improved processing of carbon nanotube-magnesium alloy composites

8/3/2019 Improved Processing of Carbon Nanotube-magnesium Alloy Composites

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Improved processing of carbon nanotube/magnesium alloy composites

Qianqian Li *, Andreas Viereckl, Christian A. Rottmair, Robert F. Singer

Institute of Advanced Materials and Processes, University of Erlangen-Nuremberg, Dr.-Mack-Str. 81, 90762 Fuerth, Germany

a r t i c l e i n f o

Article history:

Received 10 December 2008

Received in revised form 11 February 2009

Accepted 15 February 2009Available online 24 February 2009

Keywords:

Melt stirring

A. Carbon nanotubes

A. Metalmatrix composites (MMCs)

A. Nanocomposites

B. Mechanical properties

a b s t r a c t

Carbon nanotubes (CNTs) are promising reinforcements for light weight and high strength composites

due to their exceptional properties. However, until now, the main obstacle is to obtain a homogenous dis-

persion of the CNTs in the desired material matrix. Quite a few methods have been studied to help

improving the dispersion of CNTs in a polymer matrix. But not much research has been conducted on

how to disperse CNTs in metal matrices. In this study, a two-step process was applied. In the first stage,

a block copolymer was used as a dispersion agent to pre-disperse multiwall carbon nanotubes (MWNTs)

on Mg alloy chips. Then the chips with the well dispersed MWNTs on their surface were melted and at the

same time vigorously stirred. The molten MWNT Mg alloy composites were poured into a cylindrical

mould to solidify quickly. For the pre-dispersion step, the microstructures of the Mg alloy chips were

studied under SEM. MWNTs were quite successfully dispersed on the surfaces of the Mg alloy chips.

The mechanical properties of the MWNT/Mg composites were measured by compression testing. The

compression at failure, the compressive yield strength and ultimate compressive strength have all been

improved significantly up to 36% by only adding 0.1 wt% MWNTs to the Mg alloy. In order to predict the

potential yield strengths of the MWNT reinforced Mg alloy composites, the contributions by load transfer,

Orowan strengthening and thermal mismatch were added up.

2009 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon nanotubes have attracted the attention of many scien-

tists worldwide since their discovery in 1991 [1,2]. Numerical sim-

ulations [36] and experimental results [710] have indicated their

extraordinary strength (up to 150 GPa) and Youngs modulus (up

to 1 TPa), which make them ideal candidates as reinforcements

for high strength, light weight and high performance composites.

The main problem for CNT composites is to obtain a homogenous

dispersion in the matrix materials. Nanoparticles are difficult to

disaggregate due to their attractive van der Waals interactions;

CNTs have larger surface area ($1000 m2/g) and much higher as-

pect ratio ($104 normally) than traditional fillers. This high specific

surface, which is desirable when it acts as an interface for an effi-cient stress transfer or when maximum resistance against disloca-

tion movement is desired, causes the strong tendency of the CNTs

to form agglomerates [11,12].

Carbon nanotubes reinforced polymer based composites have

been widely synthesized by repeated stirring, solution evaporation

with high energy sonication, surfactant assisted processing and

interfacial covalent functionalizations [1316]. Only limited

research has been done on carbon nanotube reinforced metal com-

posites. Zhou et al. [17] has reinforced Al composites with carbon

nanotubes by pressureless infiltration technique. The hardness of

the composites was increased 40% by adding carbon nanotubes.

Esawi et al. [18] tried to disperse carbon nanotubes in aluminium

powder by mechanical alloying. They also reinforced Al strips with

carbon nanotubes by a powder can rolling technique [19]. The

Youngs modulus of the composites was increased by 20% after

adding 0.5 wt% carbon nanotubes. Carreno-Morelli et al. [20] pro-

duced multiwall carbon nanotube/pure magnesium composites

by a powder metallurgical method. The results showed that the

Youngs modulus was about 9% higher compared to pure Mg metal

by adding 2 wt% CNTs. Shimizi et al. [21] fabricated 1 wt% of short

and straight carbon nanotubes reinforced Mg alloy composites by a

vacuum hot pressure method followed by extrusion. The yield

strength, the tensile strength and the Youngs modulus of theCNT/Mg composites have all been improved by about 23%.

