1. ARTICLE IN PRESS
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MSA-28410; No. of Pages 9
Materials Science and Engineering A xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A
journal homepage: www.elsevier.com/locate/msea
Creep behavior of commercial FeCrAl foils: Beneficial and detrimental
effects of oxidation
Sebastien Dryepondt ∗ , Bruce A. Pint, Edgar Lara-Curzio
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States
a r t i c l e i n f o a b s t r a c t
Article history: Creep tests were performed at 875 ◦ C and 1050 ◦ C on commercially available FeCrAl foils (∼50 ␮m, 2 mil
Received 6 January 2012 thickness) over a wide range of stress and duration. The oxide scales formed on the creep specimens were
Accepted 13 March 2012 analyzed and compared to those that formed on unstressed specimens to assess the effect of stress and
Available online xxx
strain on oxide growth behavior. Below a specific stress threshold, the creep rate becomes moderately
dependent on the applied load, the lifetime independent of that load, and rupture occurs due to the
Keywords:
onset of breakaway oxidation. A creep rate model based on the strengthening of the FeCrAl foils due
Creep test
to load-bearing by the thermally grown alumina scale was observed to be in good agreement with the
Ferritic steels
Oxidation
experimental results.
Modeling Published by Elsevier B.V.
High temperature deformation
1. Introduction Commercial RE-doped FeCrAl foils have a unique oxidation
behavior because they experience little or no scale spallation. The
FeCrAl alloys are of great interest due to their excellent cor- growth of the oxide generates stresses in the oxide scale that are
rosion behavior in aggressive environments, especially when they balanced by stresses in the substrate according to the ratio of the
are doped with reactive elements (RE) [1] such as Y and Zr. Except respective thicknesses [7]. Stress levels in thin foils can be high
for oxide dispersed strengthened (ODS) FeCrAl, most ferritic FeCrAl enough to induce creep deformation of the alloy substrate, thus
alloys offer poor mechanical properties at high temperature and, relaxing this stress and reducing the strain energy needed for spal-
therefore, their use is limited to low stress applications [2,3]. lation or cracking of the scale [8,9]. Because of the absence of
Models have been proposed to predict the lifetime of FeCrAl com- spallation, foil failure is due to intrinsic chemical failure (ICF), with
ponents based on the available reservoir of Al being consumed to breakaway oxidation (at t = tb ) appearing after the entire consump-
form an alumina scale. In the case of an idealized oxidation process tion of Al (Cb = 0) and the beginning of the formation of a chromia
described by parameters k and n, the lifetime can be expressed as layer [10–12] followed by the rapid formation of Fe-rich oxide and
follows [4]: complete metal consumption.
A limited amount of data has been published on the mechan-
n
1 ·d ical properties of FeCrAl and on the durability of these materials
tlife = (C0 − Cb ) (1)
k ˛ when they are subjected to mechanical stresses [13–18]. This arti-
cle presents results from creep testing performed at 875 ◦ C on two
where C0 , is the original alloy Al concentration, d is the ratio different commercial FeCrAl foils, with duration ranging from a
between the volume and the surface of the specimen (i.e. few minutes to 13,000 h (13k h). The rupture modes relative to
d ∼ thickness in the case of foils), is the density of the alloy, ˛ is the stress level are discussed with respect to the extent of oxida-
a stoichiometric factor and Cb is the Al concentration below which tion degradation and the transition from stress-limited lifetime to
breakaway oxidation occurs. Models have been developed to take Al consumption-limited lifetime. While the 875 ◦ C temperature is
into account more complex situations, such as multiple oxidation relevant for car catalyst substrates, the mechanical properties of
stages [5] or spallation of the oxide scale [6]. the growing alumina layer are unknown at that temperature, and
likely to change with time because of the progressive transforma-
tion from transient (cubic) alumina phases to ␣-Al2 O3 [19–22]. A
third commercial FeCrAl foil was therefore creep tested at 1050 ◦ C,
∗ Corresponding author at: ORNL, PO Box 2008, MS 6156, Oak Ridge, TN 37831-
temperature at which only ␣-Al2 O3 is expected to be present
6156, United States. Tel.: +1 865 574 4452.
