Materials Science and Engineering A 428 (2006) 314–318

Influence of iron powder particle size on the microstructure andproperties of Fe3Al intermetallics prepared by mechanical

alloying and spark plasma sintering

Qing He ∗, Chengchang Jia, Jie MengSchool of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China

Received 17 March 2006; received in revised form 10 May 2006; accepted 11 May 2006

Abstract

Fe3Al based intermetallics were prepared by mechanical alloying and spark plasma sintering (SPS), influences of iron powder particle size onthe microstructure and properties of samples were investigated. The phase identification was investigated by X-ray, the surface morphology andfractography were observed by SEM. The mechanical properties such as bending strength and strain were tested. The results show that the particlesize has evident effects on the sintered materials. Homogeneous and fine microstructure was obtained by super fine (2–3 �m) iron powder. Thedte©

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ensities of samples are nearly the theoretical value of Fe3Al. The mechanical properties of sintered samples were improved. Comparing withhe coarse powder, the quantity of Fe3AlC0.5 in the samples by fine powder increased, the sintered density increased 2–3%, the bending strengthnhanced about twice and strain about treble.

2006 Elsevier B.V. All rights reserved.

eywords: Fe3Al intermetallics; Mechanical properties; Mechanical alloying and milling; Powder metallurgy

. Introduction

Fe3Al intermetallics have been widely studied among thiseld because of their low cost, low density, good wear resis-

ance, ease of fabrication, resistance to oxidation and corrosion.owever, there are some inferior properties to be conquered for

ommercial applications, such as low ductility exhibited at lowemperature and limited workability [1–3]. There were some

ethods to improve the material properties, such as composites,ddition of other elements, heat treatment, control of grain size,nd so on [4]. Especially, the control of grain size to nanometer orub-micrometer scale and addition of other elements will effec-ively improve the properties of Fe3Al intermetallics, becausehe fine grain size can increase the yield strength and improve theuctility [5], and the addition of other elements can strengthenatrix phase and grain boundaries, and may suppress the grain

rowth.Conventional methods of processing Fe3Al intermetallic,

ncluding melting and casting, tradition powder metallurgy, have

been investigated till now. In recent years, some efficient meth-ods were reported to fabricate the fine grain size metals andalloys, such as mechanical alloying, spark plasma sintering(SPS) [6,7]. The mechanical alloying process, which involvesa repeated cold-working, fracturing, and welding, makesmicrostructure refinement and alloy formation. Microstructurerefinement may easily result in fine grains within micrometersize particles [8]. The SPS is a rapid solidification processingmethod, uniform, dense and fine grain materials can be obtainedby applying pressures and passing electric pulse current to thecompact. The process, due to spark plasma formed between thepowder particles, enhanced the distorted energy of the particlesand the rate of the diffusion between the particles [9,10].

Addition of other elements was another effective method toimprove the mechanical properties of Fe3Al intermetallic. Theadditional elements such as nickel, chromium, boron and carbonhave been used [11–13].

In this study, the fabrication of Fe3Al by SPS from mechan-ically activated powders has been investigated. The purposeof this work is to produce near full dense and fine grain sizespecimen by mechanical alloying and spark plasma sintering

∗ Corresponding author. Tel.: +86 10 62334271; fax: +86 10 62325983.E-mail address: htqmoon@yahoo.com (Q. He).

(SPS). Excellent mechanical properties attributed to uniformityof microstructures and fineness of grain sizes. Parameters of

921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.05.024

Q. He et al. / Materials Science and Engineering A 428 (2006) 314–318 315

Table 1the parameters of the powders

Powders Particle size (�m) Purity (wt.%) Preparation method Impurity (main, wt.%)

Coarse Fe <74 99.5 Reduction method C (≤0.01%), N (≤0.1%), O(≤0.3%)Fine Fe 2–3 99.5 Carbonyl process C (≤0.02%), S (≤0.02%), Mn (≤0.3%)B 2–3 99.9 Fe (≤0.0027%)Al <74 99.5 Atomization Fe (≤0.2%), Si (≤0.1%)

mechanical alloying and SPS were investigated but this reportfocuses on the influence of iron powder particle size on the microstructure and properties of samples.

