Internal stresses and textures of nanostructured alumina scales growing
on polycrystalline Fe
3
Al alloy
Pedro Brito
a
Institut für Angewandte Materialforschung, Helmholtz-Zentrum Berlin für Materialien und Energie,
Albert-Einstein-Str. 15, Berlin 12489, Germany
Haroldo Pinto
Departamento de Engenharia de Materiais, Aeronáutica e Automobilística, Universidade de São Paulo,
Av. Trabalhador São Carlense 400, São Carlos 13566-590, Brazil
Manuela Klaus, Christoph Genzel, and Anke Kaysser-Pyzalla
Institut für Angewandte Materialforschung, Helmholtz-Zentrum Berlin für Materialien und Energie,
Albert-Einstein-Str. 15, Berlin 12489, Germany
Received 19 March 2010; accepted 25 March 2010
The evolution of internal stresses in oxide scales growing on polycrystalline Fe
3
Al alloy in
atmospheric air at 700 ° C was determined using in situ energy-dispersive synchrotron X-ray
diffraction. Ex situ texture analyses were performed after 5 h of oxidation at 700 ° C. Under these
conditions, the oxide-scale thickness, as determined by X-ray photoelectron spectroscopy, lies
between 80 and 100 nm. The main phase present in the oxide scales is -Al
2
O
3
, with minor
quantities of metastable -Al
2
O
3
detected in the first minutes of oxidation, as well as -Fe
2
O
3
.
-Al
2
O
3
grows with a weak 0001 fiber texture in the normal direction. During the initial stages of
oxidation the scale develops, increasing levels of compressive stresses which later evolve to a steady
state condition situated around 300 MPa. © 2010 International Centre for Diffraction
Data. DOI: 10.1154/1.3402764
Key words: oxidation, alumina, energy-dispersive diffraction, internal stress, texture
I. INTRODUCTION
Iron aluminides are considered as candidate materials for
high-temperature applications due to their low cost, elevated
strength to weight ratio, and excellent oxidation resistance.
The oxidation resistance of these alloys relies upon the for-
mation of a stable and protective -Al
2
O
3
scale that adheres
to the metal surface and acts as a diffusion barrier for the
underlying substrate against potentially hazardous corrosive
environments Prescott and Graham, 1992; Grabke, 1999.
The important factors that affect the integrity of the protec-
tive oxide scale are growth stresses that develop within the
scale, intrinsic to the oxidation process, and also the residual
stresses that result after cooling to room temperature from
the differences in thermal expansion between the metal sub-
strate and the oxide Tolpygo and Clarke, 1999. For this
reason, much effort has been put forth in the past years to
characterize the evolution of growth stresses in oxide scales
during oxidation Schumann et al., 2000; Messaoudi et al.,
2000; Mennicke et al., 2001; Clarke, 2002; Eschler et al.,
2004; Huntz et al., 2007. In the specific case of Al
2
O
3
form-
ing alloys, recent studies have focused on the in situ deter-
mination of growth stresses during high-temperature oxida-
tion above 1000 °C of Ni-Al and Fe-Cr-Al alloys using
synchrotron radiation Specht et al., 2004; Veal et al., 2006;
Reddy et al., 2007; Veal and Paulikas, 2008. However, in
spite of these successful attempts to determine internal
stresses in growing oxide scales, the mechanisms of strain
formation during oxide growth are manifold Evans, 1995
and not fully understood for a number of metal-oxide sys-
tems Veal et al., 2006; Clarke, 2003; Panicaud et al., 2006.
Another important issue that affects the oxidation resis-
tance of iron aluminides is the appearance of less protective
Al
2
O
3
polymorphs, generally monoclinic -Al
2
O
3
or cubic
-Al
2
O
3
, which only later transform into the stable -Al
2
O
3
.
These phases are formed when iron aluminides are subjected
to low oxidation temperatures below 1000 °C and have a
detrimental impact on the oxidation resistance of the alloy
Grabke, 1999; Levin and Brandon, 1998. The development
of transition Al
2
O
3
may also modify the stress state in the
oxide layer since the transformation to -Al
2
O
3
is accompa-
nied by a volume contraction Rybicki and Smialek, 1989
which can induce tensile stresses into the first formed
-Al
2
O
3
grains. Nevertheless, the evolution of phase compo-
sition in thermally growing alumina scales, especially in the
early oxidation stages of Fe-Al alloys, remains yet to be
clarified Pöter et al., 2005.
In order to further enhance the understanding on the
mechanisms of internal strain formation during oxidation,
the present work aims at the study of the microstructure in
terms of chemical composition, phase development, and
crystallographic texture and the stress evolution in oxide
scales forming in atmospheric air on an intermetallic Fe–26
at. % Al alloy. A low oxidation temperature 700 °C was
applied in an attempt to favor the formation of metastable
Al
2
O
3
polymorphs.
II. EXPERIMENTAL
The specimens used in the oxidation experiments were
8-mm-diameter disks of 1 mm thickness cut from a polycrys-
a
Author to whom correspondence should be addressed. Electronic mail:
ppbrito@gmail.com
114 114 Powder Diffraction 25 2, June 2010 0885-7156/2010/252/114/5/$30.00 © 2010 JCPDS-ICDD