Al Fe Interaction 2003

Materials Science and Engineering A363 (2003) 53–61

Kinetic interactions between solid iron and molten aluminium

A. Bouayada,b, Ch. Geromettaa,∗, A. Belkebirc, A. Ambarib

a LPMI/Laboratoire Industriel de Fonderie, ENSAM CER d’Angers, 2 Bd du Ronceray, 49035 Angers Cedex, Franceb LPMI/EMT, ENSAM CER d’Angers, 2 Bd du Ronceray, 49035 Angers Cedex, France

c Fonderies du Poitou Fonte, TEKSID Group, 86220 Ingrandes sur Vienne, France

Received 15 October 2002; received in revised form 3 June 2003

Abstract

In order to improve the metallurgical bond between ferrous inserts and aluminium matrix in castings, the interaction between moltenaluminium and solid iron is studied by immersion tests. Morphology evolution and growth kinetics of the intermetallic layers Fe2Al5 andFeAl3 were established as a function of time and temperature. For medium interaction time (<45 min), a diffusion regime controlled the Fe2Al5

growth, while FeAl3 grew under a kinetic regime. The apparent activation energy was evaluated in the range of 700–900◦C for the diffusionalgrowth of the Fe2Al5 layer. A theoretical approach based on a reaction diffusion model of simultaneous growth of the two intermetallic layerswas successfully used to describe growth kinetics. The parabolic and linear rate constants of Fe2Al5 and FeAl3 were identified. This approachrationalizes the experiments.© 2003 Elsevier B.V. All rights reserved.

Keywords: Kinetic interactions; Intermetallics; Reaction diffusion; Solid iron; Liquid aluminium

1. Introduction

The incorporation of reinforcements in aluminium alloycastings is a technique increasingly used to obtain partscombining resistance and lightness[1–3]. Bimetallic pieces,which combine the good mechanical properties of a ferrousmetal with the low density of aluminium alloys, are of partic-ular interest for automotive applications. However, achiev-ing high performance requires good bonding. A continuousmetallurgical bond, made up of intermetallic layers, is idealto guarantee the sealing and heat transfer and to ensure aperfect mechanical connection. The study of the formationand the growth of intermetallic compounds in the Fe–Al sys-tem is also important for other processing techniques, suchas hot dip aluminizing, cementing, CVD and liquid metalcorrosion[4–6].

∗ Corresponding author. Tel.:+33-2-4120-7374;fax: +33-2-4120-7394.

E-mail addresses: a [email protected],[email protected] (A. Bouayad),[email protected] (Ch. Gerometta),[email protected] (A. Belkebir),[email protected] (A. Ambari).

Reaction diffusion laws govern the formation and thegrowth of intermetallic layers at the interface between asolid and a liquid metal. Several authors have dealt withthe theoretical models that describe these phenomena[7,9].In a strict ‘diffusional’ perspective, the growth of these in-termediate phases follows parabolic kinetics. However, this‘diffusional’ approach is not sufficient to explain certainexperimental observations. The consideration of a chem-ical reaction step allowed Dybkov[10] to propose morerealistic models for binary systems. These models clar-ify the reasons for the absence of certain phases expectedfrom the phase diagram and which are not observed exper-imentally. The properties of the entire intermetallic layerdepend on the types of phases formed, their morphologiesand thicknesses. Consequently, the knowledge of thesephenomena and the associated growth mechanisms areimportant.

The aim of this work is to study the interaction of puresolid iron with pure liquid aluminium in order to charac-terize the types of interfacial phases formed, their mor-phology and growth kinetics. The model of Dybkov[10]is used to quantify the growth kinetics. The Al–Fe systemhas been selected because of its importance in the field ofinsertion in die casting. Denner et al.[11] and Bahadurand Mohanty[12] undertook studies to ensure a perfect

0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0921-5093(03)00469-6

54 A. Bouayad et al. / Materials Science and Engineering A363 (2003) 53–61

adherence in the case of hot dip aluminizing of sheets oflow and medium carbon steel. The phases formed in thecase of the interaction of pure liquid aluminium (or satu-rated with iron) with pure solid iron (or a mild steel) areFeAl3 near aluminium and Fe2Al5 near the ferrous substrate[13,14].

