High temperature oxidation resistance of alumina-forming materials is connected to the
growth of dense, stable and protective alumina scales. Depending on temperature, impurities
present in the base alloys, presence of water vapour in the oxidizing atmosphere, the alumina scales
are composed of alpha-alumina (which is the stable phase obtained for temperatures over 1000°C)
or of transient alumina (γ,θ,δ obtained for lower temperatures). It is generally considered that γ-
Al2O3 grows when T<850°C, that θ-Al2O3 is present for 850°C<T<1000°C and that α-Al2O3 is
stable when T exceeds 1000°C.
The exact role played by transient alumina formation and/or transformation on the high temperature
performances of alumina-forming materials is not exactly defined. Many works proposed that
transient alumina phases grew during the first steps of the oxidation process and transformed into
the stable phase after further oxidation. The transformation of transient phases in the stable alphaphase
is generally accompanied by a volume contraction of around 14 %.
In order to get better oxidation resistance, the formation of transient alumina is not wished, because:
1) their growth rate is generally higher than that of alpha-alumina with, as a consequence, a huge Al
consumption, detrimental for the material resistance after long exposures, 2) the change in volume
during the transformation of transient phases into alpha-alumina can generate stresses in the oxide
scale and can weaken its adherence to the underlying substrate, leading to massive spallation.
The present study deals with the coupling of different characterization tools in order to precisely
identify the transient phases grown on FeCrAl materials. The use of scanning electron microscope
(SEM-FEG), transmission electron microscope (TEM), Photoluminscescence Spectroscopy(PLS),
X-ray photoelectron spectrometry (XPS) and X-ray diffraction at different glancing angles (XRD)
on model materials oxidized at two temperatures (850 and 1100°C) could help the identification of
transient phases. These techniques gave a better understanding of the alumina scale growth
mechanism.
Stress Determination in Thermally Grown Alumina Scales Using Ruby Luminescence