2004 High temperature oxidation behaviour of Fe–Al–C alloys an overview

Materials Science and Engineering A364(2004)

232–239

High temperature oxidation behaviour of Fe–Al–C alloys:an overview

V .Shankar Rao ?

Corrosion Science and Engineering Program,Indian Institute of Technology,Mumbai 400076,India

Received 6June 2003;received in revised form 8August 2003

Abstract

Carbide containing dual phase iron aluminide offers two-fold bene?ts;it posses high temperature strength and high resistance to probability of hydrogen embrittlement compared to single-phase iron aluminide.In this paper,we summarise the recent developments related to the oxidation behaviour of Fe–Al–C alloys.Emphasis is placed on the each variables of the system C,Al and temperature,and tried to differentiate the oxidation behaviour between Fe 3Al/?and Fe 3AlC phases while reviewing the oxidation resistance of Fe–Al–C alloys.?2003Elsevier B.V .All rights reserved.

Keywords:Iron aluminides;Carbides;Oxidation;Sul?dation;Hot corrosion

1.Introduction

In the development of materials for high temperature ap-plications,Fe 3Al based intermetallics are considered as one of the potential candidates for various reasons:they consti-tute low-cost elements,show excellent corrosion resistance at elevated temperatures and exhibit high-strength to weight ratio [1,2].However,these materials could not be fully com-mercialised so far to its potential due to its inadequate high temperature strength above 550?C and poor room tempera-ture ductility [3].In the process of development of these al-loys,it has long been recognised that the addition of carbon in Fe 3Al improves the high temperature strength by form-ing a second-phase as Fe 3AlC [4–8].In addition,it is also claimed that the probability of hydrogen embrittlement (HE)in the dual-phase alloy (Fe 3Al–Fe 3AlC)is lower than that of the single-phase Fe 3Al alloy.[9–11].Despite increasing number of studies in last two decades on the development of Fe–Al–C alloys [4–14],no self-contained description about high temperature corrosion behaviour of these alloys exit in literature though it is mainly developed for high tempera-ture applications.In this article we tried to provide a com-prehensive outline on the oxidation behaviour of Fe–Al–C alloys owing of its practical and scienti?c importance for the researchers working in this ?eld.

?

Present address:Department of Material Science and Engineering,Korea Advanced Institute of Science and Technology,373-1KuSong-dong,YoSong-gu,Taejon 305-701,South Korea.

E-mail address:vshankarao@yahoo.co.in (V .S.Rao).This review is mainly concerned with the studies carried out on Fe–Al–C alloys since the early 21st century;some of these alloys are listed in Table 1.The alloys are grouped in Table 1such a way that comparison of oxidation behaviour of alloys 1–4,in which Al content remains the same cor-responding to Fe 3Al intermetallic provides an understand-ing of the in?uence of carbon in the range 0.05–1%in Fe–16Al alloys.On the other hand,comparison of oxidation behaviour of alloys 3,5and 7provides an understanding of the role of Al in Fe–0.5C alloys,while alloys 4,6and 8on the role of Al in Fe–1C alloys.

Oxidation resistance of an alloy can depend upon its mi-crostructure and presence of phases;therefore,a brief de-scription about the microstructures and phase analysis of Fe–Al–C alloys is presented here,whereas a detailed analy-sis about these can be found elsewhere [15,16].Alloys 1–4contain two-phases namely Fe 3Al and Fe 3AlC 0.69.The size and volume fraction of the carbides increases with increas-ing carbon content.In alloys 5–8the phases present are Fe–Al (?)and Fe 3AlC 0.69,respectively.The stoichiometry of the carbides remains unchanged with variation in Al and C content within the composition range given in Table 1.

