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Materials Science and Engineering A 528 (2011) 2143–2148

Contents lists available at ScienceDirect

Materials Science and Engineering A

journa l homepage: www.e lsev ier .com/ locate /msea

ffect of particle size on microstructure and mechanical properties of compositesroduced by ARB process

oohollah Jamaati ∗, Sajjad Amirkhanlou, Mohammad Reza Toroghinejad, Behzad Niroumandepartment of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

r t i c l e i n f o

rticle history:eceived 14 October 2010eceived in revised form8 November 2010ccepted 18 November 2010

eywords:

a b s t r a c t

In the present work, Al/10 vol.% SiC metal matrix composite (MMC) was manufactured by accumulativeroll bonding (ARB) process. The silicon carbide particles with two various particle sizes of 40 and 2 �mwere used. Effect of particle size on microstructure (by scanning electron microscopy) and mechanicalproperties (tensile strength and elongation) at various ARB cycles was investigated. It was found that themicrostructural evolution in MMC with 40 �m particle size was more salient compared to the MMCs with2 �m particle size. Also, the composite strip with 40 �m particle size became uniform with high bondingquality and without any porosity sooner than the strip of 2 �m particle size. Moreover, when the number

etal matrix compositeccumulative roll bondingicrostructureechanical properties

of cycles was increased, the tensile strength for both samples was improved. The tensile strength of thecomposite strip with 40 �m particle size was more than the composite strip with 2 �m up to the seventhcycle. By increasing the number of cycles after the seventh cycle, the value of tensile strength of MMCwith 40 �m particle size became saturated and then decreased, and its tensile strength became less thanthat of the composite with 2 �m particle size for the ninth and eleventh cycles. Up to the seventh cycle,when the number of ARB cycles was increased, the elongation of composite strips was decreased, but

tensil

after the ninth cycle, the

. Introduction

The attractive physical and mechanical properties that cane obtained with metal matrix composites (MMCs) such as highpecific modulus, strength and thermal stability have been docu-ented extensively [1–4]. The MMCs have higher strength in shear

nd compression and higher temperature capability because theyre a combination of metallic properties (ductility and toughness)nd ceramic properties (high strength and modulus). Interest inMCs for aerospace, automobiles and other structural applica-

ions has grown over the last few years, as a result of availabilityf relatively inexpensive reinforcement as well as the develop-ent of new processing routes [1,3,4]. Although several processingethods have been developed to produce MMCs such as pow-

er metallurgy and spray deposition, casting is the cost-effectivene. In spite of that, cast aluminum alloy composites have seri-us problems for practical applications. The tensile ductility and

racture toughness at room temperature of the cast MMCs areignificantly poor and their relatively high content of ceramic par-iculates makes them too hard. These problems encourage thestablishment of new fabrication processes which can efficiently

∗ Corresponding author. Tel.: +98 911 2124023.E-mail address: r.jamaatikenari@ma.iut.ac.ir (R. Jamaati).

921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2010.11.056

e elongation for both samples was improved.© 2010 Elsevier B.V. All rights reserved.

produce engineering aluminum alloy composites [5]. Recently,present authors have developed accumulative roll bonding (ARB)[6–8] process and also invented a new technique, namely, contin-ual annealing and roll-bonding (CAR) [9,10] process, in order toovercome the aforementioned problems. It should be noted thatthe ARB was invented by Saito et al. [11,12]. Accumulative rollbonding has been established as an effective means to fabricateultrafine grained sheet metals by severe plastic deformation (SPD).Compared to other SPD techniques, such as equal channel angularextrusion and high pressure torsion, the ARB technique can be moreeasily scaled up for large production quantities of sheet metals[11,12].

The present authors [6–10] have reported much more improvedmicrostructure and mechanical properties of MMCs by ARB andCAR processes. This improvement is associated with better par-ticulate distribution and uniformity of the ARBed and CARedmicrostructures. However, the effect of particle size on microstruc-ture and mechanical properties of produced composites has notbeen studied. Alizadeh and Paydar [13] have also fabricated thenanostructure Al/SiC composites by ARB process, but they have notinvestigated the effect of particle size.

Therefore, the aim of the present work is to produce Al/10 vol.%SiC MMCs by ARB process and investigate the effect of the siliconcarbide size on the microstructure and mechanical properties ofmanufactured composites.

