TRANSMISSIOX ELECTRON MICROSCOPY OF METALS 



attack after this and obtain other fairly good material, especially the distribution of dis- 



specimens. Surveys of electrolytic polishing locations, the question arises how far this 



methods, with detailed information con- situation has been affected by the prepara- 



cerning the polishing solutions and condi- tion. Our impression is that the effect on the 



t ions for various metals and alloys, are given distribution of dislocations is small, though 



by TomUnson (15), and Kelly and Nutting minor local changes, as for example straight- 



(16). Other variations, such as the "Window ening of dislocations or a weak relaxation 



Method," the "Figure of Eight Alethod," of piled up groups (Fig. 19) may take place, 



and the "Uniform Field Method" are also It should be said that the dislocations usually 



described in reference 16. are pinned at the surface of the foil and that 



Another technique was introduced by dislocation networks are very stable. An- 



Mirand and Saulnier (17), who treated other case arises when dislocations start to 



specimens, first mechanically thinned down move after a certain irradiation or after 



to 0.04 mm, in a commercial elect ropolishing heating or stressing. Here the situation 



apparatus for metallographic purposes (Disa- changes and the velocities of movements of 



Electropol). The specimens were attacked dislocations, as filmed by the group Hirsch, 



alternately from both sides over a diameter Whelan et al. (6, 10) are expected to be quite 



of about 5 mm and then pohshed do\\ai until different in thin foils from those in the bulk 



nothing remained of the original area, except material. 



for a few small flakes of metal swimming A preparation technique for semicon- 



in the solution; this was subsequently de- ductors, such as germanium and silicon by 



canted and the flakes were rinsed several chemical etching has been developed by 



times and then fished out with a specimen Innng (18). For different crystal faces dif- 



grid. A certain danger of this method may ferent etchants have been used (e.g. 1 % 



be that a flake, having remained a short time sodium hypochlorite solution for the (111)- 



in the polishing solution without current can face of germanium). Also bismuth telluride 



become etched, but this will depend on the has been chemically etched by Geach and 



metals and solutions. Phillips (19). 



There are a lot of possible electropohshing A completely different way of obtaining 



techniques but normally it takes some time thin metal specimens is microtome cutting 



to master any one of them. It is not im- with a diamond knife, introduced by Fer- 



portant which way one originally goes, but nandez-Moran (20). Metals have been cut 



it is very important to see from a result in in this way by Haanstra (21), Tsuchikura 



which sense the conditions have to be cor- and Ichige (22) and Reimer (23, 24). This 



rected. For electropohshing it is essential to method can be useful for obtaining informa- 



know that pohshing, i.e., the non-specific tion on different phases but the metal is 



uniform attack, is only achieved at high strongly distorted by the cutting process, 

 current density in a relatively small range. 



At low current density the attack is metal- Dislocations 



lographic which means that grain boundaries Definition. In a perfect ideal crystal all 



etc. are specifically attacked. On the other the atoms are arranged in a periodic lattice 



hand when the specimen shows a crust on structure. After a plastic deformation of the 



the surface or a spotty attack, the current crystal this periodicity is disturbed. For 



density or the temperature of the bath is too reasons of energy content, the disturbances 



high. of the lattice are concentrated locally in fines 



As the electropofished specimens are used while the great part of the lattice is periodic 



to study the internal situation of the bulk as before, except that it is elastically 



293 



