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
