ELECTRON TRANSMISSION THROUGH THIN METAL SECTIONS 875 



can be had by considering the expression for the diffracted intensity (equa- 

 tion (1)). If dislocations are introduced into a crystal the effect is that of 

 producing a mosaic or block structure^ the units of which will scatter 

 incoherently. If there are a sufficient number of dislocations to reduce the 

 coherent penetration path z to a value z' such that AiTz' is small, then equa- 

 tion (1) will become 



(fj 



The important part in considering equation (6) is the disappearance of the 

 periodic, dynamic term. If the model obtained by introducing dislocations 

 into the crystal even roughly approximates the actual situation then it would 

 be expected from equation (6) that the extinction contours will vanish. 

 Actually, starting with a perfect crystal and adding dislocations, the dynam- 

 ical effects will not be noticeably effected until the coherent penetration %' 



\P 

 becomes much smaller than — . For 50 KV electrons in aluminum, z' would 



have to be something of the order of 150A or less before the dynamical 

 effects would disappear. If the model is carried still further, it can be spec- 

 ulated that z' is the mean separation of dislocations in the crystal indicating 

 that a distance of separation of the order of 150 A is required to extinguish 

 the extinction contours. This would correspond to a dislocation density of 

 about 5 X 10" lines/cm^. This is admittedly a very crude approximation 

 and, although the dislocation density is of the right order of magnitude, 

 too much significance should not be attached to it. It does seem fairly safe 

 to conclude that the extinction contours will disappear when the dislocation 

 density reaches a high enough value. This fact in itself greatly broadens 

 the interpretation of the electron micrographs to be presented. 



Preparation of Thin Aluminum Sections 



The details of the preparation of the thin sections used for electron mi- 

 croscopy have been published^ and are not vital to the discussion. Suffice 

 it to say that the sections are produced from 0.005" sheet by an electro- 

 poUshing technique using a special holder. The central portion of the metal 

 disc is thinned down to several hundred Angstroms or less while maintain- 

 ing a smooth surface. A rinsing procedure is necessary to prevent the forma- 

 tion of corrosion layers. 



^ R. D. Heidenreich and W. Shockley, Report of a Conference on Strength of Solids, 

 p. 57 (Physical Society, London, 1948). 



