The detection and quantification of carbon by conventional energy dispersive X-ray spectroscopy (EDXS) performed under standard conditions is not feasible due to occurring contaminations in common electron microscopes. In contrast, novel ultra high vacuum EDXS (UHV-EDXS) was used to acquire elemental mappings of carbon on dual phase (DP) steel, which exhibits a microstructure consisting of ferrite and martensite. These phases differ in hardness, local dislocation density and, most importantly, in their carbon content.
Since the UHV conditions in combination with a customized windowless EDXS detector ensured a minimization of hydrocarbon contamination during the measurements and a maximization of the sensitivity for the detection of light elements, UHV-EDXS carbon mappings could successfully be obtained, which clearly reflect the ferrite and martensite microstructure of the investigated DP steels, as confirmed by electron back scatter diffraction (EBSD). Most importantly, it was possible to quantify the carbon content of individual grains with concentrations as low as 0.2 wt.%. Furthermore, nano-hardness tests were performed on the very same grains which were characterized by UHV-EDXS and EBSD. It is shown that the hardness of the martensite grains is correlated to their carbon content, as directly determined by UHV-EDXS. With this new method an additional tool for advanced material characterization of multiphase materials on the level of individual grains is now available.