JOURNAL OF MATERIALS SCIENCE35 (2000 )4695– 4703

High-temperature oxidation of Fe3Al

containing yttrium

INSOO KIM, W. D. CHODepartment of Metallurgical Engineering, University of Utah,Salt Lake City, UT 84112, USAE-mail: w.cho@m.cc.utah.edu

H. J. KIMDepartment of Mechanical Engineering, Yuhan College, Puchon City,Kyungki-Do, 422-749, South Korea

The effect of yttrium addition on the oxidation behavior of Fe3Al alloys was investigated interms of oxidation rate and oxide adhesion in the temperature range of 800 to 1100 ◦C. Theoxidation rate of the alloys, Fe-14.3 wt% Al and Fe-14.1 wt% Al-0.3 wt% Y, was nearlyidentical, and the parabolic rate constant as a function of temperature is found to beKp= 5128 exp[−39506 (cal/mol)/RT ] mg2/cm4 hr. While the alumina scale formed on theY-free Fe3Al alloy was observed to be fragile and spalled easily, the oxide layer formed onthe Fe3Al-Y was protective, dense, and adhesive. Based on the microstructural,morphological, and compositional studies, the adhesion improvement due to the yttriumaddition was discussed in terms of growth stress, the formation of pegs and scale growthmechanism. C© 2000 Kluwer Academic Publishers

1. IntroductionIron aluminides have attracted the attention of manyresearchers as candidates for high-temperature struc-tural materials mainly because of their excellent ox-idation resistance due to the formation of protectivealumina scale. In order to resist aggressive environ-ments at elevated temperatures, oxide scale formed oniron-aluminum alloy must satisfy the following threerequirements: (1) thermodynamic stability, (2) slowgrowth rate, and (3) adherence on the substrate. Accord-ing to Wallwork and Hed [1], the most suitable alloysfor high-temperature applications are Fe, Ni, and Coalloys with the respectively protection of SiO2, Cr2O3,and A12O3 scales. While chromium oxide can vaporizeas CrO3 at the temperatures above 1000◦C and siliconoxide may form low melting silicate [2, 3], A12O3 isthermodynamically stable at high temperatures and hashigher melting point. In this respect, the alloys form-ing alumina, such as iron aluminides, seem to be mostsuitable for use at high temperature. However, aluminascale formed on the iron-aluminum alloys tends to beeasily spalled during oxidation due to the growth stressand thermal stress.

It has been known that the addition of small amountsof reactive element, such as Y or Ce, to chromia- andalumina forming alloys improves the adhesion of theoxide to the substrate. Ramanarayananet a1. [4–6]investigated the influence of yttrium on oxide scalegrowth and adherence of alumina formed on variousalloys. They reported that the presence of yttrium re-duced growth stresses arising from Al2O3 nucleationwithin an existing scale and, thus, greatly improved the

scale adhesion. Foxet al. [7] studied about the effects ofyttrium on oxidation of Fe-5% Al alloy. They showedthat convolution of oxide scale due to growth stress wasnot found when yttrium was present. Golightlyet al.[8] compared the oxidation behavior of Fe-27% Cr-4%Al and Fe-27% Cr-4% Al-0.82% Y in 1 atm oxygenat 1200◦C. According to their results, the addition ofyttrium to the alloy apparently prevent the formationof oxide within an existing oxide layer, improving theadherence of alumina scale.

Many investigators have studied to find mechanismsfor the improved oxide scale adherence and mechanicalproperties due to the presence of reactive elements suchas Y, Ce, and Hf, etc. The mechanisms reported previ-ously can generally be categorized into seven groups:(1) pegging mechanism [9–11], (2) vacancy sink mech-anism [12–14], (3) graded seal [15], (4) improvementof bond strength at the interface [16–18], (5) modifi-cation of growth process [16, 19–20], (7) and enhance-ment of scale plasticity [4, 6]. However, there existsno consensus view on the role of reactive elements inhigh-temperature oxidation of alloys.

The purpose of the present study is to investigate theeffects of yttrium addition on the oxidation behaviorof iron-aluminum alloys in terms of the oxidation rate,scale adhesion and microstructure.

