Journal of Alloys and Compounds 826 (2020) 154075

Contents lists available at ScienceDirect

Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom

Mechanical properties and strain hardening behavior of aluminum
matrix composites reinforced with few-walled carbon nanotubes
B. Chen a, *, Z. Li a, J. Shen b, c, S. Li d, L. Jia d, J. Umeda b, K. Kondoh b, J.S. Li aaState Key Laboratory of Solidiﬁcation Processing, Northwestern Polytechnical University, Xi’an, 710072, Shaanxi, China
Joining and Welding Research Institution, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka, 567-0047, Japanc
School of Aeronautics, Northwestern Polytechnical University, Xi’an, 710072, Shaanxi, Chinad
School of Materials Science and Engineering, Xi’an University of Technology, Xi’an, 710048, Chinab

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 17 October 2019
Received in revised form
17 January 2020
Accepted 26 January 2020
Available online 30 January 2020

In this study, for the ﬁrst time few-walled carbon nanotubes (FWCNTs) with ~3 walls were used as reinforcements in fabricating high performance aluminum matrix composites (AMCs). FWCNTs/Al composites and referential Al materials were prepared by a powder metallurgy route consisting of highenergy ball milling, spark plasma sintering (SPS) and subsequent hot extrusion. It is found that, bydecreasing SPS temperature and time, FWCNTs/Al composites showed reduced grains with an increased
dislocation density and improved structural integrity of FWCNTs, leading to an increased tensile strength.
Meanwhile, comparatively high strain hardening rates and long strain softening behavior were observed
after necking in the sample sintered at 500  C, which resulted in a tensile elongation of 11.7% with a highyield strength of 382 MPa. The experimental results suggested that, compared with traditional multiwalled carbon nano-tubes (MWCNTs), FWCNTs showed a noticeably enhanced strengthening effectand provided a good balance of strength and ductility in Al composites. It may make FWCNTs a good
reinforcement candidate for metal matrix composites to achieve improved mechanical properties.
© 2020 Elsevier B.V. All rights reserved.

Keywords:
Metal matrix composites (MMCs)
Carbon nanotubes (CNTs)
Strength
Ductility
Strain hardening

1. Introduction
Aluminum (Al) and its alloys are key materials in aerospace andother transportation ﬁelds due to their excellent properties such as
high speciﬁc strength, high thermal conductivity and excellent
corrosion resistance. With the increasing properties required bymodern aircrafts and cars, traditional Al alloys gradually fail tosatisfy the demand of high performances. The Al alloys-based
composites not only inherit the excellent properties of Al alloys,
but also can have high speciﬁc modulus, good high-temperature
performances, low thermal expansion coefﬁcient, good fatigue
resistance and wear resistance that contributed by reinforcements[1]. Thus, Al matrix composites (AMCs) progressively become an
important class of materials in aerospace and automobile areas [2].
A simple and easy path to improve the properties of AMCs is towisely select the reinforcing phase. Carbon nanotubes (CNTs) are
known for their low density, large aspect ratio, high speciﬁc surface
area, ultra-high strength (up to ~100 GPa), ultra-high modulus* Corresponding author.
E-mail addresses: chen@nwpu.edu.cn, biao.chen521@gmail.com (B. Chen).
https://doi.org/10.1016/j.jallcom.2020.154075
0925-8388/© 2020 Elsevier B.V. All rights reserved.

(~1 TPa) and excellent thermal/electrical properties [3], which thenare popularly used and studied in Al composites [4,5]. The everobtained results have implied that the CNT-reinforced AMCs are
expected to become the next generation of lightweight and highstrength structural materials.
According to the shear-lag theory [6,7], to achieve a highstrength of AMCs, CNTs with high strength and large aspect ratios
are desired. As revealed in previous studies [8,9], the strength andaspect ratio of CNTs can be simultaneously increased by reducing
the number of walls. Therefore, from the viewpoint of theoreticalprediction, the most promising CNTs to show strengthening effectmost efﬁciently shall be single-walled CNTs (SWCNTs). Yet, in reality it is rather difﬁcult to produce SWCNTs with high aspect ratios
in massive production. Moreover, SWCNTs are quite difﬁcult todisperse uniformly that may cause agglomeration and reduce theirstrengthening efﬁciency. A previous study reported that thestrengthening effect of SWCNTs in the 5083 Al alloy matrix waseven inferior to multi-walled CNTs (MWCNTs) [10]. As a result,
MWCNTs with much larger diameters (several tens of nanometers)
were used as reinforcements in most of studies on CNTs/Al composites [11e14], mainly owing to their cost-efﬁcient production andeasy dispersion. To demonstrate the effect of CNT diameters, Esawi

