TRANSMISSION ELECTRON MICROSCOPY OF METALS 



Fig. 16. Splitting of dislocations in stainless steel into stacking fault ribbons {Whelan, Hirsch, Home 



and Bollman^", Courtesy Royal Society) 



that the fringes are shifted by half a period, 

 when two stacking faults are present, and 

 disappear completely when three or a mul- 

 tiple of three stacking faults follow each 

 other (Fig. 20). 



Condensed Vacancies 



A theory of Kuhlmann-Wilsdorf (35) con- 

 cerns the condensation of vacancies, when a 

 metal is quenched from a high temperature. 

 These features have been observed by 

 Hirsch ei al. (36). In aluminum the vacancies 

 condense in a close packed plane as small 

 disks, which collapse. Such a collapsed disk 

 would form a stacking fault surrounded by a 

 so-called sessile partial dislocation with a 

 Burgers vector of the type (a/3) (111). As 

 the stacking fault energy of aluminum is 

 high, the disk does not collapse perpendicu- 

 lar to the plane but inclined, adding a ghssile 

 partial dislocation (a/3) (111) + (a/6) 

 (112) = (a/2) (110) thus forming a disloca- 

 tion ring (Fig. 21). As the Burgers vector 



Fig. 17. Cross slip trace in aluminium (Silcox 

 (in 27) Courtesy Institute of Metals) 



sticks out of the plane this dislocation ring 

 can only glide on a cylinder parallel to the 

 Burgers vector. Under shear stress, the dis- 

 location ring expands over the cylinder, one 

 part gliding downward, the other upward. 



In a metal with low stacking fault energy, 

 like gold, the disks of condensed vacancies 

 collapse reallj^ to a stacking fault, but to 



301 



