TRANSMISSION ELECTRON MICROSCOPY OF METALS 



(as many steps to the right as to the left, 

 etc.). The vector pointing from the end to 

 the beginning of the circuit is the "Burgers 

 vector" b. Examples are shown in Fig. 1. 

 The sign of the Burgers \-cct()r has only a 

 meaning in connection with the direction of 

 the dislocation line. Changhig the one also 

 changes the other (right-hand screw!). 



Frank has given another definition of the 

 Burgers vector, where a closed Burgers 

 circuit in a dislocated crystal is imaged onto 

 an ideal crystal where the closure failure 

 there is the Burgers vector. The result is 

 essentially the same but the procedure is 

 academically more correct. 



The Burgers vector in magnitude and 



Fig. 3. Interaction of dislocations of different 

 orientations but the same Burgers vector in stain- 

 less steel during cold-working. (Whelan^^ Courtesy 

 Royal Society) 



bj.b^ 



»^ 



Fig. 4. Interinlion of dislocations forming a 

 network during cold-working. {Whelan,^^ Courtesy 

 Royal Society) 



direction is constant along a dislocation 

 line, even when the line changes its direction 

 (Fig. Ic). That part of a dislocation Une 

 perpendicular to its Burgers vector is said 

 to be in an "edge" orientation, that parallel 

 to its Burgers vector in a "screw" orienta- 

 tion. A dislocation line can be curved and 

 follow all intermediate stages between edge 

 and screw orientations. 



Dislocations can interact and form nodes. 

 The law here is that the sum of the Burgers 

 vectors of all dislocations entering the node 

 is equal to the sum of the Burgers vectors of 

 those emerging from the node (analogous to 

 Kirchoff's law). Complicated network can 

 be formed, of which examples are given in 

 Figs. 3, 4 and 5. A detailed analysis of dis- 

 location interactions in stainless steel has 

 been given by Whelan (29). 



Movement. Essentially two kinds of 

 movement of dislocations have to be dis- 

 tinguished — -"glide" and "climb"; these are 

 schematically shown in Fig. G and 7. Glide 

 is a movement parallel to the Burgers vector, 

 while climb is perpendicular to it. Glide does 

 not need material transport, as it is the 

 movement of a configuration analogous to a 



295 



