Materials and Design xxx (2014) xxx–xxx

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Materials and Design

journal homepage: www.elsevier .com/locate /matdes

Effect of the heating rate on the microstructure of in situ Al2O3

particle-reinforced Al matrix composites prepared via displacementreactions in an Al/CuO system

http://dx.doi.org/10.1016/j.matdes.2014.06.0230261-3069/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Address: 49 Aimin Street, Hohhot 010051, China.Tel./fax: +86 (0)471 6575752.

E-mail address: zhaoge@imut.edu.cn (G. Zhao).

Please cite this article in press as: Zhao G et al. Effect of the heating rate on the microstructure of in situ Al2O3 particle-reinforced Al matrix comprepared via displacement reactions in an Al/CuO system. J Mater Design (2014), http://dx.doi.org/10.1016/j.matdes.2014.06.023

Ge Zhao a,⇑, Zhiming Shi a, Na Ta a, Guojun Ji b, Ruiying Zhang a

a School of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, Chinab Chemical Engineering College, Inner Mongolia University of Technology, Hohhot 010051, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 3 March 2014Accepted 10 June 2014Available online xxxx

Keywords:Aluminum matrix compositesChemical synthesisMicrostructureScanning electron microscopyHeating rate

In this study, an in situ Al2O3 particle-reinforced Al(Cu) matrix composite was successfully synthesizedusing a displacement reaction between Al and CuO powders. The powders were mixed at a weight ratioof 4:1 Al to CuO, cold-pressed and holding time at 900 �C for 1 h using varying heating rates. The effectsof the heating rate on the microstructures of the composites were investigated using differential scanningcalorimetry (DSC), X-ray diffraction (XRD), optical microscopy (MO), scanning electron microscopy (SEM)and energy dispersive spectrometry (EDS). The results indicate that all of the composites contain Al, Al2O3

particles and Al2Cu phases. Although the heating rate does not significantly affect the phase compositionsof the composites, it has a significant effect on their microstructures, most likely because it strongly influ-ences the diffusion rates of the Cu and O atoms. As the heating rate is increased, the Al2O3 particles becomemore dispersed, and they have a more uniform particle size distribution. Meanwhile, the Al2Cu structuretransforms from the network (Al + Al2Cu) eutectic to the block-like Al2Cu phase. The�2 lm Al2O3 particlesand the block-like Al2Cu phase are distributed uniformly in the Al matrix when the sample is placeddirectly into a 900 �C furnace. This sample has a relative higher Rockwell hardness B (HRB) value of 87.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction Al and metal oxide powders are commonly used in displace-

Particle-reinforced aluminum matrix composites are of interestbecause of their high specific strength and stiffness, good wear resis-tance, and low thermal expansion coefficients. Further, they can beprepared via traditional preparation processes and used for variousapplications in the aerospace, military, electronic device andautomobile industries [1–3]. The most common particles used forreinforcing Al matrices include those composed of Al2O3, SiC andTiC. Among these, Al2O3 particles have proven their suitability foruse in Al-based alloys because they have high hardness, good wetta-bility and enhanced chemical stability when coupled with Al-basedmatrices. In situ reaction processes involving particle-reinforcedcomposite systems eliminate interfacial compounds in favor ofnucleation and growth from the parent matrix phase to form ther-modynamically more stable reinforced compounds. At the sametime, the composites possess contaminant-free reinforcement/matrix interfaces, and the in situ Al2O3 particles are fine and canincrease the strength and ductility of the composite [4].

ment reactions. Previous studies have shown that when the tem-perature is increased, the reaction accelerates. Additionally, thesize of the Al2O3 particles increase with increasing temperature,holding time and reinforcement content [5–7]. Rapid solidificationand ball milling lead to more uniformly distributed Al2O3 particlesand produce composites with a homogeneous fine microstructure[8–10]. Fine and homogeneous in situ Al2O3 particles have beensynthesized using the hot pressing process [7,11]. These studiesindicate that the microstructures of the composites are dictatedby the process parameters. It is well-known that small, closelyspaced particles pin boundaries and dislocations. Therefore,whether the strength and ductility of the composites are influ-enced by in situ particles depends on the interparticle spacingand particle size. Furthermore, the properties of the compositeswill be affected by the inclusion of a coarse, continuous, net-work-like intermetallic compound. Thus, it is necessary to performa systematic study of the effect of the process parameters on themicrostructures of in situ Al2O3P/Al composites.