The parabolic kinetics of growth found by Denner andJones[8] and Bouché et al.[15] contradicts the works ofYeremenko et al.[16] and Eggeler et al.[17] who observednegative deviations from the parabolic law after long re-action times. The latter affirm that this deviation is dueto the iron enrichment in the aluminium melt by dissolu-tion and not to the spalling of the intermetallic layer asDenner and Jones[8] concluded. This needs confirmation.In fact, Dybkov [18], in a more recent investigation, us-ing the rotating disc technique to study the interaction ofa 18Cr–10Ni stainless steel with liquid aluminium, pointedout the effect of the presence of a chemical reaction step onthe layer growth. He experimentally confirmed the presenceof a linear growth stage after the parabolic one (paralineargrowth kinetics) of the layers formed during the interac-tion of this stainless steel with an aluminium melt saturatedwith iron. This gives a rational explanation for the negativedeviation from the parabolic kinetics for long interactiontime.

The interface between the ferrous substrate and theFe2Al5 layer is serrated. Eggeler et al.[13] showed thatthis tongue-like morphology cannot be explained by apreferential nucleation because no influence of ferrous sub-strate’s grain size was observed. According to the sameauthor, this interface irregularity seems to conform to aplanar shape when the temperature of the aluminium meltincreases. Shyam et al.[19] and Bouché et al.[15] also con-firmed this observation. However, contrary to Bouché et al.[15], Eggeler et al.[13] state that this morphology is nottime-dependent. Heumann and Dittrich[20] give a plausibleexplanation for the Fe/Fe2Al5 interface irregularity. Thistongue-like interface is a result of favorable possibilities foraluminium atoms to diffuse along thec-axis direction of theFe2Al5 orthorhombic structure on structural vacancies. Thisshows that there is still a need to clarify the morphologyand the growth kinetics of the intermetallic phases that formwhen Fe is in contact with liquid Al, both experimentallyand theoretically. Therefore, the present work investigatesthis interaction for the binary model system Fe–Al. It isexpected that the results will provide some guidance for theinterpretation of more complex reactions when Fe-basedalloys are in contact with Al melts of industrial purity.

Table 1Chemical analysis of iron and aluminium specimens used in this investigation

Impurity (ppm) C Si Fe Cu Co Ca S Ni Mn Mg Cr

Iron ≤5 ≤10 Bal. 6.5 8.5 – ≤5 0.9 0.35 – 0.4Aluminium – 400 300 30 – 30 – – – 4 –

Fig. 1. Device used for immersion tests.

2. Experimental

Iron samples of high purity (seeTable 1) were machined tocylinders of 8-mm diameter and 7-mm height. The averagesurface roughnessRa was 2.2�m. Samples were fixed toa specimen holder by their tapping. They were pickled inan ultrasonic bath in a chemical solution made up of 50%nitric acid (5%) and 50% hydrofluoric acid (2%). A smallquantity (17 g) of pure aluminium (99.9%, seeTable 1) wascleaned of its oxide film in a 5% NaOH solution for 1 minbefore melting, followed by washing, then decontamination(neutralization) for 3 min with diluted nitric acid (d = 1.33).The aluminium was melted in an alumina crucible underargon atmosphere as shown inFig. 1. The temperature of thealuminium melt was measured by a thermocouple in contactwith the alumina crucible. The furnace was heated graduallyand controlled by a regulator that ensured a temperature ofthe melt equal to the required temperature (T = 700, 800 or900◦C). Rings and elastomer seals prevented leaks in thefurnace.

