TRANSMISSION ELECTRON MICROSCOPY OF METALS 



between two atoms in the lattice surrounded 

 by atoms in the same symmetry. In a f.c.c. 

 lattice it is of the type (a/2) (110) (connec- 

 tion between the corner to the centre of the 

 face in the unit cell). At the same time it is 

 the connection between two spheres in a 

 close packed plane (Fig. 15). When a dis- 

 location moves between a layer A and a 



Twins (cub. f.c. ) 



ABC ABC ABC AB 



Fig. 13a. 



layer B, the atoms in the layer B have to 

 jump from the starting position to the 

 neighboring position the Burgers vector h 

 away. This jump can be achieved in two 

 steps Bi ^ C and C — > B2 . C in this case is 

 not a lattice site but the site occupied when 

 a stackhig fault is at this place. Such a 

 splitting of a total dislocation into two 

 "partial dislocations" or "partials" is de- 

 scribed by the equation in Fig. 15 for the 

 Burgers vectors. A partial dislocation is not 

 a dislocation line in the usual sense but the 

 boundary of a stacking fault. 



As the Burgers vectors of the two partial 

 dislocations include an angle smaller than 

 90° they repel each other. By separating, 

 the two partial dislocations have to do work 

 to produce a stacking fault in between them. 

 WTien the stacking fault energy is high, the 

 stacking fault ribbon between the two 

 partials becomes narrow and vice versa. The 

 splitting of dislocations into stacking fault 



f-r^- 



Fig. 13b. 

 Fig. 13. Twin boundary in stainless steel. (Whelan, Hirsch, Home and Bollman^'> Courtesy Royal 

 Society) 



299 



