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Journal of Colloid and Interface Science 285 (2005) 201–205www.elsevier.com/locate/jcis

Interface characteristics in diffusion bonding of Fe3Al with Cr18-Ni8stainless steel

Juan Wanga,∗, Yajiang Lia,b, Yansheng Yina

a Key Lab of Liquid Structure and Heredity of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University,Jinan 250061, People’s Republic of China

b National Key Lab of Advanced Welding Production Technology, Harbin Institute of Technology, Harbin 150001, People’s Republic of China

Received 16 September 2004; accepted 27 October 2004

Available online 8 January 2005

Abstract

Fe3Al and Cr18-Ni8 stainless steel were diffusion-bonded in vacuum and a Fe3Al/Cr18-Ni8 interface with reaction layer was formeMicrostructure in the reaction layer at Fe3Al/Cr18-Ni8 interface was analyzed by means of scanning electron microscope (SEM) and eprobe micro-analyzer (EPMA). The growth of reaction layer with heating temperature (T ) and holding time (t) was researched. The resuindicate that FeAl, Fe3Al, Ni 3Al, and α-Fe (Al) solid solution are formed in the reaction layer. These phases are favorable to promelement diffusion and to accelerate the formation of the reaction layer at Fe3Al/Cr18-Ni8 interface. The growth of reaction layer obeysparabolic law and its thickness (X) is expressed byX2 = 7.5× 10−4 exp(−83.59/RT )(t − t0). 2004 Elsevier Inc. All rights reserved.

Keywords: Fe Al/Cr18-Ni8 interface; Diffusion bonding; Reaction layer; Characteristics

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1. Introduction

Fe3Al intermetallics have high hardness and excellentsistance to abrasion, oxidization, and corrosion becausits special DO3 ordered superlattice structure[1–3]. It isexpected that Fe3Al intermetallics can be applied to petrchemical industry, pressure vessel, electric power, anon. In recent years, the plasticity and toughness of Fe3Alintermetallics have been promoted greatly and the percage elongation at room temperature has been up to 8–10controlling alloy composition and improving heat machinitechnology[4,5]. If the joining of Fe3Al intermetallics andcommonly used Cr18-Ni8 austenitic stainless steel is rized and a composite structure with above materials is mfactured successfully, advantages in economy and propewill be attained fully and mutually.

* Corresponding author.E-mail address: jwang@sdu.edu.cn(J. Wang).

0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2004.10.071

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The vacuum diffusion bonding technique has progresincreasingly with the development of computer and vacutechniques, and it is used in the joining of brittle mateals and dissimilar materials[6,7]. Fe3Al intermetallics andCr18-Ni8 austenitic stainless steel are bonded by diffusbonding technology, and cracks due to welding couldavoided. This will be the key to promoting the applicationFe3Al intermetallics in aviation, petrochemistry, and electpower.

During diffusion bonding, an interface with a reactilayer can be formed between substrates. The reacted pin the interface and the thickness of the reaction laare the key to determine the performance in the inface[8,9]. In this paper, Fe3Al and Cr18-Ni8 stainless steewere diffusion-bonded in vacuum. Microstructure and ccentration in the reaction layer at Fe3Al/Cr18-Ni8 interfacewere analyzed by means of a scanning electron micros(SEM) and an electron probe micro-analyzer (EPMA). T

relation between the thickness of reaction layer and tech-nological parameters during bonding and the growth of re-action layer were studied. The results will provide experi-

202 J. Wang et al. / Journal of Colloid and Interface Science 285 (2005) 201–205

Table 1Chemical compositions and thermophysical properties of Fe3Al intermetallics and Cr18-Ni8 steel

Chemical compositions (wt%)Base metals Al Cr Nb Ni Zr B Ce S P Fe

Fe3Al 16.5 2.5 1.0 – 0.1 0.04 0.15 – – Bal.Cr18-Ni8 – 18.21 – 9.43 – – – 0.03 0.03 Bal.