Fixed on a stainless steel holder, the iron sample was pre-heated to 200◦C before dipping. A thermocouple placed onthe specimen holder close to the substrate controlled the tem-

A. Bouayad et al. / Materials Science and Engineering A363 (2003) 53–61 55

perature. When the temperature of molten aluminium wasstabilized (rolled up resistance ensures good temperature ho-mogeneity), the sample was dipped into static molten alu-minium for a fixed timet (t = 1–45 min). The solid–liquidinteraction was interrupted by switching off the heating sys-tem and quenching the aluminized substrate in water to roomtemperature (>400 K s−1). The cylindrical sample was cutparallel to its axis (i.e. normal to the Fe–Al interface) with adiamond disc saw in order to minimize damage at the inter-face. It was then polished down to 1�m for microstructuralexamination.

The chemical reagents used for etching depend on thephases to reveal. Hydrofluoric acid (0.5%) was used for alu-minium, Nital (2%) for pure iron and NaOH (10%) for theintermetallic phases. The bimetallic samples were examinedby both optical and scanning electron microscopy (JEOL6301F). Compositions of the phases were obtained by elec-tron probe microanalysis (EPMA). An X-ray energy disper-sive microprobe (Link-Isis-Oxford 1994) was used.

3. Results and discussion

3.1. Morphology and types of intermetallic phases

In all experiments carried out, independently of immer-sion time and melt temperature, two phases were observedat the interface of pure iron with pure liquid aluminium. Thefirst intermetallic layer (adjacent to iron) was thick and pre-sented a tongue-like shape oriented towards the iron. Thesecond (adjacent to aluminium) was thin and of darker colorbecause it was etched more by the metallographic reagent.This layer presented also some irregularities on the side ofthe aluminium (seeFig. 2). The morphology of layer 1 (ad-

Fig. 2. Microstructure of the interface zone after etching (5% NaOH)revealing the presence of two phases (T = 900◦C, t = 10 min).

Fig. 3. Microstructure of the intermetallic layer formed: morphology ofthe interface with iron (T = 800◦C, t = 5 min).

jacent to pure iron) consisted of a polycrystalline structurein the part close to layer 2, while long monocrystals werepresent near the iron. These long grains seemed to grow ei-ther in transgranular or intergranular directions (Fig. 3).

Iron enrichment in the aluminium matrix zone close to theinterface with the second layer was observed (seeFig. 2).Fig. 4 shows polyhedric crystals peeled off from the inter-metallic layer in the aluminium matrix. The presence of thesecrystals can also be explained by the decrease of the ironsolubility limit in the aluminium during cooling. The occur-rence of these crystals is more favored when the cooling rateis increased, the wetting surface is large and the interactiontime is long. The origin of the tongue-like shape remainsunclear. Anisotropic diffusion (high vacancy concentrationin the c-axis of the orthorhombic structure of the formedphase) is a possible explanation for this irregularity. Indeed,

Fig. 4. Scanning electron micrograph showing crystals spalling (interactiontime 45 min at 800◦C).


the existence of cavities and pores in the intermetallic layerformed, as shown inFig. 4, corroborates this ‘directed’ dif-fusion postulate.

The intermetallic phases formed were identical to thoseobserved by other authors[15,19,21]. Indeed, the EDX mi-croprobe gave a major phase composition in the existinginterval of Fe2Al5. The composition of the minor layer cor-responded to FeAl3, regardless of the interaction time.

For the polyhedric crystals, which were observed in thealuminium matrix, several compositions were found such asFeAl2, Fe2Al5 and FeAl3. This confirmed that a number ofthese crystals had become detached from the intermetalliclayer.

3.2. Kinetics of growth of the intermetallic layers

In view of the irregularity of the interface between theintermetallic layer and the iron substrate, the thicknessquantification of the layers required the determination ofa maximum and mean layer thickness, as shown inFig. 5.Table 2 lists the various thicknesses of the intermetalliclayers.Fig. 6 shows the variation of the Fe2Al5 layer meanand maximum thickness with time at 800◦C. This curvedemonstrates a parabolic law of growth for the maximumthicknessXmax and mean thicknessXmean, except in theinitial stage of growth, which was also observed by Bouchéet al. [15].

