06) 1527–1532www.elsevier.com/locate/matlet

Materials Letters 60 (20

A study on the thermal expansion characteristics of a dissimilar fusion jointby high temperature X-ray diffraction

Aritra Banerjee a, S. Raju a,⁎, R. Divakar a, E. Mohandas a, C. Sudha a, A.L.E. Terrance a,P. Parameswaran a, G. Panneerselvam b, M.P. Antony b

a Physical Metallurgy Section, Materials Characterisation Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Indiab Fuel Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India

Received 6 April 2005; accepted 22 November 2005Available online 12 December 2005

Abstract

The lattice thermal expansion behaviour of the constituents of a dissimilar fusion joint between a magnetic iron alloy and Inconel-82® 1 hasbeen investigated by high temperature X-ray diffraction in the temperature range 300–1273 K. It is found that the thermal expansivity of theinconel phase in the fusion zone of the weld is considerably lower than that of pure Inconel-82® filler metal. On the other hand, the thermalexpansivity of the soft-iron phase is rather close to that of the inconel phase in weld, at least up to moderate temperatures. In addition, thetemperature sensitivity of its thermal expansion is also rather weak. It is found that in the temperature range 300–773 K, the thermal expansionmismatch between the weld constituents is not really drastic. Metallographic characterisation of the fusion zone of the weld using SEM-EDX andEPMA techniques revealed the presence of a compositionally modulated zone on the inconel side of the fusion line. It is reasoned that the changein composition of the inconel phase in weld as compared to pure Inconel-82® filler metal, is what is responsible for its reduced thermalexpansivity in weldment.© 2005 Elsevier B.V. All rights reserved.

Keywords: Metals and alloys; Thermal property; Thermal expansion; X-ray diffraction; Dissimilar weld

1. Introduction

It is a well-known fact that the highly aggressive environ-ment characteristic of a liquid metal cooled fast reactor placesstringent requirements on the part of material properties, com-ponent fabrication techniques and qualification procedures [1].An in-depth understanding of various material related issues istherefore essential from the point of view of successful design,construction and subsequent safe operation of a nuclear reactor.Among the various components of the first Indian prototypefast breeder reactor PFBR, the Diverse Safety Rod Drive Mech-anism (DSRDM) constitutes a crucial sub-system. The majorfunction of this system is to shut down the reactor uponreceiving a scram signal [2]. Stating briefly, the DSRDM con-

⁎ Corresponding author. Tel.: +91 44 27480 306; fax: +91 414 27480 081.E-mail address: sraju@igcar.ernet.in (S. Raju).

1 Inconel is a registered trademark of International Nickel Company.

0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2005.11.064

sists of an electromagnet assembly, which serves to hold firmlythe Diverse Safety Rod (DSR) during the course of normaloperation of the reactor. This electromagnet also ensures theprompt drop of DSR into the reactor core when it is deener-gised [2].

The electromagnet assembly has two cylindrical pole pieces,machined in the form of truncated conical ends that are placedone within the other [2,3]. The material of choice for theelectromagnet is basically a dilute iron alloy of compositionFe—0.35 wt.% Mn—0.05 wt.% C, supplied by MIDHANI,India. In the rest of this paper, this iron-based alloy will bereferred to as soft-magnetic iron. The actual composition of thismaterial determined by direct reading optical emission spec-trometry, is listed in Table 1. The two pole pieces of theelectromagnet are held in position by a separating annularspacer made out of a suitable non-magnetic material. Themain function of the spacer is to prevent the influx of hotsodium into the electromagnet assembly as well as to shortenthe response time of the electromagnet by providing a

Table 1Nominal compositions in wt.%, of Inconel-82® and soft magnetic iron alloy,used in the fabrication of the DSRDM electromagnet are tabulated

Element Inconel-82® Soft magnetic iron alloy

Ni (+Co) 72 b0.14C 0.02 0.05±0.005Mn 3.00 0.3±0.05Fe 1.00 BalanceSi 0.20 b0.060Cu 0.04 b0.040Cr 20.0 b0.3Ti 0.55 b0.003Nb (+Ta) 2.50 Not analysedS 0.007 b0.007Mo – b0.03Al – b0.005P – b0.007

The composition of iron alloy is determined in the present study using directreading optical emission spectrometry.

Fig. 1. An expanded schematic of the fusion joint giving the details of the weldgeometry. The dimensions are in mm.

