Materials Sciences and Applicatio ns, 2011, 2, 20-29
doi:10.4236/msa.2011.21004 Published Online January 2011 (http://www.SciRP.org/journal/msa)
Carbon Nanotube Addition to Simultaneously
Enhance Strength and Ductility of Hybrid
AZ31/AA5083 Alloy
Muralidharan Paramsothy1, Manoj Gupta1*, Jimmy Chan2, Richard Kwok2
1Department of Mechanical Engineering, National University of Singapore, Kent Ridge, Singapore; 2Singapore Technologies Kinet-
ics Ltd (ST Kinetics), Boon Lay, Singapore.
Received December 13th, 2010; revised January 4th, 2011; accepted January 10th, 2011.
ABSTRACT
AZ31/AA5083 hybrid alloy nanoco mposite contain ing CNT nanopar ticle reinforcement was fab ricated using solid ifica-
tion processing followed by hot extrusion. The AZ31/AA5083 hybrid allo y nanocomposite exh ibited similar grain size to
monolithic AZ31/AA5083 hybrid allo y, reasonab le CNT na nopar ticle distribu tion, non -dominant (0 0 0 2) texture in the
longitudinal direction, and 20% higher hardness than monolithic AZ 31/AA5083 hybrid alloy. Compared to monolithic
AZ31/AA5083 hybrid alloy (in tension), the AZ31/AA5083 hybrid alloy nanocomposite exhibited higher 0.2% TYS, UTS,
failure strain and work of fracture (WOF) (+ 9%, + 4%, + 38% and + 44%, respectively). Also, compared to mono-
lithic AZ31/AA5083 hybrid alloy (in compression), the AZ31/AA5083 hybrid alloy nanocomposite exhibited similar
0.2% CYS (+ 1%), and higher UCS, failure stra in and WOF (+ 7%, + 23% and + 23%, respectively). The effect of CNT
nanoparticle add ition on the enhanced tensile and compressive respo nse of AZ31/AA5083 hybrid alloy is investiga ted in
this paper.
Keywords: AZ31/AA5083 Hybrid Alloy, Carbon Nanotube Nanocomposite, Microstructure, Mechanical Properties
1. Introduction
AZ31 is a very commonly used Al-containing (or Zr-free)
Mg alloy in today’s engineering world. Its use comes
with the following engineering advantages: 1) Low cost,
2) Ease of handling and 3) Good strength and ductility.
Using the friction stir processing technique recently,
AZ31 has been surface-reinforced with SiC micropar-
ticulates [1], C60 molecules [2], and multi-walled carbon
nanotubes [3]. Here, it was reported that the base matrix
was consequently hardened due to good dispersion.
Similar findings along with grain refinement were also
reported for AZ31 reinforced with SiC and B4C mi-
croparticulates using gas-tungsten arc (GTA) with si-
multaneous reinforcement powder feeding processing
technique [4-6]. Adherent and defect-free particle-matrix
interface in the AZ31/SiC microcomposite has been re-
ported [5,6]. Pulsed current hot pressing (PCHP) has
been used to incorporate TiNi shape memory alloy (SMA)
fibers in AZ31 matrix without significant interfacial re-
action [7]. The yield stress and elongation in the
AZ31/TiNi microcomposite increased with temperature
(strength significantly exceeded that of AZ31 matrix).
This was a consequence of residual compressive stress in
the AZ31 matrix due to phase change induced shrinkage
of the TiNi fiber. Technically, AZ31 may be alloyed with
more pure aluminium to obtain the other magnesium
alloys in the AZ series (AZ61, AZ81, AZ91 etc.). How-
ever, the use of aluminium alloy (as opposed to pure
aluminium) to metallurgically upgrade AZ31 has not
been reported. There may be certain advantages in this
approach based on the lower liquidus temperature of the
aluminium alloy compared to pure aluminium. The car-
bon nanotube (CNT) possesses many unique properties
[8,9] which enable it to be useful in selected applications.
It has superior strength (30 GPa) and stiffness (1 TPa) in
tension [10]. Also, the CNT bends reversibly [11]. Its
electrical properties are comparable to those of metals
and semi-conductors [12,13]. Furthermore, the thermal
conductivity of the CNT has been predicted to be unusu-
ally high [14]. Additionally, the CNT has a high aspect
ratio which implies that it has a high surface area to
volume ratio [15]. The unique properties and high SA/V
ratio of the CNT have resulted in its combination with

Carbon Nanotube Addition to Simultaneously Enhance Strength and Ductility of Hybrid AZ31/AA5083 Alloy21
other materials in attempts to create useful composite
materials [16-19]. However, open literature search has
revealed that no successful attempt has been made to
simultaneously increase tensile as well as compressive
strength and ductility of wrought AZ31/AA5083 hybrid
alloy with well dispersed CNT or any other nanoparticles,
using a high volume production spray-deposition based
solidification processing technique.
Accordingly, the primary aim of this study was to si-
multaneously increase strength and ductility in tension
and compression of wrought AZ31/AA5083 hybrid alloy
using CNT as reinforcement. Disintegrated melt deposi-
tion (DMD) [20,21] followed by hot extrusion was used
to synthesize the wrought AZ31/AA5083/CNT hybrid
alloy nanocomposite.