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B. Chen et al. / Journal of Alloys and Compounds 826 (2020) 154075

et al. [15] compared the strengthening effect of two multi-walled
CNTs with different diameters of 40 nm and 140 nm. The results
showed that the CNT diameter has a great effect on dispersion
quality and consequent strength of composites. Therefore, an
improved strengthening effect shall be achieved by a careful selection of CNT types with optimized balance of CNT diameter andstrength.
As a compromise, few-walled CNTs (FWCNTs, with a wall
number of <10) is a wise choice since they have a strength comparable to that of SWCNTs [16] while having an aspect ratio for easy
dispersion. Yet, the strengthening efﬁciency of FWCNTs has been
hardly studied yet, not to speak of the optimized processing routefor FWCNTs/Al composites. In this study, an attempt was made toinvestigate the strengthening effect of FWCNTs in AMCs, as well as
the appropriate parameters for materials production. FWCNTs of ~3walls (~5 nm in diameter) were uniformly dispersed into Al powders by high energy ball milling, and then the powder mixture wasconsolidated by spark plasma sintering (SPS) under different conditions followed by hot extrusion. The results showed that
FWCNTs/Al composites had an enhanced balance of strength andductility. It is also found that the CNT type and SPS condition bothhave a great effect on the strengthening and strain hardening
behavior of CNTs/Al composites. The underlying mechanism wasdiscussed in relating to their microstructures with an assistance ofproper strengthening and strain hardening models.

Fig. 1. High-resolution TEM images of raw FWCNTs (a, b) and MWCNTs (c, d) at
different magniﬁcations. Marked parallel lines in (b) suggest CNT walls.

2. Experimental methods
2.3. Microstructure characterization
2.1. Powder processing
In this study, FWCNTs (~3 walls, ~5 nm in diameter and ~1 mm inlength) powders and pure Al powders (120 g, ~20 mm in diameter,
99.9% in purity) were mixed by high energy ball milling (HEBM)
using a planetary ball milling machine (PULVERISETTE 5, Fritsch,
Germany). The FWCNTs (1 wt%) powders and process control agent(stearic acid, 1 wt%) were sealed with Al powders in a ZrO2 jar(500 mL in volume), where ZrO2 milling balls of 10 mm in diameterwith a ball to powder ratio of 5:1 were used. The revolution speed
was 200 rpm and the milling time was 24 h. To suppress extensiveoxidation of powders, the milling jar was aerated with a ﬂow ofargon gas and a cooling-down break of 5 min in every 15 min to
avoid overheating. One sample doped with commonly used
MWCNTs (~20 walls, ~15 nm in diameter and ~1 mm in length) wasused as reference. The morphologies of FWCNTs powders and
MWCNTs powders are shown in Fig. 1. The structure of walls andhollow channels of FWCNTs and MWCNTs can be clearly observedin the high-resolution TEM view (Fig. 1b, d). There is an obvious
difference in the wall number and outer-diameter between the two
types of CNTs.

2.2. Composite fabrication
The as-received FWCNTs/Al powder mixture (60 g) was
consolidated by spark plasma sintering (SPS) in vacuum (<5 Pa) at
two conditions, 600  C for 60 min and 500  C for 30 min, for which
the corresponding samples are named as SPS600C and SPS500C for
simplicity hereafter. To further improve the composite density, ahot-extrusion process was applied to the sintered billet. Referential
materials made of raw pure Al powders, milled Al powders and
MWCNT-Al powders were also consolidated under the same conditions, hereafter named as pure Al, milled Al and MWCNT/Al
composite, respectively. More details of the HEBM process can befound elsewhere [17,18].