2. Experimental

Three elemental powders Al, Fe and B were mixed by thecommon ball mill for two hours in the stoichiometric ratio ofAl (30 at.%), B (0.5 at.%). The parameters of the powders areshown in Table 1. The mechanical alloying was performed for0–10 min by three-dimensional vibratory mill with about 1 mlabsolute alcohol as the process control agent (PCA). Stainlesssteel ball and vial were used, and the ball-to-powder ratio was10:1.

Graphite mould was used during the SPS process, the innersurface was covered with a thin carbolic paper. The apparatusis the DR.SINTER model SPS-1050 sintering system producedby Sumitomo coal Mining Co. Ltd. The mixed powders werecompacted into the graphite mould. The graphite mould withthe powder was placed inside the SPS reaction chamber, then thesystem exhausted from ordinary atmospheric pressure to 15 Pain the reaction chamber approx for 5 min. The following stepwas to set the sintering temperature at 1050 ◦C for 5 min and thepressure to the 50 MPa.

Phase analyses were made by XRD (Rigaku D/MAX—RB)a

bend strength (MTS810) was measured by the three-point bend-ing method at a constant strain rate of 0.5 mm/min at ambienttemperature exposed in the air. The SEM (Cambridge S-360)was used to analyze the surface morphology and the fractogra-phy features.

3. Results and discussions

3.1. Milled powder with different particle size of ironpowder

Figs. 1 and 2 give the SEM images of coarse and finepowder ball milled at various time. For coarse powder, theapparent particle size slightly became larger for milled 5 minand went to a steady one about 50 �m. This is because that themechanical alloying process includes deformation, rupture andcold welding of powder, which conduce to a homeostasis statefor along milling time. In this work, both iron and aluminumparticles have excellent ductibility and coalescent, so the effectof cold welding is stronger than rupture in the initial stage ofball milling, and after some milling time they become equipoise.For the fine powder, however, the particle size of iron powdergenerally keeps the same, but the coarse particles of aluminumpowder inchmeal decrease and disappear. Finer particle size isgenerally favorable to get advanced microstructure of sintereds

pow

powd

nd densities were measured by the Archimede’s method. The

Fig. 1. SEM images of coarse powder: (a) raw

Fig. 2. SEM images of fine powder: (a) raw

amples.

der, (b) milled 5 min, and (c) milled 10 min.

er, (b) milled 5 min, and (c) milled 10 min.

316 Q. He et al. / Materials Science and Engineering A 428 (2006) 314–318

Fig. 3. The XRD patterns of the powder: (a) coarse Fe-30Al-0.5B powder and(b) fine Fe-30Al-0.5B powder.

Fig. 3 shows XRD patterns of the powder. For both coarseand fine powder, the diffraction patterns show the evolution ofelemental mixture powders with prolongation of milling time.X-ray diffraction peaks are broadened with the increase of themilling time. It indicates that the grains were refined and defectswere caused by large local strains in the powder particles. Asthe milling time is increased, diffraction peaks of aluminum arenoted to decrease in intensity, especially the (1 1 1) one. This isbecause that an iron-based solid solution is formed.

3.2. The phase analysis and density of sintered samples

Fig. 4 shows the XRD patterns of the sample by SPS pro-cess with 5 min milled both coarse and fine powder. The Fe3Al,Fe3AlC0.5 are formed during the SPS. The forming of carbidemaybe attributes to the graphite C diffusing into the samplesand a little CO reacts with Fe3Al at high temperature. The car-bon comes from the mould and carbolic paper. A little residualatmosphere reacts with the carbon, and the CO was generated.Comparing the diffraction peaks of two kinds of powder indi-

Fig. 4. The XRD patterns of the sample.

cates that the peak of fine powder broadens more than coarsepowder, which show that the crystal grain size become finer.Contemporary, highness (strength) of IFe3Al/IFe3AlC0.5 indicatesthat the quantity of Fe3AlC0.5 in the sample by fine powder ismore than coarse one.

The density of the sintered samples is presented in Fig. 5.All of the samples (with coarse and fine iron powder) are nearlythe theoretical value of Fe3Al (6.7 g/cm3). For both two kindsof powder, the density of the sample with 5 min milled powderis higher than others. These phenomena result from that the SPSprocess makes it easy to sinter milled powders. Furthermore,the density of sample by fine iron powder is higher than that bycoarse one for all milling time. Consider the theoretical of theFe3AlC0.5 (6.214 g/cm3) is lower than the Fe3Al and the quantityof Fe3AlC0.5 in the sample by fine powder is more than coarseone, the results show that fine iron powder is favorable to accel-erate the combination (reaction) between iron and aluminum,and increase the sintered density.

