Visualization of carbon in steels using energy filtering TEM

Abstract

Energy filtering TEM [Reimer L (1984) Transmission Electron Microscopy, Physit of Image Formation and Microanalysis, pp. 391–393, (Springer, Berlin)] i potentially capable of visualizing the distribution of a particular element a the nanometre scale. However, visibility of the energy-loss image depend on the combination of the atomic species of interest and the matrix. In thi paper we discuss experimental conditions required for obtaining carbo: ‘mapping’ in steels. Three types of common carbides cementite, (Fe3C), (Ti, Nb)C and M23C6, in both thin foil and extraction replica specimens wer examined in a high voltage high resolution electron microscope equippe with a post-column energy filtering system. The peak to background (P/B ratio at the C–K edge is the determining factor for obtaining a carbon image Carbon mapping was relatively easy for cementite in pearlite when th electron beam specimen geometry was such that the carbide was present a through the foil thickness. In contrast, in the case of M23C6 in a stainles steel, carbon imaging was possible when the carbide was extracted but nc when it was embedded in the matrix. Based on the statistic theory of Berge and Kohl [(1993) Optik 92: 175–193], we estimated the minimum carbid thickness tmin and the maximum total foil thickness as a function of the Mi type carbide thickness for obtaining the carbon image. The total foil thicknes to should be thinner than about 44 nm when the MC carbide, whose thicknes is 5 nm, is to be imaged with the C–K edge.