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Grain boundary wetting as a preliminary stage for zinc-induced grain boundary weakening and embrittlement in a Zn-coated press-hardened 20MnB8 steel was analyzed using electron backscatter diffraction, Auger electron spectroscopy, energy dispersive X-ray analysis, atom probe tomography and transmission electron microscopy. Microcracks at prior austenite grain boundaries were observed. Structures that developed after microcrack formation were identified: for example, Zn/Fe intermetallic phases with grain sizes smaller than 100 nm were present at the crack surfaces and the wedge-shaped crack tips. An electrolytically coated reference sample that underwent the same heat treatment as the press-hardened material but without the application of tensile stress was investigated in order to find the initial cause of the microcracks. On this sample, Zn, in the order of one atomic layer, was found along prior austenite grain boundaries several micrometers away from the actual Zn/Fe phases in the coating. The resulting grain boundary weakening with the Zn wetting of prior austenitic grain boundaries during austenitization and/or hot forming is a necessary precondition for microcrack formation. [ABSTRACT FROM AUTHOR]

Activated carbons derived from viscose fibers were prepared using potassium hydroxide, carbon dioxide, or water vapor as activation agents. The produced activated carbon fibers were analyzed via scanning electron microscopy and energy dispersive X-ray spectroscopy, and their porosity (specific surface area, total pore volume, and pore size distribution) was calculated employing physisorption experiments. Activated carbon fibers with a specific surface area of more than 2500 m2 g−1 were obtained by each of the three methods. Afterwards, the suitability of these materials as electrodes for electrochemical double-layer capacitors (supercapacitors) was investigated using cyclic voltammetry, galvanostatic measurements, and electrochemical impedance spectroscopy. By combining CO2 and H2O activation, activated carbon fibers of high purity and excellent electrochemical performance could be obtained. A specific capacitance per electrode of up to 180 F g−1 was found. In addition, an energy density per double-layer capacitor of 42 W h kg−1 was achieved. These results demonstrate the outstanding electrochemical properties of viscose-based activated carbon fibers for use as electrode materials in energy storage devices such as supercapacitors. [ABSTRACT FROM AUTHOR]

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 0.25 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. • Quantification of low carbon concentrations is obtainable by EDXS under UHV conditions. • The carbon content of single martensite grains was determined by UHV-EDXS in DP steels. • Nanoindentation was equally performed on individual martensite and ferrite grains of the DP steels. • Correlation between the hardness of the grains, the martensite volume fraction and the carbon content in the martensite was proven. [ABSTRACT FROM AUTHOR]