Materials Science and Engineering A 419 (2006) 144–147

Superplastic deformation studies in Fe–28Al–3Cr intermetallic alloy

Garima Sharma a,∗, R. Kishore a, M. Sundararaman a, R.V. Ramanujan b

a Material Science Division, BARC, Trombay, Indiab School of Materials Engineering, Nanyang Technological University, Singapore

Received in revised form 3 December 2005; accepted 3 December 2005

Abstract

The superplastic deformation behaviour of coarse grained (600–650 �m) Fe–28Al–3Cr alloy was studied by strain rate change test at 873–1273 Kunder a strain rate of about 10−2 to 10−4 s−1. The alloy showed superplastic behaviour above 1073 K with strain rate sensitivity values (m) above0.3. The detailed microscopic studies showed recovery and dynamic recrystallization as micromechanism responsible for superplastic behaviour.© 2006 Elsevier B.V. All rights reserved.

Keywords: Iron aluminides (based on Fe3Al); Intermetallics; Ordered structures; Superplasticity; High temperature deformation

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. Introduction

Intermetallics often exhibited high ambient temperature hard-ess and high strength at elevated temperature. Fe3Al basedlloys represent a class of intermetallic compounds which areotential replacement for stainless steel and nickel base super-lloys [1]. These low cost alloys possess good strength andxcellent oxidation and sulphidation resistance. Applicationsnclude jet engine compressor blade and housing, heat exchang-rs, furnace fixtures as well as piping and tubing for powerenerations and chemical industries. Two factors limit the fur-her use of these alloys: their poor room temperature ductilitynd low strength above 873 K [2]. Several approaches have beeneveloped to overcome these limitations including additions oflloying element such as Cr and a combination of thermome-hanical and heat treatments designed to produce metastable B2t room temperature [3].

Superplasticity has emerged as a very attractive method fororming these inherently brittle materials. For intermetallics, itas been found that their superplastic behaviour is similar to thatf conventional metals and alloys inspite of their ordered crystaltructures. In the recent years, several superplastic studies on

FeAl alloys mainly with Ti additions [8–13]. Recently, high tem-perature deformation behaviour of Fe–30Al–4Cr alloy modifiedby TiB2 additions has also been studied in detail [14]. As Cris added mostly in these alloys to improve the ductility, it isimportant to study the high temperature deformation of thesealloys with Cr additions. In this paper, we report the high tem-perature deformation behaviour of coarse grain Fe–28Al–3Cr(at%) (600–650 �m) alloy under a strain rate range of 10−2 to10−4 s−1 at high temperatures.

2. Experimental

Tensile specimens with gauge sections of 12.5 mm × 4 mm ×2 mm were cut from the rolled sheets of iron aluminides(Fe–28Al–3Cr) by electric sparking. Specimens were heattreated at 1123 K for 1 h followed by furnace cooling. Theinitial grain size of alloy specimen before deformation was600–650 �m. Tensile testing was performed on an Instron Uni-versal Testing machine (Instron, model 1158) under a constantcrosshead velocity in the range of 0.05–5 mm min−1. Differ-ential strain rate tests were conducted on a single specimenin the temperature range of 873–1273 K. At a given test tem-

ne grain TiAl alloys, NiAl and some other intermetallics haveeen reported [4–7]. However, superplasticity in iron aluminidesas not been studied widely. There have been only few recenttudies on the superplasticity of iron aluminides, i.e. Fe Al and

perature, strain rates were increased from 10−4 to 10−2 s−1

in steps and the flow stress was determined after attaining thesteady state of flow at each strain rate. Constant initial strainrate tests were also carried out in air in the temperature rangefspd

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∗ Corresponding author. Tel.: +91 22 25590457; fax: +91 22 5505151.E-mail address: garimas@yahoo.com (G. Sharma).

921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2005.12.015

rom 1123 and 1223 K till failure. Subsequent to failure, thepecimens were quenched immediately in water to room tem-erature. Microstructural investigations were performed on theeformed samples by optical, SEM and TEM techniques.

G. Sharma et al. / Materials Science and Engineering A 419 (2006) 144–147 145

Fig. 1. The plot showing the variation of flow stress with strain rate at differenttemperatures, the slope yields strain rate sensitivity exponent (m).