Thermophysical propertiesBasemetals

Structure Order criticaltemperature (K)

Young’smodulus (GPa)

Meltingpoint (K)

Coefficient of heatexpansion (10−6 K−1)

Density(g cm−3)

Tensilestrength (MPa)

Elongation(%)a

Hardness

Fe Al DO 813 140 1813 11.5 6.72 455 3 �30 HRC

3 316.7

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tgr to

Cr18-Ni8 – – – –

a Elongation refers to the ratio of the increase in length (subtracting

Fig. 1. Assemble and position of samples in the vacuum chamber

mental and theoretical basis on joining of Fe3Al with othermaterials.

2. Experimental

2.1. Materials and interface preparation

Materials used in the test are Fe3Al intermetallics andCr18-Ni8 austenitic stainless steel. Fe3Al intermetallics wasmelted by the vacuum induction furnace and then fabricainto plate used for bonding with Cr18-Ni8 steel. The cheical compositions and thermophysical properties of Fe3Alintermetallics and Cr18-Ni8 steel are listed inTable 1. Fe3Alintermetallics sample was machined into the dimensio100× 30× 20 mm3 and Cr18-Ni8 steel sample was 100×30× 10 mm3.

The oxide film on the sample surface should be elinated by a series of treatments including grounding by spaper, acetone dipping, alcohol cleaning, then water fling and dry. The samples were put into the vacuum chber and the position in the vacuum chamber is shownFig. 1.

The diffusion bonding was immediately carried out in4.5 × 10−4 Pa vacuum at heating temperatures (T ) rang-ing from 1000 to 1080◦C for a holding time (t) from 15to 80 min. The heating rate is 12◦C/min in each run. The

◦

chamber temperature wan cooled 100C by circulating wa-ter, followed by furnace cooling. After the diffusion bond-ing, the interface sample was cut from a diffusion-bonded

8.03 520 40 70 HRB

al length from the total length of broken specimen) to the original lengt.

joint for microstructure observation and the researchgrowth behavior.

2.2. Characterization

The interface samples were ground by a series of tyof sandpaper and then polished and finally etched wisolution consisting of 70% HCl and 30% HNO3. The mi-crostructure in the reaction layer at the Fe3Al/Cr18-Ni8 in-terface was observed by means of JXA-840 scanning etron microscopy (SEM). Element concentration from Fe3Alintermetallics to Cr18-Ni8 steel across the interface wmeasured with JXA-8800R electron probe micro-analy(EPMA). The growth of reaction layer was researchedparabolic equation.

3. Results and discussion

3.1. Microstructure in the interface

Atoms from substrates diffuse continuously towardFe3Al/Cr18-Ni8 interface during bonding under the actiof heating and concentration grads. When elements contration is to certain extent, the elements will react mututo produce new phase forming reaction layer, in whichstructure is different from that in substrates.Fig. 2 showsfeatures of the reaction layer at Fe3Al/Cr18-Ni8 interface.

With the enhancing of heating temperature and holdtime, the thickness of reaction layer increases and thecrostructure in the layer is becoming coarse. In order toalyze the reacted phase, element concentration from F3Alintermetallics to Cr18-Ni8 steel across the Fe3Al/Cr18-Ni8interface was measured by means of EPMA. The measlocation and results are shown inFig. 3.

Al concentration increases abruptly due to the effecCr and Ni in the reaction layer near the side of Fe3Al. Alconcentrates in the small region of Fe3Al side and its contenis more than that of Fe3Al. Thus, FeAl is produced accordinto iron–aluminum phase diagram. From the reaction laye

Cr18-Ni8 steel, Al, Fe concentrations decrease and Ni, Crconcentrations increase gradually to form Ni3Al and α-Fe(Al) solid solution. Ni3Al is the reacted phase of Ni from

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Fig. 2. Feature of the reaction layer at the Fe3Al/Cr18-Ni8 interface:(a) 1020◦C× 60 min, (b) 1040◦C× 60 min.

Cr18-Ni8 steel and Al from Fe3Al. The diffusion coefficientof Al is small, so that Al concentration in the region of Cr1Ni8 steel side is quiet low. Finally,α-Fe (Al) solid solutioncan be formed after cooling.