At 800◦C, the thicknessY of FeAl3 (layer 2) is very lowcompared to that of Fe2Al5. Omitting the initial stage ofinteraction (t < 1 min), the FeAl3 layer thickness is seento increase linearly with time (seeFig. 7). This indicates aconstant growth rate.

With regard to the Fe2Al5 layer, Xmax is fitted withthe curveXmax= 13.452

√t (�m), from which a parabolic

evolution of the type (dx/dt) = (k1/x) with a constantk1 = 9.05× 10−11 m2 s−1 is obtained.

Fig. 6. Time dependence of the Fe2Al5 intermetallic layer thickness at 800◦C (maximum thicknessXmax (�) and mean thicknessXmean (�)).

Fig. 5. Schematic diagram showing the measurement method of thethickness of intermetallic layers on micrographs.

Table 2Experimental data at 800◦C

Time (min) 1 5 10 15 30 45

Xmean (�m) 90.5 159.5 207.6 287 422 512Xmax (�m) 159 257 326 423 572 672Z (dimensionless) 1.07 0.87 0.8 0.67 0.5 0.44Y (�m) 8 6.5 6.4 8 13 20

For the FeAl3 layer, a linear regression of the curveafter the transient stage of growth gives the equationy = 6.4× 10−9t + 2.217× 10−6 (m). Being linear, the ki-netics growth rate is then (dy/dt) = k2 = 6.4× 10−9 m s−1

(seeFig. 7).The evolution of the morphology of the wavy Fe/Fe2Al5

boundary with interaction time can be characterized by the


Fig. 7. Variation of the FeAl3 layer thickness with time at 800◦C.

non-dimensional ratioZ = (Xmax−Xmin)/Xmoy. This ratio al-lows us to follow the interface fluctuations evolution propor-tionally to the mean thickness.Fig. 8 displays the variationof Z at different times of interaction.

At the beginning of the interaction the tongue shapepresents a lengthened profile. The interface shows, for shortimmersion times, significant distances between peaks andvalleys. These distances tend to decrease, in proportionto the mean thickness, with increasing interaction time(Fig. 8). However, this reduction in tongue size seems tolessen with time and to stabilize around a ratioZ = 0.6.This shows that the tongue-like shape does not disappearfor long interaction times and consequently is characteristicof this interface between pure iron and the Fe2Al5 layer.

Fig. 8. Morphology evolution of Fe/Fe2Al5 interface at 800◦C: Z characterizes the wavy boundary.

The results of this study on Fe/Fe2Al5 interface morphol-ogy, rarely mentioned in literature, are in agreement withthe results obtained by Bouché et al.[15]. The latter affirmthat the disturbance of this interface, appearing from thebeginning of the interaction, spreads continuously duringthe growth of the layer.

The intermetallic layer growth kinetics, as a function oftemperature, was studied for a fixed immersion duration of10 min. The thicknesses of the layers formed were measuredat 700, 800 and 900◦C. The Fe2Al5 layer thickness wasfound to increase with temperatureT according to the lawof Arrhenius:

D = D0 exp

(− Q

RT

), (1)


Fig. 9. Relationship between ln(X/t1/2) (Xmax (�) and Xmean (�)) and 1/T for the Fe2Al5 layer at a fixed immersion time of 10 min.

whereD is the diffusion coefficient,D0 a frequency factor,Qthe apparent activation energy of the process,T the absolutetemperature andR the gas constant.