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. Experimental procedure

The materials used in this study were fully annealed strips ofA1050 aluminum alloy and SiC particles with two different aver-ge sizes, 2 and 40 �m. To produce a satisfactory metallurgicalond by the ARB process, it is essential to remove any contami-ations that may be present on the surfaces of the metals to be

oined. A number of authors have claimed degreasing followedy scratch brushing with a rotating steel brush to be the bestethod for surface preparation [14–19]. Therefore, two strips of

50 mm × 50 mm × 1.5 mm were degreased in acetone bath andcratch brushed with a stainless steel wire brush 0.26 mm iniameter. After surface preparation, SiC particles were uniformlyispersed between the two strips by a brush. In fact, in each cycle,he silicon carbide particles were weigh and added between theluminum strips. Then the strips stacked over each other and fas-ened at both ends by steel wires. The roll bonding process wasarried out with no lubrication, using a laboratory rolling mill, withloading capacity of 20 tons. The roll bonding process was carriedut with a specific amount of reduction equal to 50%. Then, the rollonded strips were cut in half. The same procedure was repeated upo five cycles at room temperature (the first step). Then, to achieveuniform distribution of reinforcement particles in the matrix andlso to remove porosities in the interfaces of aluminum–silicon car-ide and aluminum–aluminum, the above procedure was repeatedgain up to eleven cycles without adding reinforcement particlesthe second step). The schematic illustration of the ARB processor manufacturing of composite is shown in our previous article7].

PHILIPS XL30 scanning electron microscopy (SEM) was used foricrostructural observation to evaluate how well the SiC parti-

les were distributed in the produced composites at various ARBycles. The tensile test samples were machined from the ARBedtrips, according to the ASTM E8M standard, to get oriented alonghe rolling direction. The gauge width and length of the tensile testamples were 6 and 25 mm, respectively. The tensile tests wereerformed at room temperature on a Hounsfield H50KS testingachine at an initial strain rate of 1.67 × 10−4 s−1. The total elon-

ation of the samples was determined as the difference betweenauge lengths before and after testing.

. Results and discussion

.1. Microstructure observation

SEM micrographs of the microstructure for ARBed Al/10 vol.%iC MMCs with various particle sizes (2 and 40 �m) after the fifth,eventh, ninth, and eleventh cycles are presented in Fig. 1. After fiveycles, it is clear that there are large particle free zones and agglom-rated and clustered SiC particles in aluminum matrix (Fig. 1(a)nd (b)). It is important that for particle size of 40 �m, the par-icle free zones are larger, while the number of agglomerated orlustered particles is smaller than that of 2 �m particle size. Gen-rally, it can be said that the distribution of silicon carbide particlesn aluminum matrix for both samples after the fifth cycle is com-letely non-uniform. Fig. 1(c) and (d) demonstrates that after theeventh cycle, the large particle free zones for 40 �m particle sizeecome smaller and the big number of agglomerated SiC for 2 �marticle size becomes lower. On the other hand, the agglomera-ion and clustering of SiC for 40 �m particle size and the size of

article free zones have not changed pronouncedly. Therefore, ithould be said that the distribution of SiC particles for both samplesfter the seventh cycle becomes better, but it is still non-uniform.t can be seen from Fig. 1(e) and (f) that after the ninth cycle, theumber of agglomerated and clustered SiC particles and the size

gineering A 528 (2011) 2143–2148

of particle free zones become again lower for both samples. But, itcannot be said that these microstructures are completely uniform.As the ARB process progressed forward and after the eleventh cycle(Fig. 1(g) and (h)), the clusters and the particle free zones almostdisappeared. However, for the sample with 2 �m particle size, thereare still clustered silicon carbide particles in the aluminum matrix.In other words, unlike the samples with 40 �m particle size, themicrostructure of the MMC with 2 �m particle size did not indicatea wholly uniform distribution of the SiC particles in the aluminummatrix after eleven cycles. Generally, it should be noted that themicrostructural evolution in samples with 40 �m particle size wasmore noticeable compared to the MMCs with 2 �m particle size. Infact, MMC with 40 �m particle size becomes uniform sooner thanthat with 2 �m particle size.

Regarding the techniques used to establish atom-to-atom bondsbetween strip layers, it has been reported [3,14–21] that the filmtheory is to be a major mechanism in the roll bonding process.During roll bonding, two opposing brittle surface oxide layers pro-duced after surface preparation break up coherently to expose theunderlying metals which are extruded under normal roll pressurethrough widening cracks in the surface oxide layers from bothsides of the interfaces. Also, during rolling, the metal plasticallydeforms and extends. In the presence of silicon carbide particlesbetween the strips in the current investigation, the regions of SiCparticles opened up into particles and, consequently, got uniformlydistributed in the aluminum matrix. Furthermore, according to thefilm theory, aluminum flowed through the widening cracks in thesurface oxide layers. The interface was, therefore, a combination ofsilicon carbide particles and bonded areas of extruded aluminum.Consequently, the opening of the surface oxide layer produced aftersurface preparation allowed for metal–metal contact and roll bond-ing to take place [6–10].

