j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 88–93

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Growth of intermetallic layer at roll bondedIF-steel/aluminum interface

Vikas Jindala,∗, V.C. Srivastavab

a Punjab Engineering College, Chandigarh 160012, Indiab National Metallurgical Laboratory, Jamshedpur 831007, India

a r t i c l e i n f o

Article history:

Received 10 October 2006

Received in revised form

1 February 2007

Accepted 18 April 2007

Keywords:

a b s t r a c t

The solid-state reactive diffusion between IF-steel and Al was experimentally studied using

IF-steel/Al/IF-steel diffusion couples. The specimens were prepared by a roll bonding tech-

nique and then annealed at temperatures 773 K for different time span. At the IF-steel/Al

interface in the annealed diffusion couple, wavy layer of Al5Fe2 was observed. The average

thickness (Tavg) of Al5Fe2 layer monotonically increases with increasing annealing time (t)

according to the equation Tavg = k(t)n, where t is time in second. Value of n = 0.5 indicates

that interdiffusion through aluminide phase is the rate controlling step. During annealing,

IF-steel matrix has undergone recrystallization. Microstructure and hardness measurement

Roll bonding

Steel/aluminum

Diffusion

Al5Fe2

of IF-steel shows that recrystallization process completes within 30 min. Effective heat of

formation theory has been applied to predict phase formation sequence during annealing

of IF-steel/Al/IF-steel diffusion couples.

© 2007 Elsevier B.V. All rights reserved.

As diffusion phenomenon depends on annealing conditions,

EHF theory

1. Introduction

Intermetallic compounds are material of considerable inter-est because of their high temperature strength, low densityand high creep resistance. However, application of inter-metallics has been limited due to their highly brittle behaviorat ambient temperature (Stoloff et al., 2000). Whereas, ifintermetallics are complemented with ductile metallic layer,together they offer a combination of good strength and tough-ness. Based on this idea, a new class of structural materialknown as metal-intermetallic laminate (MIL) composites havebeen developed (Harach and Vecchio, 2001). MILs based onTi–Al3Ti and Ni–Al3Ni have attracted considerable attentionin the recent past (Harach and Vecchio, 2001; Peng et al., 2005;

Luo and Acoff, 2004; Kim et al., 2005). However, steel being oneof the important and widely available cheap structural mate-rial, can be a potential candidate for the development of cost

∗ Corresponding author. Tel.: +91 172 2753957; fax: +91 172 2745175.E-mail addresses: vijindal@gmail.com (V. Jindal), vcsrivas@yahoo.co

0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2007.04.118

effective MIL composite. No significant amount of research insteel–Al based MILs has been reported so far.

Generally, synthesis of MILs is accomplished by a sequenceof temperature and pressure steps on the alternate stacks ofM (Ti, Ni) and Al layer (Harach and Vecchio, 2001; Peng et al.,2005; Luo and Acoff, 2004; Kim et al., 2005). Thickness of Allayer is so chosen that all the aluminum is consumed in thechemical reaction leading to the formation of intermetalliclayer. The thickness of the intermetallic layers in the MILsplays an important role in obtaining optimum properties. Itis established that the growth of the intermediate phases canbe governed by chemical reactions at the interfaces and byinterdiffusion of reacting species through different phases.

m (V.C. Srivastava).

i.e. temperature and time, type of aluminide phase and itsthickness can be tailored by employing appropriate annealingconditions.

g t

mibag1i

2

TsIiTitaIatottow7iwarls5reHrts

tfAnttto

2

SiSmie

ther, they coalesce forming a continuous layer. Fig. 2d–f clearlyindicate this tendency. Similar trend continues after 150 and240 min as shown in Fig. 2g and h. The aluminum layer, whichis sandwiched initially between two IF-steel layers, continue

j o u r n a l o f m a t e r i a l s p r o c e s s i n

In the present work, interaction between IF-steel and alu-inum is examined and the kinetics of diffusion driven

ntermetallic compound layer formation, which are grownetween IF-steel and aluminum, is studied. There exist severalluminides of iron that can form based on the Al–Fe phase dia-ram. Effective heat of formation (EHF) theory (Pretorius, 1984,990, 1996) has been applied to predict phases that may formn such diffusion controlled situation.