2. Experimental Procedures
2.1. Materials
In this study, AZ31 (nominally 2.50-3.50 wt.% Al,
0.60-1.40 wt.% Zn, 0.15-0.40 wt.% Mn, 0.10 wt.% Si,
0.05 wt.% Cu, 0.01 wt.% Fe, 0.01 wt.% Ni, balance Mg),
supplied by Tokyo Magnesium Co. Ltd. (Yokohama,
Japan) was used as shell matrix material. AA5083
(nominally 4.0-4.9 wt.% Mg, 0.40 wt.% Si, 0.40 wt.% Fe,
0.10 wt.% Cu, 0.40-1.00 wt.% Mn, 0.05-0.25 wt.% Cr,
0.25 wt.% Zn, 0.15 wt.% Ti, 0.15 wt.% others, balance
Al), supplied by Yan San Metals Pte. Ltd. (Singapore)
was used (addition of 3 wt.% relative to AZ31 weight) to
metallurgically upgrade AZ31. AZ31 and AA5083 blocks
were sectioned to smaller pieces. All oxide and scale
surfaces were removed using machining. All surfaces
were washed with ethanol after machining. CNT powder
(vapor grown, 94.7% purity, 40-70 nm outer diameter, up
to 100 aspect ratio [22]) supplied by Nanostructured &
Amorphous Materials Inc (Texas, USA) was used as the
reinforcement phase.
2.2 Processing
Monolithic AZ31/AA5083 hybrid alloy (3 wt.% AA
5083 addition relative to AZ31 weight) was cast using
the DMD method [20,21]. This involved heating AZ31
and AA5083 blocks to 750˚C in an inert Ar gas atmos-
phere in a graphite crucible using a resistance heating
furnace. The crucible was equipped with an arrangement
for bottom pouring. Upon reaching the superheat tem-
perature, the molten slurry was stirred for 2.5 min at 460
rpm using a twin blade (pitch 45˚) mild steel impeller to
facilitate the uniform distribution of heat. The impeller
was coated with Zirtex 25 (86%ZrO2, 8.8%Y2O3, 3.6%
SiO2, 1.2%K2O and Na2O, and 0.3% trace inorganics) to
avoid iron contamination of the molten metal. The melt
was then released through a 10 mm diameter orifice at
the base of the crucible. The melt was disintegrated by
two jets of argon gas oriented normal to the melt stream
located 265 mm from the melt pouring point. The argon
gas flow rate was maintained at 25 lpm. The disinte-
grated melt slurry was subsequently deposited onto a
metallic substrate located 500 mm from the disintegra-
tion point. An ingot of 40 mm diameter was obtained
following the deposition stage. To form the AZ31/
AA5083/1.0 vol.% CNT hybrid alloy nanocomposite,
CNT powder was isolated by wrapping in Al foil of
minimal weight (< 0.50 wt.% with respect to AZ31 and
AA5083 total matrix weight) and arranged on top of the
AZ31 and AA5083 alloy blocks, with all other DMD
parameters unchanged. All billets were machined to 35
mm diameter and hot extruded using 20.25:1 extrusion
ratio on a 150ton hydraulic press. The extrusion tem-
perature was 350˚C. The billets were held at 400˚C for
60 min in a furnace prior to extrusion. Colloidal graphite
was used as a lubricant. Rods of 8 mm were obtained.
2.3. Heat Treatment
Heat treatment was carried out on all extruded sections at
200˚C for 1 hour using a resistance heating furnace. This
selection of temperature and time was made in order to
relax the monolithic AZ31/AA5083 hybrid alloy (3 wt.%
AA5083 addition relative to AZ31 weight) without re-
crystallization softening. The recrystallization tempera-
ture of AZ61 magnesium alloy (as the nearest matching
alloy in terms of composition) following 20% cold work
after 1 hour is 288˚C [23]). Prior to heat treatment, the
sections were coated with colloidal graphite and wrapped
in aluminum foil to minimize reaction with oxygen pre-
sent in the furnace atmosphere.
2.4. Microstructural Characterization
Microstructural characterization studies were conducted
on metallographically polished monolithic and nano-
composite extruded samples to determine grain charac-
teristics as well as nanoparticle reinforcement distribu-
tion. Hitachi S4300 Field-Emission SEM was used. Im-
age analysis using Scion software was carried out to de-
termine the grain characteristics. XRD studies were con-
ducted using CuKα radiation (λ = 1.5406 Å) with a scan
speed of 2˚/min in an automated Shimadzu LAB-X
XRD-6000 diffractometer to determine intermetallic
phase(s) presence and dominant textures in the transverse
and longitudinal (extrusion) directions.
2.5. Hardness
Microhardness measurements were made on polished
monolithic and nanocomposite extruded samples. Vick-
ers microhardness was measured using Matsuzawa
MXT50 automatic digital microhardness tester using 25

Carbon Nanotube Addition to Simultaneously Enhance Strength and Ductility of Hybrid AZ31/AA5083 Alloy 22
gf-indenting load and 15 s dwell time.
2.6. Tensile Testing
Smooth bar tensile properties of the monolithic and
nanocomposite extruded samples were determined based
on ASTM E8M-05. Round tension test samples of 5 mm
diameter and 25 mm gauge length were subjected to ten-
sion using an MTS 810 machine equipped with an axial
extensometer with a crosshead speed set at 0.254 mm/min.
Fractography was performed on the tensile fracture sur-
faces using Hitachi S4300 FESEM.
2.7. Compressive Testing
Compressive properties of the monolithic and nanocom-
posite extruded samples were determined based on
ASTM E9-89a. Samples of 8 mm length (l) and 8 mm
diameter (d) where l/d = 1 were subjected to compression
using a MTS 810 machine with 0.005 min-1 strain rate.
