materials characterization, april 2012..pdf

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Characteristic of copper matrix simultaneously reinforced with

nano- and micro-sized Al2O3particles

Viseslava Rajkovic, Dusan Bozic, Aleksandar Devecerski, Milan T. Jovanovic

Materials 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 A B S T R A C T

Article history:

Received 25 June 2011

Received in revised form

23 February 2012

Accepted 27 February 2012

The effect of the simultaneous presence of nano- and micro-sized Al 2O3particles on the mi-

crostructure and properties of copper matrix was the object of this study. The mixture of

inert gas-atomized prealloyed copper powder (with 1 wt.% Al) and 0.6 wt.% commercial

Al2O3 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 for

up to 20 h in the planetary ball mill. During milling of the prealloyed powder, finely

dispersed nano-sized Al2O3particles 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 microhardness

of compact obtained from 10 h-milled powder was 3 times higher than the microhardness

of 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 synergetic

effect of nano- and micro-sized Al2O3 particles. Recrystallization, which occurred during

prolonged 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 alloying

Internal oxidation

Nano- and micro-sized Al2O3particles

Strengthening

Microhardness

Electrical conductivity

1. Introduction

Strength and softening temperature of copper matrix may be

increased by finely dispersed oxide particles, whereas ade-

quate thermal and electrical conductivity are maintained at

room and elevated temperatures. These properties depend

on the amount, size, and uniformity of thedispersed particles.

Copper matrix, reinforced by mechanical alloying or internaloxidation, has been extensively studied in recent years due

to its attained better properties compared to pure copper

and precipitation, or solid solution hardened copper. A unique

combination of high strength and conductivity at elevated

temperatures makes copper-based composites the best candi-

date for high temperature electric materials, such as spot

welding electrodes, lead wires, connectors, and other elec-

tronic devices. These materials are also ideal for the ITER

(International Thermonuclear Experimental Reactor) as high

heat flux components, like divertor and first wall[1].

High-energy milling is a very common and often applied

technique in powder metallurgy for the processing of copper

matrix strengthened with the fine dispersion of various

sized Al2O3 particles. Nano-scaled grain structure may be

retained even during compaction. This fine-grained struc-

ture together with Al2O3particles contributes to copper ma-trix strengthening. Depending on the method, nano-sized

Al2O3 particles formed in situ by internal oxidation ranged

in 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 Al

powders, Al2O3 particles ranging in size from 30 to 50 nm

were produced. On the other side, the size of Al2O3particles

produced by mechanical alloying was between 14 nm [7]

and 2m[8,9].

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

Corresponding author.Tel.: +381 11 3804 593; fax: +381 11 224.E-mail address:[email protected](V. Rajkovic).

1044-5803/$ see front matter 2012 Elsevier Inc. All rights reserved.doi:10.1016/j.matchar.2012.02.022

A v a i l a b l e o n l i n e a t w w w . s c i e n c e d i r e c t . c o m

w w w . e l s e v i e r . c o m / l o c a t e / m a t c h a r
http://dx.doi.org/10.1016/j.matchar.2012.02.022http://dx.doi.org/10.1016/j.matchar.2012.02.022http://dx.doi.org/10.1016/j.matchar.2012.02.022mailto:[email protected]://dx.doi.org/10.1016/j.matchar.2012.02.022http://dx.doi.org/10.1016/j.matchar.2012.02.022mailto:[email protected]://dx.doi.org/10.1016/j.matchar.2012.02.022


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internal oxidation of 1 wt.% Al. This calculation was made

using the simple equation:

4Al 3O2 2Al2O3: 2

Given that 4 27= 108 g of aluminum oxide produces 204 g

of Al2O3, i.e.2(227+316)=204 g, then oxidation of 1 g Al,

contained in the prealloyed copper, will generate 1.9 g of

Al2O3. Considering this result, it is supposed that the milled

powder mixtures with a total amount of 2.5 wt.% Al2O3parti-

cles have been obtained.

Full width at half maximum (FWHM) measured from XRD

patterns of Cu1Al+Al2O3 powders shows a progress in line

broadening 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. The

most intensive grain refinement occurs up to 10 h, when the

grain size decreases from 550 to 78 nm. With prolonged time

the 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

powders crystal lattice distortion during the first 10 h of mill-

ing. When milled for a longer period of time, the lattice distor-

tion becomes less evident. The distortion appears as a result

of plastic deformation which is due to a decrease in the

grain size. It was shown that the contributions to the lattice

distortion may arise from internal stresses imposed by dislo-

cations and inhomogeneously distributed point defects[15].

