The reinforcement of an age-hardenable Al±Cr matrix alloy in situand by SiC/Al2O3 particles: tailoring of the interface

S. Dattaa, N.R. Bandyopadhyaya, S. Bandyopadhyayb,*, M.K. Banerjeea

aDepartment of Metallurgy, B.E. College (Deemed University), Howrah 711103, IndiabSchool of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia

Received 30 June 1997; received in revised form 30 August 1999; accepted 7 October 1999

Abstract

Al±Cr alloys containing less than 0.75% Cr give rise to an age-hardening e�ect and the alloys also exhibited a good combinationof strength and ductility properties. The achievable high toughness of these alloys led to an attempt to produce in situ composites

with an Al±Cr solid solution as the matrix phase and excess insoluble intermetallics of chromium aluminides as the reinforcingconstituent. The development of composites based on the Al±Cr matrix alloy with SiC and/or Al2O3 as the reinforcing phase wasalso attempted by a casting route. It was observed that the increase in strength was quite considerable even with a relatively good

ductility. The precipitation behaviour and its kinetics in this newly developed composite were also studied by di�erential scanningcalorimetry (DSC) and transmission electron microscopy (TEM). # 2000 Elsevier Science Ltd. All rights reserved.

Keywords: A. Metal-matrix composites (MMCs); A. Intermetallics; B. Toughness; Al±Cr alloy; In situ composites; B. Age hardening; Activation

energy of transformation

1. Introduction

The sloping nature of the solvus curve in binary Al±Cr phase diagram has prompted researchers to use it asan age-hardenable alloy. But chromium is known toform large-sized intermetallic particles with aluminiumrendering the alloy quench sensitive [1]. In Al±0.5 Cralloy the precipitation of chromium aluminides was seento take place at lattice defects and the nature and shapeof the ageing precipitates were also seen to vary withageing temperature [2,3]. This alloy system was alsoreported to be associated with excellent corrosion andoxidation resistance [2]. Following the documentede�ect of trace additions on the age-hardening behaviourof aluminium alloys in a number of publications [3,4],the e�ect of minor additions on the toughness of an Al±0.75 Cr alloy was studied by two of the authors [5].Again, there are several reports on the use of an age-hardenable matrix alloy in forming particulate metal±matrix composites [6±10]. Inmany of these pieces of work,

the ageing kinetics are said to have been appreciablyin¯uenced by the formation of a precipitate at disloca-tions. These precipitates are known to be formed as aconsequence of the di�erential coe�cients of thermalcontraction (CTE dislocations) between the matrix andthe reinforcing particles [11].In view of the inherent characteristics of dislocation-

aided precipitation in this alloy it appears that reinfor-cing the alloy with particles would have a signi®cantin¯uence on the overall strengthening. It appearsworth while, therefore, to probe into the possibility ofusing this alloy to form such a particulate compositewhich will possess a suitable strength/ductility balancewhile maintaining a high level of corrosion and oxidationresistance. Moreover, the existence of a two-phase ®eld(liquid + chromium aluminides) in the phase diagramenables the formation of an age-hardenable aluminium/chromium solid solution, dispersed within which, willremain the chromium aluminides. These dispersoids donot su�er from any wettability problems like the rein-forcing particles in conventional metal-matrix compo-sites (MMCs), which ensures better interfacial bonding.So it is equally interesting to look into the e�ect of suchinherent dispersoids within an age-hardenable matrix onthe ®nal properties of alloys. In the present investigation

0266-3538/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PI I : S0266-3538(99 )00145-1

Composites Science and Technology 60 (2000) 451±456

* Corresponding author. Tel.: +61-2-9385-4509; fax: +61-2-9385-

5956.

E-mail address: s.bandyopadhyay@unsw.edu.au (S. Bandyopadhyay).

the suitability, if any, of such composite microstructure,here referred to as in situ composites, is also investigated.