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nonmagnetic interleaving path [2]. In the current design ofDSRDM electromagnet, Inconel-82® has been identified asthe spacer material from the point of view of its adequatemetallurgical compatibility with soft magnetic iron [2]. Theelectromagnet assembly is fabricated in such a way that aftersuitable edge preparation procedures, the spacer separating thetwo poles is formed in situ by fusing an Inconel-82® filler wireby standard TIG welding process [4]. In Fig. 1, a schematic ofthe cross-sectional geometry of the dissimilar fusion joint isillustrated.

From a materials-engineering point of view, it is clear thatthere is a thermal expansion mismatch at the weld interface dueto the dissimilar nature of the weld constituents. Because ofthis, there is also a possibility of development of thermalstresses in the fusion zone of the weld. These considerationsprompted a study on the differential thermal expansion char-acteristics of the dissimilar constituents of the weld. Consider-ing the nature of the sample under study, namely the fusionzone of the weld, it is rather difficult to adopt dilatometry forprobing the thermal expansion disparity between the dissimilarconstituents. Instead, high temperature X-ray diffraction tech-nique (HTXRD) has been chosen for investigating the differ-ential lattice thermal expansion behaviour. The results of thisstudy are reported in this paper. The experimental details aregiven in the ensuing section.

2. Experimental details

The normal ambient temperature around the electromagnetinside the reactor is about 773 K, with the possibility that it cango up to 1000 K under transient conditions [2]. In view of thisfact, high temperature X-ray diffraction studies are carried outin the temperature interval 300 to 1273 K. The X-ray diffrac-tion experiments are carried out on foil samples. For thispurpose, thin slices from the fusion zone containing both softiron and inconel constituents of the weld are precision cut.These transverse sections of the weld are subsequently coldrolled to obtain thin foils of dimension, roughly of the order of0.5×2 cm, with an average section thickness of about 100 μm.

Although the cold rolling introduced considerable amount ofdeformation induced strain in the sample, this did not pose anyproblem from the point of view of obtaining good diffractionpatterns at high temperatures. This is due to the fact thatconsiderable strain relieving took place in situ in the heatingstage of the XRD equipment by virtue of gradual heating andone hour soaking schedule at each temperature step that isinherent of our experimental schedule. Since, the details ofHTXRD experiments have already been reported in some ofour recent thermal expansion studies [5–7], we skip here anelaborate account of this aspect. The experiments are per-formed in a Philips-MPD® X-ray diffraction system equippedwith a high vacuum (better than 10−5 mbar) high-temperaturestage consisting of a tantalum resistance heater. The tempera-ture is monitored by a Pt–Pt/Rh thermocouple that is spotwelded to the bottom of the tantalum heater. Experiments arecarried out with nickel filtered Cu–Kα radiation in the θ–θgeometry. The heating stage is flushed with argon before thestart of each experimental run. A heating rate of 1 K perminute with a soaking time of 60 min at each temperaturestep is followed. Individual runs are taken at every 50 Kinterval. The HTXRD studies have been carried out on pureInconel-82® filler metal, soft-magnetic iron and on the weld-ment. Since, the results on Inconel-82® have already beenpublished [6], we focus here only on the results of experimentsperformed with the weldment.

During the course of this experiment, we have also co-recorded the reflections arising from the substrate tantalumheater. This is done in order to account for the temperaturedrop between the bottom portion of the tantalum heater and thethin foil sample placed on it. This temperature difference arisesdue to the finite thickness of the sample foil and any attempt togo in for very thin foils resulted in sample buckling and therebya distortion of diffraction geometry as well. Since effecting arigorous correction factor to these effects is rather difficult, weresorted to an empirical correction procedure [5]. This isachieved by comparing the lattice parameter data of tantalumcalculated from the co-recorded heater reflections against the

Table 2The change in room temperature lattice parameter upon ageing of the inconelphase in the fusion zone of the weldment is presented as a function of agingtime

Phase designation Lattice parameter (nm)

Pure Inconel-82® filler metal [6] 0.3546Inconel in the as welded state 0.3579Inconel aged at 823 K; 100 h 0.3590Inconel aged at 823 K; 500 h 0.3596

The quoted lattice parameters are accurate up to ±5%.

Fig. 2. The room temperature XRD profiles of as welded and aged samples. Thelattice parameters calculated from these XRD profiles are listed in Table 2.

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critically assessed lattice parameter data on tantalum by Reeberand Wang [8].

