Correlative Atom Probe Tomography and Transmission Electron Microscopy Analysis of Grain Boundaries in Thermally Grown Alumina Scale

We employed correlative atom probe tomography (APT) and transmission electron microscopy (TEM) to analyze the alumina scale thermally grown on the oxide dispersion-strengthened alloy MA956. Segregation of Ti and Y and associated variation in metal/oxygen stoichiometry at the grain boundaries and triple junctions of alumina were quantified and discussed with respect to the oxidation behavior of the alloy, in particular, to the formation of cation vacancies. Correlative TEM analysis was helpful to avoid building pragmatically well-looking but substantially incorrect APT reconstructions, which can result in erroneous quantification of segregating species, and highlights the need to consider ionic volumes and detection efficiency in the reconstruction routine. We also demonstrate a cost-efficient, robust, and easy-handling setup for correlative analysis based solely on commercially available components, which can be used with all conventional TEM tools without the need to modify the specimen holder assembly

Effect of gas composition on the oxide scale growth mechanisms in a ferritic steel for solid oxide cell interconnects

The oxidation behavior of a ferritic steel Fe-23Cr-0.5Mn-0.6 Nb-0.1Ti (at%) considered for application in solid oxide cell (SOC) stack interconnects was studied at 800 °C. The oxidation kinetics and oxide scale microstructure formed in Ar-20%O2, Ar-4%H2-4%H2O and Ar-1%CO-1%CO2 atmospheres, simulating the SOC operation environments, were investigated by thermogravimetry (TG) in conjunction with electron microscopy (SEM/TEM) and atom probe tomography (APT). In all three environments multilayered oxide scales formed, consisting of Mn-Cr spinel on top of Cr2O3 and an additional Nb-rich oxide layer at the chromia-alloy interface. The initially faster oxidation in the low pO2 gases was attributed to formation of porous chromia scales compared to a dense scale formed in the high pO2 (Ar-O2) atmosphere. APT revealed segregation of minor alloying elements (Mn, Nb and Ti) to chromia grain boundaries in all three simulated SOC environments in quantitatively similar amounts, suggesting their similar effect on the ionic transport through the oxide scale. The findings indicate that oxygen activity in the test gas plays a dominating role in governing the oxidation kinetics and the oxide scale microstructure of the studied ferritic steel

A Nanoscale Study of Thermally Grown Chromia on High-Cr Ferritic Steels and Associated Oxidation Mechanisms

Fe-22Cr-0.5Mn based ferritic steels are used as interconnect materials for solid oxide fuel/electrolysis cells. Four steel samples, including the commercial steel Crofer 22 H, were oxidized at 800 °C in a model Ar-4%H2-4%H2O atmosphere simulating the fuel side of the cells and investigated by atom probe tomography (APT) in conjunction with electron microscopy and thermogravimetry. All steels form an oxide scale mainly consisting of MnCr2O4 spinel on top of Cr2O3. APT revealed segregation of minor alloying constituents (Nb and Ti) to chromia grain boundaries and highlighted their effect on mass transport through the chromia scale. Relationships between segregation activity of individual elements (in terms of Gibbsian interfacial excess), oxide scale microstructure and alloy oxidation rate have been established based on the APT results. Comparison of segregation activities revealed that vacancies formation due to Wagner-Hauffe doping with aliovalent Ti and Nb impurities cannot be solely responsible for faster oxidation, assuming alteration of the grain boundary structure and associated changes of their mass transport properties. Controlled Si addition to the alloy (about 0.4 at%) suppresses the detrimental effect of Nb on the oxidation resistance but results in formation of a thin, although still discontinuous, SiO2 layer at the metal-oxide interface. © 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited

Non-equilibrium solid solution of molybdenum and sodium: Atomic scale experimental and first principles studies

We report a combined experimental and first principles study of an extremely immiscible alloy of Mo with 1 and 2 at.% Na, which was produced by high-energy ball milling. The microstructure of the as-milled and annealed state were examined by various methods, including atom-probe tomography (APT), transmission electron microscopy, and energy-dispersive (EDX) analysis. Despite the complete immiscibility of the Mo-Na system in the solid and even in the liquid state, APT measurements clearly evidence the formation of a true nanocrystalline solid-solution microstructure with insignificant Na clustering for samples with 1 at.% Na. In agreement with our x-ray diffraction experiments, first principles calculations expose that the Na atoms do not expand the Mo lattice, which is in contrast to predictions using Vegard's rule. Heating at 700 °C induces only slight grain growth while the solid solution remains remarkably stable without any decomposition. On the contrary, after annealing at 900 °C first Na segregations at triple junctions and significant grain growth are observable, although the solid solution still retains most of the dissolved Na