Correlation between the Microstructure, Growth Mechanism, and Growth Kinetics of Alumina Scales on a FeCrAlY Alloy
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TRODUCTION
ALUMINA scale formation on Fe- and Ni-based high-temperature materials has been the subject of research for more than 30 years. The importance of grain-boundary transport for the growth of the alumina as well as for other oxide scales has been well established in early studies.[1–4] A large number of investigations have particularly concentrated on the effect of reactiveelement (RE) additions, such as Y, Hf, and Zr at typical levels of about 0.5 at. pct, in improving the protective properties of the scale, especially adherence.[5] Several investigations using surface-sensitive characterization techniques (e.g., X-ray photoelectron spectometry (XPS) and auger electron spectroscopy (AES) have revealed the positive RE effect on the scale adherence to be related to the prevention of deleterious impurity (sulfur) segregation to the scale/metal interface[6] or to free surfaces[7] of interfacial pores and microdelaminations. By impurity gettering, the RE prevents the formation and growth of such interfacial defects. D. NAUMENKO, E. WESSEL, L. SINGHEISER, and W.J. QUADAKKERS are with Forschungszentrum Ju¨lich GmbH, IEF-2, 52425 Ju¨lich, Germany. Contact e-mail: d.naumenko@ fz-juelich.de B. GLEESON, formerly with the Materials Science and Engineering Department, Iowa State University, Ames, IA, USA, is with the Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA, USA. Manuscript submitted April 13, 2007. Article published online November 1, 2007 2974—VOLUME 38A, DECEMBER 2007
Another finding, claimed to be of importance for the scale adherence, is that the REs change the scale-growth mechanism from the mixed Al and O transport to nearly exclusive inward oxygen diffusion.[8,9] This was demonstrated by two-stage oxidation tests, using an oxygen tracer with subsequent secondary ion mass spectrometry (SIMS) depth profiling of the formed oxide scales.[10] These results were supported by high-resolution transmission electron microscopy (TEM) observations of RE segregation to the grain boundaries of the alumina scale.[11] The segregation has been inferred to be as RE ions rather than as fine precipitates.[11,12] The amount that is segregated has been quantified to about 0.2 monolayer.[12] As reviewed and analyzed by Hou,[13] REs significantly reduce the grain-boundary diffusion coefficient of Al in the alumina. As a consequence, the RE addition tends to reduce the amount of Al outward transport through the alumina scale, which in turn reduces the thickening kinetics of the scale. The atomic mechanisms associated with this apparent ‘‘RE blocking effect’’ are currently not well understood. It is interesting to note that similar effects of segregated REs (Y, Zr, and La) on hindering Al grain-boundary diffusion were observed for bulk alumina samples, in which a significant decrease of the creep rate was measured due to the RE additions.[14] In the discussion of their results, Cho et al.[14] considered whether the modifications of the grain-boundary diffusion properties are due to site blockin
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