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J. Mater. Sci. Technol., 2013, -(-), 1e4

Thermal Expansion of Al Matrix Composites Reinforced with Hybrid Micro-/

nano-sized Al2O3 Particles

Zhibo Lei1), Ke Zhao1), Yiguang Wang1)*, Linan An2)*

1) School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China2) Advanced Materials Processing and Analysis Center, Department of Materials Science and Engineering, University of Central

Florida, Orlando, FL 32816, USA[Manuscript received September 21, 2012, in revised form November 22, 2012, Available online xxx]

* Corresnwpu.ed1005-03JournalLimited.http://dx

Please

The thermal expansion behavior of aluminum matrix composites reinforced with hybrid (nanometer andmicrometer) Al2O3 particles was measured between 100 and 600 �C and compared to theoretical models.The results revealed that the nanoparticle concentration had significant effect on the thermal expansionbehavior of the composites. For the composites with lower nanoparticle concentration, their coefficient ofthermal expansion (CTE) is determined by a stress relaxation process. While for the composites with highernanoparticle concentration, their CTE is determined by a percolation process.

KEY WORDS: Metal-matrix composites (MMCs); Thermal properties; Hybrid particles

1. Introduction

Ceramic particle reinforced aluminum metal-matrix compos-ites (Al-MMCs) have received extensive attentions for theirexcellent properties and widespread applications. Due to theirlightweight, high specific mechanical strength and modulus, andbetter wear and creep resistance, Al-MMCs are replacing con-ventional alloys as structural materials for automotive andaerospace related applications[1e5]. The materials are alsoconsidered as one of promising and versatile materials forelectronic packaging[6e8] due to their good thermal conductivityand low coefficient of thermal expansion (CTE)[9,10]. Recently, ithas been reported that the properties of Al-MMCs can be furtherimproved by reinforcing them with high volume fractions ofnanometer-sized ceramic particles. For example, the wear resis-tance of Al reinforced with 15 vol.% of Al2O3 nanoparticle waseven better than that of stainless steel[11]. The surface of Al-alloytreated with ceramic nanoparticles also exhibited significantlyimproved wear resistance[12]. Such improvements can furtherbroaden the applications of the materials.For most applications of Al-MMCs, CTE is a key parameter

that needs to be carefully considered and tailored. Thereby,thermal expansion behavior of Al-MMCs has been extensively

ponding authors. Prof., Ph.D.; E-mail addresses: wangyiguang@u.cn (Y. Wang), lan@mail.ucf.edu (L. An).02/$e see front matter Copyright� 2013, The editorial office ofof Materials Science & Technology. Published by ElsevierAll rights reserved..doi.org/10.1016/j.jmst.2013.04.022

cite this article in press as: Z. Lei, et al., Journal of Materials Science

studied in the last decades. Previous studies primarily focused onmaterials reinforced with micrometer-sized particles, long fibersand whiskers[13e17]; little attention has been paid to the materialswith nanometer-sized reinforcing particles. In particular, the CTEof the composites with a high volume fraction of nanometer-sized reinforcements has been rarely reported.In this paper, we report the thermal expansion behavior of a

set of Al matrix composites reinforced with hybrid (nanometer-and micrometer-sized) alumina particles. It is demonstrated thatthe concentration of nanometer-sized particles has significanteffects on the CTE. Such effects have been discussed in term ofthe effect of nanoparticles on the microstructures and residualstresses of the composites.

2. Experimental Procedure

Pure Al powder of w100 mm with 99.9% purity (ShanghaiShan Pu Chemical, Shanghai, China) and Al2O3 powders of3 mm and 50 nm (Buehler, Bluff, Illinois, USA) were used asthe starting materials. Pure powders of Al and Al2O3, in thedesired sizes and volume fractions, were mixed together byhigh-energy ball milling for 20 h to ensure the uniform mixing.The detailed mixing procedure has been reported in litera-ture[18]. The entire procedure was carried out in argon atmo-sphere to minimize the contamination resulting from handlingpowders in air. The obtained powder mixtures were then sin-tered to bulk specimens by hot pressing at 780 �C with apressure of 50 MPa in vacuum, followed by quickly cooling toroom temperature in 30 min. In this study, four differentcomposites were prepared (Table 1).

