Published: Jan 1968
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Application of the electron microscope hot stage can provide unequivocal evidence for the nature of thermal processes. The difficulties inherent in the technique, however, have minimized its use up to the present. In particular these difficulties are: (1) the intervention of surface diffusion (not characteristic of bulk processes) due to the large surface area of thin foils, (2) the differences in mechanisms of both sheartype and diffusional processes which sometimes develop between thin foils and bulk materials, (3) the contamination of reactive materials by the environment provided by present electron microscopes, (4) the changes in contrast conditions of thin foils congruent with expansion of their lattices and with buckling due to thermal strains, and (5) the inaccuracy of measuring the temperature of the foil while being observed. Methods of circumventing these difficulties are described. Criteria for the successful application of the hot stage are established in view of these difficulties. It is concluded that the type of phenomenon best treated by means of the hot stage is one occurring on the scale of hundreds of angstroms but not greater than 0.2 μ to prevent interference by the foil surface, and repeating at intervals of the order of microns in order to increase the probability of obtaining results, particularly when using ciné techniques. Thus, very fruitful studies of continuous and discontinuous precipitation processes, ordering reactions, and recovery phenomena may be made with ease, while investigations into recrystallization or certain martensitic transformations are not likely to be successful unless extensive preliminary studies are made.
metallography, electron microscopes, temperature measurement, surface properties, diffusion, phase transformations, martensite, precipitation, stacking fault energy
Scientific Laboratory, Ford Motor Co., Dearborn, Mich.