X-RAY STUDY OF DIFFUSE-SCATTERING IN NI-20 AT-PERCENT CR

A single crystal of Ni-20 at% Cr aged at 744 K for 480 h, similar to t he case of a previous neutron scattering experiment, is investigated b y diffuse X-ray scattering using X-rays at three different wavelengths , lambda = 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 separa ted based on the 3lambda method recently introduced as well as on the Georgopoulos-Cohen method. Both evaluation schemes yield similar Warre n-Cowley short-range order parameters with less agreement in the indiv idual static atomic displacements. The nearest-neighbor displacement o btained by the 3lambda method for Ni-Cr pairs, is consistent with a si mple phenomenological model in which the Cr atoms expand the Ni lattic e in a short-range ordered alloy. Effective pair interactions calculat ed from model crystals using the inverse Monte Carlo method are found to be larger than previously deduced from neutron scattering experimen ts. If these pair interactions are used in Monte Carlo simulations to calculate the transition temperature T0 of Ni2Cr, T0 is underestimated compared with direct experimental findings.