ELECTRON MICROSCOPY 



Fig. 5. Hexagonal network forming a twist 

 boundary between two (lll)-planes during poly- 

 gonization in aluminium. (Hirsch, Home and 

 Whelan,^ Courtesy Philosophical Magazine) 



wave, while climb does. Glide is produced by 

 a shear stress, while climb needs hydrostatic 

 pressure or tension. Glide is essentially re- 

 sponsible for cold working, while climb con- 

 tributes to high temperature creep because 

 diffusion is only possible at high tempera- 

 tures. 



The glide of dislocations is shown in the 

 films of the group Hirsch, Whelan et al. (6, 

 10) on aluminum and stainless steel (Fig. 8). 

 The specimen is locally heated by the elec- 

 tron beam inside the electron microscope. 

 The thermal stresses produced by this heat- 

 ing perhaps in connection with the surface 

 contamination through oil vapor act so as to 

 move the dislocations. This movement can 

 be slow because of pinning of the disloca- 

 tions at the oxide layer on the surface or it 



can be so quick that only the slip traces left 

 by the moving dislocations mark their path. 

 Wilsdorf (30) and also Berghezan and 

 Foudreux (31) have studied the movement 

 of dislocations due to externally applied 

 stresses. 



A moving dislocation cuts the volume into 

 two domains, one of which is displaced (dis- 

 located!) with respect to the other. Which 

 one is displaced and how much can be 

 recognized in the following way. 



(vX I) = m 



where v is the velocity vector of the dis- 

 location, / a vector in the direction of the 



GLIDE 



W 



Fig. 6. Glide movement of a dislocation in edge 

 orientation {Courtesy Schweizer Archiv) 



CLIMB 



I 



7 



t7. 



D 



i 



t 1 



Fig. 7. Climb movement of a dislocation in 

 edge orientation {Courtesy Schweizer Archiv) 



296 