2.Oxidation studies

Many excellent reviews exist on the oxidation behaviour of binary Fe–Al alloys [17–19].A review on the oxida-tion behaviour of multi component two-phase alloys by Gesumundo and Gleeeson [20]provide a comprehensive

0921-5093/$–see front matter ?2003Elsevier B.V .All rights reserved.doi:10.1016/j.msea.2003.08.031

V.S.Rao/Materials Science and Engineering A364(2004)232–239233

Table1

Chemical compositions of Fe–Al–C alloys

Alloy Composition(wt.%)Reference 1Fe–16Al–0.05C[8]

2Fe–16Al–0.0.14C[5,16]

3Fe–16Al–0.5C[5,16]

4Fe–16Al–1C[6,16]

5Fe–10Al–0.5C[7]

6Fe–10Al–1C[7,15]

7Fe–8Al–0.5C[11]

8Fe–8Al–1C[6,15] theoretical treatment on the similarities and differences between single-phase and two-phase alloys.Therefore,in this article we limited our discussion only on the ternary Fe–Al–C alloys and no theoretical treatment is repeated here.The three variables in the present system C,Al and temperature,each of them will be discussed individually along with the collective effect of C and Al on the oxidation behaviour of Fe–Al–C alloys.We will end the section with the discussion on high temperature corrosion behaviour of these alloys in mixed oxide environments.

2.1.Effect of C

In pure iron oxidation rate reported to be decreases with addition of carbon[21].However,this bene?t could not be manifold when it was alloyed with Al to form better protec-tive scale and it makes understanding of oxidation behaviour of ternary Fe–Al–C more complicated.The?rst ever work on the oxidation behaviour of carbon containing Fe–Al al-loy reported in literature is by Boggs[22].According to his studies there is no apparent difference in oxidation resis-tance of Fe–5.5Al and Fe–5.5Al–0.1C alloys at500?C, whereas at above700?C evolution of CO2interfere with protective oxide?lm and it was shown that at800?C oxi-dation rate increases with increase in carbon content from 0.005to0.1%in Fe–4.94Al alloys.This was attributed to the growth of nodular iron oxide through the alumina?lm, which increases with increase in carbon content.Kao and Wan[23]have also reported the harmful effect of carbon on the oxidation behaviour of Fe–5.5Al alloy as they found oxidation rate of Fe–5.5Al–0.55C alloy is higher than that of Fe–5Al alloy at800?C and a similar explanation the formation of iron oxide rich nodules in carbon containing alloys was attributed.A comparative study on the oxidation behaviour of Fe–Al–C alloys with that of the Fe–Al alloys for nearly the same Al content under similar oxidation con-ditions reported in yet another study,in which the weight gain in Fe–8Al–1C alloys is an order of magnitude higher than that of the Fe–8Al alloys[15].Prescott and Graham [19]brie?y summarised the oxidation behaviour of Fe–Al–C alloys while reviewing the oxidation of Fe–Al alloys.Pub-lished literature till nineties on the effect of carbon on ox-idation behaviour of Fe–Al alloys are concerned only with low-Al content alloys and for very short duration.It is not

W

t

.

C

h

a

n

g

e

(

m

g

/

c

m

2

)

Time (h)

Fig.1.Oxidation kinetics of alloys1–4and310SS at1000?C[16].

very clear the purpose of previous researchers,it may be academic,however the low-Al with high-C content alloys are nowhere suitable for high temperature applications.Our experience on this shows these alloys are even dangerous in application because they give some misleading judgement. Such as these materials may perform well at higher tempera-ture(800?C)but will give poor performance at temperature below than this,which is called as inversion of oxidation in technical terms.

Shankar Rao et al.studied the effect of carbon on long-term oxidation behaviour of Fe3Al based iron alu-minides[16].In their study,unlike in low-Al content alloys, no adverse effect of carbon was found on the oxidation ten-dency of Fe3Al between700and900?C when C lied in the range of0.05–1wt.%.However,on increasing temperature to1000?C two changes are evident.Firstly,an inversion of oxidation behaviour of Fe3Al–Fe3AlC alloys,this will be discussed in Section2.3.Secondly,the effect of carbon be-comes obvious,which is illustrated in Fig.1.With increase in the carbon content duration of oxidation resistance de-creases,as there is increase in spallation tendency of oxide scales.Furthermore,these authors compared the oxidation resistance of these alloys with310stainless steel,as a ref-erence alloy.Up to800?C carbon alloyed iron aluminides were comparable with310stainless steel,whereas above this temperature310stainless steel was found inferior to iron aluminides.As it could be seen from Fig.1,310stainless steel fails within25h of exposure,whereas iron aluminide continued to perform well beyond500h of exposure. Thermal stability of the carbides at oxidation temperature has also been studied in Fe3Al based iron aluminides.In Fig.2,cross-sectional view of Fe–16Al–0.14C alloy after oxidation at900?C for1000h shows the carbides from the matrix get coagulated along the grain boundaries,which is against the?ne precipitation of carbide in the matrix before oxidation.This indicates the higher solubility of carbon in Fe3Al at high temperatures and unoxidised carbon might be driven into this phase,whereas on cooling it precipitates