2. Experimental procedure

In order to select a suitable Al±Cr alloy as the matrixalloy for the production of metal±matrix composites,the following techniques were adopted to produce a fewAl±Cr alloys. Requisite quantities of aluminium andchromium were compacted green. In some powdermixes, pre-weighed quantities of pure silicon powderwere mixed and green compacts were produced. Purealuminium was melted under suitable ¯ux cover in a pottype electrical resistance furnace. The powder compactswere added to the liquid aluminium melt at 750�C. Themelts were then allowed to become semi-solid. Thesemi-solid masses were further melted at 850�C, held for15 min under gentle stirring and then resolidi®ed. Theseresolidi®ed masses were remelted at 750�C for achievinguniformity in composition. Finally, they were cast to13 mm�25 mm�250 mm samples. The chemical com-positions of the alloys are given in Table 1. Based onexperiments on the ageing behaviour of the binary andternary alloys, Al±0.65 Cr±0.2 Si alloy was chosen as thebase alloy. This alloy was melted at 850�C and wasstirred mechanically to form a vortex. Particles ofSiC/Al2O3 were gradually added within the vortex soformed. In order to minimise the possible reactionbetween SiC and aluminium [12,13], the SiC particleswere added slowly and the temperature of the melt wasallowed to come down to 780�C. Degassing was carriedout under continuous stirring so as to avoid preferentialsegregation of particles, if any, due to probable wett-ability problems. These melts were ®nally cast to13 mm�25 mm�250 mm specimens.All cast samples were hot-rolled carefully at 375�C.

The rolled alloys were suitably cut into pieces and thecut samples were solutionised at 600�C for 5 h followedby ice±brine quenching. The quenched samples wereaged at various temperatures for 90 min.The ageing hardness was measured in Rockwell `F'

scale and also by a micro-hardness tester. Optical,

scanning and transmission electron microscopy (TEM)along with di�erential scanning calorimetry (DSC) werecarried out for structural characterisation of the matrixalloys and the composites. The mechanical properties ofthe alloys and composites were evaluated in an InstronTesting Machine at a strain rate of 5�10ÿ4/s.

3. Results and discussion

The isothermal ageing curves for the binary and tern-ary alloys aged at 375�C are shown in Figs. 1±3. Fig. 1shows that the age-hardening response of the binary Al±0.78 Cr is higher than the Al±0.65 Cr alloy. However, inthe Al±0.9 Cr alloy, in which chromium content exceedsits primary solubility limit in aluminium (�0.78 wt%),the ageing response is found to be poor. Moreover,before the onset of age-hardening, an incubation periodof 120 min is noticed. When silicon is added, as inAl±0.9 Cr±0.6 Si, the ageing response is found to beincreased with no incubation period. The e�ect of add-ing 0.2 wt% Si in binary alloys is seen to improve theageing behaviour of Al±Cr alloys (Fig. 2). However, thebene®cial e�ect of 0.2 wt% Si is found to be more in theAl±0.78 Cr alloy than in the Al±0.9 Cr alloy whichcontains excess chromium. It is further noticed, in Fig.3, that the increase in chromium content to 1.20 wt%has led to a signi®cant change in its ageing behaviour.This alloy contains a good amount of insoluble inter-metallics which may be treated as in situ compositewhere CrAl7 is embedded uniformly in the matrix of a±Al±Cr solid solution. Owing to quench sensitivity, anincubation period of 210 min is seen in its ageing curve,although the hardness values of this in situ compositeare found higher at all ageing times than the binary Al±0.65 Cr alloy.Chromium is known to have strong binding energy

with quenched-in vacancies [4]. As a result the di�usionof chromium in aluminium is slow. Also, the presence ofinsolubles in high chromium alloy leads to annihilationof vacancies at the particle/matrix interfaces.Due to the paucity of vacancies, di�usional precipita-

tion of chromium-aluminides becomes di�cult. Rather,

Table 1

Chemical composition of the alloys (wt%)

