M A T E R I A L S C H A R A C T E R I Z A T I O N 6 7 ( 2 0 1 2 ) 1 2 9 – 1 3 7

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Characteristic of copper matrix simultaneously reinforced withnano- and micro-sized Al2O3 particles

Viseslava Rajkovic⁎, Dusan Bozic, Aleksandar Devecerski, Milan T. JovanovicMaterials Science Laboratory, Institute of Nuclear Sciences “Vinca”, University of Belgrade, P.O. Box 522, 11001 Belgrade, Serbia

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +381 11 3804 59E-mail address: visnja@vinca.rs (V. Rajko

1044-5803/$ – see front matter © 2012 Elseviedoi:10.1016/j.matchar.2012.02.022

A B S T R A C T

Article history:Received 25 June 2011Received in revised form23 February 2012Accepted 27 February 2012

The effect of the simultaneous presence of nano- andmicro-sized Al2O3 particles on themi-crostructure and properties of copper matrix was the object of this study. The mixture ofinert gas-atomized prealloyed copper powder (with 1 wt.% Al) and 0.6 wt.% commercialAl2O3 powder (serving as micro-sized particles) was used as the starting materials.Strengthening of the copper matrix was performed by treating the powders in the air forup to 20 h in the planetary ball mill. During milling of the prealloyed powder, finelydispersed nano-sized Al2O3 particles were formed in situ by internal oxidation. The approx-imate size of these particles was between 30 and 60 nm. The highest values of microhard-ness were reached in compacts processed from 10 h-milled powders. The microhardnessof compact obtained from 10 h-milled powder was 3 times higher than the microhardnessof compact processed from as-received and non-milled prealloyed powder. At the maxi-mum microhardness the grain size reaches the smallest value as a result of the synergeticeffect of nano- and micro-sized Al2O3 particles. Recrystallization, which occurred duringprolonged milling, was the main factor influencing the decrease in microhardness. The in-crease in electrical conductivity of compacts after 15 h of milling is the result of the de-crease in microhardness and activated recrystallization processes.

© 2012 Elsevier Inc. All rights reserved.

Keywords:Mechanical alloyingInternal oxidationNano- and micro-sized Al2O3

particlesStrengtheningMicrohardnessElectrical conductivity

1. Introduction

Strength and softening temperature of copper matrix may beincreased by finely dispersed oxide particles, whereas ade-quate thermal and electrical conductivity are maintained atroom and elevated temperatures. These properties dependon the amount, size, and uniformity of the dispersed particles.Copper matrix, reinforced by mechanical alloying or internaloxidation, has been extensively studied in recent years dueto its attained better properties compared to pure copperand precipitation, or solid solution hardened copper. A uniquecombination of high strength and conductivity at elevatedtemperatures makes copper-based composites the best candi-date for high temperature electric materials, such as spotwelding electrodes, lead wires, connectors, and other elec-tronic devices. These materials are also ideal for the ITER

3; fax: +381 11 224.vic).

r Inc. All rights reserved.

(International Thermonuclear Experimental Reactor) as highheat flux components, like divertor and first wall [1].

High-energy milling is a very common and often appliedtechnique in powder metallurgy for the processing of coppermatrix strengthened with the fine dispersion of varioussized Al2O3 particles. Nano-scaled grain structure may beretained even during compaction. This fine-grained struc-ture together with Al2O3 particles contributes to copper ma-trix strengthening. Depending on the method, nano-sizedAl2O3 particles formed in situ by internal oxidation rangedin size from 10 to 15 nm [2,3] to 50 nm [4,5]. It was reported[6] that by internal oxidation in the air of prealloyed Cu–Alpowders, Al2O3 particles ranging in size from 30 to 50 nmwere produced. On the other side, the size of Al2O3 particlesproduced by mechanical alloying was between 14 nm [7]and 2 μm [8,9].

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The influence of Al2O3 particle size on the strengthening ofmatrix is important since it affects the deformation and re-crystallization of the copper matrix. Small and closely spacedparticles exert a pinning effect on the movement of grainboundaries, resulting in retardation or even complete sup-pression of recrystallization [7]. On the contrary, the matrixwith larger and widely spaced particles is prone to acceleratedrecrystallization [10]. Thus, the presence of variously sizedparticles and interparticle spacing can cause either retarda-tion or acceleration of recrystallization of the matrix.

