Investigation of short-range order in Ni–10 at.% Al single crystals by diffuse X-ray scattering

The diffuse scattering from single crystals of nominally Ni-10 at.% Al, quenched after heat treatment at 973 K, has been measured with Mo Kα radiation. The data were analysed for the contributions due to short-range order and static atomic displacements. Consistency of the structural parameters as determined by three different methods, the separation methods of Georgopoulos-Cohen and Boric-Sparks and the least-squares method of Williams, is demonstrated for the first time. The variation of the first few short-range-order parameters α_lmn with the coordination shell Imn strongly resembles that of the L1₂ superstructure. Employing these parameters to model a short-range-ordered computer crystal with 13 104 atoms, a preference for configurations of the L1₂ superstructure type is found, though no Ni₃Al embryos are observed. The values of α_lmn rapidly reach those for a random solid solution.