A ﬁeld emission scanning electron microscopy (FE-SEM, JEM6500F, JEOL, Japan) was used to characterize the morphology of
Al-FWCNT powders and composites. In addition, a high-resolution
transmission electron microscope (HRTEM, JEM-2010, JEOL, Japan)
operated at 200 kV was used to examine the microstructure characteristics of the starting FWCNT powder, the Al-CNTs powder andthe Al-CNTs composite. TEM samples of CNTs/Al composites wereprepared using a focused ion beam (FIB) system (FB-2000S, HITACHI, Japan). Grain structure of each sample was also evaluated byelectron backscatter diffraction (EBSD) using a TSL camera (TSLDigiView IV) attached to the FE-SEM. The texting sample surface isvertical to the axial direction of the extruded samples. The phasestructures were identiﬁed by X-ray diffraction (XRD-6100, Shimazu, Japan). The instrument-induced XRD peak widening wasinvestigated by the diffraction spectrum of a single-phase siliconplate (99.99% in purity). The full width at half maximum (FWHM) of
the silicon (111) peak was measured to be 0.097 。
2.4. Tensile test
To evaluate the mechanical properties, room temperature tensile tests were carried out for each material along the extrusion
direction. Tensile specimens were machined out from the extruded
rods with a gauge length of 15 mm and a diameter of 3 mm. The
tensile tests were performed on a universal testing machine(AUTOGRAPH AG-I 50 KN, Shimadzu Co. Ltd., Japan) at a strain rateof 5  104 s1. At least three specimens for each material were
tested to assure reproducibility.
3. Results and discussion
The morphologies of the composite powders after HEBM for
24 h are shown in Fig. 2. It can be observed that the obtained
powder particles have an average dimeter of ~20 mm (Fig. 2a),

B. Chen et al. / Journal of Alloys and Compounds 826 (2020) 154075

3

Fig. 2. Morphology of Al-1 wt.% FWCNTs composite powder after ball milling at different magniﬁcations.

which is equivalent to that of starting powders. However, thecomposite powder particles exhibited a special shape, which isdistinct from the tear-like shape of the starting powder [17]. Froman enlarged view of the powder (Fig. 2b), it is seen that eachcomposite powder particle consists of several parts. It suggested
that the cold-welding phenomenon [19] occurred at the present
HEBM condition. By previous experience, it can also be deduced
that the Al-CNTs powder particles must experience the ﬂattening,
fracturing and cold-welding process during HEBM, which is similar
to that of Al-MWCNTs powders at similar conditions [20].
To illustrate the effect of consolidation conditions on ball milled
powders, the XRD patterns of the FWCNTs/Al composites (SPS500C
and SPS600C) as well as the milled FWCNTs-Al powder are displayed in Fig. 3. Two signiﬁcant characteristics could be found fromthe XRD patterns. First, a small amount of Al4C3 phase formed in thecomposites, as seen from the low intensity of Al4C3 peaks. The
morphology of Al4C3 will be shown later. Second, the dislocation
density (r) decreased from the as-milled powder to the SPS500C
sample and then to the SPS600C sample, evidenced by the decreaseof the FWHM of the Al (111) peak (Fig. 4). Since the grain size of thecomposites was measured as ~400 nm which was much larger than
100 nm, r can be estimated from Ref. [21].

r¼

b2
4:35b2;(1)

where b is the true FWHM of Al(111) peak and b is the Burgers
vector (0.29 nm) of Al. Moreover, b ¼ bmeasured-binstrumental. The
dislocation density of FWCNTs/Al (SPS500C) and FWCNTs/Al

Fig. 3. XRD patterns of FWCNTs-Al powder and FWCNTs/Al composites with nominalized Al (111) peaks. FWHM is full width at half maximum of Al (111) peaks.

