Nanophase identification via correlative transmission electron microscopy: A case study of galvannealed advanced high-strength steel

The unambiguous identification of nanoscaled phases is an extensively challenging task, which stretches conventional analytical methods to the limit. It holds especially true for nanophases embedded in a matrix with a similar structure and/or chemistry. This hurdle necessitates the use of mutually reinforcing characterization techniques. Here we present the power of correlative transmission electron microscopy utilizing industrial Zn-coated steel with high Si content as a suitable test specimen. Diffusion of Si from the substrate into the coating during annealing leads to the formation of Si-rich nanoprecipitates (NPs) of $30-50$ nm in size surrounded by a Zn-Fe matrix near the steel/coating interface. Application of our characterization approach, which involves a synergy of high-resolution transmission electron microscopy (TEM) and scanning TEM energy-dispersive X-ray spectroscopy complemented by transmission Kikuchi and precession electron diffraction, allows refining NPs as $D{O}_3$-type structured AlFe₂Si surrounded by the ${\Gamma }_1$ fcc Zn-Fe intermetallic compound. Additionally, the correlation between the structural orientation of the NPs and the matrix was revealed. Considering the dimensions of the entities under study, dependable and high-quality thin TEM sample preparation is of principal importance. We addressed this via low-temperature cutting of specimens with a Xe plasma focused ion beam.