The change of Cu1Al+ Al2O3particles morphology with in-

creasing milling time is shown inFig. 4. During high-energy

milling the powder particles change morphology and size as

a consequence of repeated deformation, fracturing and weld-

ing processes. According to these micrographs powder size

increases for up to 5 h of milling due to the welding predomi-

nance in the milling process (Fig. 4a). With longer milling time

the powder size decreases since the fracturing predominates

in the milling process. After 20 h of milling Cu1Al+Al2O3par-

ticles are rather small, but not equiaxed in shape (Fig. 4b). Dif-

ferent morphologies of these particles indicate that the

balance between fracturing and welding processes was not

achieved.

The composition of Cu1 wt.% Al powders changes during

milling. It was recently reported [6]that during high-energy

milling of prealloyed powders, the Al2O3 particles formed

through the reaction of aluminum with oxygen from the air

are of nano-sized dimensions, i.e. most of the particles have

the approximate size of 50 nm or less. At higher magnification

SEM micrograph of 10 h-milled powder illustrates the presence

of particles with different morphologies (Fig. 5). Lamellae (L)

representing traces of previous individual powder particles

may 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 nm are also present in the matrix (Fig. 5b).

Stresses imposed by larger particles are the main reason for the

appearance of microcracks (C) in the matrix (Fig. 5b).

3.2. Compacts

3.2.1. Microstructure

Fig. 6 illustrates the microstructure of compacts obtained

from milled Cu1Al+Al2O3 powders. The compacts retainedFig. 2 Effect of milling time on full width at half maximum

(FWHM) of Cu1 wt.% Al+0.6 wt.% Al2O3powders.

Fig. 3 Effect of milling time on grain size and lattice

distortion of Cu1 wt.% Al+0.6 wt.% Al2O3powders.

Fig. 1 Lattice parameter vs.milling time of Cu1Al+Al2O3powders. (In this and all following cases 0on the X-axis

denotes as-received and non-milled condition regarding to

powders and corresponding compacts).

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lamellar structure, a characteristic of high-energy milled pow-

der particles. Although lamellae are retained in compacts

processed from powders milled for 12 h, some changes in

the microstructure may be distinguished, i.e. the light areas

(denoted by arrows) indicate recrystallization which occurred

during hot-pressing (Fig. 6a). Note that recrystallization was

mostly initiated at the boundaries of the powder particles,

but in a lesser extent, is also visible at the corners of particles

where the concentration of stresses imposed during compac-

tion was highest. In compact processed from powder milled

for 20 h (Fig. 6b), the extent of recrystallization was extensive

and unrecrystallized particles were surrounded by recrystal-

lized areas.

A SEM micrograph of compact processed from 10 h-milled

powders is shown in Fig. 7. In the backscattered electron

image (BSE) small and large particles may be seen. The

inserted EDS spectrum shows the presence of aluminum and

oxygen in the small particle. Small amounts of iron probably

originated from the steel balls of the high-energy mill. Since

it was estimated that commercial Al2O3 particles could not

be fractured during milling[12], then the structure of compact

processed from 10 h-milled Cu1Al+Al2O3powders consists of

nano- and micro-sized Al2O3particles embedded in the cop-

per matrix.

A SEM micrograph of the compact processed from 20 h-

milled powders is illustrated inFig. 8. A wide recrystallized

area free of particles with annealing twins may be seen in

the BSE image, whereas the distinction between nano- and

micro-sized particles was difficult to establish.

Fig. 5 SEM micrographs of 10 h-milled Cu1Al + Al2O3particles. Arrows denote: (a) Lamellae (L); (b) nano-sized

particle (N); micro-sized particle (M); microcrack (C).

Fig. 6 Light micrographs. Microstructure of compacts

processed from Cu1Al+ Al2O3powder after different milling

times. (a) 12 h; (b) 20 h. Arrows denote recrystallized regions.Fig. 4 SEM micrographs. Morphology of Cu1Al+ Al2O3particles after different milling times. (a) 1 h; (b) 20 h.

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The effect of milling time on XRD pattern of Cu1Al+ Al2O3powder 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) and

compacts (Fig. 9b) is relatively small, it may be observed that

the 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 was

not 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 in

Table 1. The grain size of compacts was calculated usingEq.

(1), based on the results of (FWHM) measured from XRD pat-

terns of compacts.

Results ofTable 1show that the grain size of both powders

and compacts decreases during shorter milling time, reaching

a 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 to

diffusion processes during hot-pressing and their influence

on the grain growth.