& Technology (2013), http://dx.doi.org/10.1016/j.jmst.2013.04.022

Table 1 Relative density and hardness measured of materials studied inthis paper

Materials Composition (vol.%) Relativedensity (%)

Hardness(GPa)

Al 50 nmAl2O3

3 mmAl2O3

AA-1 90 5 5 98.8 1.8AA-2 85 5 10 98.6 1.9AA-3 85 10 5 98.7 2.5AA-4 80 10 10 98.3 2.3

2 Z. Lei et al.: J. Mater. Sci. Technol., 2013, -(-), 1e4

The density of the obtained composites was measured usingArchimedes displacement method according to ASTM C-20standard. The hardness of the obtained composites was measuredusing a micro-hardness tester (HX-1000TM/LCD, ShanghaiTaiming Optical Instrument Co., Ltd, Shanghai, China), andresults are listed in Table 1. It is seen that regardless the size andthe volume fraction of the reinforcements, the density of allsamples is closed to their theoretical density.The specimens of 25 mm � 3 mm � 3 mm in dimensions

were cut from the bulk samples using a wire electrical dischargemachine for CTE measurement. The thermal expansion was thenmeasured with a dilatometer (DIL 402C, Netzsch, Selb, Ger-many) between 100 and 500 �C at heating and cooling rates of5 �C min�1 in argon. The instantaneous CTE at a given tem-perature was calculated using the following equation:

CTE ¼ v

vT

�DLL

�(1)

where L is the length of the specimen and T the temperature.

Fig. 1 Instantaneous coefficient of thermal expansion as a function

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3. Results and Discussion

In Table 1, it is seen that the hardness of the samples con-taining 10 vol.% nanometer-sized reinforcements is much higherthan that of samples containing 5 vol.% nanometer-sized parti-cles, regardless the concentration of the micrometer-sized rein-forcement. This phenomenon becomes even clear by comparingsamples AA-2 and AA-3, which contain the same amount ofreinforcement. The result is consistent with previous observa-tions that nanometer-sized particle was more effective thanmicrometer-sized one for improving the properties of Al-basedcomposites[11].The instantaneous CTE obtained from the first and second

cycles as a function of temperature is plotted in Fig. 1 for allmaterials. The effect of the concentration of the nanometer-sizedreinforcement on the thermal expansion behavior is obvious. Forthe samples containing 5 vol.% nanoparticles (AA-1 and AA-2),the CTE measured during the first heating cycle increases line-arly with increasing temperature between 100 and 300 �C. Athigher temperatures, the CTE continuously increases at a rela-tively high rate until w470 �C (w410 �C for material AA-2) toreach its maximum; with further increasing temperature, the CTEdecreases at a fairly high rate. However, the CTE measuredduring the second heating cycle has no such a maximum.Instead, it increases continuously with increasing temperature atthe rate similar to that measured in the first heating cycle be-tween 100 and 300 �C. On the other hand, the CTE of the ma-terials containing higher concentration nanoparticles (AA-3 andAA-4) increases continuously within the entire testing tempera-ture range without the maximum. In addition, there is no sig-nificant difference between CTEetemperature curves measuredin the first and the second heating cycles.

of temperature for: (a) AA-1, (b) AA-2, (c) AA-3, (d) AA-4.

& Technology (2013), http://dx.doi.org/10.1016/j.jmst.2013.04.022

Fig. 2 Compare the CTE of different composites.

Z. Lei et al.: J. Mater. Sci. Technol., 2013, -(-), 1e4 3

Fig. 2 compares the thermal expansion behavior of the foursamples obtained from the second heating cycle. It can be seenthat the CTE value for samples AA-1, AA-2 and AA-3 aresimilar, but much higher than that for sample AA-4.For composite materials, residual stresses, resulted from

thermal mismatch between the matrix and the reinforcement,play a key role in determining the thermal expansion behavior.Fei et al.[17] have observed that thermal plastic deformationinduced by fast relaxation of residual stress could cause themaximum value in CTEetemperature curves of whisker rein-forced pure aluminum composites. It is believed that themaximum values observed for samples AA-1 and AA-2 duringthe first heating cycle were also caused by the fast relaxation ofresidual stresses. The maximum value occurred at a lower tem-perature for AA-2 than AA-1, likely because the residual stress

Fig. 3 Instantaneous coefficient of thermal expansion as a function of tempera(c) AA-3, (d) AA-4.