3.2. Growth of diffusion reaction layer

Atom diffusion in the interface is a nonstabilized dynamprocess during diffusion bonding[10,11]. The diffusion dis-tance of Al, Fe, Ni, and Cr in Fe3Al/Cr18-Ni8 interface withdifferent heating temperature and holding time was msured with EPMA.Fig. 4 shows the relation between diffusion distance and square root of holding time at temperaranging from 1000 to 1060◦C.

The diffusion distance and square root of holding timeFe3Al/Cr18-Ni8 interface are approximately linear relatioship. The good linear relationship indicated the growthreaction layer obeys the parabolic law

(1)x2 = Kp(t − t0),

in which x is diffusion distance (µm),Kp is parabolic con-stant (µm2/s), t is holding time (s),t0 is latent period (s).

Parabolic constantKp increases from 1000 to 1060◦C.It is well known that parabolic constantKp is related to theelement diffusion coefficient at certain temperature. Difsion distance is also expressed by the diffusion coefficienD

according to the empirical formulas put forward by Arrhnius[12],

(2)x2 = 2

C�CD(t − t0),

i

in which x is diffusion distance (µm),�C is concentra-tion difference on the sides of interface (%),Ci is element

erface Science 285 (2005) 201–205 203

Fig. 3. EPMA for the reaction layer at Fe3Al/Cr18-Ni8 interface: (a) measured location (1060◦C× 60 min), (b) element concentration.

Table 2Diffusion coefficients of Al, Fe, Ni, and Cr in the reaction layer at Fe3Al/Cr18-Ni8 interface

Temperature (◦C) 1000 1020 1040 1060

D (µm2/s)Al 0.49 0.8 1.6 2.1Fe 0.5 0.7 1.2 1.5Ni 0.49 0.5 0.8 1.4Cr 0.40 0.47 0.49 0.7

lnD (µm2/s)Al −0.71 −0.22 0.47 0.74Fe −0.69 −0.36 0.18 0.41Ni −0.71 −0.69 −0.22 0.34Cr −0.92 −0.76 −0.71 −0.36

concentration (%),D is diffusion coefficient (µm2/s), t isholding time (s), andt0 is latent period (s).

On the basis of Eqs.(1) and (2), diffusion coefficients ofAl, Fe, Ni, and Cr in the reaction layer at Fe3Al/Cr18-Ni8 in-terface are shown inTable 2. The relation between diffusiocoefficient and heating temperature is shown inFig. 5.

According to the slope and intercept of linear relation-ship between diffusion coefficient (natural logarithm value)and heating temperature (reciprocal value), the diffusion ac-

204 J. Wang et al. / Journal of Colloid and Interface Science 285 (2005) 201–205

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Fig. 4. Relation between diffusion distance and holding time in t

Fig. 5. Relation between diffusion coefficient (lnD) and temperature (1/T )in the reaction layer.

tivation energy (Q) and diffusion factor (D0) of Al, Fe, Ni,Cr are calculated and the result is shown inTable 3.

It can be seen inTable 3that diffusion activation energat the Fe3Al/Cr18-Ni8 interface is less than that in sustrates including Fe3Al intermetallics and Cr18-Ni8 steeThis indicates that microstructure in the reaction layeFe3Al/Cr18-Ni8 interface is more favorable to the elemediffusion and the growth of the reaction layer.

Since diffusion distance of every element at the interfis different due to its distinct diffusion factor and activtion energy, the thicknesses of the reaction layer are

resented by the diffusion distances of Al, Fe, Ni, and Cr,respectively. By comparisons of the calculated and EPMA-measured thickness of reaction layer, the result indicates tha

action layer at Fe3Al/Cr18-Ni8 interface: (a) Al, (b) Fe, (c) Ni, and (d) Cr.

the difference is least when the thickness of reaction layrepresented by diffusion distance of Cr. Thus based ondiffusion factor and activation energy of Cr, the thicknessreaction layerX is calculated to be

(3)X2 = 7.5× 10−4 exp

(−83.59

RT

)(t − t0).