E-mail address: dryepondtsn@ornl.gov (S. Dryepondt). after the first few hours of exposure. Calculations based on the
0921-5093/$ – see front matter. Published by Elsevier B.V.
doi:10.1016/j.msea.2012.03.031
Please cite this article in press as: S. Dryepondt, et al., Mater. Sci. Eng. A (2012), doi:10.1016/j.msea.2012.03.031

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MSA-28410; No. of Pages 9
2 S. Dryepondt et al. / Materials Science and Engineering A xxx (2012) xxx–xxx
4
875ºC
20
2
15 F1 6.9 MPa
Deformation (%)
0
0 1000 2000 F1 12.5 MPa
10
F1 15 MPa F1 3.5 MPa
FAl 6.9 MPa
5
FAl 3.5 MPa
0
0 2000 4000 6000 8000 10000 12000 14000
Duration (hr)
Fig. 1. Al, Cr and Fe line-scan profiles through the thickness of the as fabricated Fig. 2. Creep curves of FeCrAl-1 (F1) and FeCrAl-Al (FAl) alloys tested at 875 ◦ C and
FeCrAl-1 and FeCrAl-Al foils measured by microprobe analysis. The Al and Cr gradi- tensile stresses ranging from 3.5 to 15 MPa. Arrows in the enlargement highlight
ents for the FeCrAl-Al foil come from a second step of aluminization. abrupt changes in the creep rates for the FeCrAl-1 foils.
strengthening of the composite oxide/FeCrAl due to the growth of testing with a 500 h cycle between cooling was carried out in lab-
the alumina scale were carried out and compared with experimen- oratory air at 875 ◦ C, and the specimen mass gains per unit surface
tal results. Cross sectional analyses were also performed to estimate area were measured on a Mettler Toledo model XP205 balance
the effect of stress on the oxidation behavior of FeCrAl foils. (±0.01 mg accuracy) every 500 h up to 10 or 50k h. Experimental
details for the cyclic oxidation testing are given in Ref. [23].
After creep or oxidation testing, foils were Cu-plated prior to
2. Experimental procedure
mounting in epoxy to protect the oxide scale during metallographic
polishing. Foils were examined by light and scanning electron
Two commercial alloys, designated FeCrAl-1 (Kawasaki Steels,
microscopy, and compositions were determined by electron probe
River Lite R20-USR alloy) and FeCrAl-Al (Durafoil, Texas Instru-
micro analysis (EPMA) using a JEOL model 8200.
ments), were examined in the 800–900 ◦ C range. Due to the small
volume of material available, bulk chemical analysis could not
be performed and the type and amount of reactive elements are 3. Results
unknown, but the Fe, Cr and Al contents were measured by elec-
tron probe micro analysis (EPMA), Fig. 1. The FeCrAl-Al foil was 3.1. Creep and oxidation testing between 800 ◦ C and 900 ◦ C
fabricated by a second step of aluminization, which explains the
Al and Cr concentration gradients measured through the thickness, 3.1.1. FeCrAl-1 (F1)
Fig. 1. The higher Al reservoir in the FeCrAl-Al foil after aluminiza- Creep curves resulting from tests performed at 875 ◦ C are
tion is expected to delay the onset of breakaway oxidation and shown in Fig. 2. The FeCrAl-1 specimens loaded with 3.5 and
thus increase the foil oxidation lifetime. The chemical composi- 6.9 MPa were tested in the same furnace and they both broke after
tion of the commercial alloy tested at 1050 ◦ C, FeCrAl-2 (Sandvik 11.9k h. An increase of the creep rates was observed after 700 and
Steel, grade OC404, Fe-bal. Cr—21.3 wt%, Al—5.6 wt%, Mn—0.25%, 1000 h respectively (arrows in Fig. 2 enlargement). The sudden
Si—0.27%, Ce—0.03%, La—0.01%) was measured by inductively cou- creep rate increase was subsequent to an unexpected shut down
pled plasma and combustion techniques. of the furnace for several hours after 670 h of testing. After 11.9k h,
Foil thicknesses were measured using an optical comparator the two foils failed suddenly and catastrophically at the same time
and range from 52 to 55 ␮m. Dog-bone test specimens with a gage and only small pieces were available for observation. Part of a cross
length of 25 mm and a cross-sectional area of 9.5 mm × 0.05 mm section of the specimen tested at 6.9 MPa is shown in SEM images in
were machined by electrical discharge machining. Nickel based Fig. 3a, b and e. For about half of the observed foil cross-section, all
superalloy foils were used to reinforce the test specimens in the tab the metal had been consumed by oxidation (Fig. 3e). In areas where
region to limit the deformation around the hole used to transfer the metal remained, the Al and Cr EPMA maps presented in Fig. 3c
applied load using pins. Alumina rods were used for the load train and d indicate that the oxide scale consisted of an outer alumina
and dead loads were hung to apply the stress. The grips displace- layer and an inner chromia layer, as has been reported previously
ment was measured using a linear variable differential transformer [10]. EMPA concentration profiles through the entire foil thickness
(LVDT) extensometer. Type S or K thermocouples were spot welded were used to evaluate the Al and Cr average contents given in
in the specimen shoulders to monitor and control the test tem- Fig. 4. In the case of the specimen tested at 6.9 MPa, no Al was left
perature. For long-term exposures, two specimens were tested in the foil before rupture occurred, Fig. 4a. The total consumption
simultaneously in each furnace using independent load trains, with of Al in the foil explains the formation beneath the alumina scale
a maximum temperature gradient of 8 ◦ C between the two speci- of a chromium scale. The Cr content in the foil also decreased
mens. The specimens were loaded before heating to avoid inducing from 20.6 wt% for the as received foil to 17.4 wt% because of the
undesirable bending or twisting deformation at high temperature. formation of the chromia layer. One of the foils tested at 12.5 MPa
At 1050 ◦ C, creep tests were interrupted after 500 or 1000 h to allow ruptured after 12,021 h of exposure. Thus, the lifetimes were simi-
oxide scale surface observation. For comparison, cyclic oxidation lar for all the specimens with an applied stress of 12.5 MPa or lower.
