IMAGING OF NANOMETER-SIZED PRECIPITATES IN SOLIDS BY ELECTRON SPECTROSCOPIC IMAGING

Electron spectroscopic imaging (ESI) in the transmission electron micr oscope (TEM) can be efficiently used to detect precipitates in solids. In this work we used a GATAN imaging filter which has been attached t o a 200 kV TEM to record elemental maps using inner-shell ionization e dges. We have investigated a niobium alloy with nanometer-sized titani um-oxide precipitates and steels with vanadium-carbide and chromium-ca rbide precipitates. These precipitates could be visualized using inner -shell ionization edges (Ti L(23), Nb M(45), Cr L(23) V L(23), V M(23) and Fe L(23)). We have compared different ESI techniques to check the ir validity for precipitate imaging. First, energy-filtered images can yield an enhanced contrast compared to the conventional TEM bright fi eld, but are very sensitive to diffraction contrast in crystalline spe cimens and to sample thickness variation. Second, elemental maps have been recorded by using the three-window method (two pre-edge images an d one post-edge image). Third, ratio images have been acquired by usin g 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 h ave successfully used ratio images to detect very small precipitates o f diameters ranging from 2 to 10 nm in the materials mentioned above. However, ratio images have to be used carefully, because they are susc eptible to artifacts.