High‐resolution electron microscopic study on long‐period superstructures with 6 H stacking sequence in Au 3 Cd alloys

Atomic arrangements of long‐period superstructures in Au–24.5at.%Cd alloy with the basic h.c.p. structure have been investigated by high‐resolution electron microscopy. The alloy shows one‐dimensional or two‐dimensional long‐period superstructures in the close‐packed plane with a 6 H stacking sequence, depending on heat treatments. It is shown that the two‐dimensional incommensurate superstructure with a period larger than 9 a 0 is composed of domains of the hexagonal commensurate structure ( M = 9) separated by the antiphase‐boundary zone in which 3 3/2 a 0 × 2 a 0 × 3 c 0 orthorhombic structure is formed ( a 0 , c 0 being the lattice parameters of the h.c.p. structure). Microstructures of the alloy annealed at 620 K with the one‐dimensional long‐period superstructure and of the alloy quenched from 770 K showing diffuse scattering have also been investigated by high‐resolution electron microscopy.