METAL SURFACES 



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surfaces. Any diffusion of the incident light prechided this method. 

 Scratches and sleeks showed well in bright contrast with a 0.12A+0.25X 

 diffraction plate. Grain boundaries were made more visible in high- 

 speed steels, and carbide segregation showed with bright and with dark 

 (0.12A— 0.25X) contrast. Stains, etching, and polishing effects could 

 be studied by this means. 



In England, Cuckow (1947, 1949) concluded, after examination of 

 metals, that more information is revealed by the phase vertical il- 

 luminator than by the study of replicas. He found it particularly good 

 for the assessment of surface levels. Martensite and transformed 



Fig. VI.5. Silica replica from a steel surface, showing pearlitic structure, ca. 1000 X : 

 Left, brightfield. Center, dark-contrast (A — ) phase. Right, bright-contrast phase. 



austenite showed well. For some problems he believed it to be more 

 valuable than replica methods would be with the electron microscope, 

 in general, more convenient to use, and sometimes a helpful guide to 

 aid in understanding the very fine detail revealed by the electron 

 microscope. Taylor (1949) has described the Cooke vertical phase 

 illuminator and recommends it for metallurgy and mineralogy. Crystal 

 boundaries could be seen as well as slight differences in level from 

 etching and polishing. 



Coarse detail may be studied in relatively thick replicas (Fig. IV. 2). 

 Finer detail requires thinner replicas, and for the usual Formvar replica 

 prepared for the electron microscope the 0.05A+0.25X and 0.2A+0X 

 plates are very good, and the 0.2Azt:0.25X plates are useful. For fine 

 detail such as dots, the 0.07 transmission is useful. Increased detail can 

 be obtained with the phase microscope if the replicas are shadowcast 

 with a dielectric. Very fine detail of replicas can be seen with the phase 

 microscope, such as the differences from path differences of 40 to 50 A 

 thickness in Formvar produced by lightly etched metal surfaces. 

 Pearlitic structure in a silica replica is shown in Fig. VI.5. Replicas for 