In addition to the uniformity of particles in matrix which wasexplained before, bonding quality between reinforcement andmatrix and also porosity are other effective factors in the MMCproperties. SEM micrographs of the composite strips after five,seven, nine, and eleven cycles at high magnification are shown inFig. 2 for both particle sizes. From this figure, the bonding qualitybetween silicon carbide particles and aluminum matrix and alsoporosity can be observed. For five and seven cycle ARBed sam-ples, the bonding of the Al/SiC is very weak for particle sizes of2 and 40 �m. In the case of 2 �m particle size, the porosity canbe observed more in the clusters of SiC particles. From Fig. 2(e)and (f), it is obvious that unlike the MMC with 2 �m particle size,the sample with 40 �m particle size has a high bonding qualitywithout any porosity in the matrix. However, when the number ofARB cycles increases again, the bonding becomes stronger and theporosity decreases so that after the eleventh cycle (Fig. 2(g) and(h)), there are no debonding and porosity in both composite strips.It can be suggested that the subsequent rolling improves the bond-ing of the Al/SiC interfaces introduced in the previous cycle andeliminates the porosity. In fact, the microstructure after eleven ARBcycles indicates a good bond since neither debonding nor poros-ity is observed. Therefore, it can be concluded that the bondingquality of the interface is very good by 50% thickness reductionin each cycle, which is considered very important in the ARBprocess.

It should be noted that the rolling process is advantageousover other metal forming processes such as extrusion. It hasbeen reported [22,23] that the extrusion process can improve themicrostructure of metal matrix composites. But, unlike the rolling

process, the extrusion cannot eliminate porosity completely andcan produce a high quality bonding between particles and matrix.In the extrusion process, hydrostatic compressive stresses exertedon the samples by reaction with die-walls assist in the closure ofpores. While the rolling process involves two types of stresses at

R. Jamaati et al. / Materials Science and Engineering A 528 (2011) 2143–2148 2145

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increased, the tensile strength for both samples improved. As seen,the tensile strength of the ARBed MMCs is 2.46 (2 �m) and 2.33

ig. 1. SEM micrographs of the Al/10 vol.% SiC composite microstructures at RD–TDycle for the particle sizes of: (a, c, e, and g) 40 and (b, d, f, and h) 2 �m.

he point of contact between the roll and the strip, the compressiveadial pressure increases by decreasing the roll gap and the tangen-ial shear stress due to the friction between the roll and the strip.oth stresses have horizontal and vertical components. Therefore,he matrix is allowed to flow in various directions and the poros-ty disappears. But, the matrix flow between higher particle size isasier than that between lower particle size. Thus, it is reasonablehat the microstructure of the composite with higher particle size

40 �m) becomes completely uniform with high bonding qualitynd without any porosity sooner than the composite with lowerarticle size (2 �m).

e after: (a and b) the fifth, (c and d) seventh, (e and f) ninth, and (g and h) eleventh

3.2. Mechanical properties

Variations in tensile strength and elongation of the alu-minum/10 vol.% SiC MMCs produced by the ARB process versusnumber of cycles for both particle sizes are shown in Figs. 3 and 4,respectively. It can be seen that when the number of cycles

(40 �m) times higher than those obtained for the annealed sam-ple. It has been reported that the tensile strength changes in ARBprocess are governed by two main strengthening mechanisms,

2146 R. Jamaati et al. / Materials Science and Engineering A 528 (2011) 2143–2148

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ig. 2. SEM micrographs of the RD–TD plane of the Al/10 vol.% SiC composite microsnd (g and h) eleventh cycles for the particle sizes of: (a, c, e, and g) 40 and (b, d, f, a

amely, strain hardening by dislocations, and grain refinement6,7,11–13]. In the initial cycles of accumulative roll bonding pro-ess, the strain hardening or dislocation strengthening play a mainole in increasing the tensile strength, while at final stages, higherensile strength is achieved by grain refinement. In other words,hen the number of cycles increases, the role of work hardening

ecreases and gradual evolution of ultra fine grains plays the mainole in strengthening. On the other hand, it is very important tobserve that in this study, strengthening of the MMCs results fromoth of the two mechanisms mentioned above, and is also affectedy other six factors as follows:

res at high magnification after: (a and b) the fifth, (c and d) seventh, (e and f) ninth,2 �m.