(SPS600C) can be estimated as 12.3  1012 m2 and 6.6  1012 m2.
The noticeable decrease of dislocation density at higher temperature and longer time was attributed to the annealing effect during
SPS on the stored dislocations in HEBM powders.
The grains and dislocation features are also investigated by EBSD
(Fig. 4). It can be observed that in the FWCNTs/Al composites, thegrain orientation is most pronounced at <111> (Fig. 4a and d),suggesting that the extruded materials exhibit a similar texture.
This is in good agreement with the texture observations on
extruded pure Al and MWCNTs/Al composites [22]. Comparing
Fig. 4a and d, it is apparent that there is no signiﬁcant difference inthe grain orientation of SPS600C and SPS500C. However, as the SPS
time and temperature increased, the ﬁnal grain size increased forthe extruded samples, where SPS600C had an average grain size of
0.46 mm and SPS500C had a value of 0.38 mm. The kernel average
misorientation (KAM) ﬁgures that reﬂect the density of geometrically necessary dislocations are displayed for both FWCNTs/Al
composites in Fig. 4c and f. It is clear that the SPS500C sample had ahigher dislocation density level compared to SPS600C, shown as
the overall decrease of the high dislocation density areas (in green
color). This result conﬁrmed the noticeable dislocation density
difference measured from XRD peaks.
The dislocation morphology of the composites were further
investigated by TEM. Fig. 5 shows the HRTEM images of FWCNTs/Al
composite (SPS500C). Some dispersed FWCNTs were visible at thegrain boundaries, as indicated by arrows in Fig. 5a. An enlarged
view of a typical FWCNTs in black contrast is shown in Fig. 5b. The
wall structure of FWCNTs (Fig. 1b) were hardly observed because of
the overlap of Al and carbon atoms. There was no physical gap
between Al and CNTs visible. From the atom planes of Al shown in
Fig. 5c, a high dislocation density was observed at the matrix near
FWCNTs.
Except for FWCNTs, Al4C3 was also detected in the SPS500C
composite under TEM observation identiﬁed by selected area
diffraction patterns (Fig. 6). The Al4C3 phases always exhibited as ananorod shape and they sporadically distributed in Al matrix,
which formed by a full consumption of FWCNTs. The TEM results on
Al4C3 conﬁrmed the small amount of Al4C3, in agreement with the
XRD result (Fig. 3). The temperature of 500  C was much lower thanthat required for Al4C3 formation, which is reported to be ~600  C at
the same route [23]. Yet, HEBM was applied in the present study
while a mechanical coating process was used in Ref. [23]. HEBM
could bring two effects to CNTs. One is an increased structural
damage for CNTs in comparison to the mechanical coating process[24]. The other effect is that the HEBM process imposed severeplastic deformation on the Al powders, inducing a higher dislocation density in the Al matrix than the one by the coating process.
These two effects promoted the interfacial reaction between Al and
CNTs and resulted in the formation of Al4C3 nanorods at a relatively
low temperature.

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B. Chen et al. / Journal of Alloys and Compounds 826 (2020) 154075

Fig. 4. EBSD analysis of FWCNTs/Al composite sintered at 500  C for 30 min (a, b, c) and sintered at 600  C for 60 min (d, e, f): inverse polar ﬁgures (a, d), grain misorientation angle
maps (b, e) and kernel average misorientation (KAM) maps (c, f).

Fig. 5. HRTEM microstructures of FWCNTs/Al composite sintered at 500  C for 30 min (SPS500C) at different magniﬁcations. (c) is an inverse fast Flourier transformation image ofthe box area in (b).

In the SPS600 sample, individual Al4C3 nanorods with similarmorphology were occasionally observed under TEM as well.
However, it is difﬁcult to compare the content of Al4C3 in these twocomposites by TEM because of the limited observation areas. As

revealed in the previous study [25], the amount of Al4C3 precipitates increases in the composites processed by HEBM withincreasing SPS temperature and time. It is thus understandable
that, compared with SPS500C, more FWCNTs in SPS600C reactedwith Al to form Al4C3. SPS conditions also have an obvious effect on
the dislocation morphology in the composite. Fig. 7 shows theFig. 6. TEM microstructures of FWCNTs/Al composite sintered at 500  C for 30 min(SPS500C).

Fig. 7. TEM microstructures of FWCNTs/Al composite sintered at 600  C for 60 min(SPS600C). (b) is an inverse fast Flourier transformation image of the box area in (a).

