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Scanning Auger electron spectroscopy study

of the oxide film formed on dendritic and

interdendritic regions of C containing

Fe3Al intermetallic

V. Shankar Rao a, M. Norell b, V.S. Raja a,*

a Corrosion Science and Engineering, Indian Institute of Technology, Bombay,

Powai, Mumbai 400 076, Indiab Department of Materials Science and Engineering, Chalmers University of Technology,

SE-412 96 Gooteborg, Sweden

Abstract

The oxide films formed during early stage of oxidation at 800 C on dendritic and inter-

dendritic regions of the cast Fe16Al1C (wt.%) alloy were studied using scanning Auger

electron spectroscopy. Microhardness measurement and elemental depth profiles by Auger

spectroscopy reveal that the carbide, Fe3AlC0:69, is the major constituent of the interdendritic

region, while dendrites are predominantly Fe3Al phase. Between the two, the interdendritic

region is found to be more prone to oxidation than the dendritic region, which was attributed

to presence of carbides with low-Al content. In spite of the difference in oxide film thickness

exhibited by both the phases, they consist of an inner aluminium oxide layer and an outer iron

oxide layer.

Keywords: Intermetallics-iron aluminide; AES; XRD; Interfaces; Oxidation

1. Introduction

In recent years, there has been increasing number of studies on the development

of carbon containing iron aluminides, for high temperature applications [15]. This is

http://mail%20to:%20vsraja@met.iitb.ac.in/

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because of two-fold benefits carbon is reported to offer for iron aluminides. By en-

abling carbide formation, it offers strength to this material on one hand and reduces

the environment embrittlement on the other [1,5,6]. Most of the studies on carbon

containing aluminides are directed towards alloy development [1,2] and examinationof their constituent phases [5,7], microstructures [3,4] and mechanical properties

[5,8]. Since these materials are primarily developed for high temperature structural

applications, they are expected to possess high temperature oxidation resistance.

Oxidation behaviour of Fe3Al and FeAl phases has been well documented by

several investigators [912], but little was known on the oxidation behaviour of

carbon containing iron aluminides until the oxidation studies of these alloys initiated

in our laboratory. In the recent communications, we have reported the effect of

carbon on long-term oxidation behaviour of Fe3Al [13] and the effect of Al on the

oxidation behaviour of FeAlC alloys [14,15]. In the first study we have shown that

the addition of carbon (0.141%) has only moderate effect in the temperature range

700900 C on the oxidation behaviour of these alloys, but it is responsible for

raising the spallation tendency of the alloys at 1000 C [13]. In the second and third

communications, we found that the carbide phase is more prone to oxidation in

high-Al content alloys, whereas in low-Al content alloys it is the matrix phase

[14,15]. Previous papers [1315] are concerned with long-term oxidation behaviour

of these dual phase iron aluminides, with variation of C and Al content from one

alloy to another. However, for a better understanding of the subject it is equally

important to know the oxidation behaviour of each phase within an alloy. The

present work examines the nature of oxides formed on dendritic and interdendriticregions of carbon containing Fe3Al during the early stage of oxidation. This has

been accomplished using scanning Auger electron spectroscopy. The different in-

terfacial chemistry between oxide-dendrite and oxide-interdendrite region in the

present alloy is believed to differentiate the role of carbon at these two phases.

2. Experimental

The alloy used in this study had a nominal composition of Fe16Al1C (wt.%). It

was prepared by air induction melting followed by electroslag refining technique.Details on production and mechanical properties of this alloy can be found else-

where in the literature [2,3]. Specimens for microscopy studies were obtained by

polishing them through successive grades of silicon carbide emery papers starting

with 220 down to 1000, followed by 1 l diamond paste and lastly etching. The et-

chant consisted of 33%CH3COOH + 33%HNO3 +1%HF+33%H2O by volume.

Phases present in the alloy were identified by X-ray diffraction (XRD) technique with

CuKa radiation. Samples for oxidation studies were prepared in the same manner as

was done for microscopy studies, except that they were not etched. Scanning Auger

electron spectroscopy (AES) was employed to examine the nature of oxides formed

during early stages of oxidation. Oxidation was carried out at 800 C under 200mbar O2 for 10 min in a furnace, it was then transferred to the Auger chamber. A

scanning Auger microscope of model PHI 660 operated with an electron beam of

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0.2 lA at 10 kV took the spectra. For depth profiling, the specimen was bombarded

by Ar ions and the sputtering rates were calibrated using Ta2O5 film of known

thickness. The differentiated Auger peaks were found to differentiate the O signals

related to the Al and Fe oxides; similarly Fe and Al signals were differ in the oxideand bulk. In the evaluation of the depth profiles, these were used for a numerical

separation 1 of the signals from the different states of these elements in the various

layers. The details about the technique is discussed elsewhere [16].