2. Experimental procedure2.1. Materials and sample preparationThe alloys used in this study were made by arc meltingfurnace, and subsequently homogenized at 1000◦C in

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Ar atmosphere for 10 hrs. These were cut into ellip-tical shapes with a dimension of∼1.7× 1.0× 0.1 cmby a sectioning saw, and a hole was drilled to hang thespecimen to microbalance using Pt wire. The sampleswere then abraded and polished through 1µm diamondpaste. After polishing, each specimen was degreasedwith soap and finally cleaned in an ultrasonic cleanercontaining acetone. DCP (Double Current Plasma) andEPMA (Electron Microprobe Analysis) analyses wereperformed to examine the composition of the speci-mens. The composition of the two alloys used in thisstudy are Fe-(14.33 wt%) Al and Fe-(14.08 wt%) Al-(0.30 wt%) Y.

2.2. Oxidation testsThe experimental set-up including TGA (Thermogravi-metric Apparatus), shown schematically in Fig. 1, con-sisted of a recording electrobalance, a kanthal-woundfurnace, and a gas train for purification and mixing ofreagent gases. The argon and oxygen were dried and pu-rified by passage through columns of P2O5 and ascarite.The alloy specimen was loaded with Pt wire into the re-action tube. Using a four-way stopcock, the argon gaswas fed into the reaction tube while the argon/oxygenmixture flowed to the gas outlet at the same rate throughthe stopcock. After the temperature in the reaction tubereached 800, 900, and 1000◦C, the stopcock was re-versed to feed the argon/oxygen gas-mixture into thereaction tube. From this point, the weight change of thesample was measured using an electrobalance (D101Cahn balance) which has 1µg sensitivity. Oxidationwas carried out in 0.2 atm O2 at 800, 900, and 1000◦Cfor 63 hrs. The parabolic oxidation rate constant was ob-tained by the least square method from the kinetic data.The adhesion of the oxide formed on the alloys wasexamined by thermal quenching and water impact inultrasonic cleaner. Microstructure characterization andmorphology examination were made by XRD (X-rayDiffraction), SEM (Scanning Electron Microscopy),EPMA (Electron Microprobe Analysis), and opticalmetallography.

Figure 1 Schematic diagram of thermogravimetric apparatus (1. As-carite (CO2 removal), 2. P2O5 (H2O removal), 3. Flowmeter, 4. Mixingchamber, 5. Ar cylinder, 6. O2 Cylinder and 7. Thermometer).

3. Results & discussion3.1. Oxidation rateOxidation experiments of the two alloys, Fe3Al andFe3Al-Y, were carried out in 0.2 atm O2 at 800, 900,and 1000◦C. The change of specimen weight with timeduring the oxidation of Fe3Al at 800, 900, and 1000◦Cis shown in Fig. 2. It was observed that under the sameexperimental conditions the oxidation rate of Fe3Al-Yalloy was almost same as that of Fe3Al.

The oxidation rates of the two alloys were calculatedusing parabolic rate equation given by Equation 1

X2 = Kpt (1)

where X is the change in weight per unit surfacearea after timet , and Kp is parabolic rate constant.The parabolic rate constant obtained at 800◦C is5.7× 10−5 mg2/cm4 hr, which is consistent with the re-sults of Devan [21], although the oxidizing atmospherewas slightly different. Table I shows the parabolicrate constants obtained at three different temperatures.As expected, oxidation rate is increased with increas-ing temperature. The temperature dependence of theparabolic rate constant (Kp) at constant oxygen pres-sure can be expressed by Equation 2.

Kp = Ko exp

(−Q

RT

)(2)

where Ko is a constant,Q activation energy,R gasconstant, andT absolute temperature. The activationenergyQ and the constantKo were determined from aplot of log Kp vs. 1/T as shown in Fig. 3. The general

TABLE I Parabolic rate constants measured for Fe-14.3 wt% Al alloyat three temperatures

Temperature (K) Parabolic Rate Constants (mg2/cm4 hr)

1073 5.7× 10−5

1173 1.3× 10−4

1273 1.1× 10−3

Figure 2 Weight-gain kinetictics for the oxidation of Fe-(14.3 wt%) Alalloy in 0.2 atm O2 at 800, 900, and 1000◦C.