. Experimental details

he study was conducted after roll bonding layers of IF-teel and Al foils. Al foils were sandwiched between twoF-steel sheets and warm rolled so as to achieve good bond-ng between the two materials. In the present investigation,i-added IF-steel was chosen to avoid blue brittleness dur-

ng rolling. The other reason for taking IF-steel is the facthat it is a very important commercial material availables thin sheets. Cold-rolled and fully annealed 1 mm thickF-steel sheets (30 mm × 90 mm) and commercially availableluminum foils (120 �m thick) of matching dimensions wereaken in the initial stack. To achieve good bonding, surfacesf sheets as well as foils were wire-brushed and degreasedo remove oxide layer and surface contamination, respec-ively. To avoid slip during rolling, leading and trailing edgesf the stack were fastened by steel wires. Stacked assemblyas soaked in a resistance furnace set at a temperature of

73 K for 300 s and then rolled. Rolling operation was donen a single pass without lubricant, using a two-high mill

ith roll diameter of 300 mm. This gave rise to good work-bility and bonding. Warm rolling (at 773 K) also avoidedecrystallization and associated recovery of the accumu-ated strain. The reduction in stack thickness per pass waslightly more than 50% (equivalent strain of about 0.8). A0% reduction in stack thickness does not indicate a 50%eduction in individual layer thickness. The Al foil experi-nces larger thickness reduction compared to IF-steel sheet.owever, two steel sheets do undergo same reduction. This

eduction was found sufficient to make proper surface con-act and to break any remaining oxide layer on the joiningurfaces.

Roll bonded samples were taken for annealing so as to syn-hesize Al–Fe intermetallic layer. Annealing was done at 773 Kor various length of time, i.e. 10, 20, 30, 60, 90, 150 and 240 min.s the surfaces had already made good contact, pressure wasot applied during annealing treatment. Annealing condi-ions were chosen keeping in view the following objectives:o study (1) reaction synthesis of intermetallic compounds athe IF-steel and aluminum interface and (2) recrystallizationf IF-steel.

.1. Characterization

pecimens of the materials were taken at various process-ng stage for microstructural study and mechanical testing.

ample surfaces were mechanically polished using standardetallographic technique. Microstructural features were stud-

ed using optical microscopy, image analysis and scanninglectron microscopy (SEM). Grain size analysis was carried out

e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 88–93 89

by the standard linear intercept method using an image analy-sis system (Metal Power, India). Scanning electron microscopicstudies were carried out on SEM (JSM-840A, JEOL, Japan). X-ray diffraction (XRD) patterns of the phase grown at interfacewere obtained using Siemens XRD system (D-500, German’smake) with Co K� radiation. Micro-hardness tests were con-ducted using Leica VMHTAUTO model using standard Vickersscale.

3. Results and discussion

3.1. Kinetics of aluminide growth

Growth kinetics of aluminide was studied by measuring thethickness of aluminide layer with annealing time. The thick-ness measurement scheme of layers is shown in Fig. 1.However, owing to the non-uniform interface morphology ofthe Fe2Al5 phase (as determined by X-ray analysis), measure-ments were done at a number of locations (more than 20) tohave an estimate of average layer thickness. In Fig. 1, T1 isthe aluminide layer thickness. The loss of aluminum (shownas T2) is defined by the difference between the initial thick-ness (T2I) and final thickness (T2F) of aluminum layer. Giventhe symmetry of stack arrangement, the loss in aluminum istaken half of the difference. Similarly, loss of IF-steel (T3) layeris defined. For the purpose of calculation, only the mean valuesof the thickness have been used.

Fig. 2 shows the growth of aluminide layer in white con-trast at IF-steel and aluminum interface with annealing time.Oxide particles are seen at the interface in the dark grey con-trast in Fig. 2a. This forms when the oxide layer on the Alfoil ruptures during rolling. With increasing annealing timeit appears that these oxide particles hinder further growth ofthe aluminide layer. This can be seen clearly in Fig. 2b andc, where aluminide layer is broken wherever there are oxideparticles. As this discontinuous aluminide layer grows fur-

Fig. 1 – Scheme of layer thickness measurement.

