Identification Of Cu-Co-Oxide Phases Of Reactive Air Brazed Ba0.5sr0.5co0.8fe0.2o3-Delta-Ag-14cuo Joints By Ebsd, Epma And Tem Diffraction

The oxygen transport membrane Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) was wetted by the reactive air brazing alloy Ag-14CuO. Triple point phases of different cobalt-copper-oxides are formed in the reaction zone and are origin of micro-cracks and stress concentration. Therefore, their identification is necessary for understanding the microstructure evolution and designing the interface structure advantageously by an adapted brazing process. However, as the cobalt-copper-oxide system can form many different oxides with solubility of Co and Cu respectively, different analytical methods are necessary for identification. TEM diffraction is used for the correct identification of the occurring crystal structures, EBSD is used for analysis of composition, arrangement and orientation of the triple point phases and EPMA is used to confirm the crystal structures identified by EBSD and to determine the elemental concentrations. As the triple point phases consist of small grains with unpredictable forms and especially, Cu-Ka can excite Co-Ka by characteristic secondary fluorescence, different evaluation procedures for EPMA are performed on FIB lamellae: 1) Approach based on standard lamellae, where FIB lamellae from standards are used to determine the k-factor in dependence of the mass thickness t/lambda, and 2) Monte Carlo simulation approach, where an iterative procedure is used to calculate the k-ratios, based on bulk standards. The use of FIB lamellae allows excluding the fluorescence effect and incorrect Monte Carlo simulations due to incorrect assumptions of the microstructure below the surface, which is within the excitation volume.