Fractography was performed on the compressive fracture
surfaces using Hitachi S4300 FESEM.
3. Results
3.1. Macrostructural Characteristics
No macropores or shrinkage cavities were observed in
the cast monolithic and nanocomposite materials. No
macrostructural defects were observed for extruded rods
of monolithic and nanocomposite materials.
3.2 Microstructural Characteristics
Microstructural analysis results revealed that grain size
and aspect ratio remained statistically unchanged in the
case of nanocomposite as shown in Table 1 and Figures
1(a,b). CNT reinforcement distribution in the nanocompo-
site was reasonably uniform as shown in Figures 1(c,d).
Texture results are listed in Table 2 and shown in
Figure 2. In monolithic and nanocomposite materials,
the dominant texture in the transverse and longitudinal
directions was (1 0 –1 1).
3.3. Hardness
The results of microhardness measurements are listed in
Table 1. The nanocomposite exhibited higher hardness
than the monolithic material.
3.4. Tensile Behavior
The overall results of ambient temperature tensile testing
of the extruded materials are shown in Table 3 and Fig-
ure 3(a). The strength, failure strain and work of fracture
(WOF) of AZ31/AA5083/1.0 vol.% CNT were higher
compared to monolithic AZ31/AA5083. The WOF was
determined by computing the area under the stress-strain
Table 1. Results of grain characteristics and microhardness of AZ31/AA5083 and AZ31/AA5083/CNT nanocomposite.
Grain characteristics a
Material CNT
(vol.%) Size (μm)Aspect ratio
Microhardness
(HV)
AZ31/AA5083 - 4.4 ± 0.7 1.4 114 ± 6
AZ31/AA5083/1.0 vol.% CNT1.00 3.8 ± 0.8 1.4 137 ± 7 (+ 20)
aBased on approximately 100 grains; ( ) Brackets indicate % change with respect to corresponding
result of AZ31/AA5083.
Table 2. Texture results of AZ31/AA5083 and AZ31/AA5083/CNT nanocomposite based on X-ray diffraction.
Material Section
aPlane Representative I/Imax b
1 0 –1 0 prism 0.44
0 0 0 2 basal 0.28
T
1 0 –1 1 pyramidal 1.00
1 0 –1 0 prism 0.32
0 0 0 2 basal 0.63
AZ31/AA5083
L
1 0 –1 1 pyramidal 1.00
1 0 –1 0 prism 0.43
0 0 0 2 basal 0.15
T
1 0 –1 1 pyramidal 1.00
1 0 –1 0 prism 0.36
0 0 0 2 basal 0.64
AZ31/AA5083/1.0 vol.% CNT
L
1 0 –1 1 pyramidal 1.00
aT: transverse, L: longitudinal; bImax is XRD maximum intensity from either prism, basal or pyramidal planes.

Carbon Nanotube Addition to Simultaneously Enhance Strength and Ductility of Hybrid AZ31/AA5083 Alloy23
Table 3. Results of tensile testing of AZ31/AA5083 and AZ31/AA5083/CNT nanocomposite.
Material 0.2% TYS
(MPa)
UTS
(MPa)
Failure Strain
(%)
WOF
(MJ/m3)
AZ31/AA5083 203 ± 4 310 ± 4 8.7 ± 1.8 25 ± 5
AZ31/AA5083/1.0 vol.% CNT 221 ± 4 (+ 9) 321 ± 1 (+ 4)12.0 ± 1.0 (+ 38) 36 ± 4 (+ 44)
(a)
Al
12
Mg
17
at grain
boundary
(b)
Al
12
Mg
17
at grain
boundary
(c) (d)
CNT
CNT
Figure 1. Representative micrographs showing grain size in monolithic AZ31/AA5083 and AZ31/AA5083/CNT nanocompo-
site: (a) Lower magnification and (b) Higher magnification. Representative micrographs showing individual CNT presence in
the AZ31/AA5083/CNT nanocomposite: (c) Lower magnification and (d) Higher magnification.
a
c
a
c
Figure 2. Schematic diagram showing textures of mono-
lithic AZ31/AA5083 and AZ31/AA5083/CNT nanocompo-
site, based on X-ray diffraction. In each case, vertical axis
(dashed line) is parallel to extrusion direction. Each cell is
made up of 2 HCP units having 1 common (0 0 0 2) basal
plane.
curve up to the point of fracture. The fractured surface of
all extruded materials exhibited mixed (ductile + brittle)
mode of fracture as shown in Figures 4(a,b). Minimal
CNT pull-out from the tensile fractured surface given the
minimum CNT (vapor grown) aspect ratio of 100 [22]
was observed in AZ31/AA5083/1.0 vol.% CNT as shown
in Figure 4(c).
3.5. Compressive Behavior
The overall results of ambient temperature compressive
testing of the extruded materials are shown in Table 4
and Figure 3(b). The strength, failure strain and work of
fracture (WOF) of AZ31/AA5083/1.0 vol.% CNT were
higher compared to monolithic AZ31/AA5083. The
fractured surface of AZ31/AA5083/1.0 vol.% CNT ap-
peared smoother than that of monolithic AZ31/AA5083
as shown in Figures 4(d,e). CNT pull-out along the

Carbon Nanotube Addition to Simultaneously Enhance Strength and Ductility of Hybrid AZ31/AA5083 Alloy 24
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
400.00
0.000 0.0200.040 0.060 0.0800.100 0.120 0.140
strai n
stress (MPa)
0.00
100.00
200.00
300.00
400.00
500.00
600.00
0.000 0.050 0.1000.150 0.200 0.250 0.3000.350 0.400
strai n
stress (MPa)
AZ31 / AA5083
(a)
TENSILE
AZ31 / AA5083 / 1.0 vol.% CNT
AZ31 / AA5083
(b)
COMPRESSIVE
AZ31 / AA5083 / 1.0 vol.% CNT
Figure 3. Representative: (a) Tensile and (b) Compressive stress-strain curves of monolithic AZ31/AA5083 and
AZ31/AA5083/CNT nanocomposite.