CuO is one of the most widely used metal oxides because finein situ Al2O3 particles are synthesized in Al/CuO systems. Thereduced Cu not only dissolves to some extent in the Al matrixbut also further reacts with the Al to form an intermetallic phase

posites

2 G. Zhao et al. / Materials and Design xxx (2014) xxx–xxx

that can reinforce the matrix of the composite [12]. The effects ofmany synthesized process parameters on composite microstruc-tures have already been studied; however, the effect of the heatingrate on the composite microstructure in Al/CuO systems has notbeen clarified completely. Zhu et al. [13] and Kou et al. [14] haveperformed relevant studies. They reported that when the heatingrate increases, the reaction temperature increases. Biswas et al.[15] and Pathak et al. [16] investigated the effect of the heating rateon combustion in the Ni–Al system and found that the heating ratestrongly influences the diffusion rate, the resultant phases andtheir microstructures. Therefore, this study investigates the effectof the heating rate on the composite microstructure, specificallythe sizes and distributions of the Al2O3 particles and the morphol-ogy of Al2Cu phase. The goal is to obtain dispersed Al2O3 particlesand avoid network-like Al2Cu intermetallic compounds.

2. Experiments

Pure Al powder (less than 50 lm) and CuO powder (from sev-eral microns to 100 lm) were used for the preparation of thein situ composites. The powders were mixed at a weight ratio of4:1 Al to CuO using a planetary ball mill (QM-BP) operating at40 rpm for 2 h. Alcohol was selected as the grinding media. Thepowder mixture was then cold-pressed under 400 MPa of pressureto form discs with diameters of 10 mm. Before synthesized, differ-ential scanning calorimetry (DSC) (Netzsch STA409 PC, heatingrates between 0.1 and 50 �C/min) was conducted on small piecesof the sample (/4.0 mm � 1.0 mm) from ambient temperature to900 �C at heating rates of 5, 10, 20, 30, 40 and 50 �C/min to deter-mine the temperatures where reactions between Al and CuOoccurred.

Green samples were holding time at 900 �C for 1 h at the aboveheating rates in argon atmosphere, and another green sample wasplaced directly into a 900 �C furnace so that its microstructurecould be compared with that of the other samples. All sampleswere furnace-cooled. X-ray diffraction (XRD, D/MAX-2500/PC,40 kV, 20 mA, Cu Ka radiation), optical microscopy (MO), scanningelectron microscopy (SEM) and energy dispersive spectrometry(EDS) were used to investigate the phase and microstructuralchanges that occurred when the composites were processed.

3. Results and discussion

3.1. DSC analysis

Following the relevant literature protocols [6,17], the sampleswere heated to 900 �C at heating rates between 5 and 50 �C/minduring the DSC analysis. The results show that the peaks inthe DSC curves were influenced by the heating rates and that the

Fig. 1. DSC curves of Al–20 wt%CuO samples obtained under flowin

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reaction temperatures were higher when elevated heating rateswere used. Thus, only the DSC curves of the Al–20 wt%CuO samplesthat were prepared using heating rates of 5 �C/min and 50 �C/minare shown in Fig. 1. Negative endothermic peaks resulting fromAl melting are observed in Fig. 1(a) and (b). Two positive peaksare located on the left and right sides of the endothermic peak;the first exothermic peak in Fig. 1(a) is the only one that doesnot partially overlap the endothermic peak. Another exothermicpeak is observed at a higher temperature. The end temperature isapproximately 870 �C in the two above DSC curves, so the sampleswere heated to 900 �C. The peaks in the DSC curves were stronglyinfluenced by the heating rate at relatively low temperatures,while they were only slightly influenced by the heating rate athigher temperatures.

Next, the reactions corresponding to the exothermic peaks inthe DSC curves were determined. Briefly, green samples possessingthe same thickness as the samples used in the DSC experimentswere heated to the temperatures selected according to the DSCcurves in a tube furnace (argon atmosphere) and then waterquenched. The temperatures were raised to between 640 and710 �C and to between 650 and 740 �C for the samples preparedusing a 5 �C/min and a 50 �C/min heating rate, respectively.

