Weak beam under convergent beam illumination

The weak beam technique is now used widely for the determination of stackin g fault energies, in particular for intermetallic alloys, and the accuracy of the approach is critically dependent upon the reliability of the relatio nship between the image and the actual position of the dissociated dislocat ions. Examining as a model case a dislocation dissociated into two Shockley partial dislocations in Cu at 100 kV for orientations ranging through the g(3g) weak beam condition, image simulations are used to explore the accura cy to which the true spacing between the partial dislocations can be determ ined from the spacing measured on the image as a function of the dislocatio n character, the foil thickness, the dislocation depth in the foil, the dif fraction condition and the beam convergence. It appears that for image simu lations and for the given conditions a beam convergence of about 5 mrad all ows to greatly improve the accuracy, and that beam convergence must be take n into account quantitatively when deducing the true partial dislocation sp acing as it is the principal parameter controlling the precision in this ty pe of measurement. (C) 2000 Elsevier Science B.V. All rights reserved.