Beyond simple reinforcement theories based on load transfer,

e.g. the rule of mixture, we expect size dependent reinforcement

mechanisms to take place. It is the fact that the CNTs have high as-

pect ratio, which makes it possible to acquire smaller interparticle

spacing in the matrix at very low concentrations compared to tra-

ditional reinforcements such as SiC. Therefore, CNTs can act as

obstacles to dislocation movement in metals. Plastic deformation

can only proceed if the dislocations circumvent the obstacle (Oro-

wan mechanism) or shear the nanotube. Because of the small

diameter of the CNT, shear appears to be the most likely mecha-

nism. It would mean that dislocations are held up at the CNT and

0266-3538/$ - see front matter 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.compscitech.2009.02.020

* Corresponding author. Tel.: +49 911 95091833; fax: +49 911 95091815.

E-mail address: [email protected](Q. Li).

Composites Science and Technology 69 (2009) 11931199

Contents lists available at ScienceDirect

Composites Science and Technology

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the stress concentration at the head of the pile-up group of dislo-

cations causes the CNT to yield by deformation or fracture. In the

case of obstacles with high aspect ratio, dislocations cannot easily

climb to circumvent the obstacle, so we expect not only improve-

ments of flow stress and toughness but also a considerable

improvement in creep resistance of the material [22].

The investigation of metal matrix composites reinforced with

carbon nanotubes has many challenges:

1. A uniform dispersion of CNTs in the metal matrix.

2. A fabrication technique suitable for up-scaling and commercial

production.

3. Control of the interfacial reaction between CNTs and the metal

matrix.

In the present study, a two-step process is designed and ap-

plied: step one is a pre-dispersing procedure of CNTs on the Mg,

in order to break down big agglomerates; step two is a fabrication

of CNT/Mg alloy composite by a melt stirring technique. At step

one a block copolymer is used as a dispersing agent to pre-disperse

CNTs on Mg alloy chips. The block copolymer was chosen because

it has already been proven in previous research that it can improve

the dispersion of CNT in ethanol [23,24].

After the CNTs are dispersed on the Mg chips, a straight-forward

process a melt stirring technique (see Fig. 1) is used to produce

samples. Some research groups have tried to use melt stirring to

produce CNT metal composites before [25]. The results showed

that the elastic modulus, tensile strength and elongation of the

CNT metal composites were all increased. However, in these previ-ous studies the technique was not optimised, CNTs were only stir-

red into Mg melts without pre-dispersion and the resulting

dispersion of the CNTs in the Mg matrix was still uneven [25]. In

our study, by combining the pre-dispersion and melt stirring to-

gether, we are expecting better dispersion of CNTs and stronger

composites.

2. Experimental

The block copolymer Disperbyk-2150 (BYK Chemie GmbH) was

first dissolved in ethanol in a small beaker. Then MWNTs (0.1 wt%

of the metal matrix, mass ratio to the block copolymer 1:1, diam-

eter of 520 nm, Baytubes

C 150P) were added to the as-preparedsolution. This mixture was put at room temperature into an ultra-

sonic bath for 15 min. Then it was stirred for 30 min at 250 rpm.

After adding Mg alloy chips (AZ91 D, ECKA), the suspension was

further stirred at 250 rpm inside a fume cupboard to evaporate

ethanol and homogenize the mixture.

After the mixture was dried, the MWNT coated chips were

placed in a cylindrical sample crucible as shown in Fig. 1. This cru-

cible was placed into an oven and heated up to 650 C under an in-

ert gas atmosphere to avoid oxidation. When the Mg alloy chipswere molten, the liquid was mechanically stirred at 370 rpm for

30 min to further disperse MWNTs. After stirring, the molten

MWNT/Mg composite was poured into a mould. The cooled sample

was machined to cylindrical shaped specimens (diameter

5 mm height 7 mm) for subsequent compression tests. Reference

samples were made using exactly the same procedure but from

pure AZ91.

