COMPOSITES

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Composites Science and Technology 67 (2007) 1275–1279

SCIENCE ANDTECHNOLOGY

Short communication

Development of ultrafine grain aluminium–graphite metalmatrix composite by equal channel angular pressing

M. Saravanan, R.M. Pillai *, K.R. Ravi, B.C. Pai, M. Brahmakumar

Materials and Minerals Division, Regional Research laboratory (CSIR), Industrial Estate PO, Pappanamcode, Thriuvananthapuram 695 019, Kerala, India

Received 14 August 2006; received in revised form 22 September 2006; accepted 4 October 2006Available online 16 November 2006

Abstract

Al–5 vol% graphite particulate composite was subjected to equal channel angular pressing (ECAP) at room temperature up to fourpasses. A significant grain refinement, down to the submicron level of �300 nm was achieved. Hardness enhancement of more than twofold was observed with only a marginal difference along the perpendicular and parallel directions of pressing. Further, the strength of thecomposites was increased by more than two folds. ECAP at room temperature has resulted in a more rapid and significant grain refine-ment and strengthening in Al–graphite composite.� 2006 Elsevier Ltd. All rights reserved.

Keywords: E. ECAP; A. Metal–matrix composites; B. Strength

1. Introduction

Equal channel angular pressing (ECAP) is a novel tech-nique for producing ultra fine grain structure in submicronlevels without change in the billet shape or dimensions byintroducing a large amount of shear strain into the materi-als [1–4]. Till date, this technique has been used in a widerange of materials including pure metals [5–14], alloys[15–20], intermetallics [21,22] and metal matrix composites[23–28]. This process is well suited for aluminum alloys andis capable of producing ultra fine grain structures withgrain sizes falling between 200 and 500 nm. However, thereappears to be only limited number of investigations carriedout on aluminium metal matrix composites (MMCs) with6061 alloy as the matrix and Al2O3 [23–27] and SiC whis-kers [28] as the reinforcement. It has been observed thatECAP of Al MMCs significantly improves the homogene-ity of reinforcement distribution in the matrix [23] andrefines the matrix grain size into submicron level [26].

0266-3538/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compscitech.2006.10.003

* Corresponding author. Tel.: +91 471 2515270: fax: +91 471 2491712.E-mail address: rmpillai_rrl@yahoo.com (R.M. Pillai).

Strength of the Al 6061–Al2O3 composite has beenobserved to increase by almost a factor of two by ECAP[25]. There has been little or no breaking of Al2O3 in6061Al–Al2O3 composites after subjecting to 12 ECAPpasses [24]. However, the average length of SiC whiskershas been found to reduce from 42 to 4.2 lm during ECAPof 6061Al–SiC whisker composites [28]. Further, all of thereported studies on Al MMCs have been done at elevatedtemperatures except one [27]. However, there is no workreported till date on ECAP of Al - graphite composite,wherein graphite is soft in nature compared to Al2O3 andSiC. Further, there is a possibility of performing ECAPof Al–graphite particle (Grp) composite at room tempera-ture due to its soft and self lubricating nature. Therefore,an attempt has been made to subject Al–5 vol.% Grp com-posite to ECAP at room temperature for developing ultra-fine grain structure.

2. Experimental details

Al–5 vol.% Grp composite prepared by stir casting route[29] was chosen for this investigation along with unrein-forced pure Al for comparison. Graphite particles used

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have an average particle size of 65 lm. Samples(12 · 12 · 55 mm) machined out of Al–5 vol.% Grp com-posites and unreinforced pure Al cylindrical castings weresubjected to ECAP at room temperature using a die havingan internal channel bent through an angle (U � 90�) andthe outer arc of curvature (W � 20�) to impose maximumstrain and obtain homogeneous microstructure in thematerial [4,30,31 and 32]. These die angles introduce astrain of �1 on each pass of billet through the die [33].Among the three deformation routes commonly usednamely, (1) route A (without rotation between passes),(2) route BC (90� rotation in the same direction betweenpasses) and (3) route C (180� rotation between passes),route BC was chosen in this investigation because of itsmaximum efficiency in generating refined equiaxed grains[4,8 and 34]. It is to be noted that Al–5 vol.% Grp couldsustain a maximum of four passes at room temperaturebeyond which cracking of the samples started. Samples(10 · 10 · 2 mm) were cut from the centre of the specimensand polished for optical metallography and atomic forcemicroscopy and later thinned by twin-jet electro polishingmethod as thin foils for TEM observations. Microhardnesswas measured using a Vickers diamond pyramidal indentorwith an imposed load of 200 g on polished 10 · 10 · 5 mmsamples. Tensile test was carried out at room temperatureand a strain rate of 2 · 10�3 s�1 on samples (gauge lengthof 18 mm and gauge diameter of 4.5 mm) machined paral-lel to the pressing direction of Al–5 vol.% Grp composites.