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Fig. 3. Cross-sectional micrographs after creep testing at 875 ◦ C for 11,900 h with a 6.9 MPa applied stress, (a and e) back scattered SEM images of the entire foil, (b)
enlargement of the oxide scale, (c and d) Al and Cr EPMA mapping of the oxide scale.
As expected, the minimum creep rates for the specimens tested at time to rupture can be correlated to the applied stress using a
12.5 MPa were higher at the beginning of the tests compared with Monkman–Grant relation [24]. For stresses ≤ 12.5 MPa, and before
the minimum creep rate for the foils tested at 3.5 and 6.9 MPa. the abrupt change in creep rate observed for the specimens tested
However, the sudden increase of the creep rates after ∼1000 h for at 3.5 and 6.9 MPa, Fig. 2, the minimum creep rate follows a Nor-
the foils tested at 3.5 and 6.9 MPa resulted in lower minimum creep ton power law as well, but with an exponent of ∼1. The minimum
rates after ∼1500 h for the specimens tested at 12.5 MPa, Fig. 5. creep rates after 1000 h of exposure were higher for the specimens
The lifetimes for the specimens creep tested at ≥15 MPa were tested at 3.5 and 6.9 MPa compared with the specimens tested
drastically lower, ranging from 363 h for 15 MPa to 0.4 h for a at 12.5 MPa. The time to rupture was independent of the applied
stress of 30 MPa (not shown in Fig. 2). Plotting the applied stress stress for stresses ≤ 12.5 MPa and post-mortem observations indi-
versus creep rate and rupture time, there are two clearly distinct cate that rupture occurred because of breakaway oxidation (Fig. 3),
regimes depending on the level of applied stress, Fig. 5. For stresses i.e. complete consumption of the metal due to depletion of the Al
between 12.5 MPa and 30 MPa, the minimum creep rate follows a reservoir.
Norton power law with a Norton exponent close to 15, and the Specimen mass gains ( m/surface) versus the square root of
time at 800 ◦ C and 900 ◦ C are shown in Fig. 5. The faster initial
transient oxidation stage, ∼4000 h at 800 ◦ C and <500 h at 900 ◦ C
25 (less than one thermal cycle), was attributed to the formation of
11kh, 875ºC, 6.9 MPa 50kh, 900ºC metastable cubic alumina phases such as the ␪ phase [19–22,25].
20 The subsequent steady-state mass gain curves appear linear up to
50k h, suggesting a parabolic oxidation rate, but the determination,
FeCrAl-1 by using a log–log plot, of the coefficients n in the power-law oxi-
15 dation kinetics m/S = k·tn , suggests two distinct oxidation regimes
at 900 ◦ C, for exposure below, and over 10k h. The different coef-
10 ficients n at 800 and 900 ◦ C and the time ranges used for their
As Fabricated calculation are reported in Table 1, as well as the parabolic rate
constants kp (n = 0.5). All of the sub-parabolic n values were below
5
10kh, 900ºC
3
10
a) 0 875ºC
Average Al wt% Average Cr wt% 4
25 10000 10
13kh, 875ºC, 6.9 MPa 50kh, 900ºC
Lifetime 5
10
20 1000
Creep rate (s-1)
Min. creep rate
Lifetime (h)
FeCrAl-Al 10 6
15 Lifetime
100
pre-oxidation 7
Min. creep rate 10
200h 1050ºC
10 As Fabricated t >1500h
10 8
10
10kh, 900ºC
5 9
1 10
b) 0 10 10
Average Al wt% Average Cr wt% 0 5 10 15 20 25 30 35
Fig. 4. Average Al and Cr content remaining in the foils after creep or oxidation Stress (MPa)
testing measured by 3 EPMA line-scans through the entire foil thickness, (a) FeCrAl-
1 foils and (b) FeCrAl-Al foils. The error bars are not represented because line-scan Fig. 5. Log–log diagram of creep rate and time-to-failure versus applied stress for
measurements differ by less than 10% in all cases. FeCrAl-1 foil at 875 ◦ C.
Please cite this article in press as: S. Dryepondt, et al., Mater. Sci. Eng. A (2012), doi:10.1016/j.msea.2012.03.031