Dislocations and Their Movement in Metal Foils 



315 



of the band of contrast left after the dislocation has 

 passed. Cine film measurements have shown that 

 these bands appear in less than .5^ second. Fig. 4 

 is a typical example of fast movement. Here a 

 dislocation has approached a boundary along AB, 

 cross-slipped several times, and eventually pene- 

 trated the boundary at C. Another fast dislocation 

 has left the boundary at C and can be seen held up 

 at the end of the slip-trace CD. The interpretation 

 of the bands is that the dislocation leaves evidence 

 of its passage on the slip plane, and that the width 

 of the band is due to the finite thickness of the foil. 

 In most areas, such as in fig. 4, the normal to the 

 foil is [100] so that the thickness of the foil is roughly 

 equal to the width of the slip-trace. Measurements 

 of the slip traces gives a thickness of roughly 500 A 

 in an average area of the foil. The directions of the 

 slip traces have been determined by difl'raction, and 

 are known to be parallel to the traces of the (111) 

 slip planes in aluminium. At points such as E the 

 dislocation is observed to transfer from one slip 

 plane to another. This is direct evidence of the cor- 

 rectness of the Mott-Frank mechanism of cross 

 slip of a screw dislocation (10). 



Slip trace contrast. — The band of contrast left 

 by a moving dislocation slowly fades, and in about 

 10 seconds has completely disappeared. The dis- 

 appearance rate is higher at the larger illumination 

 intensities obtained under fine-focus illumination 

 conditions. The contrast may be either lighter or 

 darker than that of the surrounding region, and is 

 more intense near the edges of the slip trace, i.e. at 

 the surface of the foil (fig. 4). This contrast is tenta- 

 tively interpreted in terms of the generation of point 

 defects by moving dislocations (12). A dislocation 

 line containing jogs may generate vacant lattice 

 sites and interstitial atoms by non-conservative mo- 

 tion of these jogs. It is thought that the short range 

 strain fields associated with a distribution of vacan- 

 cies or interstitials on the slip plane may give rise 

 to Bragg contrast in a manner similar to the strain 

 field of a dislocation. Some preliminary observations 

 indicate that the contrast is also reversed in dark 

 field illumination. The disappearance of the slip 

 trace contrast would then be due to the diffusion 

 of the defects away from the slip plane. A calculation 

 shows that the right order of magnitude for the 

 persistence time is obtained for vacancies if the tem- 

 perature is of the order of 100 to 200 C. The more 

 intense contrast at the edges of the band may be due 



to the larger number of defects generated near the 

 oxide film, or due to the stresses set up by the interac- 

 tion of the oxide film with a dislocation trying to 

 penetrate the surVdce. (Note in proof. — Recent obser- 

 vations have shown that the slip trace contrast is 

 due to this latter mechanism, and not to point 

 defects.) 



Conclusion. — There is little doubt thai il is possible 

 to examine the dislocation structure of metals directly 

 by transmission electron microscopy. This study of 

 aluminium foils has shown that the ideas of disloca- 

 tion theory applied to networks and surface arrays 

 are essentially correct. Moreover it has shown in the 

 case of aluminium, typical of a well polygoniscd 

 metal, that most of the dislocations are in the 

 boundaries even in the cold worked state, and very 

 few dislocations remain inside the subgrains. It 

 should be possible to extend these observations to 

 other metals. Some preliminary observations on Pd, 

 Au and Ag foils show that similar dislocations are 

 visible, and il is also possible to see their movement. 



Many of the effects observed during movement of 

 dislocations are complicated and not completely 

 understood. It has not yet been possible to locate 

 dislocation sources, although expanding loops are 

 often observed. It is however clear that dislocations 

 come out of the boundaries. Some slip traces are 

 observed to terminate at the boundaries while others 

 penetrate them. The detailed movement of the dis- 

 locations in aluminium has been recorded on 16 mm 

 cine film. 



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