STRUCTURE OF TWIN BOUNDARIES IN L12 ORDERED ALLOYS

The first results of a transmission electron microscopy study on the structure of grain boundaries in ordered alloys are reported. The existence of two structures is demonstrated for SIGMA equals 3 left brace 111 right brace twin boundaries in ordered Cu//3Au, consisting of a symmetric and an asymmetric structure, separated by an antiphase boundary on one side of the boundary plane. Contrary to earlier theoretical work, it is found that these structures are the only possible simple structures obtained for rigid translations of the type LT AN BR 110 RT AN BR /2 parallel to the boundary plane. A translation of the type LT AN BR 211 RT AN BR /6 parallel to the boundary plane, generated by a dislocation in the boundary, was found to lead to the same structures.