Diffuse x-ray scattering of Ag–13.4 at. % Al

A single crystal of Ag–13.4 at. % Al aged within the α phase at 673 K was investigated by diffuse x-ray scattering. The separated short-range-order scattering is only weakly modulated and shows two types of diffuse maxima simultaneously, at 2k_F¹¹⁰ (where k_F is the Fermi wave number along 〈110〉) and at 1/2,1/2,1/2 positions. Such a topology has previously been observed only in the Cu-Pt system. A detailed analysis of nearest-neighbor configurations, however, exhibits large differences between the two systems; the data suggest the A₅B structure as a possible ground-state structure of the fcc Ag-Al lattice, i.e., none of the structures observed in the Cu-Pt system. For this structure an order-disorder transition temperature of ∼135K is estimated on the basis of Monte Carlo simulations with effective pair-interaction parameters of the first 18 shells.