Detection and quantification of precipitates in 11–12 wt% Cr steels using in-column secondary electron detectors

In order to quantify precipitates such as Laves phase, M23C6 carbides and modified Z-phase in 9 – 12 wt. % Cr power plant steels, a range of microscopy techniques, including scanning electron microscope (SEM) and focussed ion beam (FIB) imaging, are currently used. This paper’s key finding reports on the novel application of a field emission SEM (FESEM) in-column upper secondary electron detector (USD) for imaging and quantification of precipitates in a thermally aged 11 - 12 wt. % Cr steel sample. The USD produces images from first order secondary electrons generated directly from primary electron beam interaction. These images were found to not only show significant contrast between the matrix and the precipitates to enable quantification, but between particles which otherwise have identical greyscale intensities in FIB images. Previously assumed to be the same phase, some of these precipitates appeared significantly darker than others. The greyscale differential between these precipitate types was sufficient for separate quantification. Energy dispersive x-ray spectroscopy and selected area diffraction analysis identified the darker particles as modified Z-phase and the lighter grey particles as M23C6 carbides. When analysing all particles together, comparison of data from FESEM USD and FIB images confirmed that both techniques produce statistically comparable precipitate analysis results. However, as modified Z-phase and M23C6 carbide distributions can be analysed separately from a FESEM in-column USD image at low accelerating voltages, it was found that this technique provides for more reliable precipitate quantification than FIB imaging alone in these steels. With future work, this could enable faster characterisation and may lead to automated larger area mapping.