ELECTRON :MI(.K<>sr.OPY 



Stacking Fault (cub. f.c.) 



ABCABCABCA B 



(a) 



****■» f- -tmt' 



m^ 



^ 



^g^dmibHsk 







(b) 

 Fig. 14. (a) Schematic representation of a 

 stacking favilt. (b) System of .stacking faults in 

 cobalt on all 4 (lll)-planes. These stacking faults 

 stabilize the observed region in the cubic phase at 

 room temperature although it normally would be 

 hexagonal . 



ribbons can be seen in the film on disloca- 

 tions in stainless steel (10) (Fig. 16). 



Dislocations glide especially easily on 

 certain planes (the so-called glide planes) 

 which in the case of the cubic face centered 

 lattice are (lll)-planes (close-packed planes 

 perpendicular to the space diagonal of the 

 unit cell). If a dislocation ribbon is narrow, 

 it can easily change from one glide system 

 to the other, when the Burgers vector is 

 parallel to both glide planes. This move- 

 ment is called cross-slip (Fig. 17). It has 

 been observed in aluminum, which is a metal 



with high stacking fault energy and thus 

 with narrow dislocation ribbons. If the dis- 

 location ribbon is wide, the Burgers vector is 

 composed of the two vectors of the partial 

 dislocations, both lying in the glide plane. 

 To change the glide plane, the ribbon has to 

 be contracted and to dissociate again into 

 two new partial dislocations lying in the new 

 glide plane. In a metal with low stacking 

 fault energy like stainless steel, this is only 

 possible at a serious obstacle as for example 

 a twin boundary (Fig. 18) but another dis- 

 location on the same slip plane is not suf- 

 ficient to introduce cross slip so that series 

 of dislocation originating from the same 

 source become piled up against each other 

 (Fig. 19) and thus form piled up groups. 



Contrast Due to Stacking Faults 



A theory explaining the observed fringes 

 from stacking faults has been given by 

 WTielan and Hirsch (33, 34) on the basis of 

 the kinematical and dynamical theory of 

 electron diffraction. These authors showed 



SPLITTING OF A DISLOCATION LINE INTO PARTIAL DISLOCATIONS 

 b, = "b, + t, 



|[,oi>i[n2].i[2iil 



C A B C ^A 



\ A A / \ / 



B — C — ■» B 



/ / / / / 

 / / ,'-/-'/ / 

 ',.../.// / V A 



->•,> \/' / 



/•--/-' / A 



y / / / 



Fig. 15. Separation of a total dislocation into 

 partials. 



300 



