Electron-microscope imaging of short-range order in disordered alloys

Dynamical diffraction effects of diffuse scattering due to point defects in crystals and the associated imaging are studied in relation to imaging of the short-range order in disordered alloys through theoretical formulations and computer image simulations. Simulations of gold crystals including one point defect without strain show that the image contrast is localized in atomic size and relatively insensitive to the defect depth when the total thickness is fixed. The contrast from a small number of defects in an atomic column can be simply related to the number of defects by a nonlinear expression. An approximate imaging theory for short-range order in disordered binary alloys is discussed. The present study shows that the important parameters for observations of defective crystals are total thickness and microscope defocus, but not the defect depth. With controlled values of these parameters and sample conditions, the images of disordered binary alloys can be interpreted semi-quantitatively.