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in AA-2 is higher than that in AA-1 due to higher ceramicreinforcing phase in the former one. Since the cooling rate duringthe first heating cycle was lower than that of material processing,much less residual stress was built up in the materials after thefirst heating cycle. The intensity of residual stress relaxationmust be lower during the second heating cycle, leading to thedisappearance of the maximum value. It is interesting to note thatthere is no maximum value for samples AA-3 and AA-4 even forthe first heating cycle. This is likely because the high concen-tration of the nanoparticles effectively suppressed the fast motionof dislocation, and no fast thermal plastic deformation occurred.Such suppression of dislocation motion by high nanoparticleconcentration is consistent with the observed high hardness andwear resistance[11].Further analysis of the thermal expansion behavior of the

composites was done by comparing the experimental resultswith theoretical models. Several models have been proposedfor estimating the CTE of the metal-matrix composite[19e21].These models are all based on the assumption that both matrixand reinforcement phases are only deformed elastically, andthus the bounds of the composite CTE are given. By assumingthat only uniform hydrostatic stresses exist in the phases,Turner proposed that the CTE of a particular composite can bedescribed by

ac ¼ amVmKm þ arVrKr

VmKm þ VrKr(2)

where a is CTE, V the volume fraction, K the bulk modulus,and the subscripts c, m, and r refer to the composite, matrixand reinforcement, respectively. On the other hand, byconsidering both hydrostatic and shear stresses, Schapery

ture compared with Schapery and Turner models for: (a) AA-1, (b) AA-2,

& Technology (2013), http://dx.doi.org/10.1016/j.jmst.2013.04.022

4 Z. Lei et al.: J. Mater. Sci. Technol., 2013, -(-), 1e4

derived the so-called Schapery upper bound and lower bound.Schapery upper bound is expressed as

ac ¼ Vrar þ Vmam þ�4Gm

Kc

��ðKc � KrÞðam � arÞVr

4Gm þ 3Kp

�(3)

where G is shear modulus and Kc the bulk modulus of thecomposite (corresponding to Hashin and strihman’s lowerbound), given by:

Kc ¼VrKr

3Kr þ 4Gmþ VmKm

3Km þ 4GmVr

3Kr þ 4Gmþ Vm

3Km þ 4Gm

(4)

Schapery lower bound is expressed by the same formula, afterinversion of the subscripts m and r in Eqs. (3) and (4). Schaperyupper bound represents that the reinforcing phase is isolated,while Schapery lower bound and Turner model represent thepercolation situation.Fig. 3 compares the experimental results with the theoretical

models for all four materials. The data of parameters used for thecomputation are taken from literature[22e24]. It is seen that forsamples AA-1 and AA-2 containing less nanoparticles, the CTEincreases much slower with temperature than Schapery’s elasticbounds. The CTE could be even lower than Schapery lower boundat high temperatures. On the other hand, for samples AA-3 andAA-4 containing more nanoparticles, the increase in CTE with tem-perature closely follows the Schapery lower bound. Such differencecaused by the nanoparticle concentration can be explained by theireffect on residual stresses. When the composites are cooled downfrom high temperatures, the thermal mismatch between the matrixand the reinforcement will result in residual stresses. These stressesare predominantly tensile in the matrix and compressive in thereinforcement. The increase in temperature during CTE measure-ment can relax such stresses, leading to the decrease inCTE. For thematerials containing a high volume fraction of nanoparticles,the CTE is controlled by a so-called percolation effect, where thereinforcement phase acts as continuous phase, leading to that theCTE of the composites follows Schapery lower bound.

4. Summary

In this research, the thermal expansion behavior of Al-basedcomposites reinforced with hybrid micro-/nano-sized Al2O3

particles has been studied. The results reveal that the concen-tration of nanoparticle can have significant effect on thermalexpansion behavior of the composites. The composites with

Please cite this article in press as: Z. Lei, et al., Journal of Materials Science

higher nanoparticle concentration have the CTE closer to thetheoretical value. The composites containing lower nanoparticleconcentration behave similarly as previously reported compos-ites, for which the CTE is controlled by the relaxation of residualstresses. On the other hand, the CTE of the composites con-taining a high volume fraction of nanoparticles is controlled by apercolation effect.

AcknowledgmentThis work was partially supported by 111 program, China

(No. B08040).

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