Structure of short-range ordered alloys. II. Ordered zones in a disordered matrix

A diffraction theory is developed for the model of ordered zones embedded in an otherwise disordered binary alloy. Evaluation of the resulting lattice sums gives an equation for the diffuse scattering everywhere in reciprocal space except under the fundamental lines. From the experimental diffuse intensity one can calculate the relative volume of the ordered zones in the whole crystal, their mean sizes along each of the crystallographic axes, and the distribution of these sizes. A set of relations between the Warren short-range-order parameters α_lmn and the size of the ordered zones is also obtained. Comparison with experimental α_lmn given in the literature show that a 50 at.% CuAu alloy quenched from 500°C contains ordered zones in a disordered matrix. Similar results were obtained for a 50 at.% CuPt alloy held at 890°C or quenched from 930°C. In a Cu₃Au alloy held at 450°C the calculations show the existence of ordered zones with a size of (3 × 3 × 3) unit cells, in accordance with the results of computer simulation work published in the literature.