3.2. XRD analysis

The diffraction patterns of the green sample and the samplesheated to varying temperatures are shown in Fig. 2. Only peaksthat correspond to Al and CuO are observed in the pattern of thegreen sample (Fig. 2a). For the samples heated at a rate of 5 �C/min, the samples were found to contain Al, CuO, Cu2O and Cuand Al, Cu2O and Cu for the samples heated to 640 �C and 710 �C,respectively. The samples heated at a rate of 50 �C/min showedsimilar XRD results as those that were heated at a rate of 5 �C/min. Thus, the representative XRD results for the sample thatwas heated to 640 �C at a rate of 5 �C/min and the one that washeated to 740 �C at a rate of 50 �C/min are shown in Fig. 2b andc, respectively.

The phase components of the products that were holding timeat 900 �C for 1 h at different heating rates were identified by XRDanalysis. The products display similar diffraction patterns, whichcontain peaks that correspond to Al, Al2O3 and intermetallic Al2Cu.Fig. 2(d) shows the XRD plot of the sample that was heated at a rateof 50 �C/min. As shown, an Al2O3P/Al composite can be synthesizedby a reaction between Al and CuO at 900 �C.

3.3. Microstructure

3.3.1. Green sampleThe microstructure of the Al–20 wt%CuO green sample is shown

in Fig. 3. An SEM micrograph is shown; the insert in the top right

g argon using heating rates of (a) 5 �C/min and (b) 50 �C/min.

e microstructure of in situ Al2O3 particle-reinforced Al matrix composites4), http://dx.doi.org/10.1016/j.matdes.2014.06.023

Fig. 2. Evolution of phases heated to different temperatures. Phase analysis of the (a) green sample and (b) a sample heated to 640 �C at a heating rate of 5 �C/min, and (c) and(d) the samples heated to 740 �C and 900 �C, respectively, at a heating rate of 50 �C/min.

Fig. 3. SEM and optical (insert) micrographs of the green sample.

G. Zhao et al. / Materials and Design xxx (2014) xxx–xxx 3

corner is an enlarged optical micrograph of the Al matrix. In theSEM micrograph, bright patches are distributed uniformly on adark Al matrix. In the optical micrograph, the bright region is theAl matrix, and the boundaries between Al particles can beobserved.

3.3.2. Water-quenched samplesThe BSE micrographs of the samples that were heated to 640

and 710 �C at a heating rate of 5 �C/min are shown in Fig. 4(a)and (b). According to the XRD results, the bright patches are com-posed of CuO, Cu2O and Cu in Fig. 4(a), and they are composed ofCu2O and Cu in Fig. 4(b). The bright network is Cu, and their sizesand shapes are similar to those of the Al particles observed in thegreen sample (Fig. 3). The number of bright networks increases asthe temperature was increases. Some black patches appear inFig. 4(b) around the bright patches. EDS was used to determine

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the elemental composition of the black patches (Fig. 4c). Thesedark patches consist mainly of O and Al. This phase could not bedetected using XRD due to its relatively low elemental content.

Micrographs of the samples that were heated to 650 and 740 �Cat a heating rate of 50 �C/min are shown in Fig. 5(a) and (b). Themicrostructures of the two samples changed in a similar manneras those that were heated at a rate of 5 �C/min; however, the blackpatches that contained Al and O were not observed. EDS indicatesthat the O and Cu atoms reduced from the CuO are located in theboundaries of the Al particles. The O atoms can bond with Al toform Al2O3; Yu et al. have observed a similar result [9].

3.3.3. The samples holding time at 900 �C for 1 h at different heatingrates

Fig. 6 shows the microstructures of the samples that were hold-ing time at 900 �C for 1 h at different heating rates. Bright dots andgray phases are embedded in a black matrix. The bright dots areeither agglomerated or dispersed, while the gray phases eitherform a network or block-like structure. EDS was conducted on rep-resentative samples that were heated at a rate of 5 �C/min and alsoon one that was put directly into the furnace at 900 �C to analyzethe chemical composition of the bright dots and gray phases.Fig. 7 shows enlarged SEM micrographs of two samples (Fig. 6aand g) and the corresponding EDS maps of the elements O, Aland Cu. From the XRD and EDS data, it can be concluded that thebright dots are Al2O3 particles and the gray phase is Al2Cu, and theyare distributed in the Al(Cu) matrix.

