mi Av

Age-hardening

rk thof homt 1peathescom

indicated the presence of precipitation phases such as CuAl2 and CuMgAl2 in the composite in both as-

attracrs aserospan stud, goodties. A

on precipitation of composite compared with unreinforced alloy[35]. It was proposed that the addition of reinforcing particlesaccelerates the aging kinetics [69], on the other hand, there weresome explanations that it decreases or poses very little alterationin the aging kinetics [10,11].

Cottu et al. [12] showed that age-hardening kinetics ofAlCuMg alloy-10 wt.% SiC ber composite was enhanced bythe presence of the reinforcement during heat treatment. They

vacancies, inadequate dislocation density and extensive interfacialsegregation of alloying elements. Similar observations of the effectof the ceramic particles on the aging kinetics were previously madefor Al alloys-Al2O3 composites [17,18]. Skibo et al. [19] reportedthat there is not obvious difference in the age-hardening kineticsbetween the 6061 Al alloy and its composite reinforced with 10and 20 wt.% SiC aged at 175 C. Although it is accepted that theaging behavior depends on the type of reinforcement and its vol-ume fraction, alloy composition, heat treatment and other process-ing parameters [16], it is known that there is still a lack ofinformation about aging behavior of Al alloys and their composites.

* Corresponding author. Tel.: +98 9133561769; fax: +98 2166165261.

Materials and Design 31 (2010) 23682374

Contents lists availab

an

elsE-mail address: smr.mousavi1364@gmail.com (S.M.R. Mousavi Abarghouie).has the highest hardness [1]. But some of the mechanical proper-ties such as low wear resistance; have limited application of thesematerials. Adding SiC reinforcing particles to these materials leadsto increase the wear resistance [2]. Aging treatment can signi-cantly increase properties of some of Al alloys and their compos-ites, especially 2xxx and 6xxx series alloys. In investigation ofage-hardening kinetics, it has been shown that the addition ofceramic particles to age-hardenable Al alloys has different effects

SiC particles. They showed that the precipitation sequence of thecomposite was similar with that of the unreinforced 6061 alloy,but the aging kinetics was altered. Aging was accelerated becausesolute diffusivity increased as dislocation density increased too.However, Pal et al. [16] showed that in the AlCuMg alloy com-posite reinforced with different percentages of SiC particles, thepresence of reinforcing particles led to decelerate age-hardeningkinetics. They attributed this behavior to lower concentration of1. Introduction

In recent years, Al alloys haveresearchers, engineers and designematerial in different industries like acial 2xxx series of Al alloys have beeof their high strength to weight rationablity and other appropriate proper0261-3069/$ - see front matter 2009 Elsevier Ltd. Adoi:10.1016/j.matdes.2009.11.063extruded and solutionized conditions. For the samples solution treated more than 2 h, hardness valuesdecreased due to the grain growth of matrix but no change occurred in the aging kinetics.

2009 Elsevier Ltd. All rights reserved.

ted attention of manya promising structuralce and automotive. Spe-ied extensively becauseformability, age harde-mong Al alloys, 2024 Al

explained this by the plastic deformation induced during heattreatment due to the difference between coefcients of thermalexpansion (CTE) of matrix and reinforcement. Thomas and King[13] stated that in powder metallurgy (PM) aluminium alloy2124/SiCp composites, the presence of reinforcing particles facili-tates the nucleation of S0 which results in the reduction of the re-quired time to achieve peak hardness. Dutta and Bourell [14] andAppendino et al. [15] studied 6061 aluminum alloy reinforced withthe maximum hardness. At the solution treating time shorter than 2 h due to incomplete dissolution ofprecipitates, the aging kinetics decelerated and the hardness values decreased. X-ray diffraction studiesAging behavior of a 2024 Al alloy-SiCp co

S.M.R. Mousavi Abarghouie *, S.M. Seyed ReihaniDepartment of Materials Science and Engineering, Sharif University of Technology, Azad

a r t i c l e i n f o

Article history:Received 8 September 2009Accepted 29 November 2009Available online 3 December 2009

Keywords:2024 AlComposite

a b s t r a c t

In the present research woallurgy method. The effectsolution treatment of the cand 3 h followed by aging aticles led to increasing theplace at shorter times thanbut took place at longer timwas about 2 h for both the