3. Results and discussion

A typical room temperature XRD pattern of Fe16Al1C alloy is shown in Fig. 1.

The d-values obtained from the patterns and the calculated lattice parameters are

summarised in Table 1. For comparison the lattice parameter of the phases reportedin JCPDS files are also shown. As brought out by Fig. 1, the XRD pattern reveals

the presence of Fe3Al and Fe3AlC0:69 carbide phases. Fe3Al has a b.c.c structure,

whereas Fe3AlC0:69 exhibits an f.c.c perovskitc-type structure. Notably, the peak

intensity of various planes does not match with the reported intensity of the corre-

sponding planes by JCPDS files (Table 1). These variations could occur due to

possible preferential orientation of phases in the as-cast alloy. Appearance of an

extra peak at the d-value of 3.89 AA is due to reflection of the superlattice plane (2 0 0)

of Fe3Al. This indicates that it is an ordered structure. Furthermore, the lattice

parameter of Fe3Al in the present alloy is higher than that reported in JCPDS files,

implying that carbon is soluble in Fe3Al phase even at room temperature. In contrast

to Fe3Al, the Fe3AlC0:69 phase of the alloy exhibits almost the same lattice parameter

value as that reported in JCPDS files and hence is expected to have the same stoi-

chiometry.

The microstructure of the alloy was examined in a scanning electron microscope

(SEM). A typical microstructure of the alloy as seen in SEM is brought out in Fig. 2.

Two distinct regions are seen in the micrograph. The alloy clearly shows dendritic

and interdendritic regions as expected in a cast alloy. Though a few authors have

reviewed the FeAlC phase diagram [17,18], phase boundaries of various phases at

different temperatures still remain uncertain. Examination of a vertical section of theFeAlC phase diagram corresponding to 15 wt.%Al, close to the present alloy,

shows that with 1 wt.%C (corresponding to 3.89 at.%) Fe3AlC solidifies as a last

liquid from the alloy [18]. Therefore, the interdendritic region must contain this

phase. This aspect was further examined using microhardness and microanalytical

techniques. Microhardness test in the interdendritic and dendritic regions, respec-

tively, yielded average values of 475 (10) and 390 (10) kg/mm2 Vickers hardness

for a 50 g applied load. The higher hardness exhibited by the interdendritic region

indicates that it consists of Fe3AlC phase.

A secondary electron image of the oxidised alloy is shown in Fig. 3. Resemblance

between this image and the one obtained from the as-cast sample (Fig. 2) is obvious.

1 Software called MultiPak from Physical Electronics was used.

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Fig. 2. Scanning electron micrograph of Fe16Al1C alloy reveals the dendritic (white contrast area) and

interdendritic (grey contrast area) regions.

Table 1

Results of XRD patterns of Fe16Al1C alloy

d-Values

[AA]

Plane

(hk l)

I/Io Cell parameter [AA] Phase (s) JCPDS

card no.

Obs. Ref. Calculated Ref.2.89 200 10.5 4 a 5:82 a 5:793 Fe3Al 06-0695

2.04 220 2.1 100

1.44 400 100 80

2.15 111 3.0 100 a 3:771 a 3:771 Fe3AlC0:69 03-0965

1.32 220 0.5 80

Fig. 1. Typical XRD pattern of the alloy identifies Fe3Al and Fe3AlC0:69 phases.

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AES analysis of oxides was performed on the dendritic and interdendritic region in

the above micrograph. AES depth profiles of the elements of oxides formed on the

dendritic as well as on the interdendritic regions are presented in Fig. 4. The oxides

of Fe and Al seem to exist as two different layers, except for some mixing at their

interface.