Figs. 6(a) and 7(a) reveal that, when the heating rate is 5 �C/min,the Al2O3 particles can range from less than 1–8 lm in size. Therelatively larger Al2O3 particles are dispersed, while the sub-micron Al2O3 particles agglomerate to form aggregates that arethan 80 lm in size. The size and distribution of these structuresare similar with those of the CuO particles in Fig. 3. It also can beobserved that the Al2Cu exists in a two-phase Al(Cu)–Al2Cu

e microstructure of in situ Al2O3 particle-reinforced Al matrix composites4), http://dx.doi.org/10.1016/j.matdes.2014.06.023

Fig. 4. BSE micrographs of the samples that were heated to (a) 640 �C and (b) 710 �C, respectively, at a heating rate of 5 �C/min and then quenched in water. (c) An EDS map ofthe sample that was heated to 710 �C.

Fig. 5. BSE micrographs of the samples that were heated to (a) 650 �C and (b) 740 �C, respectively, at a heating rate of 50 �C/min and then quenched.

4 G. Zhao et al. / Materials and Design xxx (2014) xxx–xxx

eutectic form. The eutectics are connected to each other, and theAl2O3 particles and aggregates are mainly trapped in the eutecticstructure.

Fig. 6 also reveals that as the heating rate is increased, the pro-portion of the dispersed Al2O3 particles increases and the size dis-tribution of the Al2O3 particles decreases. Further, they becomemuch more uniformly distributed in the matrix, the size andamount of the Al(Cu)–Al2Cu eutectic phase decreases and the con-nection between them weakens. When a heating rate of 30 �C/minis used, a block-like Al2Cu phase forms. The Al2Cu phase graduallytransforms from network to block-like as the heating rate is furtherincreased.

The aggregates and eutectics are hardly observed in the samplethat was placed directly into the furnace and holding time at900 �C. The block-like Al2Cu and Al2O3 particles are dispersed.Some Al2O3 particles are located around or inside the block-likeAl2Cu phase, while others are located in the Al matrix. These parti-cles have a uniform size of �2 lm. The EDS map shows that thesize and shape of the O distribution is similar to the boundariesbetween the Al particles observed in the green sample (Fig. 3).Thus, most of the O is located at the boundaries between the Alparticles. It is reasonable to expect that the Al2O3 particles aremainly located in the grain boundaries of the Al that transformedfrom the surface of the Al particles. The uniform distribution ofAl2O3 particles and dispersed Al2Cu suggests that the compositeshould have superior mechanical properties.

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3.4. Hardness

Mechanical hardness tests were performed on all of the com-posites using a HRB-150 micro-hardness tester. For each, the aver-age hardness was obtained using a 100KG-F load indenter overmany different sample regions. Each hardness value reported wasan average of the hardness of five points that were selected at ran-dom. The hardness values of the samples are shown in Table 1. Theaverage HRB values for the samples that were heated at rates lessthan 40 �C/min were between 52 and 56, while the average HRBvalue for the sample that was placed directly into the furnacewas 87.

3.5. Discussion

Oxygen atoms diffuse along the powder surfaces and the latticeand grain boundaries during synthesized [6]. It is well-known thatthe existence of an oxide film on the surface of a metal powder hasa significant effect on the bonding properties of the powder [17].The oxide film can accelerate the bonding between particles whenthe film thickness is less than a critical thickness; however, if thefilm is too thick, bonding between the particles is suppressed.

The oxygen atoms in the CuO likely incorporate themselves intothe Al lattice as a result of volume diffusion and ultimately attachto reaction fronts, while the copper atoms diffuse mainly alongthe Al particle surfaces during synthesized when a heating rate

e microstructure of in situ Al2O3 particle-reinforced Al matrix composites4), http://dx.doi.org/10.1016/j.matdes.2014.06.023

Fig. 6. SEM micrographs of samples holding time at 900 �C for 1 h at different heating rates: (a) 5 �C/min, (b) 10 �C/min, (c) 20 �C/min, (d) 30 �C/min, (e) 40 �C/min, and (f)50 �C/min. (g) SEM micrograph of the sample that was placed directly into furnace at 900 �C and then furnace-cooled.

Fig. 7. SEM micrographs of the samples that were holding time at 900 �C for 1 h and the corresponding EDS maps of the elements O, Al and Cu in each sample. (a) A samplethat was heated at a rate of 5 �C/min. (b) A sample that was placed directly into the furnace.

Table 1HRB values of samples holding time at 900 �C for 1 h.