Table 4. Results of compressive testing of AZ31/AA5083 and AZ31/AA5083/CNT nanocomposite.
Material 0.2% CYS
(MPa)
UCS
(MPa)
Failure Strain
(%)
WOF
(MJ/m3)
AZ31/AA5083 141 ± 8 478 ± 1 19.9 ± 1.8 77 ± 4
AZ31/AA5083/1.0 vol.% CNT 143 ± 12 (+ 1)512 ± 6 (+ 7)24.5 ± 5.8 (+ 23)95 ± 6 (+ 23)
compressive fractured surface was observed in AZ31/
AA5083/1.0 vol.% CNT as shown in Figure 4(f).
4. Discussion
4.1. Synthesis of Monolithic AZ31/AA5083 and
AZ31/AA5083/CNT Nanocomposite
Synthesis of monolithic and nanocomposite materials,
the final form being extruded rods, was successfully ac-
complished with: 1) No detectable metal oxidation and 2)
No detectable reaction between graphite crucible and
melts. The inert atmosphere used during DMD was ef-
fective in preventing oxidation of the Mg melt. No stable
carbides of Mg or Al formed due to reaction with graph-
ite crucible.
4.2. Microstructural Characteristics
Microstructural characterization of extruded samples is
discussed in terms of: 1) Grain characteristics and 2)

Carbon Nanotube Addition to Simultaneously Enhance Strength and Ductility of Hybrid AZ31/AA5083 Alloy 25
(d)
(b)
TENSILE
COMPRESSIVE
(c)
TENSILE
(a)
TENSILE
(e)
COMPRESSI VE
microcracks
CNT pull-out
(f)
CNT pull-out
COMPRESSIVE
Figure 4. Representative tensile fractographs of: (a) Monolithic AZ31/AA5083, (b) AZ31/AA5083/CNT nanocomposite and (c)
Pull-out in AZ31/AA5083/CNT nanocomposite. Representative compressive fractographs of: (d) Monolithic AZ31/AA5083, (e)
AZ31/AA5083/CNT nanocomposite and (f) Pull-out in AZ31/AA5083/CNT nanocomposite. Insets in (c) and (f) show tensile
and compressive fractured samples, respectively.
CNT reinforcement distribution.
Nearly equiaxed grains were observed in monolithic
material and nanocomposite as shown in Table 1 and
Figures 1(a,b). Grain size was statistically insignificant
in the case of nanocomposite, suggesting the inability of
CNT to serve as either nucleation sites or obstacles to
grain growth during solid state cooling. It was observed
that
β -Al12Mg17 intermetallic particles decorated the
grain boundaries in the monolithic material and nano-
composite (X-ray diffraction (XRD) analysis revealed
the presence of
β -Al12Mg17 phase [24]).
The reasonably uniform distribution of CNT as shown in
Figures 1(c,d) can be attributed to: 1) Minimal grav-
ity-associated segregation due to judicious selection of
stirring parameters [20], 2) Good wetting of CNT
nanoparticles by the alloy matrix [25-28], 3) Argon gas
disintegration of metallic stream [29], and 4) Dynamic
deposition of composite slurry on substrate followed by
hot extrusion. Similar reasonably uniform distribution of
CNT nanoparticles in magnesium alloy AZ31 has also
been recently reported [28].
4.3. Mechanical Behavior
4.3.1. Hardness
A significant increase in microhardness by 20% was ob-
served in the nanocomposite when compared to mono-
lithic material as listed in Table 1. This was consistent
with earlier observations made on Mg/Al2O3, AZ31/C60
and AZ31/MWCNT nanocomposites [30-32]. The in-
crease in hardness of the nanocomposite in the present

Carbon Nanotube Addition to Simultaneously Enhance Strength and Ductility of Hybrid AZ31/AA5083 Alloy 26
study can be attributed to: 1) Reasonably uniform distri-
bution of harder CNT in the matrix and 2) Higher con-
straint to localized matrix deformation during indentation
due to the presence of nanoparticles [30,31,33].