Materials

journal homepage: www.ll rights reserved.posite

e., Tehran P.O. Box 11155-9466, Iran

e 2024 aluminum alloy was reinforced with SiC particles via powder met-eat treatment conditions on articial aging kinetics was investigated. Theposite sample and the unreinforced alloy was carried out at 495 C for 1, 291 C for various aging times between 1 and 10 h. The existence of SiC par-k hardness of the alloy. The peak hardness of the composite sample tookat of the unreinforced alloy for the samples solution treated for 2 and 3 h,for the samples solution treated for 1 h. The suitable solution treating timeposite and the unreinforced alloy that led to the fastest aging kinetics and

le at ScienceDirect

d Design

evier .com/locate /matdes

Fig. 1. SEM (SE) micrograph of the 2024 Al alloy-20 vol.% SiC composite in the as-extruded, showing a uniform distribution of SiC particles in the extrusiondirection(ED).

ani /Materials and Design 31 (2010) 23682374 2369Considering strict importance and widespread applications ofthese materials, in this research the effect of heat treatment condi-tions and also SiC reinforcing particles on the articial aging kinet-ics and behavior of a 2024 Al-SiCp composite produced by powdermetallurgy technique was studied.

2. Materials and experimental procedure

2.1. Materials

The 2024 Al alloy powder with average particle size of 80 lmwas used in this research that supplied by Khorasan Powder Met-allurgy Company, Mashhad, Iran. The composition of this powderis given in Table 1. Commercial a-SiC particles with average parti-cle size of 20 lm and density of 3.2 g/cm3 were used as thereinforcement.

2.2. Processing

In this research, the aluminum matrix composite was manufac-tured by powder metallurgy technique. In this process the 2024 Alpowder and 20 vol.% SiC particulates were blended in a Turbula.The mixture was cold pressed into Al cans with a load of250 MPa using a STS hydraulic press. Then the pressed sampleswere extruded with the extrusion ratio of 16:1 after pre-heatingat 495 C for 30 min in the argon environment. The ram speedwas 2.5 mm/s in this experiment. The unreinforced 2024 Al alloywas produced by the same method. The as-extruded 2024 Al al-loy-20 vol.% SiC composite and its unreinforced alloy were solutiontreated at 495 C for 13 h and then quenched in the cold water.The specimens were placed inside the furnace, after the requiredtemperature was reached, and time was measured after the tem-perature stabilized. The cooling rate of specimens is expected tobe in the range of 470475 C/s during the quenching, supposingan initial decrease about 2025 C had happened in the interval be-tween the exit from the furnace and quench medium. Some of thesolution treated specimens were kept inside the freezer in the tem-perature about 18 C in order to prevent the natural aging. Othersamples were subsequently articially aged at 191 C for varioustimes.

The chemical composition of microstructural features wasdetermined by use of a scanning electron microscope (SEM) of typeVEGA2 TESCAN equipped with a RONTOC energy dispersive X-ray(EDX) detector. SEM micrographs of the as-extruded and solutiontreated samples were acquired using back-scattered electron(BSE) and secondary electron (SE) detectors. A PHILIPS PW3710 dif-fractometer was used to identify the present phases in the as-ex-truded and solution treated composites using CuKa radiation. Inorder to investigate the aging kinetics, hardness measurementswere performed using an INSTRONWOLPERT Vickers hardness tes-ter with the load of 30 kg. At least 5 hardness measurements weredone on each sample and then averaged.

Table 1Chemical analysis (wt.%) of 2024 aluminum alloy used in this research.

Element Cu Mg Mn Si Fe Al

Chemical composition (wt.%) 4.3 1.4 0.55 0.01 0.01 Balance

S.M.R. Mousavi Abarghouie, S.M. Seyed Reih3. Results and discussion

3.1. Microstructural characterization

Fig. 1 shows a SEM (SE) micrograph of 2024 Al-20 vol.% SiCcomposite in the as-extruded condition. One can see the uniformdistribution of SiCp arranged in the direction of extrusion (ED) at

Fig. 2. (a) SEM (BSE) micrograph of the 2024 Al alloy-20 vol.% SiC composite in theas-extruded; (b) EDX spectrum of a precipitate shown in the SEM image of Fig. 2a.

