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Materials Letters 61 (20

Wear resistance of Fe–28Al–3Cr intermetallic alloy under wet conditions

Garima Sharma a,⁎, P.K. Limaye b, M. Sundararaman a, N.L. Soni b

a Material Science Division, BARC, Mumbai, 400 085, Indiab Refuelling Technology Division, BARC, Mumbai, 400 085, India

Received 1 September 2006; accepted 13 November 2006Available online 4 December 2006

Abstract

Wear behaviour of iron aluminides (Fe–28Al–3Cr at.%) alloy has been investigated under wet conditions using ball on plate sliding weartester. Wear resistance was examined against tungsten carbide (WC) ball sliding over the iron aluminide plate at room temperature. Wear tests werecarried out at 3 N and 5 N load conditions at different sliding frequency of mating ball. The micromechanisms responsible for wear were identifiedto be microcutting, micropitting, and microcracking of deformed subsurface zones under wet conditions.© 2006 Elsevier B.V. All rights reserved.

Keywords: Iron aluminide; Sliding wear; Wear; Intermetallics; Fe3Al; Ordered structures

1. Introduction

Iron aluminides based on D03 or B2 ordered structure arenow receiving extensive attention as materials with goodpotential for industrial applications and replacement for hightemperature oxidation and corrosion resisting stainless steel[1,2]. In addition, these alloys offer low density and lower costthan many stainless steels. Furthermore, these compounds showhigh hardness and work hardening rate which mean that theycan perform in severe wear and erosion conditions. The orderedstructure inherent to these intermetallic alloys possess severalattractive properties, among them are strength, stiffness andenvironmental resistance. In addition, the long range orderedsuperlattice reduces dislocation mobility, and at high temper-ature diffusional processes. This resulted in low ductility atroom temperature and reduction in strength above 873 K whichhad retarded their development as structural materials. Howev-er, recent few studies have shown that the addition of Crresulted in improving the ductility of iron aluminides im-mensely [3,4]. Addition of Cr also resulted in the improvementin corrosion resistance of these alloys [5]. Research efforts so

⁎ Corresponding author.E-mail address: garimas@yahoo.com (G. Sharma).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.11.069

far have been focused mainly on the enhancement of roomtemperature ductility together with the high temperature creepproperties of these alloys. However, in wear related applica-tions, loads are compressive in nature, therefore, tensileductility is not as critical a mechanical property parameter ashardness, strength, and work hardening ability. A few recentstudies have been done to study the dry abrasive wear resistanceof nickel and iron aluminides [6–12]. The effect of Cr on the dryabrasive wear resistance of iron aluminides has already beenstudied in detail [13]. The abrasive wear resistance of Fe3Alalloy has been shown to be comparable with the wear resistanceof AISI 1060 carbon steel and SS 304 [8,9]. The abrasive wearof Fe3Al compares favourably with that of Hadfield steel, a hightoughness material used for the mining applications. The dryabrasion wear rate of iron aluminides varies slightly for B2 orDO3 structures and Fe3Al with DO3 structure possessesmarginally lower wear rate than those with B2 structure [8,9].The wear resistance of iron aluminides with abrasive slurry hasalso been studied [14]. It has been well reported that the majorreason for poor ductility of iron aluminides at ambienttemperature is environment embrittlement involving moisturein air [15–19]. The presence of Cr in iron aluminides has beenfound to reduce the hydrogen embrittlement due to formation ofelectrochemical passive film on the surface [5,18]. Therefore,the present paper is an attempt to study the sliding wearresistance of Fe–28Al–3Cr alloy under pure water and to

Fig. 1. 3-D profilometry of the wear groove of Fe–28Al–3Cr alloy at 3 N load and 21 Hz.

3346 G. Sharma et al. / Materials Letters 61 (2007) 3345–3348

delineate the wear mechanisms during abrasion under wetconditions.

2. Experimental

The wear tests were performed on a micro-friction machine(TE 70 Plint) offering friction evaluation and wear testingfacilities. Rolled sheets of iron aluminide of composition Fe–28Al–3Cr were heat treated at 540 °C for 170 h followed byfurnace cooling to achieve DO3 ordering at room temperature.Room temperature XRD analysis was performed to confirmDO3 ordering in the sample. The sheets were cut into plates of22 mm×40 mm cross-section. These plates were metallograph-ically polished to have an average surface roughness (Ra) of0.23 μm. Wear tests were carried out in water at room tem-perature using a tungsten carbide ball of 6 mm diameter as amating material. The abrasive tungsten carbide ball has ahardness of approx. 22 GPa and a fracture toughness of 7.0 MPam1/2. In order to retain uniform test conditions, a new ball wasused for each test. The ball was made to slide on the platesample with three frequencies (9, 15 and 21 Hz) keeping thesliding amplitude of 1 mm constant. The tests were performedwith plate submerged in water all the time during testing. Thetests were performed at 3 N and 5 N load conditions. Each testwas repeated three times and wear volume was found to bewithin ±1%. Wear track profiles were formed on the plate due toball sliding. The wear profiles were studied by 3-D profilometry

Fig. 2. Variation of coefficient of friction of Fe–28Al–3Cr alloy with slidingdistance at different load.

as shown in Fig. 1. Wear rate was calculated by dividing thevolume of the wear groove by the sliding distance. The micro-mechanisms responsible for sliding wear were studied in detailby SEM.