Sample 5 �C/min 10 �C/min 20 �C/min 30 �C/min 40 �C/min 50 �C/min Placed directly into the furnace

HRB 56 54 52 53 53 68 87

G. Zhao et al. / Materials and Design xxx (2014) xxx–xxx 5

of 5 �C/min is used. The boundaries between Al particles are notobserved in Fig. 6(a) because the oxygen atoms in the CuO powdersdo not diffuse along the Al particle surfaces. Thus, the Al particlesshow good bonding. The Al particles melt to form aluminum liquidat temperatures above the Al melting point, and the unimpededmotion of molten aluminum into the interconnected network ofchannels and crevices occurs between CuO particles via capillaryaction. Thus, the boundaries between the Al particles disappear.

It can be observed that most of the Al2O3 particles and aggre-gates were trapped in the eutectic structure (Fig. 6(a)). This phenom-enon has also been observed by many other researchers [8,9,18,19].However here, the distribution of Al2O3 particle aggregates in

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Fig. 6(a) and that of the CuO particles in Fig. 3 is similar, likelybecause the aggregates are too large to redistribute during thegrowth process of the solid phase. The dispersed Al2O3 particlessegregate in the eutectic phase is a result of their redistributionduring the solidification of the Al-rich molten phase. The interfacialenergy of Al2O3/solid-Al is greater than that of Al2O3/liquid-Al. Thisdifference in interfacial energies provides a driving force for thealumina particles to move in the liquid phase during the solidifica-tion of the furnace-cooled sample. Alumina particles were trappedin the eutectic phase because it was the last phase to solidify.An interconnected network of eutectic phases was formed tominimize the overall interfacial energy of the system [14,16].

e microstructure of in situ Al2O3 particle-reinforced Al matrix composites4), http://dx.doi.org/10.1016/j.matdes.2014.06.023

6 G. Zhao et al. / Materials and Design xxx (2014) xxx–xxx

Al2O3 particles appear in the regions near the bonding inter-faces between the Al particles in the sample placed directly intothe furnace at 900 �C. The oxygen atoms in the CuO likely diffuseprimarily along the Al particle surfaces and oxide films are likelyformed on the surfaces during synthesized. The oxide film is thickenough that it can suppress the diffusion of Al atoms between Alparticles. The copper atoms in the CuO diffuse into the Al particlesthat are in intimate contact with the CuO, but the oxide film acts asan obstacle to the further diffusion of the copper atoms to other Alparticles. The oxide film transforms into discrete fine crystallineAl2O3 particles during synthesized, impeding the connectionsbetween Al particles, the flow of liquid aluminum formed by themelting of the Al particles and the diffusion of the copper atoms[20]. Thus, the suspended Al2O3 particles in the molten phase tendto be stationary, and the block-like Al2Cu phase appears in thesample during the solidification of the furnace-cooled samples.As a result, the Al2O3 particles are distributed along the grainboundaries transformed from the Al particle surfaces and theblock-like Al2Cu phases.

4. Conclusions

It can be concluded that the heating rate does not influence thephase compositions (Al, Al2O3 and Al2Cu) of the samples holdingtime at 900 �C for 1 h, but it does affect the diffusion of oxygenatoms and copper atoms. Thus, the microstructures of the compos-ites are determined by the competition between the rates of diffu-sion of O and Cu in CuO particles in the heating stage duringsynthesized. More specifically,

1. Oxygen atoms diffuse into the Al particles that are in intimatecontact with CuO and then attach themselves to the reactionfronts. The copper atoms diffuse along the Al particle surfacein the samples that are heated at a rate 5 �C/min. The Al2O3 par-ticles were observed to range from less than 1–8 lm in size. Therelatively large particles were dispersed, while the sub-micronparticles were aggregated. Furthermore, network Al2Cu–Al(Cu)eutectic phases appeared and were connected to one another.The Al2O3 particles and aggregates were mainly located insidethe eutectic structure.

2. The oxygen atoms diffused along the Al particle surface, and thecopper atoms diffused into the Al particles that were in intimatecontact with the CuO in the sample placed directly into the fur-nace at 900 �C. The Al2O3 particles were less than �2 lm in sizeand uniformly distributed throughout the sample. Block-likeAl2Cu phases were also observed. The Al2O3 particles are dis-tributed along the Al2Cu phase, and the grain boundaries trans-formed from the Al particle surface.

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Acknowledgements

The authors are grateful to the Natural Science Foundation ofInner Mongolia (No. 2012MS0801) for grant and financial support.

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