On the application of energy-filtering TEM in materials science: III. Precipitates in steel

Energy-filtering transmission electron microscopy (EFTEM) has been used for imaging of precipitates and grain boundary phases in advanced ferritic-martensitic 10% Cr steel containing W and Mo (G-X12CrMoWVNbN 12 1 1). The investigations have been performed with a Philips CM20 equipped with a GATAN imaging filter at 200 kV acceleration voltage. The secondary phases have been imaged by recording jump-ratio images at the Fe-M-2,M-3 edge under rocking beam illumination. This method reduces unwanted diffraction contrast in crystalline specimens completely, thus allowing one to visualize all compositional heterogeneities from micrometre to a nanometre scale. Due to a heat treatment of 10000 h at a temperature of 480 degrees C the steel contains many different precipitate and grain boundary phases such as chromium carbides (M23C6), vanadium nitrides, niobium carbonitrides (MX) and Mo-W Laves phase precipitates. The distribution of the secondary phases is shown by recording jump-ratio images and elemental maps of the elements Cr, V, Nb, Mo and W. Since, especially, the heavy elements are difficult to image, the optimum instrumental settings for reliable data acquisition are presented. Possible artifacts of the technique are also discussed. In addition, the quantitative composition of the precipitate phases has been determined using EEL and EDX spectrometry. (C) 1998 Elsevier Science Ltd. All rights reserved.