TEM investigation of the oxide scales formed on a FeCrAlRE alloy (Kanthal AF) at 900°C in dry O2 and O2 with 40% H2O

Abstract

The base oxide scales on a commercial FeCrAl alloy oxidized isothermally at 900°C in dry O₂ or O₂ with 40% H₂O were studied in detail using analytical electron microscopy. Electron transparent cross-section foils prepared with a FIB/SEM in-situ lift-out technique were investigated using STEM/EDX and CBED. The oxide scales on the samples exposed to dry O₂ are slightly thinner than the scales formed in O₂+H₂O. The oxide scales exhibit a multilayered structure, with a Cr-rich layer in the middle, indicating the original metal/gas interface. An almost pure inner α-Al₂O₃ layer, containing columnar grains, was formed by inward oxygen diffusion, after exposures in both the dry and wet atmospheres. The outer oxide layer consisted of γ-Al₂O₃ in the wet case and of α-Al₂O₃/MgAl₂O₄ in the dry case. It is suggested that the α-Al₂O₃/MgAl₂O₄ phases resulted from a phase transformation of initially grown γ-Al₂O₃. The observations indicate that water vapour may stabilize the γ-Al₂O₃ phase.