The Center for Materials Science and Engineering at MIT, a Materials Research Science and Engineering Center sponsored by the National Science Foundation, maintains and supports, amongst others, an Electron Microscopy Shared Experimental Facility. The purpose of this paper is to highlight selected recent research results for high-resolution investigations performed in that facility.
The facility owns the first VG HB603 intermediate-voltage FEG-STEM, which operates at 250KeV and is equipped with a high-solid-angle x-ray detector and a Gatan Digi-Peels. It was intended to be, and has been, used for high sensitivity, high spatial resolution microanalysis. It is well-known that the “resolution” of an x-ray analysis is intimately (and inversely) related to its sensitivity; one extreme situation occurs when analyzing, for example, a diffusion profile, when the need is to determine the composition to the highest precision. An example of such an analysis is given in fig. 1. In this case, the sample is a 1.4Cr-0.8C pearlitic steel, and the chromium analysis is carried out across a cementite plate. During the growth of the pearlite, the chromium, which is not thermodynamically required to redistribute, nevertheless diffuses along the growth interface towards the cementite, resulting in a comparatively wide depletion profile in the ferrite, and a very narrow enrichment in the cementite.