Alloy Cr Si Cu Zn Mg Zr Fe Ni Ti Mn

1 0.65 0.08 0.001 0.002 0.003 0.003 0.21 0.001 0.008 0.006

2 0.78 0.13 0.003 0.002 0.003 0.001 0.19 0.002 0.004 0.004

3 0.90 0.09 0.001 0.001 0.004 0.004 0.25 0.001 0.019 0.005

4 0.77 0.20 0.001 0.003 0.003 0.003 0.21 0.001 0.007 0.004

5 0.90 0.19 0.002 0.003 0.008 0.001 0.18 0.002 0.009 0.003

6 0.78 0.60 0.001 0.003 0.007 0.003 0.20 0.001 0.008 0.004

7 1.20 0.76 0.002 0.003 0.006 0.001 0.19 0.001 0.007 0.003

8 0.65 0.20 0.001 0.001 0.003 0.001 0.19 0.001 0.004 0.003

452 S. Datta et al. / Composites Science and Technology 60 (2000) 451±456

the growth of the existing particles takes place in orderto relieve the local super saturation of the matrix. Sili-con reduces the quench sensitivity of the alloy andcauses enhancement of its precipitation hardening due,probably, to its nucleation stimulating e�ect. Thisallows the smaller critical nucleus size of higher numberdensity. Hence, higher precipitation hardening results insilicon bearing alloys.

3.1. DSC

A DSC study was carried out to understand the in¯u-ence of particle reinforcement onto the age-hardeningbehaviour of the present alloy system. The DSCresponse curves are shown in Figs. 4 and 5. The DSCcurve of the unreinforced alloy (Al±0.65 Cr±0.2 Si)exhibits a weak exothermic peak at 355�C. This peak isindicative of precipitate formation and denotes that themaximum rate of precipitation is achieved at 355�C,the peak temperature. On the contrary, the formation

peak is found to occur at a lower temperature, viz.315�C, where the base alloy is reinforced with 10 vol%SiC particles. A similar observation on the enhancementof ageing kinetics in the reinforced 6061 Al-alloy hasbeen made by other workers [7,11].This observed improvement in the ageing behaviour

largely stems from the fact that the di�erence in thecoe�cient of thermal contraction between the SiC parti-cles and the matrix has led to generation of dislocationsaround the particles upon quenching from the solutio-nising temperature. Since the present alloy system isseen to age-harden by heterogeneous precipitation ondislocations (Fig. 6), the extra number of CTE disloca-tions so formed, would increase the number of e�ectivenucleation sites for precipitation. Hence, an accelerationin the precipitation reaction has been noted in the rein-forced alloy. This kind of behaviour has been quitecommon in a number of MMCs based on precipitation-hardenable matrix alloys.The area under the DSC peaks is proportional to the

amount of transformation. The activation energy forthe formation of precipitates may be found from theDSC heating curves by following the techniques used byprevious workers [14]. From such measurements, avalue of 250 kJ/mol is recorded as the activation energy

Fig. 2. Hardness/time curves of the alloys aged at 375�C.

Fig. 3. Hardness/time curves of the alloys aged at 375�C.

Fig. 4. DSC response curve for the base alloy (alloy 8).

Fig. 1. Hardness/time curves of the alloys aged at 375�C.

S. Datta et al. / Composites Science and Technology 60 (2000) 451±456 453

for the formation of precipitates during the ageing ofthe base alloy. Its value, therefore, is quite similar tothat the interdi�usion of chromium in aluminium in abinary Al±Cr alloy [15,16]. This means that the pre-cipitation in the base alloy is kinetically controlled bythe di�usion of chromium through aluminium.On the contrary, the activation energy for the pre-

cipitate formation in the SiC reinforced alloy has beenfound to be 221 kJ/mol. Though marginal, the reduc-tion in activation energy may be attributed to theincrease in the e�ective number of heterogeneousnucleation sites at the CTE dislocations. The avail-ability of a higher number of dislocations envisages afaster precipitation due to the reduction in the di�usionpath for the chromium atom. Thus, there is an increasein the precipitate number density with correspondingdecrease in both the precipitate size and interparticledistance. Hence, higher age-hardening results. It, there-fore, becomes clear that the particulate reinforcement inthe Al±Cr alloy improves the ageing kinetics and it alsoincreases the extent of overall age-hardening.