1. Reinforcing role of the SiC particles: Silicon carbide particles canincrease the threshold stress for dislocation glide and cause thegeneration of additional dislocations around SiC particles, thusdecreasing the mobility of dislocations during plastic deforma-tion. These effects lead to increases in the strength and decreasesin the ductility of ARBed MMC. Also, the presence of SiC parti-

cles in aluminum matrix generates the Al/SiC interfaces. Duringplastic deformation, these interfaces are very good sites for ini-tiation and propagation of cracks. Therefore, the ductility ofMMC decreases. In addition, it is well known that higher vol-ume fraction of particles causes more strain hardening during

R. Jamaati et al. / Materials Science and En

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ig. 3. Variation of the tensile strength versus the number of ARB cycles for theomposite strips with 40 and 2 �m particle size.

plastic work, and as a result, the tensile strength improves whileductility decreases.

. Uniformity of the SiC particles: This factor has an important effecton both tensile strength and elongation. The distribution of thesilicon carbide particles in the aluminum matrix changes to bemore uniform and homogeneous when the number of ARB cyclesincreases. Therefore, the distance between the Al/SiC interfacesincreases and during plastic deformation the cracks initiatedin the interfaces will propagate and link up with other crackslater. Thus, the tensile strength and elongation values of MMCimprove.

. Size of the SiC particles: The fine dispersion of silicon carbide par-ticles (fine particle size) serves as a barrier to the movement of

subgrain or grain boundaries, retarding the recovery, recrystal-lization, and grain growth of the MMC’s structure, even at hightemperatures [24], and thus thereby increasing the strength. But,using the fine particle size generates more Al/SiC interfaces, and

ig. 4. Variation of the elongation versus the number of ARB cycles for compositetrips with 40 and 2 �m particle size.

gineering A 528 (2011) 2143–2148 2147

therefore, according to the later factor, the ductility decreases.4. Bonding quality of the Al/SiC interfaces: This main factor has an

important effect on tensile strength and elongation of the MMCs.In fact, composite interface strength plays a vital role in improv-ing the strength and ductility. In case of the MMCs, the fracturemechanism dominated by the crack initiation at the interfacepropagates through the interface and links up with other cracksor reinforcement/matrix interface causing the failure. When thenumber of ARB cycles increases, the bonding between the rein-forcement (silicon carbide) and the matrix (aluminum) becomesstronger due to the greater rolling pressure applied, resulting inthe enhancement of the tensile strength and elongation value.

5. Porosity: Like above factors, porosity has a major effect onmechanical properties of MMCs, too. In fact, the presence ofporosity in composites, especially the porosity surrounding thereinforcement particles, results in a lower tensile strength andelongation. When the number of ARB cycles increases, theporosity of the samples, due to good formability of the alu-minum matrix and rolling pressure, decreases. Therefore, tensilestrength and ductility increase.

6. Mismatch between the coefficients of thermal expansion (CTE): Ithas been reported [25] that the sample temperature increasesduring plastic deformation. The calculated temperature rise forcommercial purity aluminum was 50 K from the first to the thirdARB cycles and 90 K at the fourth cycle, and it was about 110 Kafter the sixth cycle [25]. In addition, these calculated values areindisputably smaller than actual temperature rise because of thetwo following phenomena: first, the frictional heat between rollsand strips is neglected, and second, it should also be noted thatsilicon carbide has a very low thermal conductivity (45 W/m K)while aluminum has a high thermal conductivity (250 W/m K)[6]. Thus, the temperature rise by plastic deformation is expectedto be mainly conducted by the aluminum matrix. That is, theplastic deformation greatly increases temperature in the alu-minum matrix. On the other hand, the CTE of silicon carbideand aluminum are 4.1 × 10−6 and 23.1 × 10−6 K−1, respectively.Therefore, after any ARB cycles and when the strip is cooling,silicon carbide particles and aluminum matrix generate disloca-tions in the sample as a result of heavily built multidirectionalthermal stress at the Al/SiC interfaces induced by the large dif-ference of the CTE between the matrix and the reinforcement.Thus, the tensile strength improves and the ductility decreases.

The values of tensile strength at the end of the first step (after thefifth cycle) for the aluminum/10 vol.% SiC composites with 40 and2 �m particle sizes were 196.8 and 191 MPa, respectively. But thoseat the end of the second step (after the eleventh cycle) were equal to205 and 216 MPa for 40 and 2 �m particle sizes, respectively. Theincreased values of tensile strength, as a result of the operationsin the first step, are 124% (40 �m) and 117% (2 �m) and those inthe second step are 4% (40 �m) and 13% (2 �m). The great differ-ence between the increased values for the first and second steps isattributed to eliminating or decreasing of the main strengtheningfactors such as strain hardening and reinforcing role at the secondstep. The slope in the curve at initial cycles is high but dwindles atfinal cycles.