In the present work, two types of particles having differentsizes, i.e. nano- and micro-sized Al2O3 particles are employedas strengtheners of copper matrix. The object of this paperwas to study the simultaneous effect of nano- and micro-sizedAl2O3 particles on the microstructure, strengthening, and elec-trical conductivity of the copper-based composites during theprocesses of internal oxidation and mechanical alloying.

2. Experimental Procedure

The inert gas-atomized prealloyed copper powder (averageparticle size — 30 μm) containing 1 wt.% Al (designation inthe further text: Cu–1 wt.% Al) mixed with 0.6 wt.% Al2O3 pow-der (commercial grade: average particle size — 0.75 μm)served as a starting material. This Cu–1 wt.% Al+0.6 wt.%Al2O3 mixture (designation: Cu1Al+Al2O3) was milled in theair for up to 20 h in the planetary ball mill. The weight ratioof powder to steel balls was 1:35. Following milling, the mix-ture was treated in hydrogen at 400 °C for 1 h in order to elim-inate the copper oxides that formed at the powder's surfaceduring milling. Compaction, executed by hot-pressing, wascarried out in an argon atmosphere at 800 °C for 1 h underthe pressure of 35 MPa. Compacts from as-received and non-milled Cu–1 wt.% Al were also synthesized under the samecondition. The approximate dimensions of all compactswere: 10 mm height and 10 mm diameter.

Milling in air of Cu–1 wt.% Al promotes the formation offinely dispersed Al2O3 particles by internal oxidation [11]. Itwas expected that during milling of Cu1Al+Al2O3 the uniformdistribution of commercial Al2O3 particles would be alsoobtained without their fracturing or coarsening [12]. Conse-quently, powders with uniformly dispersed Al2O3 particles ofvarious sizes would be obtained in the refined copper matrixafter the process of milling.

The powder mixtures and corresponding compacts werecharacterized by X-ray diffraction (XRD) analysis which wasperformed using an X-ray powder diffractometer with CuKα

Ni filtered radiation. The lattice parameter was determinedusing the least square root method. The average lattice distor-tion, i.e. the relative deviation of the lattice parameters fromtheir mean value (Δd/d) [13] and the grain size (D) were deter-mined from the broadening (β) of the first four diffractionlines (111, 200, 220 and 311) using the approach developed byWilliamson and Hall [14]:

β cos θ ¼ kλD

þ kΔdd

sinΘ ð1Þ

where the shape factor k=0.9 and the radiation wave lengthλ=0.15405 nm.

The morphology and microstructure of the powder mix-ture and compacts were characterized by light microscope,Zeiss Axiovert 25. Two scanning electron microscopes (SEM)were used: JEOL JSM 35 for microstructural investigationsand JEOL JSM-6610LV, equipped with an electron dispersiveX-ray spectroscope (EDS), for chemical analysis of particlespresent in the copper matrix. Powder particle samples forSEM microscope were mounted in acrylic resin. Polishing ofpowders and compacts was performed using the standardmethod, whereas a mixture of 5 g FeCl3 and 50 ml HCl in100 ml distilled water was used for etching.

Only a few compacts (processed from 10 to 20 h-milledpowders) corresponding to different levels of microhardnesswere investigated with the transmission electron microscope(TEM) JEOL JEM-7. For TEM characterization, thin slices (about1 mm) of the material were cut from the central cross-sectionof each compact. The slices were further thinned to 100 μmby conventional grinding. 3 mm disks were punched from thecentral part of the slices and electropolished in a twin-jet elec-tropolisher. The electropolishing was performed in the solu-tion mixture CH3OH–HNO3 with the ratio 3:1 at −35 °C, U=9 Vand I=20 mA.