Auger spectra taken on the dendritic and interdendritic regions after complete

removal of oxide layer are shown in Fig. 5. No detectable carbon could be found

from the dendritic region (Fig. 5a). The interdendritic region accounts for 14 at.% C(Fig. 4b) which corresponds to a carbide phase with formula of Fe 3AlC0:69 as de-

tected by XRD pattern. Moreover, SEM could not reveal the presence of Fe 3Al in

the interdendritic region even at higher magnification. Experimental data show that

the interdendritic regions are predominantly carbides, although it is not possible

to rule out the existence of Fe3Al phase in the interdendritic region. Hence, the

oxidation behaviour of this region is approximated to the oxidation behaviour of

carbide phase. Normally cast alloys exhibit segregation of impurities at the inter-

dendritic areas. As revealed by the comparison of Auger spectra of Fig. 5a and b, the

interdendritic region does not show the detrimental impurities such as Mn, S, P,

which adversely affect the oxidation behaviour of alloy. The absence of these im-purities in the interdendritic region could be attributed to the ability of electroslag

refining process to minimise them. The thickness of each oxide layer formed on the

two phases obtained from Fig. 4 is given in Table 2. The sputtering rate of iron oxide

is closer to that of Ta2O5, while for aluminium oxide it is about 30% slower than

Ta2O5 [19]. In making estimation we assume that any variations in the sputter rate

will affect both the dendritic and interdendritic regions in the same way.

The data show that the interdendritic region forms as much as four times thicker

oxide than the dendritic region. On the other hand, oxides formed on both the

regions appear similar in the following ways: (a) both show layers of well separated

oxides, (b) both exhibit a transition layer where both cations are intermixed and (c)oxide thickness ratios (Al oxide/Fe oxide) are almost the same (Table 2). Existence of

both the cations in the narrow zone between the two oxide layers of aluminium and

Fig. 3. Secondary electron image of the alloy, showing oxides morphology after oxidation at 800 C for 10

min. Auger analysis carried out at dendritic and interdendritic regions.

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iron oxides could mean either the iron and aluminium oxides co-exist or an oxide

containing both these cations form. The present study does not resolve this aspect,

though it is likely that peak shapes would be different if a new phase having both Fe

and Al co-exist. To some extent, the appearance of this intermediate layer is also dueto the depth resolution of the ion sputtering. Notably, during sputtering the oxide

layers of the carbide phase show no signals of carbon until they are removed com-

pletely. This corroborates that carbon does not interfere with oxide properties, either

by ionic or electronic transport process. Whereas S is reported to be interfere with

oxides in the form of FeS during oxidation of Fe 3Al alloy [20].

Velon and Olefjord reported the formation of a thin 15.9 nm bilayer oxide on iron

aluminide, where cations of Fe and Al respectively exist on outer and inner layers,

when Fe3Al (28 at.% Al) was oxidised at 300 C for 50 h [16]. Unfortunately, these

results cannot be compared with the present work, as their study was concerned with

behaviour of the aluminide at 300 C, a temperature much lower than that wasapplied in the present study. This is because of the fact that below 500 C, unlike at

higher temperatures, Al diffusion towards the surface of the alloy becomes slow.

0 200 400 600 800

0

20

40

60

80

O outer

O innerAl inner

Al bulk

Fe outer

Fe bulk

0 500 1000 1500 2000 25000

20

40

60

80

100Carbide

O surf

O inner

Al

Albulk

Fe outer

Fe bulk

AtomicConcentration(%)

AtomicConcentration(%)

Sputter Depth (nm)

Sputter Depth (nm)

inner

(a)

(b)

Fig. 4. AES depth profiles of oxide film formed after 10 min at 800 C at (a) dendritic and (b) inter-

dendritic region.

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Nonetheless, it is interesting to note that iron aluminide gives rise to similar bilayer

oxides at these two different temperatures, though the thickness of these oxides

depends on the temperature of exposure. According to these authors [16], Fe2O3being an n-type conductor allows migration of oxygen ions, which react with alu-

minium on the surface of Fe3Al to form a protective scale of Al2O3. In contrast tothis, Natesan [21] reported high Al/O ratio in the oxide film of 100 AA formed on

Fe3Al (Fe15.9 wt.%Al) when it was oxidised in air at 1000 C for 10 min. However,

200 400 600 800 1000 1200 1400

Al

Fe

Fe Fe

N(

E)/dE

Kinetic Energy (eV)

200 400 600 800 1000 1200 1400

AlC

Fe

Fe

Fe

N(

E)/dE

Kinetic Energy (eV)

(a)

(b)

Fig. 5. Typical Auger electron spectra of (a) dendritic and (b) interdendritic regions, after complete re-

moval of the oxide film by sputtering.