the matrix structure. Fig. 2a shows a SEM (BSE) micrograph of theas-extruded composite sample. The precipitates which formedduring the extrusion treatment can be seen clearly. It can be seenthat there is a considerable volume fraction of these precipitates inthe matrix. The brightness of these precipitates is due to higheratomic number of Cu in comparison with aluminum, silicon andcarbon. EDX spot analyses can conrm the enrichment of solutes,including Cu and Mg at the bright spots shown in the SEM imageof Fig. 2a. These results have been shown in Fig. 2b. Also the X-ray diffraction (XRD) pattern of this specimen (Fig. 3) can provethe existence of these precipitates as CuAl2 and CuMgAl2 phases.Fig. 4ac shows SEM (BSE) images of 2024Al alloy-20 vol.% SiCpcomposite in three different conditions of solution treatment at495 C for 13 h, respectively. It can be seen from Fig. 4a that thereare much undissolved precipitates in the structure (the brightphases in the microstructure). The typical XRD pattern of the sam-ple solution treated at 495 C for 1 h (Fig. 5) can conrm the exis-tence of these remanent undissolved precipitates in the structure.These phases are often CuAl2 and CuMgAl2. The EDX spot analysesof this sample, with spectra resembling has been shown in Fig. 4d.We can propose that the enrichment of solutes like Cu and Mg atthe structure shown in the SEM image of Fig. 4a can be conrmed.Therefore, the time of solution treatment (1 h) had not been suf-cient for complete dissolution of precipitates because many ofthese precipitates have remained undissolved in the composite

tion treated at 495 C for 3 h. It is clear that the amounts of precip-itates in this condition are almost similar to those in the 2 hsolution treated condition (Fig. 4b). Therefore, the solution treatingtime has a signicant effect on dissolution of the precipitates. Itis important to note at high cooling rates (in this research about470475 C), the possibility of segregation of the solutes and theformation of the intermediate precipitates during quenching fromsolution treating temperature is unlikely. Therefore the precipi-tates and the enrichment zones of solutes in the structure thattheir existence was proved are the precipitates that have existedin the structure before the heat treatment and have not been dis-solved during the solution treatment.

3.2. Aging kinetics

Fig. 6ac shows the hardness changes of the composite and itsunreinforced alloy as a function of aging time after the solutiontreatment at 495 C for 13 h, respectively. It can be seen that withincreasing of the solution treating time to 2 h, the aging kinetics inthe composite and the unreinforced alloy accelerates (the requiredtime for reaching the peak hardness decreases). We can supposethat the solution treating time for complete dissolution of precip-itates was insufcient for the samples solution treated for 1 h. Thistheory can be proved by obtaining much more remained precipi-tates compared with the sample solution treated for 2 h (Fig. 4b).

2370 S.M.R. Mousavi Abarghouie, S.M. Seyed Reihani /Materials and Design 31 (2010) 23682374matrix. Fig. 4b shows the SEM image of the composite sample solu-tion treated at 495 C for 2 h. As can be seen, a few undissolvedprecipitates (bright phases) have remained in the matrix. It is clearthat the undissolved precipitates in the sample solution treated for2 h (Fig. 4b) are much less than those in the sample solution trea-ted for 1 h (Fig. 4a). Image Analysis of the samples solution treatedfor 1 and 2 h with the same interfaces of matrix-particles showedthat the remnant precipitates in the composite matrix in the lattersample (solution treated for 2 h) equals almost one fth of those inthe former sample (solution treated for 1 h). In fact, the dissolutionof precipitates in the sample solution treated for 2 h is much closerto its completion than that in the sample solution treated for 1 h.Fig. 4c shows the SEM micrograph of the composite sample solu-Fig. 3. XRD pattern of the as-extruded 2Therefore, the lower aging kinetics of the composite sample atshorter solution treating time can be attributed to the presenceof undissolved precipitates. This can lead to reduction of alloyingelements content in the matrix and will subsequently reduce thesupersaturation of solute. It leads to reduce the chemical drivingforce for the precipitation and subsequently causes the sloweraging kinetics. So in the samples solution treated for 2 h due tosuitable solution treating time, a considerable volume fraction ofprecipitates has dissolved. Therefore, the aging kinetics increases.The reason of hardness changes in the unreinforced alloy is similarto the composite sample. We can see from Fig. 6a and b that thepeak hardness values have increased for both the composite andthe unreinforced alloy on solution treatment for 2 h compared to024 Al alloy-20 vol.% SiC composite.