The strengthening of the copper matrix was estimated bymicrohardness measurement with applied load of 50 g andtime of indentation 20 s. The electrical conductivity (% IACS;where IACS20°C=0.5800microhm−1 cm−1) of the polished com-pacts was measured using an apparatus Sigmatest operatingat 120 kHzwith an electrode diameter of 7 mm,whilst the den-sity of the compacts (ρ) was determined by the Archimedesmethod. The theoretical density of the compacts was calculat-ed from the simple rule of mixtures, taking the fully densevalues for copper and Al2O3 8.96 and 3.95 g cm−3, respectively.

Values of density, microhardness, and electrical conduc-tivity represent the mean value of five measurements per-formed on the same compact.

3. Results and Discussion

3.1. Powders

The relationship between Cu1Al+Al2O3 powder lattice param-eter and milling time is shown in Fig. 1. During milling, latticeparameter decreases withmilling time. The decrease of latticeparameter is the result of internal oxidation of aluminum,which precipitates from the prealloyed copper, forming a finedispersion of Al2O3. The lattice parameter continuously de-creases almost till the end of themilling process. This behavioris due to facilitation of the oxidation process through the de-formation progress of the copper matrix with the milling pro-cess. The difference in the lattice parameters of prealloyedCu–1 wt.% Al powders before (a Cu–1 wt.% Al=0.36197 nm) andafter 20 h of milling (a Cu–1 wt.% Al=0.36159 nm) is 0.10%. Thisdifference, similar to the difference (0.13%) in theoreticallattice parameters of the prealloyed powder (a Cu–1 wt.% Al=0.36210 nm) and the copper powder (a Cu=0.36152 nm), indi-cates that after 20 h ofmilling almost all the aluminumprecip-itated from the copper matrix. Assuming that the completeamount of aluminum was oxidized, it was calculated that1.9 wt.% of Al2O3 was produced in the copper matrix by

Fig. 3 – Effect of milling time on grain size and latticedistortion of Cu–1 wt.% Al+0.6 wt.% Al2O3 powders.

Fig. 1 – Lattice parameter vs. milling time of Cu1Al+Al2O3

powders. (In this and all following cases “0” on the X-axisdenotes as-received and non-milled condition regarding topowders and corresponding compacts).

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internal oxidation of 1 wt.% Al. This calculation was madeusing the simple equation:

4Al þ 3O2 ¼ 2Al2O3: ð2Þ

Given that 4×27=108 g of aluminum oxide produces 204 gof Al2O3, i.e. 2×(2×27+3×16)=204 g, then oxidation of 1 g Al,contained in the prealloyed copper, will generate 1.9 g ofAl2O3. Considering this result, it is supposed that the milledpowder mixtures with a total amount of 2.5 wt.% Al2O3 parti-cles have been obtained.

Full width at half maximum (FWHM) measured from XRDpatterns of Cu1Al+Al2O3 powders shows a progress in linebroadening with milling time (Fig. 2), as a result of a severelattice distortion and grain size refinement [13].

The effect of milling time on the grain size and lattice dis-tortion of Cu1Al+Al2O3 powders is presented in Fig. 3. Themost intensive grain refinement occurs up to 10 h, when thegrain size decreases from 550 to 78 nm. With prolonged timethe grain size of milled powders decreases quite slowly,being 78 and 76 nm after 10 and 20 h of milling, respectively.Fig. 3 also illustrates a strong increase of Cu1Al+Al2O3

Fig. 2 – Effect of milling time on full width at half maximum(FWHM) of Cu–1 wt.% Al+0.6 wt.% Al2O3 powders.

powders crystal lattice distortion during the first 10 h of mill-ing. Whenmilled for a longer period of time, the lattice distor-tion becomes less evident. The distortion appears as a resultof plastic deformation which is due to a decrease in thegrain size. It was shown that the contributions to the latticedistortion may arise from internal stresses imposed by dislo-cations and inhomogeneously distributed point defects [15].

The change of Cu1Al+Al2O3 particles morphology with in-creasing milling time is shown in Fig. 4. During high-energymilling the powder particles change morphology and size asa consequence of repeated deformation, fracturing and weld-ing processes. According to these micrographs powder sizeincreases for up to 5 h of milling due to the welding predomi-nance in themilling process (Fig. 4a). With longer milling timethe powder size decreases since the fracturing predominatesin the milling process. After 20 h of milling Cu1Al+Al2O3 par-ticles are rather small, but not equiaxed in shape (Fig. 4b). Dif-ferent morphologies of these particles indicate that thebalance between fracturing and welding processes was notachieved.