3. Results and discussion

3.1. Microstructure

Fig. 1(a) and (b) show the typical microstructures ofunreinforced pure Al and Al–5 vol.% Grp composite priorto ECAP respectively. Large grains were observed in unre-inforced pure Al (�150 lm) and Al–5 vol.% Grp composite(�35 lm). Fig. 2(a) and (b) show the AFM image of unre-inforced pure Al and Al–5 vol.% Grp composite after eightand four passes of ECAP, respectively. Fig. 3(a) and (b)

Fig. 1. Optical microstructures of (a) unreinforced pure A

show the TEM microstructures together with the SAEDpatterns for unreinforced pure Al and Al–5 vol.% Grp com-posite samples perpendicular to the pressing direction aftereight and four passes respectively. It is important to notethat there is a significant difference in the microstructuresof unreinforced pure Al and Al–5 vol.% Grp composites.In unreinforced pure Al, there are well defined grainboundaries and a relatively small number of intergranulardislocations after eight passes. On the other hand, in Al–5 vol.% Grp composite, grain boundaries are not welldefined but rather poorly delineated and irregularly shapedor curved in general after four passes. Further, the non-uni-form contrast with in the grains in Fig. 3(b) is indicative ofa highly strained structure. SAED pattern (Fig. 3(b)) of theAl–5 vol.% Grp composites subjected to four ECAP passesalso reveals an array of many very small grains having ran-dom distribution of orientations. Figs. 2 and 3 also showthat grain size of the Al–5 vol.% Grp has been refined toaround 300 nm perpendicular to the pressing directionafter four ECAP passes against 620 nm in unreinforcedpure Al even after eight ECAP passes. Fig. 3(c) shows thatthe grain size of the Al–5 vol.% Grp has been refinedaround 400–500 nm parallel to the pressing direction afterfour ECAP passes. Grain refinement achieved in Al–5 vol.% Grp composite after four ECAP passes is equalto or higher than that observed in earlier studies [25 and26] conducted in 6061 Al–10 vol.% Al2O3 composites.Han and Langdon [26] have found in 6061Al–10 vol.%Al2O3 composites that the grain size has been reduced to300 nm after eight ECAP passes at 533 K. Valiev et al.[25] have observed a reduction in grain size up to 600 nmafter 10 ECAP passes at 673 K. These results clearly indi-cate that room temperature ECAP of Al–5 vol.% Grp com-posites has resulted in more rapid formation of submicrongrains (�300 nm) with only four ECAP passes and lesservolume fraction reinforcement (5 vol.%). Since annihilationand absorption of dislocations into grain boundaries arehigher at elevated temperatures, room temperature press-ing ensures the potential for achieving finer grains morerapidly [35].

l and (b) Al–5 vol.% Grp composite prior to ECAP.

Fig. 2. AFM microstructures of (a) unreinforced pure Al and (b) Al–5 vol.% Gr composite after eight and four passes, respectively.

Fig. 3. Transmission electron micrograph and SAED pattern of (a) unreinforced pure Al after eight passes and Al–5 vol.% Grp composite after four passes(b) perpendicular and (c) parallel to the pressing direction.

M. Saravanan et al. / Composites Science and Technology 67 (2007) 1275–1279 1277

0

200

400

600

800

Parallel to thepressing direction

Perpendicular tothe pressing direction

Har

dn

ess

(MP

a)

Fig. 5. Comparison of hardness perpendicular and parallel to the pressingdirection after four passes in Al–5 vol.% Grp composites.

50

100

150

200

250

Tens

ile s

tren

gth

(MP

a)

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3.2. Hardness

The increase in hardness for Al–5 vol.% Grp com-posite after ECAP is shown in Fig. 4 and comparedwith that of unreinforced pure Al. It can be observedthat Al–5 vol.% Grp composite work hardens rapidlyduring the first ECAP pass and thereafter at a slowerrate. It also shows that the microhardness value hasincreased from 400 to 850 MPa after four ECAPpasses, registering 112% increase. Moreover, only amarginal difference exists between the hardness valuesmeasured perpendicular and parallel to the pressingdirection (Fig. 5).

3.3. Tensile properties

The strength of Al–5 vol.% Grp composite after fourECAP passes is shown in Fig. 6 and compared with thoseof unpressed Al–5 vol.% Grp and 6061Al–10 vol.% Al2O3

composites after eight ECAP passes at 533 K [26]. Thestrength has increased from 97 to 249 MPa after fourECAP passes with approximately two and half foldincrease. Despite the soft nature and less amount of thereinforcement in Al–Grp composites compared to6061Al–Al2O3 composites [26], the strength achieved ishigher than that of the latter. Further, it is achieved atroom temperature. Hence, a rapid high degree of strength-ening is possible in Al–graphite composite after ECAP atroom temperature. These results show that room tempera-ture ECAP of Al–5 vol.% Grp composite imparts bettergrain refinement and strengthening with in four passescompared to other studies in composites carried out withhigh volume percent reinforcement and at higher tempera-ture pressing.

0 2 4 6 80

100

200

300

400

500

600

700

800

900

99.5 % pure Al Al - 5 vol % Gr

p compositesV

icke

rs H

ardn

ess

(MP

a)

Number of passes

Fig. 4. Comparison of variation in hardness with number of passes in Al–5 vol.% Grp composite and unreinforced pure Al.

06061 Al - 10 vol %Al

2O

3 composites

after 8 ECAPpasses [26]

Al - 5 vol % Grp

compositesafter 4 ECAPpasses

UnpressedAl - 5 vol % Gr

p

composites

Fig. 6. Comparison of strength in unpressed and ECAP processed Al–5 vol.% Grp and ECAP processed 6061Al–10 vol.% Al2O3 composites.

4. Conclusions

In the present work, ECAP of Al–5 vol.% Grp compos-ite has been successfully carried out up to four passes atroom temperature. A significant grain refinement, downto the submicron level of �300 nm, is achieved in Al–GrP

composite. Hardness enhancement of more than two foldwas observed with only a marginal difference along the per-pendicular and parallel directions of pressing. Further, thestrength of the composites is increased more than two foldsby ECAP. ECAP results in more rapid and significant grainrefinement and strengthening in Al–GrP composite at roomtemperature.

M. Saravanan et al. / Composites Science and Technology 67 (2007) 1275–1279 1279

Acknowledgements

The authors thank the Director, Regional ResearchLaboratory, Thiruvananthapuram, for the award of CSIRdiamond jubilee Research Internship to the first author andthe members of the mechanical engineering section for thefabrication of die.

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