ani /S.M.R. Mousavi Abarghouie, S.M. Seyed Reihthose on solution treatment for 1 h. This can be attributed to highervolume fraction of precipitates in the former (solution treated for2 h). In the solution treatment condition for 1 h, incomplete disso-lution of the precipitates leads to decrease the volume fraction ofthe strengthening precipitates formed during the aging in compar-ison with those formed during the aging treatment after solutiontreated for 2 h. Also the hardness of the composite sample andthe unreinforced alloy that both were solution treated for 2 h withno aging treatment, decreased compared to the samples solutiontreated for 1 h with no aging treatment. This can be attributed todecrease of the undissolved precipitates and the grain growth ofthe matrix in 2 h solution treated condition. Therefore, the increaseof the solution treating time leads to more dissolution of the pre-cipitates and will accelerate aging kinetics, but it decreases thehardness value (in solution treated condition with no agingtreatment).

It can be seen from Fig. 6a that the required time to reach thepeak hardness of the unreinforced alloy and the composite is 7and 8 h, respectively. So in this condition (1 h solution treated)the aging kinetics of the unreinforced alloy is faster than that of

Fig. 4. SEM (BSE) micrograph of the 2024 Al alloy-20 vol.% SiC composite solution treatedSEM image of Fig. 4a.Materials and Design 31 (2010) 23682374 2371the composite. This is contrary to the expected results. Frequentlyit is expected that the aging kinetics of composite be faster thanthat of the unreinforced alloy. This is due to the high dislocationdensity at the interface of matrix-particle in the composite. Thesedislocations provide suitable sites for the heterogeneous nucle-ation of the second phase precipitates and subsequently lead tothe acceleration of the aging kinetics. When the composite iscooled from the solution treating temperature, mismatch strainsoccur due to difference between the coefcients of thermal expan-sion of the matrix and the ceramic particles (CTE2024Al = 24 106/C and CTESiC = 4 106/C) [16]. These thermal strains can be cal-culated by Eq. (1) as follows [20]:

eth Da DT 1

where Da is the difference between the coefcients of thermalexpansion of the matrix and the reinforcement and DT is the tem-perature changes.

Because of the thermal mismatch strains, plastic deformationwill take place, and this produces a high density of dislocations,

at 495 C for: (a) 1 h; (b) 2 h; (c) 3 h, (d) EDX spectrum of a precipitate shown in the

ni /2372 S.M.R. Mousavi Abarghouie, S.M. Seyed Reihaespecially in the vicinity of SiC particles. The density of dislocationsq can be calculated as follows [20]:

q B Vf eb t1 f 2

Fig. 5. XRD pattern of the 2024 Al alloy-20 vol.% Si

Fig. 6. Variation of hardness vs. aging time for 2024Al-20 vol.% SiC composite and 20Materials and Design 31 (2010) 23682374where B is a geometric constant that is theoretically between 4 (forreinforcement aspect ratio =1) and 12 (for reinforcement aspectratio = 1), e the thermal strain given by Eq. (1), Vf the volume frac-tion of reinforcement, b the Burgers vector and t the smallest

C composite solution treated at 495 C for 1 h.

24 Al alloy aged at 191 C and solutionized at 495 C for: (a) 1 h; (b) 2 h; (c) 3 h.

omp

ani /dimension of the reinforcement. During the aging, these disloca-tions are the suitable sites for heterogeneous nucleation of precipi-tates; furthermore they act as paths for pipe diffusion and thusincreasing the kinetics of aging [2].

However, in this research the aging kinetics of the compositewas slower than that of the unreinforced alloy according toFig. 6a. The slower kinetics of aging in the Al alloys-SiC compositescompared to that in their unreinforced alloys can be attributed tothe following theories [16]:

(a) Excessive segregation of alloying elements at the AlSiCinterfaces and presence of undissolved and coarse precipi-tates in the structure lead to depletion of the solute insidethe matrix. This will lead to the lower supersaturation of sol-ute. A lower degree of supersaturation of solute would

Fig. 7. (a) SEM (BSE) micrograph of the solutionized 2024 Al alloy-20 vol.% SiC c

S.M.R. Mousavi Abarghouie, S.M. Seyed Reihreduce the chemical driving force for precipitation in thecomposite. It will reduce the rate of process, consequently.

