The room-temperature dissociation of the compound Au 3Si

Abstract

Three gold-18.6 at.% silicon specimens which were solidified by quenching from above the liquidus temperature have been re-examined after being stored at room temperature for about 8 years. It was found that the “metastable” compound Au 3Si formed on quenching had begun to dissociate by a surface nucleated, solid-state reaction. The reaction front had advanced about 0.2 mm from the surface, leaving a rim of dissociation products. These dissociation products were studied using optical and scanning electron microscopy, electron beam microprobe analysis and an X-ray micro-diffraction technique. It was concluded that the reaction produces elemental gold and silicon, as at higher reaction temperatures, but that the scale of dispersion is so extremely fine that the resulting microstructure cannot be resolved by scanning electron microscopy. Examination of a number of similarly quenched specimens containing 25 at.% silicon prepared about 2 years earlier revealed that the reaction had occurred to a limited extent in these also. Nucleation of the reaction was found to have occurred at primary silicon particles, cracks, cavities, and internal interfaces of Au 3Si with the eutectic mixture, as well as at free surfaces.