The TEM contrast of faceted voids

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Abstract
The measurement of microscopic voids, as produced in fast-reactor structural materials during irradiation, requires an understanding of the TEM contrast at the edges of the smaller voids. The 1 MeV and 100 keV underfocus contrast from spherical and faceted voids has been computed for kinematical imaging conditions (i.e. not close to a Bragg reflection) using the theory of Rühle and Wilkens (1975). The position of the first dark Fresnel fringe, which is commonly used to delineate the void edge, is calculated as a function of void shape, size, orientation and defocus distance, and the results compared with TEM images of voids in 316 stainless steel. Both theory and experiment produce the surprising result that the dark fringe is accurately centred on the edge of a faceted void, when the linear tilt of the side face (relative to the electron beam) exceeds the width of the fringe.

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