3. Results and discussion

The high temperature deformation behaviour was studied inthe temperature range (873–1273 K) by strain rate change test.Fig. 1 showed the variation of flow stress with change in strainrate at constant strain and temperature. Strain rate sensitivity(m) was determined from the slope of each plot at a particu-lar temperature. In the present study, strain rate sensitivity, mwas found to vary with temperature and that above 1073 K, mwas above 0.3. These results showed that Fe–28Al–3Cr alloyfollow the superplastic behaviour in the temperature range of1123–1273 K and strain rate of 10−4 to 10−2 s−1.

The effect of temperature on the flow behaviour of this alloywas examined by true stress–true strain curves at 1123, 1223 and1273 K under a constant initial strain rate of 1.33 × 10−2 s−1

(Fig. 2). This plot showed that maximum flow stress decreasedwith increase in temperature at a constant strain rate. The steady

Fc

Table 1A summary of the tensile properties of Fe–28Al–3Cr alloy tested at variousstrain rates and temperature

Temperature(K)

Strain rate(s−1)

Maximum flowstress (MPa)

Y.S.(MPa)

(%)Elongation

1123 6.6 × 10−3 41.44 37 1901.33 × 10−2 53.61 50 1802.6 × 10−2 71.9 67 176

1223 1.33 × 10−3 28.47 18 1256.6 × 10−3 30.49 25 2101.33 × 10−2 35.0 33 190

flow stress in certain range of strain results in fairly large uni-form deformation prior to necking. The effect of strain rateat constant temperature was also studied at 1123 and 1223 K.The maximum uniform deformation of 190% was obtained ata strain rate of 6.6 × 10−3 s−1 at 1123 K and 210% at a strainrate of 6.6 × 10−3 s−1 at 1223 K. Table 1 provides summary ofthe tensile properties of this alloy tested at 1123 and 1223 K.Fig. 3 showed the micrograph of the original specimen and thattested at 1223 K at different strain rates. Lin and co-workers[8–11] have also shown a maximum elongation of 297% forFe–36.5Al–2Ti alloy with a grain size of 600 �m and 620% forFe–28Al–2Ti with a grain size of 100 �m. The maximum elon-gation to failure was found to be relatively modest in this study,i.e. 210% as compared to those reported in the literature. Thismay be due to initial large grain size used in the present study.The fact that such high strain rate superplasticity was observed inthis coarse grained structure is technologically very significant.

The activation energy for high temperature deformation wasalso determined from the slope of ln σ versus 1/T, plotted atconstant strain rate and m values, as:

Q = mR

(∂ln σ

∂(1/T )

)(1)

where R is gas constant, T the temperature and Q is the activationenergy of the rate controlling process. Fig. 4 shows the linearv

Fa

ig. 2. True stress–true strain curves plotted for different temperatures at aonstant strain rate for Fe–28Al–3Cr alloy.

ariation of ln σ versus 1/T. The mean value of m = 0.34 was

ig. 3. Micrograph of undeformed specimen and specimen deformed at 1223 Kt different strain rates.

146 G. Sharma et al. / Materials Science and Engineering A 419 (2006) 144–147

used to determine activation energy in 1123–1273 K temperaturerange. The activation energy at such high temperature was foundto be approximately 174 kJ mole−1. This value of activationenergy was low as compared to the activation energy for latticediffusion in these alloys [2]. This indicates that interfacial dif-fusion such as subgrain boundary diffusion may be the respon-sible micromechanism. Lin and co-workers [9–11] have carriedsuperplasticity investigations on the Fe–28Al–2Ti and Fe–28Alalloys and reported the Q values of 263 and 191 kJ mole−1,respectively, in the strain range of 10−4 to 10−3 s−1. Recently,Malek et al. [14] have reported an activation energy of approx-imately 280 kJ mole−1 in Fe–30Al–4Cr alloy modified withTiB2. The difference in the value of activation energy alreadyreported and that in the present study was due to different com-position and experimental conditions used.