90 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 88–93

and

Fig. 2 – Cross-section micrographs of specimens (a) initially60 min, (e) 90 min, (f) 150 min and (g) 240 min.

to shrink due to chemical reaction at IF-steel/aluminide/Alinterfaces. Almost the entire aluminum layer is consumedafter 240 min (Fig. 2h), where growing aluminide layers fromboth the sides came in contact with each other. The IF-steel layer is also consumed in the process, as evident fromoutward (towards IF-steel) growth of aluminide layer. A quan-titative analysis of this growth/consumption is presentedin Table 1.

The growth of aluminide layer with time is shown inFig. 3. Within the limits of experimental error, this curvefollows a power law. Aluminide layer thickness (in �m) is

0.2602t0.5, where t represents time in seconds. This con-firms that aluminide growth is controlled by interdiffusionthrough intermetallic phase. The growth constant was calcu-lated using the aluminide layer thickness growth with time.

Table 1 – Fe2Al5 layer Thickness, Al layer consumed, Fe layer co

Temperature (K) Time (s) Fe2Al5 layerthickness T1 (�m)

773

0 0600 7.02

1,200 9.241,800 10.963,600 17.185,400 21.929,040 25.32

14,400 28.1

annealed at 773 K for (b) 10 min, (b) 20 min, (c) 30 min, (d)

This follows the parabolic equation (Jindal et al., 2006):

d �x

dt= k

�x

This can also be written as (for the case that �x = 0 at t = 0):

(�x)2 = 2kt

where �x is the aluminide layer thickness, k is the parabolicrate constant and t is annealing time. The value of k was foundto be 3.38 × 10−14 m2/s. This low value of growth constant canbe attributed to lower diffusion temperature. An interesting

conclusion can be drawn from the growth of aluminide withrespect to aluminum consumption. This gives an idea of therelative rate of aluminum consumption with respect to alu-minide layer growth. As can be seen in Fig. 4, for initial 30 min

nsumed with time

Al layer consumedT2 (�m)

Fe layer consumedT3 (�m)

T2/T1

0 06.065 0.955 0.867.395 1.845 0.808.91 2.05 0.819.505 7.675 0.55

12.315 9.605 0.5615.475 9.845 0.6118.165 9.935 0.65

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 88–93 91

Fig. 3 – Relationship between thickness of aluminide layerand square root of diffusion time for the specimensannealed at 773 K.

Fig. 4 – Ratio of aluminide layer formation and loss inaluminum layer with time.

Fig. 5 – Microstructure of IF-steel matrix in (a) as rolled condition30 min, (d) 60 min and (e) 90 min.

Fig. 6 – Variation of hardness of IF-steel matrix with time.

this ratio varies between 0.80 and 0.85, whereas after 30 minthis gives an average value of 0.60. The drop in aluminum con-sumption is an indication of a change in reaction mechanism.This aspect of interdiffusion mechanism will be studied in ourfuture experiments.

3.2. Recrystallization and hardness variation withannealing time

IF-steel matrix has undergone recrystallization during anneal-ing because of stored strain energy. This is evident from themicrostructural analysis and also from the decrease in hard-ness as shown in Figs. 5 and 6, respectively. Fig. 5a shows

elongated grains in rolling direction. Due to recrystallizationafter 10 min (Fig. 5b), new grains start growing at the cost ofelongated grains (Fig. 5c). After 30 min (Fig. 5d), most of therecrystallization process is complete and the microstructure

and after annealing at 773 K for (b) 10 min, (b) 20 min, (c)

92 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 88–93

Fig. 8 – XRD pattern taken from intermetallic compoundlayers formed by annealing treatment.

sented in Fig. 10. It can be seen that Al13Fe4 has lowest effectivefree energy of formation. However, XRD analysis did not showthe presence of Al13Fe4. This can be attributed to the second

Fig. 7 – SEM micrograph of IF-steel/aluminum interface.

remains unchanged thereafter (Fig. 5e and f). The hard-ness value of IF-steel follows similar pattern, its value hasdecreased from 270 to 140 HV0.05 within 30 min and remainednearly constant with further increase in annealing time.