The composition of Cu–1 wt.% Al powders changes duringmilling. It was recently reported [6] that during high-energymilling of prealloyed powders, the Al2O3 particles formedthrough the reaction of aluminum with oxygen from the airare of nano-sized dimensions, i.e. most of the particles havethe approximate size of 50 nm or less. At higher magnificationSEMmicrograph of 10 h-milled powder illustrates the presenceof particles with different morphologies (Fig. 5). Lamellae (L)representing traces of previous individual powder particlesmay be distinguished (Fig. 5a, b). A number of very small globu-lar particles (N) are precipitated on these lamellae (Fig. 5b),whereas coarse particles (M) of different morphologies with ap-proximate size of 700 nmare also present in thematrix (Fig. 5b).Stresses imposed by larger particles are themain reason for theappearance of microcracks (C) in the matrix (Fig. 5b).

3.2. Compacts

3.2.1. MicrostructureFig. 6 illustrates the microstructure of compacts obtainedfrom milled Cu1Al+Al2O3 powders. The compacts retained

Fig. 5 – SEM micrographs of 10 h-milled Cu1Al+Al2O3

particles. Arrows denote: (a) Lamellae (L); (b) nano-sizedparticle (N); micro-sized particle (M); microcrack (C).

Fig. 6 – Light micrographs. Microstructure of compactsprocessed from Cu1Al+Al2O3 powder after different millingtimes. (a) 12 h; (b) 20 h. Arrows denote recrystallized regions.Fig. 4 – SEM micrographs. Morphology of Cu1Al+Al2O3

particles after different milling times. (a) 1 h; (b) 20 h.

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lamellar structure, a characteristic of high-energymilled pow-der particles. Although lamellae are retained in compactsprocessed from powders milled for 12 h, some changes inthe microstructure may be distinguished, i.e. the light areas(denoted by arrows) indicate recrystallization which occurredduring hot-pressing (Fig. 6a). Note that recrystallization wasmostly initiated at the boundaries of the powder particles,but in a lesser extent, is also visible at the corners of particleswhere the concentration of stresses imposed during compac-tion was highest. In compact processed from powder milledfor 20 h (Fig. 6b), the extent of recrystallization was extensiveand unrecrystallized particles were surrounded by recrystal-lized areas.

A SEM micrograph of compact processed from 10 h-milledpowders is shown in Fig. 7. In the backscattered electronimage (BSE) small and large particles may be seen. Theinserted EDS spectrum shows the presence of aluminum andoxygen in the small particle. Small amounts of iron probablyoriginated from the steel balls of the high-energy mill. Sinceit was estimated that commercial Al2O3 particles could notbe fractured during milling [12], then the structure of compactprocessed from 10 h-milled Cu1Al+Al2O3 powders consists ofnano- and micro-sized Al2O3 particles embedded in the cop-per matrix.

A SEM micrograph of the compact processed from 20 h-milled powders is illustrated in Fig. 8. A wide recrystallizedarea free of particles with annealing twins may be seen inthe BSE image, whereas the distinction between nano- andmicro-sized particles was difficult to establish.

Fig. 9 – XRD pattern of (a) Cu1Al+Al2O3 powders and(b) corresponding compacts after different milling times. Allpeaks correspond to the copper matrix.

Fig. 7 – SEM micrograph. BSE image of compact processedfrom 10 h-milled powders with inserted EDS spectrum of asmall particle.

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The effect of milling time on XRD pattern of Cu1Al+Al2O3

powder and corresponding compacts is illustrated in Fig. 9.The intensity of peaks decreases as milling time increases. Al-though the difference in peak intensity of powder (Fig. 9a) andcompacts (Fig. 9b) is relatively small, it may be observed thatthe peak intensity is somewhat higher in compacts, suggest-ing increased grain size as a consequence of diffusion pro-cesses during hot-pressing. Applying XRD analysis, it wasnot possible to detect Al2O3 due to very small particle size;the same problem was also mentioned by other authors [16].

