ELECTRON-MICROSCOPE IMAGE-CONTRAST FROM SMALL DISLOCATION LOOPS .3. THEORETICAL PREDICTIONS FOR NON-EDGE DISLOCATION LOOPS IN A BCC CRYSTAL

For pt.II see ibid., vol.7, p.1370 (1977). Computer image simulation has been used to study the influence of loop normal, n, Burger's vector, b, diffraction vector, g, and image plane normal, z, on the distribution of black-white intensities in images from loops located in layer 1. The main conclusion from a survey of non-edge perfect loop images is that modifications are predicted to the images from edge loops on going to the non-edge configuration, but that in the majority of cases the scheme of image classification introduced for edge loops based on mod g.b mod is still appropriate. Generally, the modifications to the edge images take the form of some skewing of the black-white lobes or rotation of the black-white interface. In particular, it was found that, firstly, the images where mod g.b mod >1 always retain their characteristic interface structure between the black and white lobes. Secondly, the g.b=0 images from loops with b normal to z may be distorted significantly from the symmetrical 'butterfly' obtained from pure edge loops and, thirdly, for some specific combinations of g, b, n and z, difficulty is encountered distinguishing between modified g.b=0 and 0< mod g.b mod <1 images.