I- and II-type superlattices in gold-rich copper-gold alloys

Elimination of quenched -in vacancies in gold-rich copper-gold alloys leads to structural states which cannot be obtained by direct annealing at the same temperatures. In this way two ordered phases are observed, a LI2 type structure (Au3Cu I) with antiphase boundaries preferentially lying in {100} planes, in stoichiometric Au3Cu and in the more copper-rich (30 at % copper) alloys, and a one-dimensional periodic antiphase structure (Au3Cu II) in stoichiometric Au3Cu and in the more gold-rich (22.5 and 20 at % copper) alloys. A two-phase region, extending over about 20 deg at the stoichiometry, widens with increasing gold content. These structural transformations are very similar to those occurring in AuCu3 and account for some previous apparently contradictory results on gold-rich copper-gold alloys. The method described may be extended to other systems with low transition temperatures and slow diffusion rates.