B. Chen et al. / Journal of Alloys and Compounds 826 (2020) 154075

HRTEM images of the SPS600C composite sample. Compared withthe 1%MWCNTs/Al composites processed by coating þ SPS þ hot
extrusion [26], the dislocation density near the CNTs-matrix
interface was much higher in the FWCNTs/Al (SPS600C) sample.
In the present FWCNTs/Al (SPS600C) sample, the processing routeis HEBM þ SPS þ hot extrusion and FWCNTs was used as thereinforcement. Therefore, the HEBM process and FWCNTs
contribute to the increased dislocation density in FWCNTs/Al(SPS500C). Compared with the SPS500C sample (Fig. 5c), thedislocation density of SPS600C was much reduced, which was ingood agreement with the measured dislocation values of these twocomposites by XRD.
Fig. 8a shows the nominal tensile stress-strain curves of
FWCNTs/Al composites as well as referential Al materials sinteredat different conditions. For both of the FWCNTs/Al SPS600C andSPS500C composites, over 100% and 60% strength increments wereobserved for yield strength (YS) and ultimate tensile strength (UTS),
respectively, indicating a good strengthening effect of FWCNTs in
AMCs. Especially, in the SPS500C composite, the improvement is
135% and 69% in YS and UTS, respectively. Compared with thosesintered at 600  C, both the samples sintered at 500  C exhibited anincreased strength. For example, SPS500C displayed an increase of
45 MPa in YS and of 40 MPa in UTS in comparison with SPS600C,
which gave rise to a YS of 387 MPa and a UTS of 396 MPa. It should
be noted that these two composites still possessed a good ductility
with an elongation-to-failure over 10% (12% for SPS500C and 15%
for SPS600C).
The tensile strength difference between the composites aremainly related to their grain size, dislocation density and thestrengthening effect of FWCNTs. Taking YS into account, thestrength difference (Ds) of 45 MPa can be expressed as

5thus calculated as 10.3 MPa. Considering dislocations strengthening, DsDD can be expressed as [29].





0:5
;
DsDD ¼ aGb r0:5
1  r2(4)

where K is a constant (0.07 MPa m0.5 for Al [28]), d1 and d2 are theaverage grain sizes of SPS500C and SPS600C, respectively. DsGS iswhere a is a constant (1.25 for Al [29]), G is the shear modulus(25.4 GPa for Al [30]), b is the Burger’s vector (0.286 nm for Al [30]),
and r1 and r2 are the dislocation densities of SPS500C and SPS600C,
respectively. DsDD is then calculated to be 11.2 MPa. Take DsGS and
DsDD values into Eq. (2), DsFWCNTs is 23.5 MPa, which suggests that
FWCNTs contributed extra 23.5 MPa more in the SPS500C than thatin SPS600C, except for the differences caused by grain reﬁnementand dislocation strengthening mechanisms. From recent studies[4,18,26], the strengthening mechanism of CNTs was attributed toload transfer and Orowan looping. Since SPS process did not change
the size of FWCNTs, Orowan looping of FWCNTs did not make muchdifference in the two composites. Then higher strength in SPS500C
implied larger load transfer effect in SPS500C. Based on the study of
Kelly and Tyson [7], load transfer effect of ﬁbrous FWCNTs isdetermined by the length to diameter ratio, interface bonding andvolume fraction of FWCNTs. With decreasing SPS temperature andtime, length to diameter ratio and the interface bonding conditions
will not be improved [23]; however, the reaction between of
FWCNTs and Al will be suppressed, leading to less FWCNTs transforming to Al4C3 and more remained FWCNTs. The results suggested that the integration of FWCNTs structure in the SPS500C
composite was the key factor for the improved strength comparedwith the SPS600C composite.
Apart from the different tensile strengths, the two composites
also had distinct strain hardening behavior, as seen from the tensilestress-strain curves in Fig. 8. To better understand the hardening
behavior, the true stress-strain curves and normalized strain
hardening rates for the two FWCNTs/Al materials were obtained, as
shown in Fig. 9. It is seen that in the SPS500C necking occurs very
early, and the true stress declined rapidly after necking, eventually
to break. However, the necking stage lasted for ~0.1 of strain. Whiletrue yield strength of SPS600C is lower than that of SPS500C, thenecking occurs much later. After the necking, the stress slowly
decreases with an increase of strain.
To under the deformation behavior of SPS600C and SPS500C
after yielding, the normalized strain hardening rate (Q) versus
strain is shown in the inset of Fig. 9. The normalized strain hardening rate (Q) is deﬁned as:

Fig. 8. Tensile properties of FWCNTs/Al composites and referential Al materials.

Fig. 9. Comparison between tensile true stress-strain curves of FWCNTs/Al composites
sintered at two conditions. Inset shows strain hardening rate (Q)-true strain curves.

Ds ¼ DsGS þ DsDD þ DsFWCNTs ;(2)

where DsGS, DsDD and DsFWCNTs are the increased strength
contributed individually by grain size, dislocation density and
FWCNTs. DsGS can be estimated according to the Hall-Petch formula [27]:;DsGS ¼ K d0:5
 d0:5
1
2

(3)

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B. Chen et al. / Journal of Alloys and Compounds 826 (2020) 154075

Fig. 10. Fracture morphologies of FWCNTs/Al composite sintered at 500  C for 30 min(a, b) and sintered at 600  C for 60 min (c, d). Inset of (d) shows a high-magniﬁcation
view.