This relationship, only valid under a diffusional regime ofgrowth, is indirectly verified by the evaluation ofQ. Fig. 9shows the relationships between the logarithm of (X/t1/2)and the reciprocal of the absolute temperature 1/T; X is themaximum Xmax or mean thicknessXmean of Fe2Al5. Theapparent activation energies obtained for the maximum andmean thickness of the Fe2Al5 layer were 73.2 and 74.1 kJmol−1, respectively. Our apparent activation energy valuesobtained forXmax andXmeanwere almost identical, contraryto the observations of Eggeler et al.[13] and Heumann andDittrich [20] (seeTable 3). These authors explained the dif-ference by the temperature dependence of the ratio of themaximum to the mean thickness. One observes also that thevalue obtained by Eggeler et al.[13] for Q with Xmax isgreater than that obtained withXmean, whereas the contraryis observed by Heumann and Dittrich[20]. The different val-ues found by these authors cannot only be caused by the pu-rity degree of the metals used (low alloyed steel by Eggeleret al.[13]). These are also dependent on the immersion timeat which the effect of temperature is studied. Actually, atlong interaction times and high temperatures, the spallingof the intermetallic layer affects the measurement of layerthickness[8,13]. Consequently, this may have an importantinfluence on the apparent activation energy evaluation.

Moreover, Denner and Jones[8] affirm that carbon pres-ence in iron (low alloyed steel) has a significant influenceon the apparent activation energy evaluation. The latter in-

Table 3Values of the apparent activation energyQ obtained by several authors[8,13,20]

Author Denner and Jones[8] Heumann and Dittrich[20] Eggeler et al.[13] Present study

With Xmax With Xmean With Xmax With Xmean with Xmax with Xmean

Q (kJ mol−1) 155 58.8 76.1 155 134 73.2 74.1

creases with increasing carbon content up to 0.17%. This ex-plains the large difference between the experimental valuesobtained in this study and those by Eggeler et al.[13], sincethese authors used a low alloyed steel containing 0.16% C.

For the FeAl3 layer, the same variation is observed con-cerning the growth of the intermetallic layer when the tem-perature increases. It is noticed that FeAl3 thickness (6.4�m) does not change when the temperature increases from700 to 800◦C. On the other hand, the thickness of this layerextends considerably (17�m) when the temperature rises to900◦C.

Irregularities in the shape of the interface Fe/Fe2Al5, char-acterized by the ratioZ, present little change with tempera-ture (1.19 for 700◦C, 1.19 for 800◦C and barely reaching1.16 for 900◦C). Hence, it seems that temperature has noappreciable effect on the Fe/Fe2Al5 interface perturbation.This observation confirms the activation energy evaluationfor Xmax andXmean.

3.3. Growth of the intermetallic phases: theoreticalapproach

Among the models, which describe growth of intermetal-lic layers formed when a liquid metal reacts with a solid sub-strate, Dybkov’s model[10] remains most suitable to be ap-plied in the case under consideration. This model deals withthe growth of two chemical compound layers at the interfacebetween two elementary substances. This corresponds to thegrowth of Fe2Al5 and FeAl3 layers at the interface betweenpure iron and pure liquid aluminium. After an initial stage


of growth (which was difficult to study experimentally andafter which saturation of the melt region near the interfacewas supposed to reach), this model predicted a ‘paralinear’stage of growth. The Fe2Al5 and FeAl3 growth kinetics aredescribed, respectively, by the equations:

dx

dt= k1B1 + k1A2

x− rg

pk0B2, (2)

dy

dt= k0B2 − q

sg

k1A2

x. (3)

If A is Fe, B is Al, ApBq is Fe2Al5 and ArBs is FeAl3, onecan interpret the various coefficients as follows:

k1A2: rate constant of the Fe2Al5 layer growth by Fediffusion at the interface with FeAl3: Fe (diffusing)+5FeAl3 → 3Fe2Al5.

k1B1: rate constant of the Fe2Al5 layer growth byAl diffusion at the interface with Fe: 2Fe+ 5Al(diffusing)→Fe2Al5.

k0B2: growth rate under conditions of reaction control ofthe FeAl3 layer by Al diffusion at the interface wherethe following chemical reaction takes place: Al (diffus-ing)+ Fe2Al5 → 2FeAl3.

