Dislocations in Stainless Steel 



317 





■% 



mm 



Fig. 3. Stacking faults. 



Fig. 4. Steel reduced 25 "„ by rolling. 



locity increases with decreasing gas pressure (because 

 the ions suffer a smaller number of collisions with 

 gas atoms). With increasing mean velocity the mean 

 energy of the ions impinging the surface also increa- 

 ses. High energetic ions strike out of the specimen a 

 large number of atoms per ion, producing in this 

 way small holes. Therefore, the lower the gas pres- 

 sure, the rougher will the treated surface become. 

 Thus the strength of the attack can be regulated by 

 varying the gas pressure. 



For our purposes, the ion bombardment did not 

 present an advantage. The smoothest surface was 

 obtained directly by electropolishing. The importance 

 of ion bombardment for the work of Castaing on 

 Al-4 "o Cu alloy seems to lie, at least partially, in the 

 fact that the Al is preferentially electropolished (as 

 pictures of Castaing show), but that the ion bom- 

 bardment released the copper preferentially from 

 the surface. It is known that it is relatively simple 

 to sputter (release by ion-bombardment) heavy met- 

 als such as copper, but that it is difficult to sputter 

 aluminium because of the oxide layer (7). 



Micrographic appearance of dislocations in crystal 

 foils. — Much theoretical work has been done on 

 dislocations in crystals to explain the behaviour of 

 metals during plastic deformation (sec 4, 6). In a 

 plastically deformed metal, the strongly distorted 

 regions are concentrated on definite lines, the so- 

 called "dislocation lines". The material around these 

 lines is more or less elastically deformed. The theory 

 shows that the dislocation lines have to be closed 



loops or have to end at the surface of the grains; 

 they can be branched into extensive networks (figs. 

 1. 2). 



Until recently, experimental evidence on disloca- 

 tions in metals was given by etching the surface, but 

 here only the end-points of dislocation lines appear 

 as etchpits (2). In salts it was possible to make 

 dislocations visible by metal precipitations (I). 



The reason why dislocations can be seen directly 

 by electron microscopy inside the material is due 

 to the fact that diffraction effects contribute markedly 

 to the image contrast in the electron microscopy of 

 transparent crystal foils. As electron diffraction 

 (Bragg condition) depends among other things on 

 the lattice constant, a local distortion of the lattice 

 becomes visible in suitable orientations. 



Sometimes, dislocation lines appear as dotted 

 lines. Using the dynamic theory of diffraction, Hei- 

 denreich showed that when a Bragg angle is nearly 

 attained by the incident electrons, the brightness of 

 the picture of a crystal foil varies periodically with 

 growing thickness. Thus, a foil with varying thickness 

 shows interference fringes. It can be shown from 

 different pictures that the dotted dislocation lines 

 might be related to these fringes. In fig. 3 interference 

 fringes are visible both at the grain boundary and 

 within the grains; at the same time the dislocation 

 lines in the upper part are dotted. If the angle be- 

 tween the object plane and the electron beam is 

 changed, the fringes disappear and the corresponding 

 dislocation lines become smooth. 



