Correlation between the Microstructure, Growth Mechanism, and Growth Kinetics of Alumina Scales on a FeCrAlY Alloy

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 manifested 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.