Therefore,r = 1, s = 3, p = 2, q = 5. g is the ratio ofmolar volumes of Fe2Al5 and FeAl3. According to Dy-bkov, the densities of the compounds areρ (Fe2Al5) = 4100kg m−3 and ρ(FeAl3) = 3800 kg m−3, hence the mo-lar volumes areV(Fe2Al5) = 6× 10−5 m3 mol−1 andV(FeAl3) = 3.6× 10−5 m3 mol−1. Thus,g = 1.7.

ThenEqs. (2) and (3)become

dx

dt= k1B1 + k1A2

x− 0.85k0B2, (4)

dy

dt= k0B2 − 0.98

k1A2

x. (5)

For largeX these two equations are reduced to

dx

dt= k1B1 + k1A2

x, (6)

dy

dt= k0B2. (7)

These simplifications, which are not obvious initially,will be justified hereafter. This implies parabolic kinetics ofgrowth for the Fe2Al5 layer (Eq. (6)) and linear kinetics forFeAl3 (Eq. (7)). The ‘paralinear’ model of Dybkov is in goodagreement with experimental observations of the growth ki-netics of these layers except for the transient regime ofgrowth (seeFigs. 8 and 9). Using the experimental data forthe Fe2Al5 growth kinetics, described by a parabolic equa-tion of the type dx/dt = k1/x, with k1 = 9.05× 10−11 m2 s−1,we deduce the value of

k1B1 + k1A2 = 9.05× 10−11 m2 s−1. (8)

For the FeAl3 layer, for which the kinetics is linear, wefind

k0B2 = 6.4 × 10−9 m s−1. (9)

At this stage, we can justify the validity of assumptions ofthis theoretical approach (for the general assumptions referto Dybkov’s study[10]).

The kinetic equation (Eq. (6)) is valid only if

k1B1 + k1A2

x rg

pk0B2,

hence

x � k1B1 + k1A2

(rg/p)k0B2.

Inserting the corresponding values givesx � 0.0166 m. Thiscondition is satisfied because the Fe2Al5 thickness neverreached the value of 1 mm.

For the second assumption, which concerns equation(Eq. (7)), we should verifyx (q/sg)(k1A2/k0B2). It is moredifficult to do because the value ofk1A2 is unknown. If wereconsider the definitions of the different coefficients, wenote thatk1B1 k1A2 becausek1A2 is the Fe2Al5 growthrate by Fe diffusion at the interface with FeAl3. The dif-fusing element iron will then combine with FeAl3 to giveFe2Al5. k1B1 is the Fe2Al5 growth rate by Al diffusion atthe interface with Fe. These two elements will then formFe2Al5.

All the preceding studies show that the element Fe dif-fuses more slowly than Al[15,22]. Thus, it is clear thatiron needs to pass across the entire Fe2Al5 layer to meetFeAl3, whereas Al (which diffuses more quickly) will tra-verse almost the same distance because of the very smallthickness of FeAl3. Hence, faster diffusion of Al thanFe allows us to conclude thatk1B1 must be much higherthank1A2. This statement is also confirmed experimentallyby the perturbation of the Fe/Fe2Al5 interface, which de-creases with interaction time. Bouché et al.[15] reporteda ratio of DAl /DFe= 10−6 at 800◦C in agreement withseveral bibliographical data mentioned. Thus,k1A2 can beoverestimated byk1B1/100. Moreover, as we only knowthe value of the sumk1B1+ k1A2 = 9.05× 10−11 m2 s−1

and k1B1 k1A2, the higher value achieved byk1A2 is(k1A2)max= k1B1/100∼=9.05× 1013 m2 s−1.

The second assumption is satisfied ifx (q/sg)(k1A2/k0B2).To ensure this, it is sufficient to havex (q/sg)((k1A2)max/k0B2), hencex 1.39× 10−4 m. That is largely satisfied bythe model validity range.

