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 conditions, are generally assumed to approximate to the compositionally weighted 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 300keV electrons, appropriate numerical integration over condenser and objective aperture configurations indicates that some thermal scattering component is still present. A Mg-Al alloy with minor additions of Zn and Mn, and to which 0.1at.% Au has been added, is shown to provide a good system for the detection of high Z (atomic number) precipitates within a low Z matrix and on which semi-quantitative calculations may be based. Correlation of absolute rather than relative intensities from small precipitates (3-10nm diameter) with calculations based on an Einstein model for (incoherent) thermal diffuse scattering show that the small precipitates consist predominantly of Au, a conclusion subsequently supported by EDX analysis and electron diffraction measurements. It is also demonstrated that this incoherent contrast mechanism is ideal for stereographic imaging in the TEM.