234V.S.Rao/Materials Science and Engineering A364(2004)

232–239

Fig.2.Cross-sectional view of the microstructure of Fe–16Al–0.14C alloy after oxidation at900?C for1000h[16].

back.Another valuable inference can get from this?gure is that due to formation of protective Al oxide layer on the surface decarburisation is prevented.The accumulation of carbides beneath the oxide scale lends credence to this fact.

A few authors suggested the enhancement of the oxidation resistance of the alloy by formation of the carbide or graphite barrier at the oxide metal interface.

Though oxidation behaviour of an alloy depends on over-all Al content in the alloy,it is possible that scale formed on one phase may be more stable than on the other phase. In order to reveal this scanning Auger electron spectroscopy (AES)was performed over the oxide?lms formed on both the phases of alloy four(Fe–16Al–1C)oxidised at800?C for10min in200mbar O2[24].A scanning Auger micro-probe of model PHI660operated with an electron beam of0.2?A at10kV took the spectra.For depth pro?ling, the specimen was bombarded by Ar ions and the sputter-ing rates were calibrated using Ta2O5?lm of known thick-ness.AES depth pro?les taken at carbides and matrix lo-cations are shown in Fig.3.This study reveals that oxide ?lm formed during initial stages of oxidation on the car-bide phase(Fig.3b)is about four times thicker than that formed on the matrix(Fig.3a).Despite the difference in ox-ide layer thickness,the oxides formed on both the regions appear similar in the following ways:both show well sep-arated oxide layers and oxide thickness ratios(Al oxide/Fe oxide)are almost the same.In both the phases the inner layer consists of Al2O3and the outer layer Fe2O3,which was con?rmed by in situ X-ray photoelectron spectroscopy [25].The sequence of oxide mechanism and difference in oxide layers thickness at the matrix and carbide are illus-trated schematically in Fig.4.The high activity of Fe and its high growth rate over Al enables iron oxides to outgrow during early stages of oxidation leaving behind the islands of aluminium oxides.Since iron activity in carbide phase is more compared to the matrix phase,the formation of oxide ?lm in the carbide phase is thicker than that of the matrix phase(Fig.4a).The oxidation of the substrate continues to occur,as oxygen ions migrate through,n-type Fe2O3con-ductor,until the aluminide develops a continuous protective alumina scale over its surface.This leaves behind a duplex oxide scale with Fe2O3on the top and Al2O3at the bottom as has been observed in the present case(Fig.4b).The for-mation of thick Al-oxide layer on the carbide phase despite its low-Al content is interesting as well as a matter of con-cern because Al reservoir in the carbide phase will become very less to subsequent maintain the protective Al2O3layer in case of spallation of the oxide scale.It is well known that despite having good oxidation resistance of Al2O3its adhesion with the substrate is very poor in iron aluminides [26].Fundamentally as discussed by Gesmundo and Glee-son[20]in two-phase alloy systems,once spallation occurs in order to maintain its mass balance and thermodynamic equilibrium between carbide and matrix phase along with its reformed oxides,C will transit towards matrix phase and Al from matrix phase towards carbide phase,which is ex-empli?ed schematically in Fig.4.The other factor that can affect the oxidation tendency of carbide phase is the release of CO/CO2on oxidation of carbon.This makes the oxide porous,which is less protective and vulnerable to oxygen diffusion.

