MODELING OF PRECIPITATED XE ATOMS IN ALUMINUM AND THE FRESNEL EFFECT IN HRTEM IMAGING

High-resolution transmission electron microscope (HRTEM) images with a [111] orientation of nanosized tetradecahedral Xe precipitates in an Al matrix have been observed and compared with simulated images. The H RTEM image simulation war carried out by changing the defocus value of the objective lens anal the size of the objective aperture. The real size of the precipitated Xe crystal is not obvious from the HRTEM imag es because of the effect of Fresnel diffraction (Fresnel fringes) at t he boundaries between the Xe atoms and the Al matrix, except when the defocus conditions are at or within 50 nm of the Scherzer defocus valu e. This means that the minimum size of the Xe crystal image does not n ecessarily show the real size. This phenomenon is independent of the s ize of the objective aperture. When two Xe bubbles with same size and orientation both lie along the beam direction in the Al matrix, the im age intensity of the two Xe crystals is enhanced and the contrast beco mes higher than the image intensity of one Xe crystal alone in the Al matrix. (C) 1998 Acta Metallurgica Inc.