Abstract
The microdiffraction mode of Scanning Transmission Electron Microscopy (S.T.E.M.) has found several applications in metallurgical problems requiring diffraction information of high spatial resolution (1,2). The purpose of the work described here was to apply the microdiffraction facility on a S.T.E.M. equipped with a field emission gun (3) to the investigation of precipitate structure.To obtain a microdiffraction pattern, the electron beam is stopped on the specimen. This results in the formation of a diffraction pattern from an area of the specimen defined by the probe cross-sectional area. To record the pattern a set of scan coils deflect the pattern past a collector aperture.The effective probe diameter was determined at the edge of a precipitate to be less than 2 nm. The spread of the diffraction spots, due to the angular spread of the beam, is defined by the angle 2αo subtended by the objective aperture, while the angular resolution is determined by the angular size of the collector aperture, 2αc.
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