4.3.2. Tensile and Compressive Behavior
4.3.2.1. Strength
The tensile and compressive strengths of monolithic ma-
terial and nanocomposite are listed in Tables 3 and 4
(and shown in Figures 3(a,b)), respectively. 0.2%TYS
and UTS were enhanced by 9% and 4%, respectively, in
AZ31/AA5083/1.0 vol.% CNT compared to monolithic
material. In comparison of compressive strengths, 0.2%
CYS and UCS of AZ31/AA5083/1.0 vol.% CNT were
higher (by 1% and 7%, respectively) compared to mono-
lithic AZ31/AA5083. The stress detected at almost any
given strain was higher for AZ31/AA5083/1.0 vol.%
CNT compared to monolithic AZ31/AA5083 as shown in
Figure 3(b). The tensile/compressive strength increase in
AZ31/AA5083/1.0 vol.% CNT compared to monolithic
AZ31/AA5083 can be attributed to the following well
known factors (pertaining to reinforcement): 1) Disloca-
tion generation due to elastic modulus mismatch and
coefficient of thermal expansion mismatch between the
matrix and reinforcement [30,33-35], 2) Orowan streng-
thening mechanism [34-36] and 3) Load transfer from
matrix to reinforcement [30,34]. The tensile/compressive
strength increase in AZ31/AA5083/1.0 vol.% CNT
compared to monolithic AZ31/AA5083 was despite
compressive shear buckling of CNT in AZ31/AA5083/
1.0 vol.% CNT as illustrated in Figure 5. Here, the CNT
(with aspect ratio as high as 100 [22]) is prone to buck-
ling followed by fracture within the AZ31/AA5083 ma-
trix during compressive deformation [37-39]. CNT buck-
ling within the AZ31/AA5083 matrix occurs more easily
with increasing CNT aspect ratio (or slenderness ratio)
[39,40]. This induces a significantly lower limit on the
well known factors pertaining to reinforcement just
listed.
In monolithic AZ31/AA5083, 0.2%TYS was about
1.44 times the 0.2%CYS. Here, the tensile/compressive
yield stress anisotropy was despite the similarity in crys-
tallographic texture compared to monolithic AZ31/
AA5083, where {1 0 1 –2} <1 0 1 –1 > -type twinning
was activated along the c-axis of the HCP unit cell in
Figure 2 with comparatively similar ease in both tension
and compression along the c-axis, based on the 45˚ angle
between the c-axis and the vertical axis [41, 42]. The
tensile/compressive yield stress anisotropy can be attrib-
uted generally to half the strain rate used (less strain
hardening) in compressive testing compared to tensile
testing. In AZ31/AA5083/1.0 vol.% CNT nanocomposite,
0.2%TYS was about 1.55 times the 0.2%CYS (slightly
higher ratio compared to monolithic AZ31/AA5083).
Here, the slightly increased tensile/compressive yield
stress anisotropy can be attributed to compressive shear
buckling of CNT as illustrated in Figure 5. The CNT is
prone to buckling followed by fracture within the
AZ31/AA5083 matrix during compressive deformation
unlike during tensile deformation [37-40].
4.3.2.2. Failure Strain
The tensile and compressive failure strains of monolithic
material and nanocomposite are listed in Tables 3 and 4
(and shown in Figures 3(a,b)), respectively. Compared
to monolithic material, tensile and compressive failure
strains were enhanced by 38% and 23% (respectively) in
AZ31/AA5083/1.0 vol.% CNT. The failure strain in-
crease in AZ31/AA5083/1.0 vol.% CNT compared to
monolithic AZ31/AA5083 can be attributed to the fol-
lowing factors pertaining to reinforcement: 1) Presence
and reasonably uniform distribution of CNT nanoparti-
cles [31,43] and 2) Compressive shear buckling of CNT
(regarding compressive failure strain only) [37-40]. In
the case of reasonably uniform distribution of CNT nano-
τ
a
τ
a
CNT
shear stress concentration plane
(p
rior to sam
p
le fracture
)
τ
b
τ
b
τ
b
τ
b
CNT buckling
further CNT buckling
CNT fracture
shear fractured
compression sample
OR
Figure 5. Schematic diagram illustrating compressive shear buckling of CNT in AZ31/AA5083/CNT nanocomposite (τa and
τb are planar shear stresses where τa < τb).

Carbon Nanotube Addition to Simultaneously Enhance Strength and Ductility of Hybrid AZ31/AA5083 Alloy 27
particles, it has been shown in previous studies that
nanoparticles provide sites where cleavage cracks are
opened ahead of the advancing crack front. This: 1) dis-
sipates the stress concentration which would otherwise
exist at the crack front and 2) alters the local effective
stress state from plane strain to plane stress in the
neighbourhood of crack tip [31,43]. In the case of com-
pressive shear buckling of CNT, CNT buckling within
the AZ31/AA5083 matrix aids in dispersing localized
stored energy during compressive deformation. This al-
lows AZ31/AA5083/1.0 vol.% CNT to globally absorb
relatively large amounts of strain energy during com-
pressive deformation [37-40]. Here, CNT buckling
within the AZ31/AA5083 matrix is a compressive tough-
ening mechanism. Tensile fracture behaviour of both
monolithic material and nanocomposite was mixed (duc-
tile + brittle) as shown in Figures 4(a,b). However, the
tensile fractured surface of the nanocomposite had: 1)
Higher occurrence of smaller dimple-like features and 2)
Absence of microcracks, compared to that of monolithic
material. The tensile cavitation resistance was lower in
the nanocomposite compared to monolithic material. For
AZ31/AA5083/1.0 vol.% CNT, compressive fracture
behavior based on viscoplastic flow [44] was relatively
more ductile (smoother fracture surface exhibited) com-
pared to monolithic AZ31/AA5083 as shown in Figures
4(d,e). This relatively more ductile compressive fracture
behavior can be attributed to increase in shear band
spacing [44,45].
4.3.2.3. Work of Fracture
The tensile and compressive work of fracture (WOF)
of monolithic material and nanocomposite are listed in
Tables 3 and 4 (and illustrated in Figures 3(a,b)), re-
spectively. WOF quantified the ability of the material to
absorb energy up to fracture under load [46]. Compared
to monolithic material, tensile and compressive WOF
were enhanced by 44% and 23% (respectively) in
AZ31/AA5083/1.0 vol.% CNT. The high increments in
tensile and compressive WOF exhibited by AZ31/AA
5083/1.0 vol.% CNT show its potential to be used in
damage tolerant design.
