868 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



techniques. In particular the loss of work hardening in pure metals by the 

 process of self-recovery is well suited to study using the electron micro- 

 scope and thin sections, as will be seen. In a previous paper^ the technique 

 of preparing thin aluminum sections and the interpretation of the diffraction 

 features were discussed in detail. The dynamical theory of electron diffrac- 

 tion has been applied to this particular problem and also extended to more 

 general cases.^ It was shown^ that, immediately after cold working, high 

 purity aluminum exhibits a sub-grain or domain structure of the order of 

 1-2 microms in size. These domains are slightly misoriented and become 

 visible in electron images through small variations in diffracted intensity in 

 passing from one domain to the next. The domains have been identified 

 with self-recovery following plastic deformation but their properties and 

 origin have not been investigated in detail. The experiments to be described 

 here include the effect of electron bombardment, and the effect of tempera- 

 ture on the size of the domains. 



It should be pointed out that the term "recovery domains" has been 

 applied by the writer to the sub-grains or units appearing in cold worked 

 aluminum. It appears now that the recovery domains are an early stage of 

 the process of "polygonization" commonly used in the literature. Poly- 

 gonization was first applied to the formation of larger blocks or polygons in 

 bent crystals that were annealed at an elevated temperature. The polygons 

 were made visible in a light microscope by the use of an etchant which 

 produced etch pits in the polygon boundaries.^ Either term, polygonization 

 or recovery domains, would be suitable although the latter is more specific 

 as to the process involved and will be used in this paper. 



Dynamical Theory Applied to Electron Images 



The wave mechanical or dynamical theory of electron diffraction is essen- 

 tial to the interpretation of electron images of crystals. The kinematic 

 theory, a simpler approximation, is not adequate. Consequently, it is advis- 

 able at this point to review briefly the salient features of the dynamical 

 theory as applied to electron images.* 



Suppose a polycrystalline film (such as a thin metal section) is mounted 

 in the conjugate focal plane of the objective lens of an electron microscope 

 as depicted in Fig. 1. Let the incident electron beam be taken as mono- 

 chromatic and plane-parallel. Regions of the specimen film oriented such 

 that a set of net planes satisfies the Bragg condition n\ = 2d sin 6 will 



»R. D. Heidenreich, J I. A pp. Phys. 20, 993 (1949). 

 «R. D. Heidenreich, Phys. Rev. 77, 271 (1950). 



' P. Lacombe and L. Beaujard, "Report of a Conference on Strength of Solids," p. 91 

 (Physical Society, London, 1948). 



* Detailed treatments are given in references 1 and 2. 