The thickness of the reaction layerX increases graduallwith the enhancement of heating temperatureT and holdingtime t . According to formula(3), the calculated thickness oreaction layer is compared with the EPMA measured vain Fig. 6. The result shows that the calculated thickneslittle larger than the measured value and the differencetween them is only less than 5% when the holding timnot longer than 60 min.

Bonding parameters can be determined by the growthof the thickness of reaction layer with heating temperaand holding time. In addition, there exists a latent periodthe formation of reaction layer. When the thickness of retion layer is certain, the latent period becomes short withincreasing of heating temperature. Therefore, holding tmay be decreased properly to raise bonding efficiency wheating temperature increases.

4. Summary

t

FeAl, Fe3Al, Ni 3Al, and α-Fe (Al) solid solution wereformed in the reaction layer at the Fe3Al/Cr18-Ni8 interfaceby controlling diffusion-bonding technological parameters.

J. Wang et al. / Journal of Colloid and Interface Science 285 (2005) 201–205 205

Table 3Diffusion activation energy and factor of elements at Fe3Al/Cr18-Ni8 interface

Parameters Fe3Al intermetallicsa Fe3Al/Cr18-Ni8 interface Cr18-Ni8 steelb

Al Fe Cr Al Fe Ni Cr Fe Ni Cr

D0 (106 µm2/s) 1.7 4 20 26,362 2650 25.87 7.34 500 1.8 20

Q (kJ/mol) 211.09 166.36 308.6 158.72 49.90 56.35 83.59 163.02 108.68 216.11

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a Data of Al, Fe, and Cr in Fe3Al intermetallics is from Ref.[13].b Data of Fe, Cr, and Ni in Cr18-Ni8 steel is from Ref.[14].

Fig. 6. Thickness of reaction layer at Fe3Al/Cr18-Ni8 interface: (a) effectof heating temperature, (b) effect of holding time.

These phases can decrease diffusion activation energpromote element diffusion and even to accelerate diffusreaction layer formation. The formation of diffusion reactilayer requires a latent periodt0 and the growth of reactio

layer obeys parabolic law. The thickness of reaction layerX is expressed to beX2 = 7.5 × 10−4 exp(−83.59/RT ) ×

(t − t0). Thus technological parameters during bondingbe determined by the relation between thickness of reaclayer and heating temperature and holding time.

Acknowledgments

This project was supported by the National Natural Sence Foundation of China (Grant 50375088) and the Sdong Province Natural Science Foundation (Y2003F0The authors express their heartfelt thanks for this suppo

References

[1] C.G. McKamey, J.H. Devan, P.E. Tortorelli, J. Mater. Res. 6 (191779.

[2] G.H. Fair, J.V. Wood, J. Mater. Sci. 29 (1994) 1935.[3] S. Zuqing, G. Dechun, Y. Wangyue, H. Xiaxu, H. Jihua, Chin. J. Ma

Res. 15 (2001) 69.[4] B.K. Kad, S.E. Schoenfeld, C.G. Mckamey, Acta Mater. 45 (19

1333.[5] G. Frommeyer, C. Derder, J.A. Jimenez, Mater. Sci. Technol.

(2001) 981.[6] S.A. David, M.L. Santella, Adv. Weld. Metall. 31 (1990) 65.[7] N.F. Kazakov, Diffusion Bonding of Materials, Mir, Moscow, 1990.[8] S.P. Garg, G.B. Kale, R.V. Patil, T. Kundu, Intermetallics 7 (1999) 9[9] Y.M. Sung, J. Mater. Sci. Lett. 20 (2001) 1433.

[10] M.A. Dayananda, Metall. Mater. Trans. A 27 (1996) 2505.[11] S.K. Kailasam, M.E. Glicksman, Acta Mater. 47 (1999) 905.[12] H. Jihua, Diffusion in Metal and Alloys, Metallurgical Industry, Be

jing, 1996.[13] Y.H. Sohn, M.A. Dayananda, Metall. Mater. Trans. A 33 (2002) 33

[14] H. Kangsheng, C. Xiongfu, Welding of Dissimilar Metal, Mechanical

Industry, Beijing, 1996.