Detection of Au precipitates in a Mg-based alloy using electronically simulated hollow cone illumination

Images from electronically simulated hollow cone illumination (or rotating dark field) conditions, obtained under plane wave (or weak) diffraction con ditions, are generally assumed to approximate to the compositionally weight ed sum of atomic number squared for sufficiently large momentum transfers. However, even for large momentum transfer encountered with a semi-angle of 5 degrees and 300 keV electrons, appropriate numerical integration over con denser and objective aperture configurations indicates that some thermal sc attering component is still present. A Mg-Al alloy with minor additions of Zn and Mn, and to which 0.1 at.% Au has been added, is shown to provide a g ood system for the detection of high Z (atomic number) precipitates within a low Z matrix and on which semi-quantitative calculations may be based. Co rrelation of absolute rather than relative intensities from small precipita tes (3-10 nm diameter) with calculations based on an Einstein model for (in coherent) thermal diffuse scattering show that the small precipitates consi st predominantly of Au, a conclusion subsequently supported by EDX analysis and electron diffraction measurements. It is also demonstrated that this i ncoherent contrast mechanism is ideal for stereographic imaging in the TEM. (C) 2000 Elsevier Science Ltd. All rights reserved.