3.3. Interface characterization (XRD and SEM)

Prior to annealing treatment, the interface of roll bondedsample has been examined in SEM. The nature of this IF-steel/aluminum interface is shown in Fig. 7. This appearsas white contrast interface between two layers. The averagevalue of EDX analysis at three points of this interface is shownin Table 2. This shows that a sound bonding between IF-steeland aluminum has been achieved during rolling treatment. AnXRD analysis was carried out to identify the phases formedat the interface after annealing treatment. Fig. 8 shows theXRD pattern revealing the presence of peaks corresponding toAl5Fe2. The microhardness of the aluminide phase has beentaken at 50 g load. The micrograph shows indentation on alu-minide and IF-steel (Fig. 9). The microhardness of aluminidephase was found to be 1050 HV0.05. Kobayashi and Yakou (2002)have also found similar value for Al5Fe2.

3.4. EHF and first phase formation related

thermodynamics

Effective heat of formation theory has been applied to predictphase formation under similar kinetics situations (Pretorius,

Table 2 – Average concentration of Al and Fe atIF-steel/Al interface

Element wt.% at.%

Al 25.3 41.21Fe 74.7 58.79

Total 100 100

Fig. 9 – Micrographs of indentations on intermetallic layerand IF-steel matrix under 50 g load.

1984, 1990, 1996). According to this theory, the first phase thatthat forms during metal–metal interaction is the phase withthe most negative heat of formation at the concentration ofthe lowest eutectic of the binary system. Furthermore phases,formed initially, react with each other to form a phase with acomposition between that of the interacting phases and clos-est to that of the lowest eutectic composition (Pretorius et al.,1993). Aluminum–Iron binary phase diagram has the lowesteutectic at 0.02% Fe. At this composition, the effective freeenergy of formation has been calculated at 773 K. This is repre-

Fig. 10 – Variation of effective free energy of formation ofFe4Al13, Fe2Al5 and FeAl2 with Fe concentration.

g t

piwpa

4

(

(

(

r

j o u r n a l o f m a t e r i a l s p r o c e s s i n

ostulate of the EHF theory. Based on this, we propose thatnitial Al13Fe4 phase (which is expected thermodynamically)

ill react with the adjacent iron rich phase to form a thirdhase whose stoichiometry will be between Al13Fe4 and �-Fend closest to the lowest eutectic, i.e. Fe2Al5.

. Conclusions

1) The solid-state reactive diffusion between IF-steel and Alwas experimentally studied using IF-steel/Al/IF-steel dif-fusion couples. At the IF-steel/Al interface in the annealeddiffusion couple, a wavy layer of Al5Fe2 was observed.

2) The average layer thickness of Al5Fe2 monotonicallyincreases with increasing annealing time t according tothe equation, Tavg (�m) = 0.2602(t)0.5, where t is time in sec-ond. Growth constant was found to be 3.38 × 10−14 m2/s.The value of time exponent (=0.5) indicates that interdif-fusion through aluminide phase is the rate-controllingstep. During annealing, IF-steel matrix undergoesrecrystallization.

3) Microstructure and hardness measurement of IF-steelshow that recrystallization process completes within30 min. The microhardness of aluminide phase was foundto be 1050 HV0.05.

e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 88–93 93

Acknowledgements

The authors would like to thank Prof. S.P. Mehrotra, Director,National Metallurgical Laboratory, Jamshedpur, for his kindpermission to publish this paper.

e f e r e n c e s

Harach, D.J., Vecchio, K.S., 2001. Metal. Mater. Trans. 32A,1493–1505.

Jindal, V., Srivastava, V.C., Das, A., Ghosh, R.N., 2006. Mater. Lett.60, 1758–1761.

Kim, H.Y., Chung, D.S., Hong, H.H., 2005. Mater. Sci. Eng. A 396,376–384.

Kobayashi, S., Yakou, T., 2002. Mater. Sci. Eng. A338, 44–53.Luo, J.-G., Acoff, V.L., 2004. Mater. Sci. Eng. A 379, 164–172.Peng, L.M., Wang, J.H., Li, H., Zhao, J.H., He, L.H., 2005. Scripta

Mater. 52, 243–248.Pretorius, R., 1984. Mater. Res. Soc. Proc. 25, 15–20.Pretorius, R., 1990. Vacuum 41, 1038–1042.

Pretorius, R., 1996. Thin Solid Films 290/291, 477–484.Pretorius, R., Marais, T.K., Theron, C.C., 1993. Mater. Sci. Eng. R10,

1–83.Stoloff, N.S., Liu, C.T., Deevi, S.C., 2000. Intermetallics 8,

1313–1320.