After the pre-dispersion step scanning electron microscopy

(SEM) was used to observe the microstructure on the surface of

CNT coated Mg chips. Raman spectrometry was used to detect

the CNTs on the Mg chips. The cast composites were cut and pol-

ished for grain size measurement by two phase linear analysis

method (Leica DMRM, Germany). The resulting samples were

tested by compression testing to determine the compression at

failure, compressive yield strength and ultimate compressive

strength. Tests were conducted at ambient temperature using

standard tensile/compression testing equipment (100 kN). Testing

was performed at a constant strain rate of 0.01 s1.

3. Results and discussion

3.1. Step 1: pre-dispersion of CNTs on Mg alloy chips

For step 1, the pre-dispersion of CNTs on the Mg alloy chips,

SEM analysis was used to study the microstructure of the raw

MWNTs and the MWNT coated Mg alloy chips. Fig. 2a and b are

SEM images of the MWNTs as received. From the images, it can

be observed clearly that the raw MWNTs are agglomerated in bigbundles. Fig. 2c and d exhibit the SEM images of dispersed MWNTs

(white arrows) on Mg alloy chips. At higher resolutions as in Fig. 2e

and f, individual MWNTs can be found (white arrows). In order to

confirm that the MWNTs we observed under SEM are not damaged,

we carried out Raman spectroscopy. In Fig. 3b, we can clearly ob-

serve the G band around 1625 cm1, which is characteristic for

sp2 bonds in MWNTs. Comparing to the Raman spectra of pristine

MWNTs as shown in Fig. 3a, the Raman spectra of MWNT/Mg

shows no changes. Therefore, we can conclude that MWNTs are

not destroyed during step one and a homogenous dispersion of

MWNTs on the Mg chips has been achieved.

3.2. Step 2: fabrication of MWNT Mg composites

The well dispersed MWNT coated Mg chips, obtained from step

one, were put into the melt stirring equipment and heated up to

650 C under an inert gas atmosphere. When the MWNT/Mg alloy

chips were molten, the liquid was vigorously stirred to further dis-

perse MWNTs. After stirring, the molten MWNT/Mg composite was

poured into a mould for rapid solidification and then machined

into cylindrical shaped specimens for subsequent compression

tests.

Kim et al. [26] reported that adding carbon to MgAl alloys

could effectively contribute to a grain-refining of the matrix. This

was explained by the formation of Al4C3 particles which enable

heterogeneous nucleation and result in a change of the microstruc-

ture. We therefore decided to check whether carbon nanotubes are

also acting as a grain refiner in the matrix. Fig. 4 shows opticalmicroscopy of the cross sectional area of the pristine AZ91 alloy

Fig. 1. Illustration of the melt stirring machine.

1194 Q. Li et al. / Composites Science and Technology 69 (2009) 11931199


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and 0.1 wt% MWNT reinforced Mg composites. As the optical

micrographs suggest, there are no obvious grain-refining effects

of MWNTs in the Mg alloy matrix.

Furthermore, grain sizes of the AZ91 alloy and the 0.1 wt%

MWNT reinforced AZ91 composite were measured by two phaselinear analysis, i.e. evaluation of the mean intercept. The differ-

ences between the two materials are statistically insignificant as

can be seen in Table 1. This result fits with other research [27],

in which no change of the microstructure was observed by adding

MWNTs. Therefore it can be claimed that the effect of grain size

modifications due to the presence of CNTs does not play a majorrole in our CNT composites. There are slight changes in the area

Fig. 2. SEM images of (a), (b) raw MWNTs as received from Bayer, and (c)(f) MWNTs homogeneously dispersed on the Mg alloy chips. The white arrows point at the MWNTs.

Fig. 3. Raman spectra of (a) pristine MWNTs as received, (b) MWNT coated chips and the pure AZ91 alloy chips.

Q. Li et al. / Composites Science and Technology 69 (2009) 11931199 1195


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percentage ofa-Mg and the eutectic phase. About 4% more eutecticphase is observed in the MWNT Mg composites compared to the

AZ91 alloy. It is not clear whether this is related to adding MWNTs

or to the statistical scatter in the process. More experiments will be

carried out.