Fig. 11. Comparison between nominal tensile stress-strain curves of FWCNTs/Al and
MWCNT/Al composites sintered at 600  C for 60 min. Inset shows average tensile
property values.

1 ds
Q¼ , :s dε(5)It can be observed that the normalized strain hardening rate ofboth materials after the yielding declined rapidly ﬁrst and thenre’s criterion [11], when the strain
slowly. According to Conside
hardening rate is equal to the true stress, ddεs ¼ s, or Q ¼ 1, necking
will occur and the true stress begins to decrease until it breaks.
Before the state of Q ¼ 1, the material undergoes uniform deformation and thus the uniform elongation can be estimated at thispoint. The Q of SPS500C composite (red curve in the inset of Fig. 9)
falls fast and reaches the position of Q ¼ 1 (horizontal dotted line inthe ﬁgure) at uniform strain εu ¼ 0.014 (inset of Fig. 9). This isconsistent with the position of the necking in the true stress-truestrain curve. After necking, SPS500C still has a comparatively
large Q stage of around 1, which causes the steady drop of truestress of the SPS500C. After the stable stage, Q started to decrease

Fig. 12. Comparison between the tensile yield strength and failure strain of FWCNTs/Aland MWCNTs/Al composites in this study and previous MWCNTs/Al composites basedon the data collected from recent review papers [4,31e33].

slowly and thus deformation could be sustained to a large deformation degree before failure (Fig. 9). With the SPS600C composite,necking occurred at around ε ¼ 0.034, after which Q decreased
slowly and still maintained at comparatively large values, whichenabled SPS600C to have a stable and long necking process (Fig. 9).
The different strain softening behaviors in the two composites wasprobably due to the observed differences in grain size and dislocations. It can be concluded that, although the two composites had
quite different strain hardening/softening behavior, the strain
hardening/softening rates after necking in two composites still
maintained at a high level. This fact resulted in good ductility of thetwo composites.
The fracture morphology of the FWCNTs/Al composites is shownin Fig. 10. On the fracture surfaces of the composites, ﬁne andshallow dimples are clearly visible, which also conﬁrms theexcellent ductility. The dimple size was in sub-micrometer scale forboth samples (Fig. 10b and d), which is basically coincident with theaverage grain size of the composites (Fig. 4). Moreover, dispersed
FWCNTs were observed at the dimple boundaries (inset of Fig. 10d).
Considering these facts, it is deduced that each dimple is corresponded to one matrix grain. Therefore, the ﬁne grain size had agreat effect on the deformation behavior and thus contributed to
the good ductility of composites.
Fig. 11 shows the direct comparison between the tensile properties of AMCs reinforced with FWCNTs and MWCNTs sintered at
600  C for 60 min. It is observed that FWCNTs/Al had higher YS andUTS than MWCNT/Al. Compared with the referential Al, the yieldstrength improvement in the MWCNTs/Al composites was
remarkably improved from 84% to 122%. Suggesting the superior
strengthening effect of FWCNT to MWCNTs. A comparison between
the tensile properties of CNT/Al composites reported in this study
and previous studies [4,31e33] is shown in Fig. 12. Compared withprevious MWCNTs/Al composites, the two FWCNTs/Al composites
in this work showed well-balanced strength and ductility. It suggests that FWCNTs are a good reinforcement candidate for MMCs
with high mechanical performances.
4. Conclusions
In this study, FWCNTs were used as reinforcement in AMCs andits strengthening effect in composites was investigated. It was
found that with decreasing SPS temperature and time, FWCNTs/AlB. Chen et al. / Journal of Alloys and Compounds 826 (2020) 154075