The morphology of Al2O3particles is illustrated in TEM mi-

crographs (Fig. 10). In general, nano-sized particles are homo-

geneously distributed within the matrix and on the grain

boundaries. In compact processed from 10 h-milled powders

nano-sized particles (mainly globular and approximately be-

tween 30 and 60 nm in size) may be seen. Some of these

particles are formed on grain boundaries preventing grain

boundary 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 in

TEM micrographs are poorly defined, it is obvious that grain

growth 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 size

values (seeTable 1) and those determined by TEM clearly ex-

ists. This difference may be fully attributed to the vaguely

defined grains in TEM micrographs. According toFig. 10c a

Fig. 8 SEM micrograph. BSE image of compact processed

from 20 h-milled powders. Annealing twins in the

recrystallized area.

Fig. 9 XRD pattern of (a) Cu1Al+Al2O3powders and

(b) corresponding compacts after different milling times. All

peaks correspond to the copper matrix.

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 98

Compact 630 295 142 90 104 138 200

Fig. 7 SEM micrograph. BSE image of compact processed

from 10 h-milled powders with inserted EDS spectrum of a

small particle.

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coarsening of some Al2O3 particles could be distinguished.

This increase of nano-sized Al2O3particles was reported sug-

gesting coarsening as a result of diffusion processes during

longer milling time[1].

3.2.2. Microhardness

Microhardness of compacts depends on the previous milling

time of Cu1Al+Al2O3powder (Fig. 11). Microhardness steeply

increases 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 of

milling time (seeFig. 3) may be regarded as a result of lattice

deformation due to successive precipitation of nano-sizedAl2O3 particlesfrom thecopper solid solution. Under theinflu-

ence of diffusion processes during milling these particles are

precipitated within the matrix and at grain boundaries.

Nano-sized particles, finely distributed in the matrix and on

the grain boundaries, act as pinning points impeding further

movement of dislocations and their propagation. Thus, the

pinning force exerted by nano-sized particles on the grain

boundary prevents the grain growth.

The microhardness of compact obtained from 10 h-milled

powder (240 HV0.05) is much higher than the microhardness

processed from as-received and non-milled Cu1 wt.% Al

powder (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 of

the fine grain copper matrix structure and the presence of

the nano-sized Al2O3 particles. These Al2O3 nano-particles

are 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 the

particle; the critical stress Or for bypassing depends on the

interparticle distance l according to Orl1. According to

the Orowan bowing mechanism and thermal mismatch be-

tween the matrix and reinforcement particles in metal matrix

composites [1821], by decreasing particulate size the strength

increases. Note that since the particles in this work are small

enough (less than 100 nm), the Orowan bowing mechanism

can be used to justify this behavior[19]. From the microhard-

ness and microscopic results it can be concluded that the flow

stress, necessary for plastic deformation of the composite in

contrast to the as-received material, is additionally increased

by dispersion-strengthening of the matrix grains.

Fig. 11 Effect of milling time on microhardness of

Cu1Al+Al2O3compacts.

Fig. 10 TEM micrographs. Compacts processed from

10 h-milled particles (a,b) and 20 h-milled powders (c).

Arrows, inFig. 10a, c denote a part of large Al2O3particle.

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The results obtained in this work reveal that at the peak

values (at 10 h of milling time), the microhardness of Cu1Al+

Al2O3compact is higher than that of the Cu1 wt.% Al (without

addition of micro-sizedAl2O3 particles), i.e. 240 vs. 220 HV0.05, re-

spectively, processed under the same conditions[11]. At this

stage, apart from the effect of nano-sized particles, the contri-

bution of micro-sized Al2O3 particles to microhardness must

be taken into account. The extent of matrix hardening is addi-tionally increased by the formation of the dislocation network

formed around these particles (seeFig. 10b). Thus, at the maxi-

mum microhardness the synergetic effect of nano- and micro-

sized Al2O3 particles exhibits a marked effect on increased

microhardness of Cu1Al+Al2O3compacts 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 Al2O3par-

ticles, under certain conditions may contribute to the increase

in grain size[10]. Namely, in the vicinity of micro-sized Al2O3particles, a cellular dislocation substructure may be created

with a markedly increased density of dislocations. The pro-

longed time of milling results in a significantly increased

subgrains number, which can be activated and become the

nucleation sites of newly created recrystallized grains. Light

(Fig. 6a, b), SEM (Fig. 8) and TEM (Fig. 10c) micrographs coupled

with the values of the grain growth with milling time (see

Table 1) indicate that the process of recrystallization, which

occurred during prolonged milling, was the main factor

influencing the microhardness decrease. On the other hand,

after reaching its maximum, microhardness of Cu1 wt.% Al

compacts remains practically unchanged[11], indirectly sug-

gesting that coarseAl2O3 particles maybe regarded as a signif-

icant parameter in decreasing microhardness of Cu1Al+Al2O3compacts 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 several

different 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 is

more pronounced during short milling time, whereas the in-

fluence of other parameters prevails with longer milling. The

twofold role of coarse Al2O3particles in matrix strengthening

must 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 the

micro-sized Al2O3particles have a significant effect.