Since the weldment sample used in this experiment isconstituted of mostly inconel, the reflections from soft-mag-netic iron are somewhat weak. Moreover, the co-recording ofreflections from tantalum, a comparatively strong scatterer ofX-rays as compared to iron, has masked some of the finefeatures associated with the HTXRD spectrum of soft iron.To alleviate this problem, we have performed two more addi-tional HTXRD runs on individual soft-iron alloy and inconelportions cut directly from the fusion zone of weld. These slicesare again rolled into foils of about 100-μm thickness. In thiscase, adequate care has been taken to mask completely thetantalum heater from contributing to the X-ray diffractionprofile. These individual HTXRD scans are processed subse-quently to obtain accurate lattice parameter estimates ofinconel and soft iron as they exist in the fusion line of theweld. For iron alloy, only two high angle reflections cor-responding to the ferrite phase are considered and the averageof the lattice parameters pertaining to these principal reflec-tions is taken to be the representative value. This is because ofthe fact that at high temperatures, certain difficulty is encoun-tered with respect to the precise peak fixation of low intensity

iron reflections. For inconel however, we made use of the threemajor fcc-reflections and an effective high angle correctedlattice parameter is obtained using Nelson–Riley interpolationscheme [9]. The HTXRD results are discussed in the followingsection.

3. Results

3.1. Room temperature X-ray diffraction studies

The X-ray diffraction study consists of two components. In thefirst, room temperature XRD profiles of the weldment in the aswelded and aged conditions are recorded. The ageing has beencarried out at 883 K for a time period of up to 500 h. The ageingtreatment has been carried out to ascertain the possibility of theformation of any new phase during a long-term hold under typicalservice conditions. The room temperature XRD patterns of aswelded and aged samples are displayed in Fig. 2. As may beseen, the profiles are self-explanatory in the sense that typicalpatterns expected of fcc-inconel and bcc-iron are readily identified.It is clear from Fig. 2, that ageing of weldment at 823 K does notresult in the formation of a new phase, at least in volume fractionsthat are within the detectable limit of present XRD studies. In Table2, the change in lattice parameter of fcc-inconel phase due to ageingis tabulated. It can be seen that ageing brings about an expansion ofthe inconel lattice. This could have arisen from a possible change inlocal composition at the fusion zone of the weld due to ageing. Sucha conclusion is also in accordance with the experimental findings ofRees et al. [10], who had found an expansion in the lattice param-eter of fcc-Ni–(Cr,Fe) solid solution with iron enrichment. Thelattice parameter of the magnetic iron phase in the fusion zone ofthe weld has not undergone any noticeable change from thecorresponding value for the pure soft iron alloy and hence is nottabulated here.

3.2. High temperature X-ray diffraction studies

In Fig. 3, the temperature dependence of the lattice parameters ofinconel and soft magnetic iron in weldment is brought out. For com-parison purposes, the corresponding data obtained on pure Inconel-82® filler metal are also displayed in the same figure [6]. A cursoryglance of Fig. 3 readily suggests that the lattice parameter of theinconel phase in weldment is systematically larger than that of thepure Inconel-82® filler metal. This aspect will be discussed at a laterpoint in this paper.

For the purpose of calculating lattice thermal expansion, the tem-perature dependence of lattice parameter is represented by a polyno-mial in temperature increment (T-300). For the inconel phase, theconsolidated data from two experimental runs conducted on samples

Table 3The temperature dependence of lattice parameter, both measured and fitted,instantaneous, relative and mean linear thermal expansivities (MLTE) for theinconel phase in the fusion zone of the weldment are listed

T (K) a 10−10

(m)a (fit)10−10 (m)

αLI 10−6

(K−1)αLr 10−6

(K−1)αLm 10−6

(K−1)MLTE(%)

300 3.5737 3.5760 4.73 4.73 – –

Fig. 3. The temperature dependencies of the lattice parameters of the inconeland the magnetic iron alloy phases of the weldment are displayed. Note that theinconel in the fusion zone of the weld shows a larger lattice parameter than thecorresponding Inconel-82® filler metal.

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of slightly different foil thickness are represented by the followingcubic polynomial,

aðmÞ ¼ 3:576� 10−10 þ 1:693� 10−15ðT−300Þ þ 6:322

� 10−19ðT−300Þ2 þ 2:812� 10−21ðT−300Þ3: ð1Þ

In Eq. (1), temperature T is given in Kelvin. A cubic fit is neces-sitated in order to capture the pronounced tendency of the latticeparameter to increase at high temperatures (see, Fig. 3). The latticeparameter at 300 K obtained from the above fit is 0.3576(7) nm. Ascan be seen from Table 2, this value is somewhat larger than the latticeparameter of pure Inconel-82® filler metal. The reason for this is ratherobvious in the sense that inconel in the fusion zone of the weld hasundergone considerable compositional modification. Further, it is alsoslightly different from the values obtained for 823 K aged sample(Table 2). This is because, prolonged isothermal hold is expected tobring in a certain stable compositional gradient across the weld inter-face, while the in situ continuous heating in the HTXRD stage,followed by natural cooling may result in a different compositionalredistribution. However, it must be said that the measured latticeparameter is only a reflection of the average composition spanning aconsiderable portion of the fusion zone. In the present study, no