5. Conclusions
1) Monolithic AZ31/AA5083 hybrid alloy and AZ31/AA
5083/1.0 vol.% CNT hybrid alloy nanocomposite can be
successfully synthesized using the DMD technique fol-
lowed by hot extrusion.
2) Compared to monolithic AZ31/AA5083, tensile and
compressive strengths of AZ31/AA5083/1.0 vol% CNT
were enhanced. This can be attributed to well known
factors pertaining to reinforcement despite compressive
shear buckling of CNT (regarding compressive strength
only).
3) Compared to monolithic AZ31/AA5083, tensile and
compressive failure strains of AZ31/AA5083/1.0 vol.%
CNT were enhanced. This can be attributed to the fol-
lowing factors pertaining to reinforcement: (a) Presence
and reasonably uniform distribution of CNT nanoparti-
cles and (b) Compressive shear buckling of CNT (re-
garding compressive failure strain only).
4) Compared to monolithic AZ31/AA5083, AZ31/AA
5083/1.0 vol.% CNT exhibited high increments in both
tensile and compressive WOF.
6. Acknowledgements
Authors wish to acknowledge National University of
Singapore (NUS) and Temasek Defence Systems Insti-
tute (TDSI) for funding this research (TDSI/09-011/1A
and WBS# R265000349).
REFERENCES
[1] Y. Morisada, H. Fujii, T. Nagaoka and M. Fukusumi,
“Effect of Friction Stir Processing with SiC Particles on
Microstructure and Hardness of AZ31,” Materials Sci-
ence and Engineering A, Vol. 433, 2006, pp. 50-54.
doi:10.1016/j.msea.2006.06.089
[2] Y. Morisada, H. Fujii, T. Nagaoka and M. Fukusumi,
“Nanocrystallized Magnesium Alloy–Uniform Dispersion
of C60 Molecules,” Scripta Materialia, Vol. 55, 2006, pp.
1067-1070.
doi:10.1016/j.scriptamat.2006.07.055
[3] Y. Morisada, H. Fujii, T. Nagaoka and M. Fukusumi,
“MWCNTs/AZ31 Surface Composites Fabricated by
Friction Stir Processing,” Materials Science and Engi-
neering A, Vol. 419, 2006, pp. 344-348.
doi:10.1016/j.msea.2006.01.016
[4] W. B. Ding, H. Y. Jiang, X. Q. Zeng, D. H. Li and S.S.
Yao, “The Surface Modified Composite Layer Formation
with Boron Carbide Particles on Magnesium Alloy Sur-
faces Through Pulse Gas Tungsten Arc Treatment,” Ap-
plied Surface Science, Vol. 253, No. 8, 2007, pp. 3877
-3883.
doi:10.1016/j.apsusc.2006.08.015
[5] W. B. Ding, H. Y. Jiang, X. Q. Zeng, D. H. Li and S. S.
Yao, “The Properties of Gas Tungsten Arc Deposited
SiCP and Al Surface Coating on Magnesium Alloy
AZ31,” Materials Letters, Vol. 61, No. 2, 2007, pp. 496
-501.
doi:10.1016/j.matlet.2006.04.113
[6] W .B. Ding, H. Y. Jiang, X. Q. Zeng, D. H. Li and S. S.
Yao, “Microstructure and Mechanical Properties of GTA
Surface Modified Composite Layer on Magnesium Alloy
AZ31 with SiCP,” Journal of Alloys and Compounds, Vol.
429, No. 1-2, 2007, pp. 233-241.
doi:10.1016/j.jallcom.2006.03.083
[7] K. Mizuuchi, K. Inoue, K. Hamada, M. Sugioka, M. Itami,
M. Fukusumi and M. Kawahara, “Processing of TiNi
SMA Fiber Reinforced AZ31 Mg Alloy Matrix Compos-

Carbon Nanotube Addition to Simultaneously Enhance Strength and Ductility of Hybrid AZ31/AA5083 Alloy 28
ite by Pulsed Current Hot Pressing,” Materials Science
and Engineering A, Vol. 367, No. 1-2, 2004, pp. 343-349.
doi:10.1016/j.msea.2003.10.286
[8] R. F. Service, “Superstrong Nanotubes Show They are
Smart too,” Science, Vol. 281, 1998, pp. 940-942.
doi:10.1126/science.281.5379.940
[9] P. G. Collins and P. Avouris, “Nanotubes for Electron-
ics,” Scientific American, Vol. 283, No. 6, 2000, pp.
38-45.
doi:10.1038/scientificamerican1200-62
[10] Y. Min-Feng, B. S. Files, S. Arepalli and R. S. Rouff,
“Tensile Loading of Ropes of Single Wall Carbon Nano-
tubes and Their Mechanical Properties,” Physical Review
Letters, Vol. 84, No. 24, 2000, pp. 5552-5555.
doi:10.1103/PhysRevLett.84.5552
[11] W. H. Knechtel, G. S. Dusberg, W. J. Blau, E. Hernandez
and A. Rubio, “Reversible Bending of Carbon Nanotubes
Using a Transmission Electron Microscope,” Applied
Physical Letters, Vol. 73, No. 14, 1998, pp. 1961-1963.
doi:10.1063/1.122335
[12] P. G. Collins, M. S. Arnold and P. Avouris, “Engineering
Carbon Nanotubes and Nanotube Circuits Using Electri-
cal Breakdown,” Science, Vol. 292, 2001, pp. 706-709.
doi:10.1126/science.1058782
[13] H. Stahl, J. Appenzeller, R. Martel, P. Avouris and B.