3.2.3. Density

Density of the Cu1Al+ Al2O3compacts decreases with milling

time (Fig. 12). The significant drop in density occurred for up

to 15 h of milling time, when the fracturing of powder parti-

cles is the predominant process during milling. According to

Fig. 4the morphology of powder particles influences the pack-

ing between particles during hot-pressing, indicating that the

better 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 powders

was not completed. The reason for such an inadequate

consolidation could also be related to insufficient applied

pressure of 35 MPa. It is quite disputable whether the precipi-

tation of nano-sized particles from the solid solution may

have any effect on the decrease of density. The measured

density of compacts processed from 10 h-milled powder

(7.75 g cm3) was 87% of the theoretical value (8.89 g cm3).

Since the measured density of the hot-extruded materials is

higher than 99.3%[23]hot-extruding seems to be a common

method of compacting. It should be noted that in this study

the theoretical density was calculated for the total amount

of 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 inTable 2.

Compact processed from as-received and non-milled

Cu1Al+Al2O3powders shows the lowest electrical conductivi-

ty. During the following milling, precipitation of nano-sized

Al2O3 particles contributes to the increase in electrical con-

ductivity, which is due to depletion of aluminum content in

solid solution. No significant change in electrical conductivity

was 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 is

obvious that the electrical conductivity of compacts is much

lower than that of pure copper or some copper based, highconductivity alloys. Nano-sized Al2O3 particles form a great

number of interfaces considered as a possible source of addi-

tionalelectron scatter,which is a significant factor in reducing

conductivity [24].

Table 2 The effect of milling time on electricalconductivity of Cu1Al+ Al2O3compacts.

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+Al2O3compacts.

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4. Conclusions

Simultaneously reinforced copper matrix with nano- and

micro-sizedAl2O3 particles was obtained by high-energy milling

of the mixture containing inert gas-atomized prealloyed copper

powder with 1 wt.% Al and 0.6 wt.% commercial Al2O3powder.

- Milling of prealloyed powder promoted an amount of 1.9 wt.%

Al2O3 by internal oxidation. Thus, thetotal amountof 2.5 wt.%

of nano- and micro-sized Al2O3particles have been obtained.

Lamellae, representing traces of previous individual powder

particles may be distinguished, whereas a number of very

small 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 h

retained lamellar structure, a characteristic for high-energy

milled powderparticles. During longer millingtime thelamel-

lar structure was somewhat changed as a result of recrystalli-

zation occurring during hot-pressing. In compact processedfrom10 h-milledpowdersnano-sizedparticles (mainlyglobu-

lar and approximately between 30 and 60 nm in size) prevent

grain boundary migration, decreasing the rate of the grain

growth. 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 processedfrom 10 h-milled powders. The microhard-

ness of compact obtained from 10 h-milled powder is 3

times higher (2400 MPa) than microhardness processed

from as-received and non-milled Cu1 wt.% Al powder

(745 MPa) compacted under the same conditions. At the

maximum microhardness the grain size reaches the smal-

lest value as a result of the synergetic effect of nano- andmicro-sized Al2O3particles.

- Prolonged milling results in a slow drop in microhardness.

This is because the micro-sized Al2O3particles under certain

conditions may contribute to an increase in the grain size.

Theprolonged milling time results in a significantly increased

subgrain number,which can be activated andbecome thenu-

cleationsites of newly formed recrystallized grains. Recrystal-

lization, which occurred during prolonged milling, was the

main factor influencing the microhardness decrease.

- Density of the Cu1Al+ Al2O3 compacts decreaseswith milling

time. The significant drop in density occurred for up to 15 h

of milling time, when the fracturing of powder particles is

the predominant process during milling. Morphologyof pow-der particles influences the packing between particles during

hot-pressing, indicating that the better packing achieved

during shorter milling time corresponds to higher density.

- The increase in electrical conductivity of the Cu1Al+Al2O3compacts 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 Project

No 172005.

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