Short-range order in h.c.p. Ag–Al

A single crystal of Ag–36.7 at.% Al aged within the hexagonal close-packed (h.c.p.) δ-phase region of the phase diagram at 366 K was investigated by diffuse X-ray scattering. Diffuse scattering maxima were found at 2 k F positions where k F is the Fermi wave number along 〈1 1 0] . The contributions from short-range order scattering, linear size-effect scattering, Huang scattering and thermal diffuse scattering were separated by least-squares fitting of the corresponding Fourier series. In crystals modeled on the basis of 40 Warren–Cowley short-range order parameters, nearest-neighbor configurations are found which are characteristic of a superstructure introduced by Neumann as a possible ground-state structure of h.c.p. Ag–Al. Effective pair interaction parameters of the first 12 shells determined by the inverse Monte Carlo method were used in Monte Carlo simulations. For the Neumann structure, an order–disorder transition temperature of ∼100 K is estimated.