The change in the grain size of powder particles and corre-sponding compacts as a function of milling time is shown inTable 1. The grain size of compacts was calculated using Eq.(1), based on the results of (FWHM) measured from XRD pat-terns of compacts.

Results of Table 1 show that the grain size of both powdersand compacts decreases during shorter milling time, reachinga minimum at 10 h, whereas with prolonged milling, an in-crease in grain size occurs. In general, compacts are character-ized by larger grains than powders; this may be ascribed todiffusion processes during hot-pressing and their influenceon the grain growth.

The morphology of Al2O3 particles is illustrated in TEM mi-crographs (Fig. 10). In general, nano-sized particles are homo-geneously distributed within the matrix and on the grain

Fig. 8 – SEM micrograph. BSE image of compact processedfrom 20 h-milled powders. Annealing twins in therecrystallized area.

boundaries. In compact processed from 10 h-milled powdersnano-sized particles (mainly globular and approximately be-tween 30 and 60 nm in size) may be seen. Some of theseparticles are formed on grain boundaries preventing grainboundary migration and decreasing the rate of grain growth(Fig. 10a). In addition to nano-sized particles, larger micro-sized individual particles also appear in the matrix (Fig. 10a,b). Dislocation network formed at large particle/matrix inter-face may be seen (Fig. 10b). Although the grain boundaries inTEM micrographs are poorly defined, it is obvious that graingrowth occurred at prolonged milling, i.e. from 100 to 150 nm(after 10 h of milling) to approximately 300 nm (after 20 h ofmilling) (Fig. 10c). The difference in calculated grain sizevalues (see Table 1) and those determined by TEM clearly ex-ists. This difference may be fully attributed to the vaguelydefined grains in TEM micrographs. According to Fig. 10c a

Table 1 – The grain size of Cu1Al+Al2O3 powders andcorresponding compacts as a function of milling time.

Cu1Al+Al2O3

Milling time (h)

0 3 5 10 12 15 20

Powder 550 287 136 78 82 90 98Compact 630 295 142 90 104 138 200

Fig. 11 – Effect of milling time on microhardness ofCu1Al+Al2O3 compacts.

Fig. 10 – TEM micrographs. Compacts processed from10 h-milled particles (a,b) and 20 h-milled powders (c).Arrows, in Fig. 10a, c denote a part of large Al2O3 particle.

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coarsening of some Al2O3 particles could be distinguished.This increase of nano-sized Al2O3 particles was reported sug-gesting coarsening as a result of diffusion processes duringlonger milling time [1].

3.2.2. MicrohardnessMicrohardness of compacts depends on the previous millingtime of Cu1Al+Al2O3 powder (Fig. 11). Microhardness steeplyincreases for up to 10 h of milling when the maximum micro-hardness is reached. A rapid increase of the crystal lattice dis-tortion reaching maximum value at approximately 10 h ofmilling time (see Fig. 3) may be regarded as a result of latticedeformation due to successive precipitation of nano-sizedAl2O3 particles from the copper solid solution. Under the influ-ence of diffusion processes during milling these particles areprecipitated within the matrix and at grain boundaries.Nano-sized particles, finely distributed in the matrix and onthe grain boundaries, act as pinning points impeding furthermovement of dislocations and their propagation. Thus, thepinning force exerted by nano-sized particles on the grainboundary prevents the grain growth.

The microhardness of compact obtained from 10 h-milledpowder (240 HV0.05) is much higher than the microhardnessprocessed from as-received and non-milled Cu–1 wt.% Alpowder (74.5 HV0.05) compacted under the same conditions.To explain this more than threefold increase in hardness,two main influencing factors should be considered. The in-crease in microhardness of compacts is a consequence ofthe fine grain copper matrix structure and the presence ofthe nano-sized Al2O3 particles. These Al2O3 nano-particlesare homogeneously located in the matrix grains having an av-erage interparticle distance of less than 100 nm (see Fig. 10a).It is known that a dislocation can bypass such particle by Oro-wan bowing [17] leaving behind a dislocation loop around theparticle; the critical stress σOr for bypassing depends on theinterparticle distance l according to σOr∝ l−1. According tothe Orowan bowing mechanism and thermal mismatch be-tween the matrix and reinforcement particles in metal matrixcomposites [18–21], by decreasing particulate size the strengthincreases. Note that since the particles in this work are smallenough (less than 100 nm), the Orowan bowing mechanismcan be used to justify this behavior [19]. From the microhard-ness andmicroscopic results it can be concluded that the flowstress, necessary for plastic deformation of the composite incontrast to the as-received material, is additionally increasedby dispersion-strengthening of the matrix grains.