Lengeler, “Intertube Coupling in Ropes of Single-Wall
Carbon Nanotubes,” Physical Review Letters, Vol. 85, No.
26, 2000, pp. 5186-5189.
doi:10.1103/PhysRevLett.84.4613
[14] S. Berber, Y. K. Kwon and D. Tomanek, “Unusually
High Thermal Conductivity of Carbon Nanotubes,”
Physical Review Letters, Vol. 84, No. 20, 2000, pp.
4613-4616.
doi:10.1103/PhysRevLett.84.4613
[15] K. K. H. Wong, M. Zinke-Allmang, J. L. Hutter, S.
Hrapovic, J.H.T. Luong and W. Wan, “The Effect of Car-
bon Nanotube Aspect Ratio and Loading on the Elastic
modulus of Electrospun Poly (Vinyl Alcohol)-Carbon
Nanotube Hybrid Fibers,” Carbon, Vol. 47, 2009, pp.
2571-2578.
doi:10.1016/j.carbon.2009.05.006
[16] P. Chen, X. Wu, J. Lin and K. L. Tan, “Synthesis of Cu
Nanoparticles and Microsized Fibers by Using Carbon
Nanotubes as a Template,” Journal of Physical Chemistry
B, Vol. 103, 1999, pp. 4559-4561.
doi:10.1021/jp983983j
[17] X. Chen, J. Xia, J. Peng, W. Li and S. Xie, “Carbon-
Nanotube Metal-Matrix Composites Prepared by Elec-
troless Plating,” Composites Science and Technology, Vol.
60, 2000, pp. 301-306.
doi:10.1016/S0266-3538(99)00127-X
[18] J. N. Coleman, S. Curran, A. B. Dalton, A. P. Harvey, B.
McCarthy, W. Blau and R. C. Barklie, “Percolation-
Dominated Conductivity in a Conjugated-Polymer-Carbon-
Nanotube Composite,” Physical Review B, Vol. 58, 1998,
pp. R7492-R7495.
doi:10.1103/PhysRevB.58.R7492
[19] V. Lordi and N. Yao, “Molecular Mechanics of Binding
in Carbon-Nanotube-Polymer Composites,” Journal of
Materials Research, Vol. 15, No. 12, 2000, pp. 2770
-2779.
doi:10.1557/JMR.2000.0396
[20] L. M. Tham, M. Gupta and L. Cheng, “Influence of Proc-
essing Parameters during Disintegrated Melt Deposition
Processing on Near Net Shape Synthesis of Aluminium
Based Metal Matrix Composites,” Materials Science and
Technology, Vol. 15, 1999, pp. 1139-1146.
[21] M. Gupta, M. O. Lai and S. C. Lim, “Regarding the Proc-
essing Associated Microstructure and Mechanical Proper-
ties Improvement of an Al-4.5Cu Alloy,” Journal of Al-
loys and Compounds, Vol. 260, 1997, pp. 250-255.
doi:10.1016/S0925-8388(97)00156-4
[22] M. H. Al-Saleh and U. Sundararaj, “A Review of Vapor
Grown Carbon Nanofiber/Polymer Conductive Compos-
ites,” Carbon, Vol. 47, No. 1, 2008, pp. 2-22.
doi:10.1016/j.carbon.2008.09.039
[23] M. M. Avedesian and H. Baker, “ASM Specialty Hand-
book: Magnesium and Magnesium Alloys,” ASM Inter-
national®, Novelty, 1999.
[24] Q. B. Nguyen and M. Gupta, “Increasing Significantly
the Failure Strain and Work of Fracture of Solidification
Processed AZ31B Using Nano-Al2O3 Particulates,” Jour-
nal of Alloys and Compounds, Vol. 459, 2007, pp.
244-250.
doi:10.1016/j.jallcom.2007.05.038
[25] B. Q. Han and D. C. Dunand, “Microstructure and Me-
chanical Properties of Magnesium Containing High Vol-
ume Fractions of Yttria Dispersoids,” Materials Science
and Engineering A, Vol. 277, 2000, pp. 297-304.
doi:10.1016/S0921-5093(99)00074-X
[26] N. Eustathopoulos, M. G. Nicholas and M. Drevet, “Wet-
tability at High Temperatures,” Pergamon, London, 1997.
[27] J. D. Gilchrist, “Extraction Metallurgy,” Pergamon, Lon-
don, 1989.
[28] M. Paramsothy, S. F. Hassan, N. Srikanth and M. Gupta,
“Adding Carbon Nanotubes and Integrating with AA5052
Aluminium Alloy Core to Simultaneously Enhance Stiff-
ness, Strength and Failure Strain of AZ31 Magnesium
Alloy,” Composites Part A, Vol. 40, 2009, pp. 1490
-1500.
doi:10.1016/j.compositesa.2009.06.007
[29] M. Gupta, M. O. Lai and C. Y. Soo, “Effect of Type of
Processing on the Microstructural Features and Mechani-
cal Properties of A1-Cu/SiC Metal Matrix Composites,”
Materials Science and Engineering A, Vol. 210, 1996, pp.
