METALS BY TRANSMISSION 



striction of a nickel rod for withdrawal dur- in multiphase alloys. Even with pure metals 



ing the return stroke, was developed by a difficulty is the formation of an etching 



Haanstra (14) for cutting thin biological substructure (17), but this can be avoided by 



specimens 50-100 A thick could be cut from proper choice of etching reagents and condi- 



a specially shaped specimen by a diamond tions. 



knife, but the resulting structure was heavily Ion Bombardment. A method of thin- 

 deformed, ning metals from the bulk state by bom- 



Reimer (15) used a diamond knife and an barding a thin disc (1-2 ^u thick) of the mate- 

 ult ramie rot ome developed by H. Fernadez- rial with a beam of ions has been developed 

 Moran to cut thin metal foils of Al, Ni, Cu, by Castaing (18, 19). In using ions to re- 

 Au, Fe, Pd, Pt, and Ag, but in this case, too, move metal atoms from a specimen, the en- 

 the foils were very heavily deformed. In spite ergy of the ions must be adjusted to give 

 of the severe deformation introduced, thin random atom removal. Too high an energy 

 sections can be readily prepared in this Avay, of the ions will result in heating and possible 

 and, in the case of alloys containing dis- damage to the structure, while too low an 

 persed phases, the results are not hkely to be energy will give an etching effect. Therefore, 

 very different from those found by examin- critical adjustment of the accelerating po- 

 ing elect ropolished foils. This technique holds tential (3000 V in the case of aluminium) is 

 great promise as a rapid method of obtaining necessary. Attempts to extend this tech- 

 foils, nique to stainless steel (21) and a-brass (18) 



have been unsuccessful. 



Dissolution Methods A difficulty in ionic thinning is the very 



low speed of metal removal — ^10 n in 24 hr. 



Chemical Etching. To produce a thin Thus, although with great care the method 



foil that is representative of bulk material, is capable of producing clean, uniform foils 



it is necessary to remove metal from a large from bulk material, the technique is compli- 



section without destroying or modifying the cated, tedious, and difficult to control in the 



structure of the material. One method of final stages. 



doing this is by chemical etching of a sheet Electropolishing. Heidenreich (20), us- 



of material 1 /j, or more thick. Hirsch, Kelly, ing an electropolishing technique, produced 



and Menter (13) examined gold by etching the first thin metal foils from bulk material 



the beaten foil in a dilute solution of potas- in a form suitable for transmission electron 



slum cyanide. The foils produced were very microscopy. His techniciue consisted in elec- 



uneven in thickness, but the heavily cold- tropolishing discs of aluminium and alumin- 



worked structure of the foil before etching ium-copper alloy, 3 mm. in dia., cut from 



w^as preserv^ed. Dislocation movements and rolled sheet 125 m thick. The electropolishing 



arrangements in aluminium were studied by produced holes at the center of the specimen 



Hirsch, Home, and Whelan (16), on thin and the areas near these holes were thin 



foils, made from sheet 0.5 /x thick by etching enough to be transparent to electrons. The 



in dilute hydrofluoric acid. The foils showed foils were often dirty and thin areas were not 



large uniform areas which were transparent obtained from every specimen, but when 



to electrons, and there was no evidence of success was achieved the results were very 



substructure due to etching. encouraging. Man}^ workers have since used 



This technique produces good results, but electropolishing techniques of different types 



its application is limited to pure metals or to prepare thin foils of a wide variety of 



single-phase alloys since it is difficult to pre- metals. These techniques are based mainly 



vent preferential attack of one of the phases on experiment, with very httle theoretical 



183 



