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It is well‐known that the addition of reactive elements (including yttrium, zirconium, lanthanum, cerium, and hafnium) improve the high‐temperature oxidation performance of alumina‐forming alloys. Less studied are strategies for optimizing these additions for developing a high‐performance, wrought FeCrAl alloy with maximum oxidation‐limited component life. Results from the literature are summarized regarding potential improvements. One promising strategy is the addition of two reactive elements, such as yttrium and hafnium, which has been effective in commercial and laboratory nickel‐based alloys and appears to impart the expected benefits, such as reducing the scale growth rate while minimizing detrimental effects, such as the formation of reactive element‐rich oxides in the scale and internal oxidation. Although there are promising data, the long‐term studies are not yet complete, and it is difficult to predict if “co‐doped” FeCrAl alloys will produce superior oxidation‐limited lifetimes in high‐temperature environments.

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A Fe-20 atom % Cr-1O%Al matrix was dispersed with a wide range of oxide particulates in order to study the effect of oxygen-active dopants on high-temperature oxidation performance and alumina scale microstructure. The effects of these various cation dopants have been correlated with dopant ion segregation to the a-A1203 grain boundaries using analytical electron microscopy. Elements such as Mn and V showed little effect on the oxide scale microstructure and were not observed to segregate. Elements such as Y, Nd, and Gd were found to have a positive effect on oxidation performance and segregated to scale grain boundaries, resulting in finei more columnar a-A1203 grains. However, Ti, Ta, Ca, and Nb also were found to segregate but had less of an effect on oxidation behavior. These results indicate that cation segregation to scale grain boundaries is not a sufficient condition for achieving beneficial oxidation performance. The driving force for diffusion and segregation in growing alumina scales is discussed. Based on comparisons with an alloy containing an A1203 dispersion, the finer scale grain sizes observed with reactive element oxide dispersions are the result of grain growth inhibition and not a heterogeneous nucleation effect. InfroductionSmall additions of reactive elements (RE5) added to chromia-and alumina-forming alloys improve their high temperature oxidation resistance. It is now widely recognized that RE dopants in alloys, added as alloy additions,11 oxide dispersions,11'8 or surface modifications,19-25 segregate to the grain boundaries of externally formed chromia and alumina scales. The segregation is not as fine precipitates but as ions.7'26 With the use of cross-sectional transmission electron microscopy, (TEM), the segregation of RE ions to the metal-scale interface also has been commonly observed.6 11,16,24,26,27 Despite the fact that RE ionic segregation has been widely observed, there is still debate as to the role these segregants play during oxidation and little understanding of the driving force for segregation.There have been a variety of models proposed to explain the RE effect, which have been reviewed elsewhere.'-4 One recent model26 based on the role of RE ion segregation in determining oxidation behavior proposed that segregation at the metal-scale interface beneficially affected scale adhesion by suppressing the driving force for interfacial segregation of indigenous 5, while segregation at scale grain boundaries inhibited outward Cr or Al boundary diffusion and grain growth. One purpose of this study was to explore the relationship of grain boundary segregation and scale morphology in co-A1203 scales formed on model oxide-dispersed FeCrA1 alloys.28'29 Using 0.2 cation % additions, thirty different dopants were added in order to assess parameters such as ion size, valence, and oxygen and sulfur affinities so as to determine why certain dopant elements produce beneficial effects on high temperature oxidation resistance.The effects of the various dopants on the oxidation performance w...