114-122.
doi:10.1016/0921-5093(95)10077-6
[30] S. F. Hassan and M. Gupta, “Effect of Different Types of
Nano-Size Oxide Particulates on Microstructural and
Mechanical Properties of Elemental Mg,” Journal of Ma-
terials Science, Vol. 41, 2006, pp. 2229-2236.
doi:10.1007/s10853-006-7178-3
[31] S. F. Hassan and M. Gupta, “Effect of Particulate Size of
Al2O3 Reinforcement on Microstructure and Mechanical

Carbon Nanotube Addition to Simultaneously Enhance Strength and Ductility of Hybrid AZ31/AA5083 Alloy 29
Behavior of Solidification Processed Elemental Mg,”
Journal of Alloys and Compounds, Vol. 419, 2006, pp.
84-90.
doi:10.1016/j.jallcom.2005.10.005
[32] S. F. Hassan and M. Gupta, “Enhancing Physical and
Mechanical Properties of Mg Using Nanosized Al2O3
Particulates as Reinforcement,” Metallurgical and Mate-
rials Transactions A, Vol. 36, No. 8, 2005, pp. 2253
-2258.
doi:10.1007/s11661-005-0344-4
[33] C. S. Goh, J. Wei, L. C. Lee and M. Gupta, “Develop-
ment of Novel Carbon Nanotube Reinforced Magnesium
Nanocomposites Using the Powder Metallurgy Tech-
nique,” Nanotechnology, Vol. 17, 2006, pp. 7-12.
doi:10.1088/0957-4484/17/1/002
[34] Z. Szaraz, Z. Trojanova, M. Cabbibo and E. Evangelista,
“Strengthening in a WE54 Magnesium Alloy Containing
SiC Particles,” Materials Science and Engineering A, Vol.
462, 2007, pp. 225-229.
doi:10.1016/j.msea.2006.01.182
[35] L. H. Dai, Z. Ling and Y. L. Bai, “Size-Dependent Inelas-
tic Behavior Of Particle-Reinforced Metal-Matrix Com-
posites,” Composites Science and Technology, Vol. 61,
2001, pp. 1057-1063.
doi:10.1016/S0266-3538(00)00235-9
[36] D. Hull and D. J. Bacon, “Introduction to Dislocations”,
Butterworth-Heinemann, Oxford, 2002.
[37] E. T. Thostenson and T. -W. Chou, “Nanotube Buckling
in Aligned Multi-Wall Carbon Nanotube Composites,”
Carbon, Vol. 42, 2004, pp. 3003-3042.
doi:10.1016/j.carbon.2004.06.012
[38] S. Namilae and N. Chandra, “Role of Atomic Scale Inter-
faces in the Compressive Behavior of Carbon Nanotubes
in Composites,” Composites Science and Technology,
Vol. 66, 2006, pp. 2030-2038.
doi:10.1016/j.compscitech.2006.01.009
[39] M. Paramsothy, J. Chan, R. Kwok and M. Gupta, “Addi-
tion of CNTs to Enhance Tensile/Compressive Response
of Magnesium Alloy ZK60A,” Composites Part A, Vol.
42, No. 2, 2010, pp. 180-188.
doi:10.1016/j.compositesa.2010.11.001
[40] J.-M. Lu, C.-C. Hwang, Q.-Y. Kuo and Y.-C. Wang,
“Mechanical Buckling of Multi-Walled Carbon Nano-
tubes: The Effects of Slenderness Ratio,” Physica E, Vol.
40, 2008, pp. 1305-1308.
doi:10.1016/j.physe.2007.08.120
[41] T. Laser, C. Hartig, M. R. Nurnberg, D. Letzig and R.
Bormann, “The Influence of Calcium and Cerium Misch-
metal on the Microstructural Evolution of Mg-3Al-1Zn
during Extrusion and Resulting Mechanical Properties,”
Acta Materialia, Vol. 56, 2008, pp. 2791-2798.
doi:10.1016/j.actamat.2008.02.010
[42] J. Bohlen, S. B. Yi, J. Swiostek, D. Letzig, H. G. Brok-
meier and K. U. Kainer, “Microstructure and Texture
Development during Hydrostatic Extrusion of Magne-
sium Alloy AZ31,” Scripta Materialia, Vol. 53, 2005, pp.
259-264.
doi:10.1016/j.scriptamat.2005.03.036
[43] S. F. Hassan and M. Gupta, “Development of Nano-Y2O3
Containing Magnesium Nanocomposites Using Solidifi-
cation Processing,” Journal of Alloys and Compounds,
Vol. 429, 2007, pp. 176-183.
doi:10.1016/j.jallcom.2006.04.033
[44] R. C. Batra and Z. G. Wei, “Instability Strain and Shear
Band Spacing in Simple Tensile/Compressive Deforma-
tions of Thermoviscoplastic Materials,” International
Journal of Impact Engineering, Vol. 34, 2007, pp.
448-463.
doi:10.1016/j.ijimpeng.2005.11.004
[45] T. S. Wang, R. J. Hou, B. Lv, M. Zhang and F. C. Zhang,
“Microstructure Evolution and Deformation Mechanism
Change in 0.98C-8.3Mn-0.04N Steel during Compressive
Deformation,” Materials Science and Engineering A, Vol.
465, 2007, pp. 68-71.
doi:10.1016/j.msea.2007.02.065
[46] R. E. Reed-Hill, “Physical Metallurgy Principles,” D Van
Nostrand Company, Princeton, 1964.