(b) Dislocation density in the composite that has a critical rolefor precipitation may be inadequate for accelerating theaging kinetics [16].

(c) The presence of SiC particles leads to the reduction in thevolume fraction of GuinierPreston (GP) zones. This is dueto the lower vacancy concentration in the matrix of the com-posite compared to that in the unreinforced alloy. Lowervacancy concentration in the composite is due to the largearea of matrix-particle interfaces that acts as vacancy sinks.Therefore, the lower vacancy concentration in the compositecompared to that in the unreinforced alloy is responsible forretarding the formation of GP zones. So delay in initial stepof aging due to lower density of GP zones may affect theremaining steps of aging and leads to the slower aging kinet-ics of composite compared to that of its unreinforced alloy.

In this study, the slower aging kinetics of the composite com-pared to the unreinforced alloy is probably due to the existenceof many matrix-particles interfaces in the composite, which aresuitable sites for segregation of the solutes and stability of the pre-cipitates. This leads to lower concentration of the solutes in thematrix of the composite and causes a decrease in the degree ofthe solutes supersaturation. This will cause a decrease in the chem-ical driving force for precipitation and therefore decelerates theaging kinetics. Fig. 7a shows a SEM (BSE) image of the compositesample. From the gure it is clear that much volume fraction ofthe undissolved precipitates (bright phases) are located at theAlSiC interfaces. This indicates that the matrix-particle interfacesare preferential sites for segregation of the solutes and stability ofthe precipitates. Also EDX spot analyses, with spectra resemblingthat in Fig. 7b, have conrmed the enrichment of solutes like Cuand Mg at many of the AlSiC interfaces shown in the SEM imageof Fig. 7a. Previously Hunt et al. [21] have reported retardation of S0

precipitation in the matrix of the composite. They have attributedthis behavior to higher intermetallic phases content in the compos-ite compared to that in the unreinforced alloy and incomplete solu-tion treatment, which makes solutes unavailable for S0 formation.However, for the composite samples solution treated for 2 and3 h, the time for dissolving of more precipitates is sufcient andso the effect of matrix-particles interfaces on stability of precipi-

osite, (b) EDX spectrum of the precipitates shown in the SEM image of Fig. 7a.

Materials and Design 31 (2010) 23682374 2373tates decreases. Therefore, as it can be seen from Fig. 6b and cthe aging kinetics of the composite is faster than that of the unre-inforced alloy. By comparison of Fig. 6b and c, it can be seen thatthe aging kinetics has not almost changed in solution treatmentcondition for 3 h compared to that in solution treatment conditionfor 2 h, in both the composite and the unreinforced alloy samples.Therefore, the solution treating time more than 2 h does not haveany obvious effect on the aging kinetics for the composite and itsunreinforced alloy. Also it is obvious by comparison of Fig. 6band c, that the peak hardness value has decreased for the samplessolution treated for 3 h, compared to that of the samples solutiontreated for 2 h. This can be attributed to grain growth of the matrixwith increasing of the solution treating time [2224]. With increas-ing of the solution treating time, the grain size of the matrix hasbeen increased which would cause fewer obstacles (grain bound-aries) to the movement of dislocation and subsequently will de-crease the hardness and the strength [2224]. Therefore, solutiontreatment for 2 h had been a suitable time for dissolution of moreprecipitates. Longer solution treating times would not accelerateaging kinetics signicantly, although lead to growth of grains andsubsequently decrease the hardness. In fact, in solution treatingtimes more than 2 h, the solution treating time on the grain growthand subsequently on decrease of the hardness has been more effec-tive than that on the aging kinetics. Thus, one can say that 2 h is acritical time for solution treatment of this composite and its unre-inforced alloy since the fastest aging kinetics and the maximumhardness value is obtained in this time.