Fig. 5. Density of the sintered samples.

Q. He et al. / Materials Science and Engineering A 428 (2006) 314–318 317

Fig. 6. Micro structure of the samples by SPS process: (a) coarse powder and(b) fine powder.

3.3. The microstructure of sintered samples

Fig. 6 shows the microstructure of the sample by SPS processwith 5 min milled both coarse and fine powder there are clearlytwo phases, the dark phase is Fe3AlC0.5, and the other (Frenchgrey) is Fe3Al. Fe3AlC0.5 distributes at the grain boundary ofFe3Al. The result also indicates that the grain size is smalland the distribution is homogeneous in the sample by finepowder.

Fig. 7. The bending strength and the strain: (a) bending strength and (b) strain.

3.4. The bending strength and the fracture surface

Fig. 7 gives the bending strength and strain of two kinds ofsamples by coarse and fine powder. For the coarse powder, thebending strength slightly increases with the milling time, butthe strain has hardly changed. On the other hand, rather higherbending strength was obtained by fine powder at the milling timelonger than 8 min, although there is little difference between thecoarse and the fine powder at the milling time within 5 min. Asfor the strain, the value by fine powder is greater than the coarse

Fig. 8. SEM imagines of fracture surface by coarse powder: (a) raw powder, (b) milled 5 min, and (c) milled 10 min.

318 Q. He et al. / Materials Science and Engineering A 428 (2006) 314–318

Fig. 9. SEM imagines of fracture surface by fine powder: (a) raw powder, (b) milled 5 min, and (c) milled 10 min.

one at any milling time (from 0 to 10 min). The improvement ofbending strength and strain is due to the fine grain size and thehomogenous distribution of Fe3AlC0.5 caused by mechanicalalloying (high energy ball milling).

Figs. 8 and 9 are the SEM images of fracture surface of sam-ples by the powder with different particle size and various milledtime. A mixed mode of brittle fracture and gliding fracture areobserved, the obviously cleavage and dimple characters in thephoto grams. For the sample by fine powder, not only is the pro-portion of gliding fracture more than the coarse one, but the grainsize is also finer. Appropriate microstructure is a foundation toobtain excellent properties.

4. Conclusions

During the mechanical alloying process, homogeneous par-ticle size was obtained by the fine iron powder, and some atomsof Al dissolve in the lattice of Fe without the appearance of ironaluminides.

The relative densities nearly 100% were got by the finepowder milled 5 min under the SPS condition of sintering tem-perature1050 ◦C, pressure 50 MPa, and holding this temperaturefor 5 min.

Bending strength of 2200 MPa and strain of 15.4% wasobtained by fine powder, and the rupture mode is a mixed oneof brittle fracture and gliding fracture.

References

[1] N.S. Stoloff, Mater. Sci. Eng. A 258 (1998) 1.[2] S.C. Deevi, V.K. Sikka, Intermetallics 4 (1996) 357.[3] S.C. Deevi, V.K. Sikka, C.T. Liu, Prog. Mater. Sci. 42 (1997) 177.[4] C.G. Mckamey, P.F. Devan, V.K. Tortorelli, J. Mater. Res. 6 (1991)

1779.[5] M. Furukawa, Y. Ma, Z. Horita, et al., Mater. Sci. Eng. A 241 (1998)

122.[6] S. Paris, E. Gaffet, F. Bernard, Z.A. Munir, Scripta Mater. 50 (2004)

691.[7] Y. Minamino, Y. Koizumi, N. Tsuji, et al., Sci. Technol. Adv. Mater. 5

(2004) 133.[8] C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1.[9] W. Chen, U. Anselmi-Tamburini, J.E. Garay, et al., Mater. Sci. Eng. A

394 (2005) 132.[10] U. Anselmi-Tamburini, S. Gennari, J.E. Garay, Z.A. Munir, Mater. Sci.

Eng. A 394 (2005) 139.[11] R.G. Baligidad, A. Radhakrishna, Mater. Sci. Eng. A 283 (2000)

211.[12] M. Cieslar, M. Karlik, et al., Mater. Sci. Eng. A 324 (2002) 23.[13] R. Balasubramaniam, J. Alloys Compd. 253 (1997) 148.