Table 2

Oxide film thickness at dendritic and interdendritic regions

Region Al-oxide

(nm)

Fe-oxide

(nm)

Total thickness

(nm)

Al/Fe oxide

thickness ratio

Dendritic/Fe3Al 200 300 500 0.67

Interdendritic/Fe3AlC0:69 850 1150 2000 0.73

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the presence of Fe2O3 in the oxide layer has not been reported. Since Al content is

compared to the total oxygen content of the oxide film, a higher Al/O ratio reported

by him does not support the possible presence of iron oxide in the oxide scale.

The present work does not address the oxidation mechanism with respect toFe3Al phase, but it seems possible to explain the bilayer oxide formation based on the

general nucleation and growth characteristics of an alloy system having two reacting

elements. The high partial pressure of oxygen and high iron activity of iron alumi-

nide enable simultaneous oxidation of iron and aluminium [22]. However, iron oxide

has very high growth rate than that of aluminium oxide. As a result, iron oxide

outgrows aluminium oxide during early stages of oxidation, leaving behind the is-

lands of aluminium oxides. The oxidation of the substrate continues to occur, as

oxygen ions migrate through, n-type Fe2O3, conductor, until the aluminide develops

a continuous protective alumina scale over its surface. This leaves behind a duplex

oxide scale with Fe2O3 on the top and Al2O3 at the bottom as has been observed in

the present case. This explains why the Fe3Al phase exhibits Fe2O3 over Al2O3. The

suggestion that O ions diffuse through Fe2O3 semiconductor is supported by the

previous study on ferritic stainless steels [23]. These authors indicated that stainless

steels form Fe-oxide layer on the alloy, through which oxygen diffuses to the alloy

oxide interface to form Cr-oxide, even when the stainless steel was oxidised at 520 C

for 3 min.

Bradford [24] reported that during prolonged oxidation Fe2O3 converts into

Al2O3 by following reaction.

Fe2O3 Al Al2O3 Fe

The above reaction is possible because of the fact that Al 2O3 is thermodynamically a

more stable species than Fe2O3. When Al comes into contact with Fe2O3 the latter

gets reduced to Fe and the former gets oxidised to Al 2O3. Though such a mechanism

can explain why Natesan [21] did not observe the presence of Fe2O3 in the oxide

layer formed at 1000 C on iron aluminide, the question arises as to how Al from the

alloy would have access to Fe2O3 when an Al2O3 barrier lies between Fe2O3/alloy

substrate exists. A possible explanation, why Natesan [21] did not observe Fe2O3 on

alloy oxidised at 1000 C even during shorter intervals of time, while Fe2O3 was

found on the iron aluminide oxidised at 300

C even for a longer duration of 15 h[16]. According to Renusch et al. [25] Fe2O3 facilitates the growth of Al2O3 by acting

as a precursor template, making its presence important at low temperatures, when it

is difficult for the latter to form. However, at 1000 C, Al diffusion and its oxidation

can be so rapid that Al2O3 may not require Fe2O3 as a precursor template and the

former is formed exclusively.

One of the important aspects of the present study is to examine the role of

Fe3AlC0:69, one of the constituent phases, on the oxidation behaviour of carbon

containing iron aluminide. The present data suggest that carbide phase forms four

times thicker oxide than the matrix. Additional microscopy evidence for the higher

oxidation tendency of Fe3AlC than Fe3Al arises from the cross-sectional image ofthe oxide scale after a prolonged oxidation of 1000 h (Fig. 11 in Ref. [13]), where

thicker oxide scale is noticed on carbide than at matrix. It needs to be emphasized

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that the despite having low-Al content in the carbide phase, the Al-oxide layer in the

carbide phase is thicker than that of the matrix phase. This observation is somewhat

similar to our previous finding, in which Al2O3 oxide forms thicker layer in Fe8Al

1C alloy than Fe16Al1C alloy, in spite of having low-Al content in the former [14].These findings lead to the dictation that the formation of Al2O3 in the FeAl alloys

depends on the relative activity of Al at oxidation temperature rather than the total

Al content in the alloy, which has been discussed in our previous communication

[14]. Nevertheless, the formation of thick Al-oxide layer on the carbide phase despite

its low-Al content is a matter of concern because Al reservoir in the carbide phase

will become very less to subsequent maintain the protective Al2O3 layer in case of

spallation of the oxide scale.