Table 2 – The effect of milling time on electricalconductivity of Cu1Al+Al2O3 compacts.

Compact Electrical conductivity (%IACS)

Milling time (h)

0⁎ 3 5 10 12 15 20

Cu1Al+Al2O3 22 30.5 30.7 31.0 32.0 37.5 47.0

Fig. 12 – Effect of milling time on density of Cu1Al+Al2O3

compacts.

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The results obtained in this work reveal that at the peakvalues (at 10 h of milling time), the microhardness of Cu1Al+Al2O3 compact is higher than that of the Cu–1 wt.% Al (withoutaddition ofmicro-sizedAl2O3 particles), i.e. 240 vs. 220HV0.05, re-spectively, processed under the same conditions [11]. At thisstage, apart from the effect of nano-sized particles, the contri-bution of micro-sized Al2O3 particles to microhardness mustbe taken into account. The extent of matrix hardening is addi-tionally increased by the formation of the dislocation networkformed around these particles (see Fig. 10b). Thus, at the maxi-mum microhardness the synergetic effect of nano- and micro-sized Al2O3 particles exhibits a marked effect on increasedmicrohardness of Cu1Al+Al2O3 compacts with respect to Cu–1 wt.% Al.

Prolonged milling results in a slow drop in microhardness.This is because the coarse particles, i.e. micro-sized Al2O3 par-ticles, under certain conditions may contribute to the increasein grain size [10]. Namely, in the vicinity of micro-sized Al2O3

particles, a cellular dislocation substructure may be createdwith a markedly increased density of dislocations. The pro-longed time of milling results in a significantly increasedsubgrains number, which can be activated and become thenucleation sites of newly created recrystallized grains. Light(Fig. 6a, b), SEM (Fig. 8) and TEM (Fig. 10c) micrographs coupledwith the values of the grain growth with milling time (seeTable 1) indicate that the process of recrystallization, whichoccurred during prolonged milling, was the main factorinfluencing the microhardness decrease. On the other hand,after reaching its maximum, microhardness of Cu–1 wt.% Alcompacts remains practically unchanged [11], indirectly sug-gesting that coarse Al2O3 particles may be regarded as a signif-icant parameter in decreasing microhardness of Cu1Al+Al2O3

compacts during prolonged milling.Unlike other papers reported in the literature, this paper

studies the effect of nano- and micro-sized particles simulta-neously embedded in the copper matrix. These results indi-cate that hardening of the copper matrix depends on severaldifferent parameters, one of them being grain size. The influ-ence of nano- and micro-sized particles is complex and diffi-cult to be resolved. The influence of some parameters ismore pronounced during short milling time, whereas the in-fluence of other parameters prevails with longer milling. Thetwofold role of coarse Al2O3 particles in matrix strengtheningmust be emphasized. During shorter milling time these parti-cles, together with nano-sized particles, contribute to the in-crease of microhardness up to its maximum value. However,the decrease in microhardness with longer milling time is re-lated to the recrystallization for which development themicro-sized Al2O3 particles have a significant effect.

3.2.3. DensityDensity of the Cu1Al+Al2O3 compacts decreases with millingtime (Fig. 12). The significant drop in density occurred for upto 15 h of milling time, when the fracturing of powder parti-cles is the predominant process during milling. According toFig. 4 themorphology of powder particles influences the pack-ing between particles during hot-pressing, indicating that thebetter packing achieved during shorter milling time corre-sponds to higher density. At the same pressures, coarser par-ticles can be consolidated to a higher density than finer

particles of the same composition [22]. The results also sug-gest that the densification by hot-pressing of milled powderswas not completed. The reason for such an inadequateconsolidation could also be related to insufficient appliedpressure of 35 MPa. It is quite disputable whether the precipi-tation of nano-sized particles from the solid solution mayhave any effect on the decrease of density. The measureddensity of compacts processed from 10 h-milled powder(7.75 g cm−3) was 87% of the theoretical value (8.89 g cm−3).Since the measured density of the hot-extruded materials ishigher than 99.3% [23] hot-extruding seems to be a commonmethod of compacting. It should be noted that in this studythe theoretical density was calculated for the total amountof Al2O3, i.e. 2.5 wt.%.