3.3. Mechanical properties of MWNT Mg composites

Typical stressstrain curves of the MWNT/Mg composite and

the pure AZ91 Mg alloy are shown in Fig. 5a. It is clear that

the compression at failure, the compressive yield strength and

the ultimate compressive strength were all increased by addition

of 0.1 wt% MWNTs. To improve the statistical significance of the

results, 24 specimens were tested following the same procedure.

The results are compiled in Fig. 5bd. By adding only 0.1 wt% of

MWNTs, the compression at failure (24.4%) has increased 36%

compared to the pristine AZ91 Mg alloy (18%) and the ultimate

compressive strength of MWNT/Mg composites (412 MPa) in-

creased 20% compared to the pristine AZ91 Mg alloy (344 MPa).

Because of the limitation of the measured sample geometry,

i.e. the top surface of the cylinder sample was not perfectly par-

allel to the bottom surface, and the short gauge length (7 mm inour case) in the compression test, 2% yield strength was mea-

sured instead of normally 0.2% yield strength to achieve more

comparable results. The 2% yield strength of MWNT reinforced

Mg composites (272 MPa) is 10% higher compared to the pristine

AZ91 Mg alloy (248 MPa).

As has been discussed before, no significant grain-refining ef-

fects were observed by adding MWNTs. Therefore the effect of

modifications in the microstructure of the matrix due to the pres-

ence of CNTs is effectively ruled out. The improvement of the

mechanical properties of the composites is contributed to excellent

mechanical properties of carbon nanotubes. Moreover, combining

the SEM observation with the results of our mechanical testing,

we attribute the improvement of the compression at failure, the

compressive yield strength and the ultimate compressive strengthto the good dispersion of MWNTs in the Mg metal matrix.

3.4. Flow strength

It is important to understand the strengthening mechanism of

CNTs in composites in order to be able to predict the strength. Ina first approach, George et al. [22] summarized three possible rein-

forcement mechanisms which might be relevant in CNT/metal

composite systems, namely load transfer, Orowan looping and

thermal mismatch. Furthermore, Zhang et al. [28] has proposed

an analytical model to predict the yield strength by incorporating

Orowan strengthening effect, enhanced dislocation density

strengthening effect due to the thermal mismatch and load bearing

effect.

Here we simply add up in a linear way all the improvements

caused by different mechanisms to predict the theoretical yield

strength. The yield strength of the composites by adding MWNTs

may be then expressed as:

ryc

rym

Drload

DrOrowan

Drthermal

1

where ryc is yield strength of the nanocomposite; rym is yieldstrength of the matrix; Drload is the improvement associated with

the load transfer effect; DrOrowan is the improvement associated

with Orowan strengthening effect; Drthermal is the improvement

associated with the increase in dislocation density due to the differ-

ent thermal expansion coefficients of the matrix and the CNTs

(thermal mismatch).

The improvements due to load transfer in the simplest form can

be written as:

Drload rre mre 2

where rre is the tensile strength of the carbon nanotubes (strength

for MWNTs is suggested in the range of 1163 GPa [29]); mre is vol-

ume fraction of the reinforcement.The Orowan strengthening effect is given by:

DrOrowan 0:8 Gm bMLp

3

where Gm is the shear modulus of the matrix; b is the value of the

Burgers vector of the matrix; Mis the Taylor factor (M is chosen to

be 3); Lp is the interparticle distance [30]. The CNTs used to rein-

force Mg alloy in our experiments have a diameter d of about

13 nm and an average length h of 1 lm, therefore they should beconsidered as rod-shaped reinforcements. According to research

by Ashby [31], Dieter [32], and Kelly [33]:

Lprods

1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiNsrod

p 4

Fig. 4. Optical microscopy (a) AZ91 alloy, (b) 0.1 wt% MWNT AZ91 composite.

Table 1

Grain sizes of 0.1% MWNT Mg composites and AZ91 alloy.

