The room temperature dissociation of Au 3Si in hypoeutectic AuSi alloys

Abstract

50 hypoeutectic AuSi alloy specimens containing 2–16 at.% Si were individually equilibrated above the eutectic temperature, quenched to room temperature, sectioned, mounted in a cold-setting resin, metallographically polished, stored for periods varying from 98 to 111 months and then examined to determine the extent to which the unstable compound Au 3Si formed on quenching had dissociated into gold and silicon. It was found that the dissociation reaction had been nucleated at the polished surfaces and that extensive surface cracking had occurred. Removal of this cracked layer by further polishing revealed that in about half of the specimens the reaction had also started at metal-resin interfaces. It is not clear why this occurred in some specimens and not others. The depth of the dissociated regions corresponds to a rate of advance of the reaction interface of about 0.03 mm year −1.