3.2.4. Electrical ConductivityThe results of electrical conductivity of compacts after differ-ent times of milling are summarized in Table 2.

Compact processed from as-received and non-milledCu1Al+Al2O3 powders shows the lowest electrical conductivi-ty. During the following milling, precipitation of nano-sizedAl2O3 particles contributes to the increase in electrical con-ductivity, which is due to depletion of aluminum content insolid solution. No significant change in electrical conductivitywas detected for up to 15 h of milling. The increase in electri-cal conductivity after 15 h of milling is connected with the de-crease in microhardness and recrystallization processes. It isobvious that the electrical conductivity of compacts is muchlower than that of pure copper or some copper based, highconductivity alloys. Nano-sized Al2O3 particles form a greatnumber of interfaces considered as a possible source of addi-tional electron scatter, which is a significant factor in reducingconductivity [24].

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4. Conclusions

Simultaneously reinforced copper matrix with nano- andmicro-sizedAl2O3 particleswas obtainedbyhigh-energymillingof themixture containing inert gas-atomized prealloyed copperpowder with 1 wt.% Al and 0.6 wt.% commercial Al2O3 powder.

- Milling of prealloyed powder promoted an amount of 1.9 wt.%Al2O3 by internal oxidation. Thus, the total amount of 2.5 wt.%of nano- and micro-sized Al2O3 particles have been obtained.Lamellae, representing traces of previous individual powderparticles may be distinguished, whereas a number of verysmall globular particles are precipitated on these lamellae.Coarse particles of different morphologies with an approxi-mate size of 700 nm are also present in the matrix

- Compacts processed from powders milled for 3 and 5 hretained lamellar structure, a characteristic for high-energymilledpowderparticles. During longermilling time the lamel-lar structurewas somewhat changed as a result of recrystalli-zation occurring during hot-pressing. In compact processedfrom10 h-milledpowdersnano-sizedparticles (mainly globu-lar and approximately between 30 and 60 nm in size) preventgrain boundary migration, decreasing the rate of the graingrowth. In addition to nano-sized particles, larger micro-sized individual particles also appear in the matrix.

- The highest values of microhardness are reached in com-pacts processed from 10 h-milled powders. Themicrohard-ness of compact obtained from 10 h-milled powder is 3times higher (2400 MPa) than microhardness processedfrom as-received and non-milled Cu–1 wt.% Al powder(745 MPa) compacted under the same conditions. At themaximum microhardness the grain size reaches the smal-lest value as a result of the synergetic effect of nano- andmicro-sized Al2O3 particles.

- Prolonged milling results in a slow drop in microhardness.This is because the micro-sized Al2O3 particles under certainconditions may contribute to an increase in the grain size.The prolongedmilling time results in a significantly increasedsubgrain number,which can be activated and become the nu-cleation sites of newly formed recrystallized grains. Recrystal-lization, which occurred during prolonged milling, was themain factor influencing the microhardness decrease.

- Density of the Cu1Al+Al2O3 compacts decreaseswithmillingtime. The significant drop in density occurred for up to 15 hof milling time, when the fracturing of powder particles isthe predominant process duringmilling.Morphologyof pow-der particles influences the packing between particles duringhot-pressing, indicating that the better packing achievedduring shorter milling time corresponds to higher density.

- The increase in electrical conductivity of the Cu1Al+Al2O3

compacts after 15 h of milling is connected with the de-crease in microhardness and recrystallization processes.

Acknowledgment

This work was financially supported by the Ministry of Educa-tion and Science of the Republic of Serbia through the ProjectNo 172005.

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