The segregation of reactive elements (REs) along thermally grown oxide (TGO) grain boundaries has been associated to slower oxide growth kinetics and improved creep properties. However, the incorporation and diffusion of these elements into the TGO during oxidation of Ni alloys remains an open question. In this work, electron backscatter diffraction in transmission mode (t-EBSD) was used to investigate the microstructure of TGO within the thermal barrier coating on a Ni-based superalloy, and atom probe tomography (APT) was used to quantify the segregation behavior of REs to α-Al2O3 grain boundaries. Integrating the two techniques enables a higher level of site-specific analysis compared to the routine focused ion beam lift-out sample preparation method without t-EBSD. Needle-shaped APT specimens readily meet the thickness criterion for electron diffraction analysis. Transmission EBSD provides an immediate feedback on grain orientation and grain boundary location within the APT specimens to help target grain boundaries in the TGO. Segregation behavior of REs is discussed in terms of the grain boundary character and relative location in TGO.

We summarize the findings from an interlaboratory study conducted between ten international research groups and investigate the use of the commonly used maximum separation distance and local concentration thresholding methods for solute clustering quantification. The study objectives are: to bring clarity to the range of applicability of the methods; identify existing and/or needed modifications; and interpretation of past published data. Participants collected experimental data from a proton-irradiated 304 stainless steel and analyzed Cu-rich and Ni–Si rich clusters. The datasets were also analyzed by one researcher to clarify variability originating from different operators. The Cu distribution fulfills the ideal requirements of the maximum separation method (MSM), namely a dilute matrix Cu concentration and concentrated Cu clusters. This enabled a relatively tight distribution of the cluster number density among the participants. By contrast, the group analysis of the Ni–Si rich clusters by the MSM was complicated by a high Ni matrix concentration and by the presence of Si-decorated dislocations, leading to larger variability among researchers. While local concentration filtering could, in principle, tighten the results, the cluster identification step inevitably maintained a high scatter. Recommendations regarding reporting, selection of analysis method, and expected variability when interpreting published data are discussed.

With its particular ability to image solute clusters in three dimensions and impurity segregation to selected interfaces and grain boundaries, atom probe tomography has provided unique insight into the effects of irradiation on materials microstructures. This article reviews the contribution of atom probe tomography to our understanding of behaviors and responses of structural materials under irradiation. Possible atom probe tomography based approaches and common data analysis methods to analyze the microstructural features often observed in irradiated materials are described. In particular, the analysis of solute clustering, solute segregation, and void imaging are discussed in the context of radiation-induced hardening of austenitic steels and reactor pressure vessel steels, and the development of oxide dispersion strengthened steels, radiation-induced solute segregation to grain boundaries for stress corrosion cracking or corrosion issues, and to understand the swelling response of irradiated materials. While highlighting the unique information that atom probe tomography can offer, common limitations, current challenges, and outstanding technical questions regarding data analysis and interpretation are also presented.

Grain boundary chemistry in an X750 Ni alloy was analyzed by atom probe tomography in an effort to clarify the possible roles of elemental segregation and carbide presence on the stress corrosion cracking behavior of Ni alloys. Two types of cracks are observed: straight cracks along twin boundaries and wavy cracks at general boundaries. It was found that carbides (M23C6 and TiC) are present at both twin and general boundaries, with comparable B and P segregation for all types of grain boundaries. Twin boundaries intercept γ’ precipitates while the general boundaries wave around the γ’ and carbide precipitates. Near a crack tip, oxidation takes place on the periphery of carbide precipitate.

Atom probe tomography is used to investigate the clustering of Y-Ti-O in a 14%Cr-2%W-0.3%Ti & 0.3% Y2O3 ODS steel. The clusters in the consolidated material are compared to clusters observed in the powder prior to consolidation. A higher density of smaller clusters is observed in the powder, and the clusters are found to contain more O and less Y.