ELECTRON MICKOSCOPY 



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Fig. 1. Schematic representations of disloca- 

 tions, (top) in edge-orientation (center) in screw 

 orientation (bottom) changing from edge to screw 

 orientation. 



strained. Such localized line defects of the 

 plastically deformed lattice are called "dis- 

 locations". Schematic representations of 

 dislocations are shown in Fig. 1. The ma- 

 terial in the core of the dislocation line, 

 where the crystal order is destroyed, is called 

 "bad" material; the crystalline material 

 around the core even if elastically strained is 

 called "good" material. From the geometry 

 of crystal lattices it follows that dislocation 

 lines cannot end inside the material. They 

 have to be closed loops or must end at the 

 surface of the crystal or at grain boundaries 

 in poly crystalline material. 



In transmission electron microscopy of 

 crystals the contrast is essentially given by 

 diffraction effects which are described by 

 Bragg's law 



2d sin d = n\ 



where d is the spacing of lattice planes, 6 

 the diffraction angle, X the wavelength of 

 the electrons, and n the order of diffraction 



(integer). Dislocation lines become visible 

 because the lattice is strained around the line 

 core which means that d varies locally and 

 thus the Bragg condition and consequently 

 the distribution of the electron intensity 

 between the direct and the diffracted beam 

 varies. As the diffracted beam does not 

 contribute to the image, because it is elimi- 

 nated by the aperture diaphragm, the in- 

 tensity of the image is given by the intensity 

 of the direct beam only. A survey of the 

 theory of the contrast due to dislocations 

 and stacking faults etc. is given by Whelan 

 (26). 



Fig. 2 shows pictures of dislocations as 

 they can he seen by transmission electron 

 microscopy. As dislocations cannot end in- 

 side the material, the ends of the lines lie on 

 the top or the bottom of the foil. 



A ciuantitative characteristic of a disloca- 

 tion line is the so-called "Burgers vector" 

 which is defined in the following way : 



First the dislocation line is given an 

 arbitrary direction. Then, around the line a 

 circuit is marked in the sense of a right-hand 

 screw far enough from the core of the dis- 

 location to be always in "good" material. 

 The circuit has to be traced in such a way 

 that it would be closed in a perfect crystal 



Fig. 2. Dislocations lines in stainless steel, 

 crossing the foil top to bottom and traversing a 

 twin boundary. 



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