The optical micrograph of Fe–28Al–3Cr alloy before super-plastic deformation is shown in Fig. 5(a). Optical microscopy

performed on the sample deformed at 1223 K showed a refine-ment in the grain size with zigzag boundaries near the fractureregion (Fig. 5b). The refined grains ranged from 20 to 130 �min size, indicating that the grain size was much smaller than theoriginal grain size. SEM examination of the deformed samplesshowed recrystallized grains some of which were as small as5 �m (Fig. 5c). TEM studies performed on the deformed sampleshowed a recovered structure with a high density of disloca-tion networks. Bright field (BF) TEM micrograph presentedin Fig. 5(d) showed hexagonal and square networks of dislo-cations. The intersection of these dislocation network leads tothe formation of subgrains as shown in Fig. 5(e and f). TEMobservations on the deformed samples showed that the sub-grain boundaries were composed of dislocations aggregated inthe form of dislocation walls or dislocation networks. Mutualinteraction of dislocations in the arrays constituting low angleboundaries led to the formation of irregular networks. Metal-

Fan

ig. 5. (a) Optical micrographs showing a coarse grained microstructure in the undet 1223 K; (c) BSE micrograph showing refined and recrystallized grains near the fretwork; (e and f) BF micrograph showing intersection of these dislocation networks

formed sample; (b) refined grains near the fractured region after deformationactured region; (d) BF micrograph showing square and hexagonal dislocationleads to the formation of subgrains.

G. Sharma et al. / Materials Science and Engineering A 419 (2006) 144–147 147

Fig. 4. Variation of flow stress with temperature at constant strain rate; the slopeyields the activation energy; mean value of m = 0.34.

lographic examination showed that the average grain size ofthe large grained iron aluminides decreased during superplas-tic deformation and that much finer grain sizes were obtainedafter such deformation. The microstructure of deformed sam-ples often appeared to be akin to a recovered structure, withlarge density of dislocation networks and subboundaries. Theseobservations suggested that continuous recovery and recrystal-lization took place during superplastic deformation, leading toa large elongation before failure in the originally coarse grainedmaterial.

4. Conclusions

The high temperature deformation studies performed onFe–28Al–3Cr alloy showed superplastic behaviour in the tem-perature range of 1123–1273 K with a maximum elongation of

210%. The microstructural investigation of deformed samplesshowed continuous recovery and dynamic recrystallization asthe responsible mechanism.

Acknowledgements

The authors would like to thank Dr. B.P. Sharma, AssociateDirector, Materials Group and Dr. S. Banerjee, Director, Mate-rials Group for their encouragement in carrying out the project.

References

[1] V.K. Sikka, S. Viswanathan, C.G. McKamey, in: R. Darolia, J.Lewandowski, C.T. Liu, P.L. Martin, D.B. Miracle, M.V. Nathal (Eds.),Structural Intermetallics, TMS, Warrendale, PA, 1993, p. 483.

[2] C.G. Mckamey, J.A. Horton, C.T. Liu, J. Mater. Res. 6 (1989) 1156.[3] P.G. Sanders, V.K. Sikka, C.R. Howell, R.H. Baldwin, Scripta Metall.

Mater. 25 (1991) 2365.[4] A. Dutta, D. Banerjee, Scripta Metall. Mater. 24 (1990) 1319.[5] H.S. Yang, P. Jin, E. Dalder, A.K. Mukerjee, Scripta Metall. Mater. 25

(1991) 1223.[6] T.G. Nieh, W.C. Oliver, Scripta Metall. 23 (1989) 851.[7] T. Takasugi, S. Rikukawa, S. Hanada, Acta Metall. Mater. 40 (1992)

1895.[8] D. Li, A. Shan, D. Lin, Scripta Metall. Mater. 33 (1995) 681.[9] Y. Liu, D. Li, D. Lin, S. Chen, Scripta Metall. Mater. 34 (7) (1996)

1095.[10] D. Lin, Proceedings for the International Symposium on Nickel and

[[

[

[

Iron aluminides: Processing Properties and Applications, vol. 187, ASMInternational Materials Park, Ohio, 1997, p. 187.

11] D. Lin, A. Shan, M. Chen, Intermetallics 4 (6) (1996) 489.12] J.P. Chu, H.Y. Yasuda, Y. Umakoshi, K. Inoue, Intermetallics 8 (2000)

1075.13] J.P. Chu, W. Kai, H.Y. Yasuda, Y. Umakoshi, K. Inoue, Mater. Sci. Eng.

A 329–331 (2002) 878.14] P. Malek, P. Kratochvil, J. Pesicka, P. Hanus, I. Sediva, Intermetallics

10 (2002) 985.