Samples Phase A (a-Mg) Phase B (eutectic phase)

Averagegrain

size (lm)

Areapercentage

(%)

Averagegrain

size (lm)

Areapercentage

(%)

AZ91 25.6 86 4.3 14

AZ91 + 0.1 wt% MWNTs 25.8 82 5.6 18



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Nsrod mres

5

where Ns is the number of particles intersecting the slip plane; s is

mean area of a slip plane intersected by the rod-shaped particles,

and with h ) d, s pd2

2[34].

Combining Eqs. (4) and (5) we find:

Lprods ffiffiffiffiffiffiffiffiffiffipd

2

2vre

s6

Finally the hardening due to the thermal mismatch can be ex-

pressed as:

Drthermal a Gm b ffiffiffiffiq

p 7where a is a geometric constant (1.25 in our case); q is the disloca-

tion density and q BmreDCTEDTb1mre

1d, in which B is a geometric constant

(4 in our case [35]); DCTE is the difference between the coefficient

of thermal expansion; DTis the difference between the process and

test temperature.

In Fig. 6, a comparison of the contributions to the improvement

in yield strength by the three strengthening mechanisms is shownas a function of the volume fraction of the MWNTs. It shows that

effective Orowan strengthening and thermal mismatch strength-

ening can already take place at low amounts of MWNTs (about

0.1 wt%) and both contribute almost equally to the improvement

of the yield strength. Strengthening due to the load transfer of

the MWNTs increases linearly and becomes more important than

other strengthening effects at higher MWNTs amounts according

to the present model. At a threshold amount of about 0.3 wt% the

load transfer effect exceeds the Orowan strengthening and the

thermal mismatch effect.

The total yield strength affected by the three mechanisms was

also plotted in Fig. 6. It demonstrates that the yield strength can

in theory be highly increased by the addition of small amounts of

MWNTs. We compared our experimental data at 0.1 wt%(0.09 vol%) to this theoretical value. It can be clearly seen that

the experimental data is below the theoretical value, which indi-

cates that the dispersion can still be improved to produce a stron-

ger composite.

Further experiments such as producing different samples con-

taining different volume fractions of MWNTs in the matrix are

required.

It is still not certain which of the mechanisms play a major role

in the CNT reinforced metal matrix composites. Eq. (1) faces certain

limitations for all three reinforcement mechanisms:

Load transfer: If interfacial shear stresses efficiently transfer loadfrom the elastically softer metal matrix to the stronger CNT, this

Fig. 6. Comparison of the strengthening mechanisms and their theoretical contri-

bution to the increase of the yield strength for MWNT/AZ91 composites as a

function of MWNT volume fraction, and the experimental 2% yield strength at0.09 vol% (0.1 wt%) of MWNTs in the Mg matrix.

Fig. 5. (a) Typical compression stressstrain curves of MWNT AZ91 alloy composites (two random samples) and AZ91 (two random samples). Comparison of (b) compression

at failure; (c) 2% yield strength and (d) ultimate compressive strength average over 24 samples.



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leads to a reduced stress level in the metallic matrix and thus to

strengthening. However, this mechanism requires a good inter-

facial bonding between the CNTs and the metal matrix, which is

said to be poor according to other research [3639].

Orowan mechanism: CNTs may act as impenetrable obstaclessimilar to non-shearable precipitates and increase the yield

stress through an Orowan mechanism. Again, there is no specific

study of the interfacial processes characterizing the interaction

between a dislocation and a CNT. In particular, it is not clear

whether dislocation motion indeed requires the formation of

Orowan loops, or whether the strain caused by the shearing of

the metal around the CNT can be accommodated by plastic

deformation or fracture of the CNT, leading to a quasi-cutting

mechanism of dislocation motion.

Thermal mismatch hardening: There is a significant mismatch in

coefficient of thermal expansion between CNT and the matrix,

and this might be accommodated by extensive dislocation

nucleation around CNT which then leads to hardening of the

metal matrix. However, it is not at all clear to which extentthe analysis of Arsenault [35], which relates to SiC platelets with

sizes and separation distances of the order of several microns,

can be applied to the quite different scale and geometry of

embedded CNT.

Due to these limitations, further understanding is clearly neces-

sary. The behaviour of CNTs in the metal matrix needs to be stud-

ied and characterization of such composites by different methods

is required.