3.2. Mechanical properties

The mechanical properties of the alloy and the com-posites are presented in Table 2. From these results it isfound that in Al±1.2 wt% Cr alloy containing a goodamount of intermetallics in the matrix of a-solid solu-tion there is about 20% increase in yield strength valuewith 6% dimination in elongation properties when age-hardened at 450�C. This is attributed to the precipita-tion in ®ne form onto the dislocation forming aroundthese insoluble intermetallics (Fig. 7). The dislocationsformed around the above-mentioned incoherent parti-cles are the mis®t dislocations, being equally e�ective asCTE dislocation in inducing ®ne precipitate formation.However, the tested alloy contains chromium just inexcess of its primary solubility at �0.77 wt%. This hasresulted in only a small volume fraction (�4±5 vol%) ofreinforcing intermetallics. Moreover, the uniformity inthe distribution of these intermetallics has not beenachieved to any great extent. But for these, the propertyimprovement could have been still better. This presentexperiment, no doubt, explores the possibility of devel-oping a high strength, high ductility Al±Cr alloy whosemicrostructure is inherently composite in character,herein called in situ composites.However, it is observed from Table 2 that a consider-

able improvement in strength properties is achievable inparticulate reinforced Al±Cr alloy. Reinforcing the base

Fig. 5. DSC response curve for the SiC reinforced alloy.

Fig. 6. TEM micrograph of alloy 2 aged at 350�C showing hetero-

geneous precipitation on dislocations.

Table 2

Mechanical properties for the alloys aged at 450�C for 90 min

Alloy Yield

strength

(MPa)

UTS

(MPa)

Knoop

hardness

(30 g load)

%el

Al±0.6 Cr (base alloy) 55 115 22 36

Al±1.2 Cr (in situ) 68 120 40 30

Al±0.65 Cr±8.0 Al2O3 76 135 35 25

Al±0.65 Cr±10.0 SiC 100 158 41 22

454 S. Datta et al. / Composites Science and Technology 60 (2000) 451±456

alloy with only 10 vol% SiC particles has raised theyield strength by around 100%, the present elongationstill remaining at a high value of 22%. In 8 vol% Al2O3

reinforced alloy, the improvement is also considerablebut not so large as in the case of SiC reinforcement.About 50% increase in yield strength at 25% elongationis recorded in this case. This improvement in strengthproperties of the particulate reinforced MMCs can beexplained from themicrostructure presented in Figs. 8±10.It appears from the photographs that reinforcing thebase alloy by particles has produced a good numberof dislocations around the particles. These dislocationshave acted as the nucleation sites for precipitationduring ageing. As a result, the precipitate density isincreased and hence the strength properties improve. Inthe case of SiC reinforcement the dislocation densityaround the particles is found to be higher (Figs. 9 and 10)than that in the case of reinforcement by Al2O3 (Fig. 8).This di�erence in the density of CTE dislocationslargely stems from the fact that the di�erence in the

coe�cient of thermal expansion between the matrixalloy and SiC is higher than that between the matrix andAl2O3 [17,18]. Owing to this, the precipitate density inthe SiC reinforced composite is found to be higher thanthat of Al2O3 reinforced composite. In the present alloy,these extra dislocations are particularly important todecide upon the ageing behaviour, as it is already men-tioned that the age-hardening in this alloy takes placeby precipitation on dislocation. This explains why theimprovement in yield stress has been more in the SiCcomposite.

4. Conclusions

Reinforcement of SiC or Al2O3 particulates by 8±10vol% can signi®cantly improve the strength propertiesof the Al±Cr age-hardenable alloy. By suitable control ofthe chemistry of this alloy system it is possible to pro-duce in situ composites of high strength and ductilityin which intermetallics act as the reinforcing dispersoids.

