TRANSMISSION ELECTRON MICROSCOPY OF METALS 



(a) 



(C) 



Fig. 8. Sequence ul ilic lilm mi .-^i;(lllle8.s -steel .showing the ghde nioveinent of dislocations. The black 

 dots in the center are marks on the fluorescent screen of the microscope. The object has been displaced 

 during observation. {Whelan, Hirsch, Home and Bollman^" Courtesy Royal Society) 



line in the defined line direction, m is a 

 vector which points to one of the two do- 

 mains. This indicated domain is displaced 

 with respect to the other by the Burgers 

 vector b when the dislocation passes. This 

 rule can be verified for the cases in Fig. 1. 

 Thus glide steps on the surface of a material 

 are produced by the sum of the displace- 

 ments due to the individual dislocations; 

 these have been studied at the edges of thin 

 fihns by Hirsch et al. (32) (Fig. 9). 



Forces Between Dislocations. The 

 energy of a dislocation, i.e., the work that 

 has to be done to produce a unit length of a 

 dislocation line is proportional to h- {h = 

 Burgers vector). This shows that a disloca- 

 tion always tends to attain the smallest 

 possible Burgers vector. The energy E of a 

 dislocation with a Burgers vector (2 6o) would 

 be twice the energy of two separate and 

 sufficient far apart dislocations with Burgers 



vector ho each: (2 6o 



2( bo' + bo'). Two 



parallel dislocation lines, the Burgers vec- 

 tors of which include an angle smaller than 



Fig. 9.- Glide steps on stainless steel. The speci- 

 men was etched only from one side. {Hirsch, Par- 

 tridge and Segall,'^^ Courtesy Philosophical Maga- 

 zine) 



90° repel each other. If this angle is larger 

 than 90° they attract each other. (The 90°- 

 case corresponds to the theorem of Pytha- 

 goras!) A detailed analysis of the stress field 

 betv.-cen dislocations shows that the forces 



297 



