Oxidation behaviour of novel ODS FeAlCr intermetallic alloys

The isothermal oxidation behaviour of novel oxide dispersion strengthened (ODS) FeAlCr intermetallic alloys was investigated at 900 and 1100 °C for up to about 150 h, with special emphasis on the oxidation kinetics, scale morphology and scale adherence. Comparisons with other alumina forming ferritic alloys and intermetallics are included. At 1100 °C, the new FeAlCr alloys show lower oxidation rates whereas at 900 °C they exhibit higher oxidation rates, which is associated with the formation of metastable θ-alumina scales. All intermetallic alloys are prone to scale spallation at 1100 °C during the cooling stage, which results from the elevated compressive residual stresses developed as a consequence of the large difference of thermal expansion coefficients between alumina and the metal substrate. In addition, the novel alloys suffer from subscale void formation. It is proposed that a decrease in the Al concentration beneath the scale yields a smaller number of thermal vacancies in equilibrium and excess of vacancies condense as voids. The absence of such oxidation induced voids in the Cr-free Fe40Al-Grade 3, having the same grain size and volume fraction of dispersoid, highlights the role of Cr in the vacancy behaviour.