alloys persisted when the alloys were rapidly quenched after casting, although microstructural changes were evident. A large decrease occurred in the Vickers hardness of the alloy with the dendritic as-cast microstructure after heat treatment at 1500°F, and in the hardness of both alloys after heat treatment at 1800°F