Connection of prismatic dislocation loop sizes and interference errorS with the techniques of determining the loop nature

Abstract

By calculating the electron microscopy (EM) contrast it is shown that the surface stress relaxation affects the EM image of loops located at the depth L < 2D (D is the loop diameter). It is found that at L < D every element of a dislocation loop interacts with the surface as a single dislocation and at L > D the loop interacts with the surface as a whole. On the basis of the results obtained validity limits of techniques used to study the sign of the Burgers vector of dislocation loops depending on their sizes are determined. The sign of contrast on the dislocation loop image is shown to remain unchanged if the interference error |S| < 0.5/ζg (ζg is the extinction length for the corresponding reflection), and at |S| > 0.5/ζg images may appear for which the sequence of black‐and‐white details of the contrast is reciprocal to that at S = 0. Hence, images with black‐and‐white contrast can be used with certainty in determining the loop nature only in the neighbourhood of S = 0.