Dislocations and Their Movcnwnl in Mclal Foils 



313 



Fig. 3(«). An example of a square cross-grid of screw disloca- 

 tions forming a iwist boundary on (100). Dislocations come 

 to an end on the surface at A. Magnification 100,000. 



Fig. 3 (ft). A hexagonal network of dislocations. 

 Magnification 200,000. 



Fig. 4. Fast dislocation slip trace showing cross-slip. Ihc 

 dislocation penetrated the boundary at C. CD is another 

 slip trace with the dislocation held up at D. Magnification 

 60,000. 



(c) Some of the arrangements of lines can be 

 explained readily on dislocation theory (see below), 

 but not on any other basis known to the authors. 



(d) On tilting the illumination or the specimen 

 through small angles, the lines remain fixed in posi- 

 tion, although the contrast changes. This shows that 

 the lines are a definite property of the specimen. 



(c) When working with large beam current den- 

 sities the lines are observed to move; the movement 

 occurs along straight lines parallel to the traces of 

 (111) slip planes, fig. 4. 



Contrast mechanism. — The diflFerences in contrast 

 between one subgrain and another are due to differ- 

 ences in intensities of Bragg refiections caused by 

 the small misorientation angles. This is easily dem- 

 onstrated by tilting the object in a stereo holder 

 through an angle of 1° or 2'', when it is possible to 

 illuminate grains which were originally dark and 

 vice versa. The thickness of the foil is sufficient 

 (about 500 A) for the scattering of electrons to be dy- 

 namical, and depending on the orientation much inten- 

 sity may be abstracted from the direct beam and the 



Obiect 



Ezzz Ezzzzz; czza 



Len s 



(a) 



Objective Aperture 



---- Low Intensity 

 — • Hiqh Intensify 



(b) 



Fig. 5((/) and (h). Illustrating Bragg contrast between sub- 

 grains. («) Grain appears light, (h) Grain appears dark. 



low angle scattering by intense Bragg reflections. 

 Owing to the physical objective aperture (30 // in 

 these experiments) these Bragg rcficctions do not 

 contribute to the image and such a subgrain appears 

 dark, see fig. 5 (a) and (/;). 



It will also be noticed that the boundaries and 

 the dislocation lines are darker than the surrounding 

 regions. The dislocation line contrast was at first 

 thought to be due to impurities, but similar observa- 

 tions on high purity foils eliminate this ctTect. It is 

 now thought that the contrast is again due to in- 

 creased Bragg refiection from the strained region 

 around a dislocation line. It is known that the intro- 

 duction of a simple dislocation into an otherwise 

 perfect lattice can produce difi'useness of the reci- 

 procal lattice points (14). The dark appearance of 

 the dislocation lines in bright field is therefore due 

 to an increased probability of refiection into several 

 Bragg reficctions from the strained region around 

 the dislocation. This is shown schematicalK in tig. 6. 



(ooo) • 



(ooo) 



(a) 



Strain-Free 



Stronq Reflection 

 Weak Reflection 



(b) 

 Strained 



Fig. 6((/) and (/>). Schematic diagram of the reciprocal lattice 

 and reflecting sphere, showing how the strain ticid of a dis- 

 location may produce increased Bragg scattering. («) A 

 strain-free region may only give rise to one or two strong 

 Bragg reflections, (h) A strained region may give rise to 

 several strong reflections. 