Since published literatures on the oxidation behaviour of carbon containing dual

phase iron aluminides are sparse, it is worthwhile to compare the oxidation be-

haviour of the present alloy with that of cast stainless steels. Interdendritic region in

cast stainless steels are known to be more prone to oxidation than that of dendrite

region, in a way much similar to the behaviour of Fe16Al1C alloy. However, the

higher oxidation tendency of interdendritic regions of cast stainless steel alloy is

attributed to the accumulation of impurities such P, S, Mn. In the present alloy, no

such impurities in the interdendritic region are enriched (Fig. 5b). Therefore, higher

oxidation tendency of interdendritic region in iron aluminide can be attributed to the

presence of carbide in this region. This is because Fe 3AlC0:69 has less Al (11 at.%)

than the Fe3Al matrix (18 at.%), as could be seen from Fig. 4a and b. The decrease in

Al content of the carbide phase, which is responsible for forming protective Al2O3,makes the interdendritic zone more prone to oxidation. The other factor that can

enhance the oxidation tendency of carbides is the presence of carbon. On oxidation,

the carbide phases release CO/CO2 [26,27]. This makes the oxide porous, which is

less protective and vulnerable to oxygen diffusion.

It is worthwhile to examine the oxide/metal interface. Simultaneous presence of

elemental and oxidised states of Al and O in addition to the expected elemental Fe,

over a small depth between the substrate and the oxide indicates that the oxide/metal

interface is non-planar. A close examination of the shapes of O inner, Alinner and Febulkprofiles of dendrite (Fig. 4a) and interdendritic region (Fig. 4b) further brings out the

subtle difference in the interface character on the carbides as compared to that ofmatrix. The oxide/matrix interface shows relatively sharp decay of Alinner and Oinner,

while oxide/carbide interface seems to exhibit a sharp decay followed by a slow decay

of these signals, indicating that this interface has higher O and Al content than that is

found at the oxide/matrix interface. The concomitant presence of Alinner and Oinner at

the interface means that Al could coexist as oxide along with metallic Fe and Al. This

implies that carbide is prone to internal oxidation at this temperature. It could be

due to relatively low-Al content of Fe3AlC0:69 and/or higher permeability of oxygen

in it in comparison with Fe3Al. The O2 permeability in many intermetallics

compound has been reported to be very low [28], while, no published data exists on

the permeability of oxygen in Fe3AlC. A detail microstructural and chemical analysisare required for an in depth understanding of the oxidation behaviour in the carbide.

Besides the compositional difference between Fe3AlC0:69 and Fe3Al, their difference

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in thermal stability at the oxidising temperatures also affect the overall oxidation

behaviour of these materials [18].

One of the authors made a detailed analysis of the oxidation behaviour of FeAl

C systems including carbon containing iron aluminide intermetallics [29]. A physical

model was developed to explain a wide range of experimental results. From the

above study a schematic diagram of the oxide layer formation over dendritic and

interdendritic regions is shown in Fig. 6. The higher C present in the alloy gives rise

to CO2. The gaseous CO2 when escapes from the surface introduces pores and

cracks, which become easy paths for O2 migration to the metal interface. In such acase the oxide needs to grow thick enough to offer an effective barrier for the mi-

gration of O2. Since, more C is present in the interdendritic region than in the

dendritic region, the oxide on the former will be more porous and thicker than that

of the latter. In the model it is assumed that during initial stage of oxidation both the

phases are oxidised independently, as the activity of the components (Al and C)

differs widely in both the phases. However, to maintain thermodynamic equilibrium

during oxidation process between elements and its ions; Al can diffuse from the

matrix phase, Fe3Al, to the surrounding Fe3AlC phase, similarly the C can diffuse

out of the carbide to the matrix phase as the solubility of C in the matrix increases

with temperature. Both these processes enhance the oxidation resistance of carbides.Nevertheless, this will depend on temperatures and time of alloy exposure, lower

temperature and shorter time being less effective.

Fig. 6. Schematic of oxide layer formation in the dendritic and interdendritic regions presented based on

Ref. [29].

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4. Conclusions

The oxide film formed on the dendritic and interdendritic regions during early

stage of oxidation consists of iron oxide in the outer layer and Al oxide in the innerlayer. The over all oxide film formed on the interdendritic phase is much thicker than

on the dendritic region. This is because presence of low-Al content carbides, in the

interdendritic region.

Acknowledgements

The authors V. Shankar Rao and V.S. Raja thank Dr. R.G. Baligidad for pro-

viding the materials and Dr. D. Banerjee, Director, Defence Metallurgical Research

Laboratory, India, for his keen interest in this work.

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