Fig. 7. TEM midrograph of alloy 7 aged at 400�C showing ®ne pre-

cipitations on the dislocations formed around the insoluble inter-

metallics.

Fig. 8. TEM micrograph of Al2O3 reinforced MMC showing disloca-

tions around the particulates.

Fig. 9. TEM micrograph of SiC reinforced MMC showing disloca-

tions around the particulates.

Fig. 10. TEM micrograph of SiC reinforced MMC showing disloca-

tions around the particulates.

S. Datta et al. / Composites Science and Technology 60 (2000) 451±456 455

Acknowledgements

This work was possible because S. Bandyopadhyay ofUNSWwas able to spend a sabbatical leave with ProfessorM.K. Banerjee at B.E. College/University. The authorsalso wish to thank the Australia±India Council, Canberra,for their continuing support in the collaboration.

References

[1] Holl HA. Investigation into the possibility of reducing quench

sensivitity in high strength Al±Zr±Mg±Cu alloys. J Inst Metals

1969;97:200±5.

[2] Nagahama K, Miki I. Precipitation during recrystallisation in

Al±Mn and Al±Cr alloys. J Inst Metals 1974;15:185±92.

[3] Banerjee MK. Age-hardening characteristics of Al±Cr alloys. J

Mater Sci 1997;32:6645±51.

[4] Polmear IJ. Role of trace elements in aged aluminium alloys.

Mater Sci Forum 1987;13(14):195±212.

[5] Banerjee MK, Datta S. Fracture toughness of Al±Cr alloys with

minor addition. Canadian Metall Quarterly 2000;39:73±9.

[6] Dutta I, Allen SM, Ha¯ey JL. E�ect of re-inforcement on the

ageing response of cast 6061 Al±Al2O3 particulate composites.

Metall Trans 1991;22A:2553±63.

[7] Papazian JM. E�ects of SiC whiskers and particles on pre-

cipitation in aluminium matrix composites. Metall Trans

1988;19A:2945±53.

[8] Appendino P, Badini C, Marino F, Tomasi A. 6061 Aluminium

alloys±SiC particulate composite: a comparison between

ageing behaviour in T4 and T6 treatments. Mater Sci Eng

1991;A135:275±9.

[9] Badini C, Marino F, Tomasi A. Kinetics of precipitation hard-

ening in SiC whisker-reinforced 6061 aluminium alloy. J Mater

Sci 1991;26:6279±87.

[10] Lee H, Lu HW, Chan SLI. E�ect of cold rolling on the aging

kinetics of Al2O3/6061 Al composite by di�erential scanning

calorimetric technique. Scripta Metall 25:2165±70.

[11] Das T, Munroe PR, Bandyopadhyay S. The e�ect of Al2O3 par-

ticulates on the precipitation behaviour of 6061 aluminium±

matrix composites. J Mater Sci 1996;31:5351±61.

[12] Lloyd DJ, Jin I. A method of assessing the reactivity between SiC

and molten Al. Metall Trans A 1988;19A:3107±9.

[13] Lioyd DJ. Particle reinforced aluminium and magnesium matrix

composites. Int Mater Rev 1994;39:1±23.

[14] Nagy S, Murgas L, Homonnay Z, Vertes A, Lakner J. Moss-

bauer investigation of Al±Fe±Si phases in aluminium. Mater Sci

Forum 1987;13/14:319±24.

[15] Fricke WG. Aluminium, vol. 1. OH, USA: American Society of

Metals, 1967.

[16] Peterson NL, Rothman SJ. Impurity di�usion in aluminium.

Phys Rev 1970;B1:3264±9.

[17] Elomari S, Boukhili R, Lioyd DJ. Thermal expansion studies of

pre-strained Al2O3/aluminium metal matrix composite. Acta

Mater 1996;44:1873±82.

[18] Ibrahim A, Mohamed FA, Lavernia EJ. Particulate reinforced

metal matrix composites Ð a review. J Mater Sci 1991;26:1137±

56.

456 S. Datta et al. / Composites Science and Technology 60 (2000) 451±456