Correlation between the Microstructure, Growth Mechanism, and Growth Kinetics of Alumina Scales on a FeCrAlY Alloy D. NAUMENKO, B. GLEESON, E. WESSEL, L. SINGHEISER, and W.J. QUADAKKERS The microstructural development of an alumina scale formed on a model FeCrAlY alloy during oxidation at 1200 °C was characterized for up to 2000 hours of growth. Quantitative scanning electron microscopy (SEM) studies revealed that the scale had a columnar microstructure, with the grain size being a linear function of the distance from the scale/gas interface. For a given fixed distance from the scale/gas interface, there was found to be no change in the oxide grain size for exposure times ranging from 24 to 2000 hours at 1200 °C, up to 100 hours at 1250 °C. Thus, there was no significant coarsening of existing grains in the scale. Through oxygen tracer experiments, the scale-growth mechanism was shown to be predominated by inward oxygen diffusion along the oxide grain boundaries. Electron backscatter diffraction (EBSD) analysis further revealed that a competitive oxide-grain growth mechanism operates at the scale/alloy interface, which is mani- fested by a preferential crystallographic grain orientation. The scale-thickening kinetics were modeled using the experimentally-derived, microstructural parameters and were found to be in excellent agreement with converted thermogravimetric (TG) measurements. The model predicted a subparabolic oxidation rate, with the time exponent decreasing with increasing exposure time. The values of the time exponent were shown to be approximately 0.35 to 0.37, at oxidation times commonly reached in the TG experiments, i.e., a few tens of hours. At longer oxidation times of a few thousand hours and with a constant rate of average oxide-grain size increase, the time exponent was predicted to approach 0.33, corresponding to an ideal cubic oxidation rate. DOI: 10.1007/s11661-007-9342-z Ó The Minerals, Metals & Materials Society and ASM International 2007 I. INTRODUCTION 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 reactive- element (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 microdelamina- tions. By impurity gettering, the RE prevents the formation and growth of such interfacial defects. 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 demon- strated 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 trans- mission 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 mono- layer. [12] As reviewed and analyzed by Hou, [13] REs significantly reduce the grain-boundary diffusion coeffi- cient 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 blocking or to structural changes. At the moment, the former mechanism seems to have more experimental and theoretical support. [15,16] 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 METALLURGICAL AND MATERIALS TRANSACTIONS A