3. Results and discussion

The variation of coefficient of friction during sliding is shown inFig. 2. The friction value was found to increase very rapidly to 0.31within few seconds of the start of the experiment. Though thecoefficient of friction fluctuates slightly with sliding time, remainconfined in the narrow range of 0.31 to 0.34. As expected, frictioncoefficient values under wet condition were much lower than thosereported values of around 0.56–0.57 under dry conditions with sameload and sliding frequency [13]. The presence of water during slidingforms a partial hydrodynamic film between the ball and the sampleresulting in the reduction of the coefficient of friction. The steady statevalues of the coefficient of friction exhibit little dependence on theapplied load, though the value at a lower load is slightly higher. The

Fig. 3. Variation of wear rate with sliding frequency at (a) 3 N; (b) 5 N load,under static water.

3347G. Sharma et al. / Materials Letters 61 (2007) 3345–3348

variation of wear rate with sliding frequency is shown in Fig. 3(a–b).These plots show that the wear rate increases as a function of slidingfrequency. The wear rate was found to be much higher under wetconditions as compared to those reported under dry conditions [13].Comparing Fig. 3(a) and (b), it was found that as the normal loadincreased from 3 N to 5 N, there was an increase in wear rate due to anincrease in wear volume with load.

The wear rate under wet conditions depends on the mass loss due toabrasion as well as interaction of hydrogen with the alloy along withabrasion. It has already been reported that iron aluminides are highlysusceptible to hydrogen embrittlement under environments containingwater [15–18]. Aluminium atoms in the aluminides react with water,resulting in the generation of atomic hydrogen. This effect is a dynamicphenomenon, which occurs when fresh unoxidized material understress is exposed to water or moisture. During wear, the plastic stressesgenerated due to wear increase the diffusion of atomic hydrogen in thealloy. This stress induced hydrogen gets accumulated at the strainedsites. The presence of hydrogen reduces the atomic bonding acrosscleavage plane resulting in reduction of cohesive strength acrosscleavage planes. This could be the reason for the increased wear rate ofiron aluminides under water as compared to dry conditions.

The micromechanism responsible for the wear in this alloy wasstudied in detail by SEM technique. Fig. 4a shows mainly microcuttingand micropitting on the wear surface after a sliding distance of 54 mtested at 3 N load condition. With further increase in sliding distance to378 m, the micropitting was found to increase as shown in Fig. 4b.Testing under 5 N load conditions also showed micropitting andmicrocutting as dominant mechanism similar to 3 N load conditionsafter sliding distance of 54 m and 216 m. Tu and Liu [14] have alsoreported micropitting, cracking and spalling as the main mechanismsfor iron aluminides when tested with abrasive slurry. However, with the

Fig. 4. SEMmicrograph showing (a) micropitting and microcutting after a sliding dist3 N normal load; (c) microcrack of brittle nature at 5 N; (d) delamination of deform

increase in sliding distance to 378 m, microcracking and initiation ofcracks at deformed subsurface layers were also found to take placealong with micropitting and microcutting. Fig. 4(c) showed microcrackof brittle nature after a sliding distance of 378 m at 5 N load. Thiscracking could be due to the combined effect of microfracture andhydrogen attack. Thus, during initial stages micropitting and micro-cutting take place. However, as the load increased, the surface suffersextensive damage and strains accumulate at the surface resulting in theformation of subsurface deformation zone. With the increase in loadcracks initiate in deformed subsurface layers and detachment ofplatelets occur (Fig. 4d). These results showed that micropitting,microcutting, and delamination of deformed platelets were thedominant mechanisms during wear under wet conditions.

4. Conclusion

The sliding wear behavior of iron aluminide has beenevaluated in this study and following conclusions are reached:

1. The sliding wear rate of iron aluminide is quantitativelymeasured and is found to increase with an increase in slidingdistance at constant sliding frequency.

2. An increase in load from 3 N to 5 N increases the wearvolume and subsequently increases the wear rate.

3. SEM examinations of the abraded surfaces indicate thatmicropitting and microcutting as the dominant sliding wearmechanisms under low load conditions. However, at higherload, delamination and microcracking of brittle nature arealso found to take place.

ance of 54 m at 3 N; (b) increased micropitting after a sliding distance of 378 m ated subsurface zones at 5 N normal load.

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Acknowledgements

The authors would like to thank Dr. B.P. Sharma, AssociateDirector, Materials Group and Dr. R.G. Agarwal, Head, Re-fuelling Technology Division for the encouragement in carryingout the project.

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