3.5. Ductility

From the compression test results, one notable observation is

that the compression at failure, i.e. the ductility, increased (asmuch as 36%) together with the strength. This is different from

what is observed when using traditional reinforcements such as

carbon fibres [40].

Fig. 7 shows SEM images of the fracture surface of AZ91 al-

loy and the 0.1 wt% MWNT reinforced AZ91 alloy. It is obvious

that the AZ91 alloy has a rather smooth fracture surface with

several sharp breaking edges, while the fracture surface of

MWNT reinforced AZ91 composite exhibits a lot of dimples,

which indicates a more ductile behaviour of the composite. In-

creased ductility has also been found in CNT reinforced Mg

composites before by Goh [41]. Their explanation for this is

the activation of prismatic slip planes in the Mg matrix by add-

ing CNTs. It implicates a minimum of five independent slip sys-

tems which are required to deform a polycrystalline metal

plastically (von Mises criterion), therefore resulting in a much

higher ductility of the composites. This idea was further con-

firmed by TEM studies [30]. A high activity of the basal slip

system and the initiation of prismatic slip at room temperature

were observed under TEM.

4. Conclusions

In this study, a two-step process was designed and applied.

Multiwall carbon nanotubes were first dispersed on the Mg al-

loy chips using a block copolymer as a dispersion agent. In this

step, the agglomerates of MWNTs were separated. Then the

MWNT coated Mg chips were used to fabricate CNT/Mg alloy

composites by a melt stirring technique. A good dispersion of

MWNT in the Mg matrix was achieved by this two-step

process.

After step one, we confirmed by Raman spectroscopy that

MWNTs were still intact on the surface of the Mg chips. Under

SEM, individual multiwall carbon nanotubes can be found on the

surface of Mg chips after the dispersion stage.Adding a small amount of multiwall carbon nanotubes signif-

icantly enhanced the mechanical properties of the AZ91 Mg al-

loy. Compared to the AZ91 Mg alloy, the compression at failure

of the MWNT/Mg composites was improved by 36%; the 2% yield

strength was improved by 10% and the ultimate compressive

strength was improved by 20%. Unlike in the case of adding tra-

ditional reinforcements, the compression at failure was improved

in addition to the other properties. We attribute the improve-

ment of the mechanical properties to the homogenous dispersion

of MWNTs in the Mg matrix. No change in grain size has been

measured between the composites and the pristine AZ91 alloy,

which indicates that MWNTs do not act as a grain refiner in

the matrix.

The contributions of load transfer, Orowan strengthening andthermal mismatch were simply added up to predict the poten-

tial yield strengths by adding different volume fractions of

MWNTs to the composites. The experimental result of 2% yield

strength at 0.09 vol% (0.1 wt%) was compared to the theoretical

value. It showed that the dispersion still needs to be improved

to produce a stronger material. It is still not certain which

mechanisms play a main role in the strengthening of the carbon

nanotube reinforced metal composites. Further understanding is

required.

Further studies such as: detailed observation on the microstruc-

ture and elemental composition together with SEM and TEM inves-

tigations of fracture surfaces and the alloyMWNT interface;

producing different samples with different volume fracture of

MWNTs, will also be undertaken in the future and presented in aseparate paper.

Fig. 7. SEM images of the fracture surface of (a) AZ91 and (b) 0.1 wt% MWNT AZ91 composite.



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Acknowledgements

A financial grant of the Bavarian State Ministry of Science, Re-

search and Arts and the Objective 2 Funding of the European Union

are gratefully acknowledged. We would like to thank Christian

Rauber for his great help and suggestions through the experi-

ments; also thanks to Florian Pyczak, Jens Schaufler and Natalie

Kmpel for their kind technical assistant. We are indebted to BayerAG, BYK Chemie GmbH and ECKA Granulate GmbH & Co. KG for

their generous offers of the materials used in this study. We greatly

appreciate Prof. Michael Zaiser and Dr. Jan Schwerdtfeger for fruit-

ful discussions and suggestions.

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improved processing of carbon nanotube-magnesium alloy composites

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