X‐ray study of diffuse scattering in Ni–20 at% Cr

A single crystal of Ni–20 at% Cr aged at 744 K for 480 h, similar to the case of a previous neutron scattering experiment, is investigated by diffuse X‐ray scattering using X‐rays at three different wavelengths, λ = 0.2075 nm (5975 eV), 0.1489 nm (8327.2 eV), and 0.1531 nm (8097 eV). The short‐range order and displacement scattering are separated based on the 3λ method recently introduced as well as on the Georgopoulos‐Cohen method. Both evaluation schemes yield similar Warren‐Cowley short‐range order parameters with less agreement in the individual static atomic displacements. The nearest‐neighbor displacement obtained by the 3A method for NiCr pairs, is consistent with a simple phenomenological model in which the Cr atoms expand the Ni lattice in a short‐range ordered alloy. Effective pair interactions calculated from model crystals using the inverse Monte Carlo method are found to be larger than previously deduced from neutron scattering experiments. If these pair interactions are used in Monte Carlo simulations to calculate the transition temperatureT0of Ni2Cr,T0is underestimated compared with direct experimental findings.