composites showed ﬁner grains, increased dislocation density andimproved structural integrity of FWCNTs, leading to increased
tensile strength. The addition of 1 wt% FWCNTs led to 135% and 69%
improvement in yield strength (YS) and ultimate tensile strength(UTS) of the referential Al material, respectively. It resulted in
387 MPa in YS, 396 MPa in UTS and 12% in elongation of a FWCNTs/
Al composite. The good ductility was related to the ultra-ﬁne grains
and resulted comparatively high strain hardening rates afternecking during tensile deformation. FWCNTs/Al composites
developed in this study have improved balance of strength andductility compared with conventional MWCNTs/Al composites.
Declaration of competing interest
The authors declare that they have no known competing
ﬁnancial interests or personal relationships that could haveappeared to inﬂuence the work reported in this paper.
CRediT authorship contribution statement
B. Chen: Conceptualization, Investigation, Formal analysis,
Writing - original draft, Supervision. Z. Li: Investigation, Writing original draft. J. Shen: Investigation, Formal analysis. S. Li: Investigation, Formal analysis. L. Jia: Investigation, Formal analysis. J.
Umeda: Resources. K. Kondoh: Formal analysis. J.S. Li: Supervision.
Acknowledgement
B. Chen would like to thank the ﬁnancial supports from the
National Natural Science Foundation of China (51901183), theFundamental Research Funds for the Central Universities ofNorthwestern Polytechnical University (G2018KY0301), and the
Research Fund of the State Key Laboratory of Solidiﬁcation Processing (2019-TS-13).
References[1] S.C. Tjong, Novel nanoparticle-reinforced metal matrix composites withenhanced mechanical properties, Adv. Eng. Mater. 9 (2007) 639e652.
[2] Y. Nishida, Introduction to Metal Matrix Composites, 2013.
[3] V.N. Popov, Carbon nanotubes: properties and application, Mater. Sci. Eng. R
43 (2004) 61e102.
[4] S.C. Tjong, Recent progress in the development and properties of novel metal
matrix nanocomposites reinforced with carbon nanotubes and graphenenanosheets, Mater. Sci. Eng. R Rep. 74 (2013) 281e350.
[5] S.R. Bakshi, D. Lahiri, A. Agarwal, Carbon nanotube reinforced metal matrix
composites - a review, Int. Mater. Rev. 55 (2010) 41e64.
[6] B. Chen, S. Li, H. Imai, L. Jia, J. Umeda, M. Takahashi, K. Kondoh, Load transfer
strengthening in carbon nanotubes reinforced metal matrix composites via insitu tensile tests, Compos. Sci. Technol. 113 (2015) 1e8.
[7] A. Kelly, W.R. Tyson, Tensile properties of ﬁbre-reinforced metals: copper/
tungsten and copper/molybdenum, J. Mech. Phys. Solid. 13 (1965),
329,in321,339-338,in322,350.
[8] R. Bacon, Growth, structure, and properties of graphite whiskers, J. Appl. Phys.
31 (1960) 283e290.
[9] A. Pantano, D.M. Parks, M.C. Boyce, Mechanics of deformation of single- andmulti-wall carbon nanotubes, J. Mech. Phys. Solid. 52 (2004) 789e821.
ty, Inﬂuence of the concentration and[10] J. Stein, B. Lenczowski, E. Anglaret, N. Fre