Fig. 4. The mean linear thermal expansivities of inconel and soft-iron phase inweldment, together with that the data on pure Inconel-82® filler metal [6] aregraphically compared.

attempt has been made to monitor the lattice parameter profile as afunction of distance from the weld interface.

The measured lattice parameter data have been used to calculate themean linear thermal expansivity (αL

m), defines as

amL ¼ ð1=a300Þ � fðaT−a300Þ=ðT−300Þg: ð2Þ

It must be mentioned that this mean linear thermal expansivity isdifferent from the thermodynamic definition of the coefficient linearthermal expansion. The latter quantity, often known as instantaneouslinear thermal expansivity, is given by the expression

aiL ¼ ð1=aT ÞðdaT=dTÞ: ð3Þ

It is obvious from their respective definitions that αLm is always less

than αLi . In addition, one also comes across in literature another

definition for characterising thermal expansion, namely, the relativelinear thermal expansivity. This is given as,

arL ¼ ð1=a0ÞðdaT=dTÞ: ð4Þ

In the above expression, a0 refers to the value of lattice parameter atthe reference temperature, taken here as 300 K. The calculated valuesof αL

m are plotted in Fig. 4 for the inconel phase in weldment togetherwith the corresponding data on pure Inconel-82® filler metal [6].These values are also listed in Table 3.

For the soft-iron phase, the temperature dependence of latticeparameter is represented by the following second order polynomial

aðmÞ ¼ 2:860� 10−10 þ 1:709� 10−15ðT−300Þ þ 1:303

� 10−18ðT−300Þ2: ð5Þ

It may be noted that the lattice parameter of soft-iron alloy at300 K, namely 0.2860(5) nm is in close agreement with the value of

373 3.5808 3.5773 5.12 5.12 4.91 0.04423 3.5763 3.5782 5.52 5.53 5.07 0.06473 3.5807 3.5793 6.05 6.05 5.28 0.09523 3.5819 3.5804 6.69 6.69 5.52 0.12573 3.5833 3.5817 7.45 7.46 5.80 0.16623 3.5824 3.5831 8.32 8.34 6.13 0.20673 3.5854 3.5847 9.31 9.34 6.49 0.24723 3.5861 3.5864 10.42 10.45 6.89 0.29773 3.5895 3.5884 11.64 11.68 7.33 0.35823 3.5894 3.5906 12.98 13.04 7.81 0.41873 3.5929 3.5931 14.44 14.51 8.33 0.48923 3.5939 3.5958 16.00 16.09 8.89 0.55973 3.5950 3.5988 17.69 17.8 9.49 0.641023 3.6026 3.6022 19.48 19.62 10.12 0.741073 3.6083 3.6059 21.39 21.56 10.80 0.841123 3.6118 3.6099 23.40 23.62 11.52 0.951173 3.6150 3.6143 25.53 25.80 12.27 1.071223 3.6239 3.6191 27.76 28.09 13.07 1.211323 3.6265 3.6300 32.55 33.04 14.77 1.51

The thermal expansion and expansivity values are rounded off to two significantdigits.

Table 4The temperature dependence of lattice parameter, both measured and fitted,instantaneous, relative and mean linear thermal expansivities (MLTE) for thesoft magnetic iron phase in the fusion zone of the weldment are listed

T (K) a (nm) a (fit)(nm)

αLm 10−6

(K−1)αLI 10−6

(K−1)αLr 10−6

(K−1)MLTE(%)

300 0.2860 0.2866 – 8.69 8.69 –373 0.2860 0.2868 7.27 9.35 9.35 0.01473 0.2864 0.2871 7.17 1.02 1.03 0.05523 0.2863 0.2873 7.31 1.07 1.07 0.12573 0.2863 0.2874 7.48 1.11 1.12 0.16623 0.2868 0.2876 7.66 1.16 1.16 0.20673 0.2868 0.2877 7.86 1.20 1.21 0.25773 0.2873 0.2881 8.28 1.29 1.30 0.29873 0.2875 0.2885 8.71 1.38 1.39 0.39973 0.2877 0.2889 9.15 1.47 1.48 0.501073 0.2880 0.2893 9.59 1.56 1.57 0.61

The thermal expansion and expansivity values are rounded off to two significantdigits.