2.2.Effect of Al

Effects of Al on high temperature corrosion behaviour of Fe–Al–C alloys have been known in studies of the oxidation behaviour of Fe–x Al–1C[15]and Fe-x Al–0.5C(x=8, 10and16)[27]alloys between700and1000?C.Oxidation kinetics of both the systems follow a similar pattern expect the weight gain is more in Fe–1C alloys which causes less oxidation resistance compared to the Fe–0.5C alloys.Ox-idation kinetics of alloys3,5and7at700and800?C are represented in Fig.5to illustrate the effect of Al on the ox-idation behaviour of Fe–0.5C alloys.A similar inference can be drawn for the Fe–1C alloys.The oxidation kinet-ics of Fe–Al–C alloy is less affected when the Al content is lowered from16to10%.The critical concentration of Al required for oxidation resistance in the present alloys seem to be around10%,as further change in Al contents from10 to8%causes signi?cant increase in oxidation tendency of the alloy.It is worth to mention that critical Al concentra-tion required for the oxidation resistance in Fe–Al–C alloys is higher than that of the binary Fe–Al alloys,due to capitu-lation of Al in carbide formation.Extending the same logic, critical Al content for oxidation resistance in Fe–1C alloys is supposed to be more than that of the Fe–0.5C alloys.Over-all,oxidation tendency in low-Al content alloy increases with temperature in the order of800<900<1000< 700?C[15,27].These results show that the weight gains in case of8%Al alloy are the highest for oxidation at700?C. They go through an inversion,with the lowest weight gain

V .S.Rao /Materials Science and Engineering A364(2004)232–239235

020

40

60

80

A t o m i c C o n c e n t r a t i o n (%)

Sputter Depth (nm)

(a)

(b)

A t o m i c C o n c e n t r a t i o n (%)

Sputter depth (nm)

Fig.3.AES depth pro?les of oxide ?lm formed after 10min at 800?C at:(a)matrix and (b)carbide region [24].

at 800?C.This inversion of oxidation was attributed to the preferential oxidation of Al at 800?C.In the case of alloys having 10%Al,inversion does occur at 800?C.However,it is less signi?cant.The inversion of oxidation has been well explained taking in account the thermodynamic and kinetic aspects of the Fe and Al at the given temperature [15].The weight gain data in alloy 7at 700?C were utilised to determine the kinetic rate constants.Two parabolic rate constants,k p 1=5.8×10?11g 2/cm 4s for the data up to 200h of oxidation and k p 2=3.1×10?12g 2/cm 4s for the data between 200and 1000h were obtained.As it is logi-cal to suggest that the outer iron-rich oxide scale is formed during early stages of oxidation,when the kinetics is rapid,k p1can be attributed to this less protective scale forma-tion.The lower parabolic rate constant (k p2)corresponding to later stages of oxidation,beyond which the rate of ox-idation becomes low indicating that it corresponds to the formation of inner layer of aluminium oxide.Notably,both

these parabolic rate constants are higher than that for the Fe–7Al binary alloys,(1.2×10?11g 2/cm 4s and 4.72×10?13g 2/cm 4s,respectively)reported by Boggs [28]despite the fact that latter was oxidised in high partial pressure of oxygen than that of the former.

A cross-sectional view of the oxide scale of Fe–8Al–0.5C alloy oxidised at 700?C for 1000h is shown in Fig.6.By a close observation two layers of the oxide scale could be seen;the outer oxide layer is porous while the inner oxide layer seems to be https://www.360docs.net/doc/fa8429518.html,ing EPMA line scans it was detected that Al is rich in the inner layer,while iron is distributed more in the outer layer than that of inner layer [15].A sign of decarburisation could be seen beneath the oxide scale.The formation of inner layer (aluminium oxide)acts as a healing layer,which prevents the alloy from further oxidation as well as decarburisation.

The surface morphology of the oxide scale developed on Fe–8Al–1C alloy after oxidation at 900?C for 1000h is

236V .S.Rao /Materials Science and Engineering A364(2004)

232–239

(a)

(b)

(c)

Fe 2O 3

Al 2O

3

Al Carbide

Matrix 2

Fig.4.Schematic representation of oxide layers formation in the carbide and matrix regions [24]

.

Fig.5.Oxidation kinetics of alloys 3,5and 7at:(a)700?C and (b)800?C.

Fig.6.Cross-sectional view of the oxide scale of

Fe–8Al–0.5C alloy after oxidation at 700?C for 1000h,shows the decarburisation in the subsurface [27].

Fig.7.Surface morphology of Fe–8Al–1C alloy after oxidation at 900?C for 1000h shows deeper attack on matrix in comparison with the carbide phase.

brought out in Fig.7.Micrograph of the oxide scale shows that a few sites are predominantly oxidised.Chemical com-positions of the oxide scale obtained through energy dis-perssive X-ray analysis (EDX)at two different locations,predominantly oxidised (location 1)and trough (location 2),are listed in Table 2.The analysis reveals that the oxide scale at predominantly oxidised location is mainly due to iron oxides and the other location consists of mixed Fe and Al oxides.The predominantly oxidised sites could possibly correspond to matrix region.