It should be noted that up to the seventh cycle, the tensilestrength for MMC strip with 40 �m particle size is more than 2 �m.According to microstructural observation, the second, fourth, andfifth strengthening factors for the composite with 40 �m particlesize improved sooner and better than that of the MMC with 2 �m

particle size. Therefore, it is reasonable that the tensile strength ofthe composite strip with 40 �m particle size is more than that of2 �m. On the other hand, regarding Fig. 3, by increasing the num-ber of cycles after the seventh cycle, the value of tensile strengthof MMC with 40 �m particle size becomes saturated and then

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ecreases. Also, its tensile strength becomes less than the com-osite with 2 �m for the ninth and eleventh cycles. This is probablyttributed to the third strengthening factor and occurrence of con-inuous recrystallization in the MMC strip with 40 �m particle size.cceleration of continuous recrystallization is attributed both to

he increased driving force arising from the dislocations generatedy the particles during deformation and to the particle stimulateducleation (PSN). Since the presence of large particles and theature of the deformation structure in the vicinity of the particlesake the particle stimulated nucleation of recrystallization possi-

le, the recrystallization kinetics and grain growth of the MMCs of0 �m particle size occur more rapidly than that of 2 �m. Thus, theensile strength of the composite of 40 �m particle size becomesess than the composite with 2 �m particle size for the ninth andleventh cycles. Also, as mentioned before, unlike the large particleize, the fine particle size serves as a barrier to the movement of sub-rain or grain boundaries, retarding the recovery, recrystallization,nd grain growth of the composites, even at high temperatures.herefore, it is probable and predictable that the value of tensiletrength of MMC with 2 �m particle size will increase after theleventh cycle.

The elongation values of the Al/10 vol.% SiC MMC with variousarticle sizes manufactured by the ARB process with respect to theumber of cycles are presented in Fig. 4. It is clear that at the firsttep, when the number of ARB cycles increased, the elongation ofomposite strips decreased. This can be related to reinforcing rolefirst), uniformity (second), bonding quality (fourth), and porosityfifth) factors. In other words, at the first step when the numberf cycles rises, the reinforcing role and porosity increase while theniformity and the bonding quality decrease. All these changes areelated to adding the SiC particle to any cycles at the first step.eanwhile, the strain hardening has a major effect on decreas-

ng the tensile elongation at the first cycle [6,7,11–13]. Therefore,he first step collectively leads to reducing the ductility. It has beeneported [1] that the elongation decreases with the increasing of theeinforcement particles due to increasing the number of reinforce-ent/matrix interfaces. In addition, higher volume fraction of SiC

auses more strain hardening during plastic deformation, and as aesult the ductility comes down by increasing the number of cyclest the first step for the produced composite strips. Elongation val-es at the end of the first step were 6% for MMC with particle sizes of0 and 2 �m. But those at the end of second step were 6.5% (40 �m)nd 5.2% (2 �m). In fact, after ninth cycle, the tensile elongation foroth samples improved. This is attributed to the second, fourth, andfth strengthening factors. It is important and interesting that theecond step greatly improves the elongation of the composite with

0 �m particle size. This can be related to the occurrence of theontinuous recrystallization and grain growth. All obtained resultsre in good agreement with the present authors’ previous findings6–10], in their investigations concerning the aluminum/aluminaomposite produced by ARB and CAR processes.

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gineering A 528 (2011) 2143–2148

4. Conclusions

In this study, aluminum/silicon carbide metal matrix com-posite was produced by accumulative roll bonding process. Themicrostructure of the composite with higher particle size becomescompletely uniform with high bonding quality and without anyporosity, sooner compared to lower particle size. In other words,particle size has a very important effect on the microstructureof the ARBed composites. The effective strengthening factors dueto increasing the number of ARB cycles included: (a) reinforcingrole of particles, (b) uniformity, (c) size of particles, (d) bondingquality, (e) porosity, and (f) mismatch between the coefficientsof thermal expansion (CTE). Up to the seventh cycle, the tensilestrength of MMC with 40 �m particle size was more than that with2 �m particle size. After the seventh cycle, when the number ofcycles was increased, the value of tensile strength of the com-posite strip with larger particle size became saturated and thendecreased, and its tensile strength became less than the compos-ite with smaller particle size for the ninth and eleventh cycles. Byincreasing the number of cycles, first, the elongation of MMCs wasdecreased, and then the tensile elongation for both samples wasimproved.

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