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Figure 3 Temperature dependence of parabolic rate constants for theoxidation of Fe-(14.3 wt%) Al in 0.2 atm O2 at 800, 900, and 1000◦C.

equation for the parabolic rate constant as a function oftemperature is as follows:

Kp = 5128 exp

[−39506(cal/mol)

RT

]mg2/cm4 hr

3.2. Oxide adhesion & microstructureThe oxides formed on Fe3Al and Fe3Al-Y were foundto be K-Al2O3 at 800◦C andα-Al2O3 at 900, 1000,and 1100◦C. Fig. 4 shows the XRD patterns for theyttrium-free Fe3Al alloys oxidized in 0.2 atm O2 at800, 900 and 1000◦C. In order to examine the adhesionof alumina scale to the substrate, the two alloy speci-mens oxidized at 800 and 900◦C were quenched inair at room temperature. The optical examination ofthe two specimens did not show any spallation of theoxide scale. The two alloys oxidized at the two temper-atures were placed in an ultrasonic cleaner to furtherexamine the degree of scale adhesion. The scale sur-face of the two oxidized specimens was impacted bywater molecules via ultrasonic waves for 5 minutes. Itwas observed that while the alumina scale formed onY-free Fe3Al was cracked and spalled, Y-doped Fe3Almaintained adhesive alumina scale without showingany cracks and spallation. Fig. 5A and B show the sur-face morphology of the two alloys oxidized at 1100◦Cfor 24 hrs. Oxide layer formed on the Y-free Fe3Al(Fig. 5A) was severely convoluted and the alloy sub-strate was partially exposed due to the spallation of theoxide layer, indicating that the oxide scale was sub-jected to growth stress during oxidation and thermalstress during cooling. Meanwhile, oxide layer formedon the Fe3Al-Y alloy was observed to be flat, dense, andadhesive. It is interesting to note that a second phase wasobserved on the surface of alumina scale formed on theFe3Al-Y as shown in Fig. 5B. The second phase willbe discussed later in detail.

Fig. 6A and B show the oxide/substrate interface nearthe substrate and the substrate surface of the Y-freeFe3Al oxidized at 1100◦C, respectively. As shown inthe figures, the interface is wavy and the surface of the

Figure 4 XRD results showing the phases of Al2O3 formed onFe-(14.3 wt%) Al at 800, 900 and 1000◦C.

substrate is deformed due to the stresses generated dur-ing the oxidation and cooing. On the other hand, theoxide/substrate interface of Fe3Al-Y alloy oxidized atthe same temperature seems to be straight without de-formation, as shown in Fig. 7. From these observations,it may be deduced that the addition of yttrium to the al-loy generates less stresses which may partly contributeto the enhancement of scale adhesion.

The surface of alumina scale formed on the Y-dopedFe3Al was examined at various oxidation periods usingoptical microscope. As shown in Fig. 8A, yttrium is ob-served mainly at the grain boundaries of Fe3Al-Y alloybefore oxidation. As soon as the specimen is exposedto air at high temperature, the yttrium is oxidized toform a complex Y-Al oxide. As the oxidation proceeds,the appearance of the surface of the oxidized specimenremains same, as shown in Fig. 8C and D. The secondphase, which is a complex Y-Al oxide, formed at the

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Figure 5 SEM micrographs showing the surface of (A) Fe-14.3 wt% Al and (B) Fe-14.1 wt% Al-0.3 wt% Y alloys oxidized in air at 1100◦C.

surface of alumina scale is shown in Fig. 9 and pre-viously in Fig. 5, and is founded to be enriched withyttrium and oxygen by X-ray map and EPMA analy-ses. According to the XRD pattern shown in Fig. 10, thesecond phase seems to be Y3Al5O12. The scale appear-ances shown in Fig. 8 indicate that the alumina scaleformed on Fe3Al-Y does not seem to grow externallybut internally by inward oxygen diffusion through alu-mina scale.