Table 2

EDX data of the oxide scale corresponding to two different locations (1)predominantly oxidised and (2)trough,as seen in Fig.7Location

Fe (at.%)Al (at.%)194.4 5.62

47.6

52.4

V.S.Rao/Materials Science and Engineering A364(2004)232–239237

2.3.Effect of temperature

Kao and Wan[29]evaluated the temperature effect on the oxidation behaviour of Fe–5.5Al–0.55C alloy at600,800 and1000?C,and showed that temperature strongly in?u-ences the carbides.The internal oxidation of Al beneath the iron oxide nodules at600?C,which was attributed to inver-sion of oxidation,is eliminated at temperatures700–900?C. In order to differentiate the effect of temperature on the oxi-dation kinetics in the alloys listed in Table1,weight changes in alloy2(Fe–16Al–0.14C)and alloy4(Fe–16Al–1C) oxidised at four different temperatures viz.,700,800,900 and1000?C are reproduced in Fig.8a and b.The results indicate that with increasing temperature up to900?C the weight gain increases,whereas further increase in tempera-ture an inversion occurs at1000?C(Fig.8a).First this de-viation was speculated to the change in phase formation in Al2O3on the available literature[15].Further study by the same author based on the physical metallurgy of Fe–Al–C,it was evaluated as the reduction in volume fraction of Fe3AlC phase at1000?C[16].The issue now becomes how much dissolution of carbides is required to render and sustain

the Fig.8.Oxidation kinetics from700to1000?C of(a)alloy2 (Fe–16Al–0.14C)and(b)alloy4

(Fe–16Al–1C).Fig.9.Vertical section of the Fe–Al–C phase diagram at constant Al content of15wt.%[30].

inversion of oxidation,as in alloy4the inversion which ap-pears during initial stage of oxidation slowly disappears as oxidation proceeds with time(Fig.8b).This was tried to ex-plain by examine the vertical section of Fe–Al–C phase dia-gram corresponding to15wt.%Al(Fig.9),which is close to the Al content in alloys1–4[30].In alloys1–3due to having C content below1.9at.%carbides precipitate from bcc FeAl (?)phase.Whereas in case of alloy4with3.9at.%C,car-bides form two ways,initially direct from the melt as a last solid and later by solid state transition from bcc FeAl phase. Since the last solidifying liquid corresponds to the interden-dritic region rich in carbides and is more prone to the oxide formation over the matrix.The other reason could arise from the fact that unlike other alloys,in alloy4its over all carbon content is higher than the solubility level of bcc FeAl phase (1.5at.%)so that all the carbides could not be dissolved even at1000?C.In addition,the presence of carbides at this temperature can further facilitate the oxidation tendency due to its low-Al content.The inversion of oxidation behaviour of these alloys is explained through a schematic diagram in Fig.10,which displays the volume fraction of the car-bides at1000?C is lower than that of900?C.As discussed in Section2.1,carbide has higher oxidation tendency than the matrix phase,therefore,the decrease in volume fraction of the carbide decreases the overall oxidation tendency of the alloy.More comprehensive discussion on the effect of temperature on other alloys can be found in a contemporary communication[31].

2.4.Synergetic effect of C and Al

As mentioned earlier,Al is responsible for the better ox-idation resistance and poor structural properties whereas in

238V .S.Rao /Materials Science and Engineering A364(2004)232–239

Al 2O 3

900o

C

1000o C

Fig.10.Schematic model for the inversion processes involved during oxidation in Fe 3Al–Fe 3AlC alloys due to dissolution of carbide phase [32].