Besides, it is difficult to compare the experimental valuesk1 andk2 with those available in the literature because themodels used are not the same. However, for the Fe2Al5layer where the kinetics of growth is almost parabolic,one can compare the data found with those of other au-thors (seeTable 4). Heumann and Dittrich[20] foundk1 = 10−10 m2 s−1 at 700◦C. Denner and Jones[8] founda value of k1 = 1.394× 10−10 m2 s−1 for an exponent


Table 4Comparison of kinetic constants obtained by several authors[8,13,14,20]

Author Heumann andDittrich [20]

Eggeler et al.[13] Denner and Jones[8] Bouch́eet al. [14]

Present study

Temperature (◦C) 700 786 771 800 800Purity degree of iron Pure iron Low-alloyed steel 99.78% 99.78% 99.999%Purity degree of aluminium Iron-saturated melt Iron-saturated melt 99.99% 99.997% 99.9%Fe2Al5 kinetic fitting

method forXmax

x = √2k1t x = √

2k1tn (n = 0.49) x = √

2k1tn (n = 0.45) x = √

2k1t x = √2k1t

k1 (m2 s−1) 10−10 5.64× 10−11 1.394× 10−10 2.645× 10−12 9.05× 10−11

n = 0.45 (instead of 0.5 corresponding to parabolic kinet-ics) at a temperature of 771◦C. Eggeler et al.[13], for aniron-saturated aluminium melt at a temperature of 786◦C,found k1 = 5.64× 10−11 m2 s−1 for an exponent of 0.49.Furthermore, Bouché et al.[14] found the growth constantk1 = 2.645× 10−12 m2 s−1 at 800◦C.

The discrepancies observed can be explained by differentexperimental conditions (iron saturated melt, metal purity,variation of melt temperatures). This is also due to the dif-ferent methods of determiningk1. Depending on whetherwe suppose a perfectly parabolic kinetics dx/dt = k1/x,hencex = √

2k1t (exponent 0.5), or we fit the data to anequation of the formx = ktn, wheren is close to 0.5, thevalue ofk1 can change.

Dybkov’s model was used to identify the Fe2Al5 growthkinetics by taking into account a chemical reaction stageomitted in the other models. For moderate interaction timesthe kinetics, which is of industrial interest in manufacturingbimetallic pieces, is parabolic. It is, therefore, in agreementwith the experimental observations and the preceding stud-ies. For the FeAl3 layer, Dybkov’s theory envisaging linearkinetics was confirmed by experimental measurements. Thisimportant result is rarely mentioned in the literature becauseof the low thickness of FeAl3.

4. Conclusion

Two intermetallic layers were observed to form duringthe interaction between pure solid iron and pure liquidaluminium. The major phase was Fe2Al5 and the otherFeAl3. The growth kinetics was determined experimentallyat temperatures between 700 and 900◦C. After a shorttransient period, the Fe2Al5 layer growth obeys a paraboliclaw, while the FeAl3 layer growth is linear. The theoreti-cal approach of Dybkov, which models the simultaneousgrowth of two intermetallic layers at the interface of twoelementary substances by taking into account both dif-fusion and chemical reaction processes, was successfullyapplied to the Fe–Al system. Parabolic and linear kineticconstants were determined for both layers formed. Theapparent activation energy for the Fe2Al5 layer growthprocess was evaluated and compared with the results ofother authors. It was found to be 73.2 kJ mol−1 for the

Fe2Al5 maximum thickness and 74.1 kJ mol−1 for the meanthickness.

The acquired knowledge on the interaction of liquid alu-minium and solid iron will allow manufacturing of bimetal-lic pieces (high-pressure die casting of motor bedplates)under well-controlled conditions.

Acknowledgements

The authors wish to thank the industrial partner of thisstudy, Fonderie Aluminium de Cléon (TEKSID group)represented by A. Valette, for financial and experimentalsupport.

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Al Fe Interaction 2003