Imaging of nanometer-sized precipitates in solids by electron spectroscopic imaging

Electron spectroscopic imaging (ESI) in the transmission electron microscope (TEM) can be efficiently used to detect precipitates in solids. In this work we used a GATAN imaging filter which has been attached to a 200 kV TEM to record elemental maps using inner-shell ionization edges. We have investigated a niobium alloy with nanometer-sized titanium-oxide precipitates and steels with vanadium-carbide and chromium-carbide precipitates. These precipitates could be visualized using inner-shell ionization edges (Ti L23, Nb M45, Cr L23, V L23, V M23 and Fe L23). We have compared different ESI techniques to check their validity for precipitate imaging. First, energy-filtered images can yield an enhanced contrast compared to the conventional TEM bright field, but are very sensitive to diffraction contrast in crystalline specimens and to sample thickness variation. Second, elemental maps have been recorded by using the three-window method (two pre-edge images and one post-edge image). Third, ratio images have been acquired by using the two-window method (one pre-edge window and one post-edge window). These ratio images show elemental contrast with lower noise than the elemental maps and are nearly free of the diffraction artifacts. We have successfully used ratio images to detect very small precipitates of diameters ranging from 2 to 10 nm in the materials mentioned above. However, ratio images have to be used carefully, because they are susceptible to artifacts.