case of C it is vice versa.Therefore,it is essential to fo-cus both the variables Al and C together for a better under-standing of the oxidation behaviour of Fe–Al–C alloys.In high Al content alloys it is seen that after complete sputter-ing off the oxide layers from the surface (Fig.3a and b ),Al content is less in the carbide phase (11%)than that of the matrix phase (18%),which is responsible for higher oxida-tion tendency of the carbide phase.In addition it was also shown that stoichiometry of the carbides remain the same as Fe 3AlC 0.69with variation of Al content at a given composi-tion range in Table 1.Therefore,on lowering of Al content from the alloy,Al content will reduce from the matrix phase rather than the carbide phase in order to maintain the sto-ichiometry of the carbides.Hence,unlike high-Al content alloys,in low-Al content alloys oxidation resistance of the carbide phase is better than that of the matrix phase.This fact was further corroborated with SEM micrograph shown previously in Fig.7,in which matrix phase was found more prone to oxidation than the carbide.Now,we will set eyes on the change in composition and phase with variation of C content and their affect on the oxidation resistance of the alloy.In Section 2.2it has been conferred that oxidation tendency of 1C alloys is higher than that of the 0.5C al-loys at all the temperature under studied.This is because with the increase in carbon content volume fraction of car-bide increases.Extending the same logic it can be argued that since the carbide stoichiometry remains the same,the same amount of Al is utilised in forming the carbide phase.This leaves less Al available to form a solid solution with Fe.As a consequence,not only is the alloy unable to form ordered ?,as it requires more Al than is available,but also makes the ?more prone to oxidation.As a consequence the oxidation resistance of low-carbon alloys is superior to that of high-carbon alloys,for a given Al content.It may be no-ticed,the effect of carbon is less signi?cant at low tempera-ture,when Al content of the alloy is more than 10%.Thus,it seems that the detrimental effect of carbon increases with decrease in aluminium content.2.5.Mixed oxide environments

In a recent study,the role of C and Al on the high tem-perature corrosion behaviour of Fe–Al–C alloys in mixed gas environments was reported in 2SO 2:O 2at 800?C [32].

The effect of carbon was less obvious at this environment as shown in Fig.11,which was attributed to the low ox-idizing power of the 2SO 2:O 2gas mixture.In addition,it was concluded that the carbide phase despite having large carbon concentration the Al content is just above the critical Al concentration (10%)required for sul?dation resistance at 800?C,therefore,overall sul?dation resistance of all the alloys in the given composition range remains the same.However,unlike air oxidation in 2SO 2:O 2atmosphere on prolonged oxidation after about 350h these alloy become unstable and irrespective of carbon content all the alloys show spallation in a similar manner.Considering the role of Al,the effect was similar to air oxide environment,in high-Al content alloy matrix as well as carbide phase are equally resistance to sul?dation,whereas in low-Al content alloy,carbide phase is more resistant to sul?dation than the matrix phase.It is generally accepted that decreasing the Al content decreases the sul?dation resistance of the Fe–Al alloys.However,in this case alloy 5,which having 10%Al compared to alloy 3having 16%,perform comparatively better to sul?dation resistance for a given carbon content of 0.5%.This was explained based on the fact that the forma-tion of mixed oxides/spinel in alloy 5offers better oxidation resistance than Al 2O 3,which forms on alloy 3.

Hot corrosion behaviour of C containing Fe 3Al al-loys was compared with the binary Fe 3Al alloys in the

W t . C h a n g e (m g /c m 2

)

Time (h)

Fig.11.Sul?dation kinetics of alloys 2–4at 800?C in SO 2+O 2envi-ronment.

V.S.Rao/Materials Science and Engineering A364(2004)232–239239

temperature range between1100and1330K after coating with Na2SO4[33].The results show the hot corrosion rate of carbon-alloyed iron aluminides is slower than that of the binary alloys and it was related to blocking effect of carbides,which are present along the grain boundaries for the penetrating sulfur.

3.Conclusions

After reviewing the oxidation behaviour of Fe–Al–C al-loys conclusions drawn are as follows.

1.In general,the addition of carbon should be as low as

possible for long run oxidation resistance of the materials.

Detrimental effect of carbon increases with lowering of Al content in the alloy.

2.The critical Al concentration in Fe–Al–C ternary alloys

should keep more than the usual binary Fe–Al alloys for better oxidation resistance and to avoid the inversion of oxidation.

3.In high-Al content alloys carbide phase is more prone to

the oxidation,whereas in low-Al content alloy it is the matrix phase.Lowering of Al content in the alloy suffers from decarburisation.

4.Effect of carbon on Fe3Al base intermetallics is less ob-

vious in mixed oxide environments.

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