The other phenomenon observed from the oxida-tion of Fe3Al-Y alloy is the formation of pegs at the

scale/substrate interface. Fig. 11 shows a typical pegformed at the alloy/scale interface. The pegs seem tobe connected to the grain boundaries of the alloy sub-strate, implying that the pegs may be grown inwardthrough the grain boundaries of the substrate. Also, thepegs anchor the oxide scale to the alloy substrate, lead-ing to the improved scale adhesion. The analysis ofX-ray map shown in Fig. 12 reveals that the pegs areenriched with O, Al, and Y, and deficient in iron. Fromthis result, it can be concluded that the pegs are de-veloped at the grain boundaries of the substrate near

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Figure 6 SEM micrographs showing (A) cross-section and (B) surface of Y-free Fe3Al alloy substrate oxidized at 1100◦C in air for 120 hrs (oxideis removed).

the scale/substrate interface due to internal oxidation.In other words, oxygen diffuses inward through thealumina scale and reacts with Al and Y at the grainboundaries of the substrate to form new complex ox-ides like pegs in the substrate. Therefore, it can be de-duced that the formation of pegs due to the presence ofyttrium is indicative of the change of oxide growthmechanism.

Many studies have been carried out on the growthmechanism of alumina scale. Pintet al. [22] conducted

sequential oxidation experiments of undoped, and Y-and Zr-dopedβ-NiA1 and FeCrAl alloys using16O and18O at 1200 and 1500◦C. According to these authors,undopedα-A12O3 was found to grow by the simulta-neous transport of both Al and O. On the other hand,Y- and Zr-dopedα-A12O3 was found to grow mainlyby the inward transport of oxygen. Schumannet al.[23] investigated the oxide growth processes and inter-facial segregation occurring inα-A12O3 scales grownonβ-NiAl containing Zr or Y at 1200◦C. They showed

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Figure 7 SEM micrograph showing the cross-section of Fe3Al-Y alloy oxidized at 1100◦C in air for 120 hrs.

Figure 8 Change of surface appearance showing grain boundaries containing complex Y-Al oxide with time during oxidation of Fe3Al-Y in air at1100◦C: (A) surface of Fe3Al-Y etched in 90 ml H2PO3-8 ml HNO3-2 ml H2O solution before oxidation (B) after oxidation for 4 min (C) afteroxidation for 80 hrs (D) after oxidation for 320 hrs.

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Figure 9 SEM micrograph showing a yttrium oxide protruded on the surface ofα-Al2O3 formed for the oxidation of Fe3Al-Y alloy in air at 1100◦C.

Figure 10 XRD results showing the phases of oxide formed on Fe3Al-Yat 1100◦C.

Figure 11 SEM micrograph showing the formation of peg at Al2O3/Fe3Al-Y interface.

that the extent of aluminum diffusion occurring dur-ing α-A12O3 growth was reduced by the presence ofalloying elements and thus the oxide grew by inwarddiffusion of oxygen. Prescottet al. [24] investigatedthe growth mechanism of alumina scale formed on Fe-25 wt% Al and reported thatα-Al2O3 grew by simul-taneous countercurrent diffusion of Al and oxygen.

The formation of alumina by the simultaneous coun-tercurrent diffusion of aluminum and oxygen indicatesthat growth stress can be generated in the scale dur-ing the growth of alumina scale, which may lead tospallation of alumina scale. On the other hand, the ad-dition of yttrium to the Fe3Al changes the oxide growthmechanism from the simultaneous countercurrent dif-fusion of aluminum and oxygen to predominant oxy-gen diffusion, which leads to the formation of scaleand pegs at the alloy/scale interface. Accordingly, the

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Figure 12 SEM and X-ray maps showing the composition of pegs formed at alumina scale/substrate interface: (A) SEM image, (B) oxygen x-raymap, (C) aluminum x-ray map, (D) yttrium x-ray map, and (E) iron x-ray map.

Fe3Al-Y alloy is subjected to less growth stress duringthe oxidation and is more resistant to thermal stress dur-ing cooling, which results in enhanced oxide adhesionand stability.

4. Conclusions1. The oxidation rates of Fe3Al and Fe3Al-Y alloys inthe temperature range of 800◦C to 1000◦C were nearlyidentical and the general equation for the parabolic rate

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constants as a function of temperature can be expressedasKp= 5128 exp[−39506 (cal/mol)/RT] mg2/cm4 hr.

2. The addition of small amount of yttrium to Fe3Alalloy modifies the oxide growth mechanism from coun-tercurrent diffusion of aluminum and oxygen to pre-dominant oxygen diffusion. The change of growthmechanism leads to the formation of pegs and less oxidegrowth stress which enhances the adhesion of aluminascale to the alloy substrate.

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Received 14 April 1999and accepted 3 February 2000

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