Phase Transformation of Al₂O₃ Scales Formed on Al-deposited Fe-Cr-Al Alloy Foil

Abstract

The phase transformation of Al₂O₃ formed on an Al-deposited Fe-Cr-Al alloy foil and an Fe-20Cr-5Al alloy foil of the similar composition, and their oxidation kinetics have been studied in air at 1073-1373K using XRD, SEM and SIMS.
It was found that the deposition of Al promotes the growth of metastable aluminas. The transformation of metastable Al₂O₃ to α-Al₂O₃ formed on the Al-deposited foil is slower than that on the alloy foil. The values of kp for the growth of α-Al₂O₃ on the Al-deposited foil is smaller than that on the alloy foil in the temperature range 1273 to 1373K. These are attributable to the purity of the Al₂O₃ initially formed on Al-deposited foil.
The phase transformation of Al₂O₃ is reflected by the scale morphology. The nucleation of θ-Al₂O₃ occurs on the surface of granular γ-Al₂O₃. θ-Al₂O₃ having a shape of whisker grows on the oxide surface. The morphology of whiskers is more round, after θ-Al₂O₃ transforms to α-Al₂O₃. We obtained the TTT diagrams for the phase transformation of Al₂O₃ scales formed on both foils in air in the temperature range 1073 to 1373K.