High-resolution electron microscopic study on long-period superstructures with 6H stacking sequence in Au₃Cd 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 6H stacking sequence, depending on heat treatments. It is shown that the two-dimensional incommensurate superstructure with a period larger than 9a₀ is composed of domains of the hexagonal commensurate structure (M = 9) separated by the antiphase-boundary zone in which 3^3/2a₀ × 2a₀ × 3c₀ orthorhombic structure is formed (a₀, c₀ 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.