ON THE APPLICATION OF ENERGY-FILTERING TEM IN MATERIALS SCIENCE - III- PRECIPITATES IN STEEL

Energy-filtering transmission electron microscopy (EFTEM) has been use d for imaging of precipitates and grain boundary phases in advanced fe rritic-martensitic 10% Cr steel containing W and Mo (G-X12CrMoWVNbN 12 1 1). The investigations have been performed with a Philips CM20 equi pped with a GATAN imaging filter at 200 kV acceleration voltage. The s econdary 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, thu s allowing one to visualize all compositional heterogeneities from mic rometre to a nanometre scale. Due to a heat treatment of 10000 h at a temperature of 480 degrees C the steel contains many different precipi tate and grain boundary phases such as chromium carbides (M23C6), vana dium nitrides, niobium carbonitrides (MX) and Mo-W Laves phase precipi tates. 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 presen ted. Possible artifacts of the technique are also discussed. In additi on, the quantitative composition of the precipitate phases has been de termined using EEL and EDX spectrometry. (C) 1998 Elsevier Science Ltd . All rights reserved.