Abstract
The addition of small quantities of certain oxygen-active or “reactive” elements (REs) such as Y, Zr, Hf and Ce produces a tremendous improvement in the oxidation behavior of both chromia and alumina-forming alloys. However, few of the hypotheses that have been put forth to explain this phenomenon have been confirmed by careful characterization of the oxidation process.The segregation theory emphasizes the role of RE segregation to oxide grain boundaries and has been successful in correlating the presence of Y and Zr segregants to changes in the oxidation mechanism (thus reducing the oxidation rate) of chromia and alumina scales. Improvements in oxide adhesion have been attributed to the segregation of REs to the metal-alumina interface on doped β-NiAl and FeCrAl alloys. All of these studies employed FEG-STEM equipped with XEDS in order to detect the interfacial segregation.The RE is also found in the scale as RE-rich particles which have been observed after high temperature oxidation using XEDS on SEM and FEG-STEM and detected using glancing-angle (0.5° incident angle) x ray diffraction.
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