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0.2866(5) nm measured for pure Armco iron at room temperature[11]. The calculated mean linear thermal expansivity (MLTE) for theiron alloy, based on Eq. (5) is also displayed in Fig. 4. In Table 4,the corresponding numerical values are listed.

From Fig. 4, it emerges that the thermal expansivity of inconelphase in weld is significantly lower than that of pure Inconel-82®filler wire [6]. This difference is in fact increasing with tempera-ture. On the other hand, the thermal expansivity of soft-iron phase

Fig. 5. The EDX spectra taken from discrete points on either side of the fusion lineindicate roughly the distance of separation the discrete points chosen for composition823 K for 500 h. The iron enrichment seen on the inconel side is noteworthy.

is rather close to that of inconel phase in weld, at least up tomoderate temperatures. In addition, the temperature sensitivity ofits thermal expansion is also rather weak. In any case, it isinstructive to note that in the temperature range 300–773 K, thelattice thermal expansion mismatch between magnetically soft ironphase and that of inconel in weld is not really drastic. But at fairlyhigh temperatures, this conclusion is rendered invalid as the inconelphase begins to manifest considerable expansion (see Fig. 4 andTables 3 and 4).

4. Discussion

It must be stated that the most significant finding of thisstudy is the fact that the disparity in the lattice thermal expan-sion characteristics of pure Inconel-82® and soft iron is certain-ly brought down in the fusion zone of the weldment (Fig. 4).This decrease in thermal expansion mismatch must in somewayarise from the chemistry of the fusion zone. A set of prelimi-nary metallography cum compositional characterisation studiesperformed using SEM-EDX (Fig. 5) and electron probe micro-analysis (EPMA) (Fig. 6) revealed that there occurs significantcompositional redistribution at and around the fusion zone ofthe weld. In specific terms, the iron content on the inconel partof the fusion zone is enhanced. In addition, a significant mi-gration of manganese from iron portion towards inconel is alsonoticed. This compositionally modulated zone seen at theinconel side of the fusion line pervades fairly extensively into

in the as welded and aged condition are displayed. The numbers in each boxanalysis, from either side of the fusion line. (a) As welded condition, (b) aged at

Fig. 6. The EPMA line scan data obtained on aged weldment samples. The EPMA results substantiate the findings of EDX data (Fig. 5). The presence of a com-positionally restructured zone on inconel side of the fusion line is clearly supported by EPMA line scan results. (a) 823 K–100 h aged sample, (b) 823 K–1075 h agedsample.

1532 A. Banerjee et al. / Materials Letters 60 (2006) 1527–1532

the weldment interior (see, Fig. 5). Similarly, but to a lessextent, there is also present a nickel enrichment on the ironside. The outcome of this significant diffusion mediated com-positional restructuring is that the change in chemistry of thefusion zone alters the thermal expansion response of the weld-ment. In fact, it is even found that a fairly long thermal anneal-ing treatment of about 1075 h at 823 K has turned out to beinadequate in fully removing the compositional gradient on theinconel side of the weld (Fig. 6b). In all probability this sug-gests that the change in composition witnessed at the inconelside of the fusion line is bound to control its overall thermalexpansivity, as compared to pure Inconel-82® filer wire.

In this connection it must also be added that our EPMA orSEM-EDX studies are only qualitative. In addition a truly long-term aging has not been performed to assess the evolution ofpossible non-equilibrium phases under a steep concentrationgradient. Therefore, more quantitative studies with electronmicroprobe or analytical electron microscope on long term-aged samples need to be carried out before a definite conclusionis drawn on this issue. Nevertheless, on the thermal expansionfront, it is certain that the mean thermal expansivity of theinconel phase has drastically come down in weldment.

Acknowledgements

We thank Mrs. R. Vijayashree and Mr. V. Rajan Babu, forbringing this problem to our notice and also for the provision ofdetails regarding DSRDM electromagnet assembly. The metal-lurgical aspects concerned with the welding procedure of dis-similar metals and other general information on inconel alloys,

were provided to us by Dr. V. Shankar. The sustained encour-agement and support that we received from Dr. M. Vijaya-lakshmi, Dr. V. S. Raghunathan, Dr. P. R. Vasudeva Rao andDr. Baldev Raj, during the course of this work are sincerelyacknowledged.

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