7nature of carbon nanotubes on the mechanical properties of AA5083
aluminium alloy matrix composites, Carbon 77 (2014) 44e52.
[11] A.M.K. Esawi, K. Morsi, A. Sayed, A.A. Gawad, P. Borah, Fabrication andproperties of dispersed carbon nanotubeealuminum composites, Mater. Sci.
Eng., A 508 (2009) 167e173.
[12] C. He, N. Zhao, C. Shi, X. Du, J. Li, H. Li, Q. Cui, An approach to obtaining homogeneously dispersed carbon nanotubes in Al powders for preparing reinforced Al-matrix composites, Adv. Mater. 19 (2007) 1128e1132.
[13] L. Jiang, G. Fan, Z. Li, X. Kai, D. Zhang, Z. Chen, S. Humphries, G. Heness,
W.Y. Yeung, An approach to the uniform dispersion of a high volume fractionof carbon nanotubes in aluminum powder, Carbon 49 (2011) 1965e1971.
[14] F. Rikhtegar, S.G. Shabestari, H. Saghaﬁan, Microstructural evaluation andmechanical properties of Al-CNT nanocomposites produced by different
processing methods, J. Alloys Compd. 723 (2017) 633e641.
[15] A.M.K. Esawi, K. Morsi, A. Sayed, M. Taher, S. Lanka, The inﬂuence of carbon
nanotube (CNT) morphology and diameter on the processing and propertiesof CNT-reinforced aluminium composites, Compos. Appl. Sci. Manuf. 42(2011) 234e243.
[16] E.M. Byrne, M.A. McCarthy, Z. Xia, W.A. Curtin, Multiwall nanotubes can bestronger than single wall nanotubes and implications for nanocomposite
design, Phys. Rev. Lett. 103 (2009), 045502.
[17] B. Chen, S. Li, H. Imai, L. Jia, J. Umeda, M. Takahashi, K. Kondoh, An approachfor homogeneous carbon nanotube dispersion in Al matrix composites, Mater.
Des. 72 (2015) 1e8.
[18] B. Chen, K. Kondoh, J.S. Li, M. Qian, Extraordinary reinforcing effect of carbon
nanotubes in aluminium matrix composites assisted by in-situ alumina
nanoparticles, Compos. B Eng. 183 (2020) 107691.
[19] A. Esawi, K. Morsi, Dispersion of carbon nanotubes (CNTs) in aluminum
powder, Compos. Appl. Sci. Manuf. 38 (2007) 646e650.
[20] B. Chen, K. Kondoh, H. Imai, J. Umeda, Effect of initial state on dispersion
evolution of carbon nanotubes in aluminium matrix composites during ahigh-energy ball milling process, Powder Metall. 59 (2016) 216e222.
€ ttcher, H. Heinke, S. Einfeldt, S. Figge, D. Hommel, Micro[21] R. Chierchia, T. Bo
structure of heteroepitaxial GaN revealed by x-ray diffraction, J. Appl. Phys. 93(2003) 8918e8925.
[22] B. Chen, S. Li, H. Imai, L. Jia, J. Umeda, M. Takahashi, K. Kondoh, Carbon
nanotube induced microstructural characteristics in powder metallurgy Al
matrix composites and their effects on mechanical and conductive properties,
J. Alloys Compd. 651 (2015) 608e615.
[23] B. Chen, J. Shen, X. Ye, H. Imai, J. Umeda, M. Takahashi, K. Kondoh, Solid-stateinterfacial reaction and load transfer efﬁciency in carbon nanotubes (CNTs)reinforced aluminum matrix composites, Carbon 114 (2017) 198e208.
[24] Z.Y. Liu, S.J. Xu, B.L. Xiao, P. Xue, W.G. Wang, Z.Y. Ma, Effect of ball-milling time
on mechanical properties of carbon nanotubes reinforced aluminum matrix
composites, Compos. Appl. Sci. Manuf. 43 (2012) 2161e2168.
[25] B. Chen, L. Jia, S. Li, H. Imai, M. Takahashi, K. Kondoh, In situ synthesized
Al4C3Nanorods with excellent strengthening effect in aluminum matrix
composites, Adv. Eng. Mater. 16 (2014) 972e975.
[26] B. Chen, J. Shen, X. Ye, L. Jia, S. Li, J. Umeda, M. Takahashi, K. Kondoh, Length
effect of carbon nanotubes on the strengthening mechanisms in metal matrix
composites, Acta Mater. 140 (2017) 317e325.
[27] N. Hansen, HallePetch relation and boundary strengthening, Scripta Mater. 51(2004) 801e806.
[28] M.A. Meyers, K.K. Chawla, Mechanical Behavior of Materials, Cambridge university press, Cambridge, 2009.
[29] R. George, K.T. Kashyap, R. Rahul, S. Yamdagni, Strengthening in carbon
nanotube/aluminium (CNT/Al) composites, Scripta Mater. 53 (2005)
1159e1163.
[30] H.J. Frost, M.F. Ashby, Deformation Mechanism Maps: the Plasticity and Creepof Metals and Ceramics, 1982.
[31] S.R. Bakshi, A. Agarwal, An analysis of the factors affecting strengthening in
carbon nanotube reinforced aluminum composites, Carbon 49 (2011)
533e544.
[32] Z. Baig, O. Mamat, M. Mustapha, Recent progress on the dispersion and thestrengthening effect of carbon nanotubes and graphene-reinforced metal
nanocomposites: a review, Crit. Rev. Solid State Mater. Sci. (2016) 1e46.
[33] A.V. Radhamani, H.C. Lau, S. Ramakrishna, CNT-reinforced metal and steel
nanocomposites: a comprehensive assessment of progress and future directions, Compos. Appl. Sci. Manuf. 114 (2018) 170e187.