4. Conclusion

The aging behavior of the 2024 Al alloy and its composite rein-forced with 20 vol.% SiC particles was studied after solution treat-ment at 495 C for 13 h. The conclusions derived from this studycan be given as follows:

(1) The precipitation phases present in the composite in bothas-extruded and solutionized conditions were CuAl2 andCuMgAl2 phases. The composite samples solution treatedfor 1 h have a larger fraction of these undissolved precipi-tates than those solutionized for 2 and 3 h.

(2) The suitable solution treating time was about 2 h for boththe composite and the unreinforced alloy that leads to thefastest aging kinetics and the maximum hardness.

(3) At the solution treating time shorter than 2 h, the agingkinetics decelerated and the hardness values decreased. Atthe solution treating time longer than 2 h, the hardnessvalue decreased but no change occurred in the aging kinet-ics. The difference in age-hardening behavior based on the

[2] Guo J, Yuan X. The aging behavior of SiC/Gr/6061 Al composite in T4 and T6treatments. Mater Sci Eng A 2009;499:2124.

[3] Chen KC, Chao CG. Effect of d alumina bers on the aging characteristics of2024-based metal-matrix composites. Metall Trans A 1995;26:103543.

[4] Bekheet NE, Gadelrab RM, Salah MF, Abdel-Azim AN. The effects of aging onthe hardness and fatigue behavior of 2024 Al alloy/SiC composites. Mater Des2002;23:1539.

[5] Nieh TG, Karlak RF. Aging characteristics of B4C-reinforced 6061 aluminum.Scripta Metall 1984;18:258.

[6] Christman T, Suresh S. Microstructural development in an aluminum alloy-SiCwhisker composite. Acta Metall 1988;36:1691704.

[7] Suresh S, Christman T, Sugimura Y. Accelerated aging in cast Al alloy-SiCparticulate composites. Scripta Metall 1989;23:1599602.

[8] Mandal A, Maiti R, Chakraborty M, Murty BS. Effect of TiB2 particles on agingresponse of Al4Cu alloy. Mater Sci Eng A 2004;386:296300.

[9] Lu L, Lai MO, Su Y, Teo HL, Feng CF. In situ TiB2 reinforced Al alloy composites.Scripta Mater 2001;45:101723.

[10] Li W, Long JP, Jing S, Shen BL, Gao SJ, Tu MJ. Aging characteristics of shortmullite ber reinforced Al4.0Cu1.85Mgmetal matrix composite. J Mater EngPerform 2003;12:1922.

[11] Janowski GM, Pletka BJ. The effect of particle size and volume fraction on theaging behavior of a liquid-phase sintered SiC/aluminum composite. MetallMater Trans 1995;26A:302735.

[12] Cottu JP, Coudere JJ, Viguier B, Bernard L. Inuence of SiC reinforcement onprecipitation and hardening of a metal matrix composite. J Mater Sci1992;27:306874.

[13] Thomas MP, King JE. Comparison of the aging behaviour of PM 2124 Al alloyand AlSiCp metal-matrix composite. J Mater Sci 1994;29:52728.

2374 S.M.R. Mousavi Abarghouie, S.M. Seyed Reihani /Materials and Design 31 (2010) 23682374time of solution treatment has been attributed to the differ-ent concentration of solutes in solid solution after the solu-tion treatment. On solution treatment for 1 h due toincomplete dissolution of precipitates, the concentration ofsolutes in the solid solution is lower than that on solutiontreatment for 2 and 3 h.

(4) The composite reached its peak hardness in shorter timecompared with the unreinforced alloy on solution treatmentfor 2 and 3 h, but reached it in longer time on solution treat-ment for 1 h.

Acknowledgments

The authors would like to express their gratefulness to Materi-als Science and Engineering Department of Sharif University ofTech. ofcials who cooperated during experiments.

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[19] Skibo M, Morris PL, Lloyd DJ. In: Fishman SG, Dhirngra AK, editors. Castreinforced composites. ASM International; 1988. p. 25761.

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[22] Chen YC, Feng JC, Liu HJ. Stability of the grain structure in 2219-O aluminumalloy friction stir welds during solution treatment. Mater Charact2007;58:1748.

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Aging behavior of a 2024 Al alloy-SiCp compositeIntroductionMaterials and experimental procedureMaterialsProcessing

Results and discussionMicrostructural characterizationAging kinetics

ConclusionAcknowledgmentsReferences