Materials Letters 264 (2020) 127323

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In-situ observation of interaction between dislocations and carbon
nanotubes in aluminum at elevated temperatures
B. Chen a,⇑, K. Kondoh b, J.S. Li aab

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
Joining & Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japana r t i c l ei n f oArticle history:
Received 31 October 2019Accepted 7 January 2020
Available online 9 January 2020
Keywords:
Metal matrix composites (MMCs)
AluminumCarbon nanotubes
Grain boundaries
Dislocationsa b s t r a c t
In this study, in-situ high-temperature transmission electronic microscopy is performed to investigate
the formation mechanism of high-fraction (>50%) low-angle grain boundaries (LAGBs) in carbon nanotubes/aluminum (CNTs/Al) composites with balanced strength and ductility. During heating up thelow-temperature-sintered composite, we find that dislocation bands first moves and then they arepinned by CNTs to form LAGBs. Abundant dislocations in the Al matrix and large-aspect-ratio CNTs areclarified as two critical factors for producing LAGBs. This study provides new insight into the effect ofCNTs on the microstructural and mechanical properties of metal matrix composites.
Ó 2020 Elsevier B.V. All rights reserved.

1. Introduction
In the past decade, increasing attention has been paid to carbon
nanotube (CNT)-reinforced aluminum (Al) metal matrix composites (MMCs) because of their promising light weight and excellent
mechanical properties [1,2]. Many studies have shown the possibility to achieve high strength over 300 MPa of CNTs/Al composites
by using powder metallurgy methods [2]. However, one criticalproblem of these composites for future applications is the limited
ductility of near or less than 5% in tensile elongation [3,4]. To
improve plasticity, some studies optimized composite microstructures related to reinforcements, such as CNT dispersion [5], interface [6] and Al4C3 amount [7]. However, limit attention has been
paid to the matrix microstructure changes induced by CNT additions [1,8].
In metals and alloys, the misorientation angle of grain boundaries is a significant microstructural feature to influence the deformation behavior and mechanical properties [9,10]. Regarding Alalloys, some researches showed that low-angle grain boundaries(LAGBs) can improve the plasticity due to the cross slip mechanism
between adjacent grains [11,12]. Recently, Chen et al. [3] reported
a large fraction of LAGBs in CNTs/Al composites fabricated at highsintering temperature of 900 K, contributing to the excellent plasticity of tensile elongation larger than 20% as well as tensile
strength over 200 MPa. The control of grain features in CNTs/Al⇑ Corresponding author.
E-mail address: chen@nwpu.edu.cn (B. Chen).
https://doi.org/10.1016/j.matlet.2020.127323
0167-577X/Ó 2020 Elsevier B.V. All rights reserved.

composites may break new ground in fabricating highperformance Al MMCs. However, the formation mechanism of
the high-fraction LAGBs was still unclear. To this end, in this study
we applied in-situ transmission electronic microscopy observations on a low-temperature-sintered CNTs/Al composite to investigate the interaction between dislocations and CNTs at elevatedtemperatures.
2. Experimental methods
Pure Al powder and multi-walled CNTs (MWCNTs, ~10 nm indiameter and ~1 lm in length, Baytubes C150P, Bayer Material
Science Co., Japan) were used as starting materials. To disperse
CNTs in Al matrix, flaky Al powders were prepared by high energy
balling and then bathed in isopropyl alcohol (IPA) based solution
with 1 wt% CNTs in a plastic bottle on a rocking ball millingmachine. To further consolidate the composite powders, spark
plasma sintering (SPS) and following hot extrusion were applied.
SPS (SPS-1030S, SPS Syntex, Japan) was conducted at two sintering
temperatures of 800 K and 900 K. Pure Al powders were also processed under the same route as reference materials. Detailed
description of the fabrication method can be found elsewhere[3,13].
Crystal grain information of each sample was analyzed using
electron backscatter diffraction (EBSD) with a TSL camera (TSLDigiView IV) attached to the field emission scanning electron
microscopy (FE-SEM, JEM-6500F, JEOL). The microstructures ofCNTs/Al composites were examined by high resolution

2B. Chen et al. / Materials Letters 264 (2020) 127323

transmission electronic microscopy (HR-TEM, JEM-2010, JEOL,Japan). For in-situ TEM testing, the FIB-TEM sample was thenwelded on the stage of a semicircle copper plates using tungsten.
A Gatan single-tilt (Model 628) heating holder with the copper disk
was put inside the chamber of Hitachi H-800 type 200-kV transmission electron microscope. The heating was carried out undera high vacuum with a pressure of lower than 106 Pa. The CNTs/
Al composite sample on the copper plate was slowly heated up
to 700 K, 800 K, 850 K and 900 K at a heating rate of ~20 Kmin1and then maintained at each temperature for 5 min. The temperature fluctuation was within ±2 K at 900 K. High-quality bright-field
TEM images were captured during the holding stages.
3. Results and discussion
Fig. 1 shows the grain boundary (GB) characteristics obtainedby EBSD of pure Al and CNTs/Al composites sintered at 800 Kand 900 K. Obviously high-angle GBs (defined as GBs with misorientation angles larger than 15°) are dominant in pure Al sintered at
800 K and 900 K (Fig. 1a) and the CNTs/Al composite sintered at
800 K (Fig. 1b). However, with the CNTs/Al composite sintered at
900 K, the fraction of low-angle GBs (LAGBs, smaller than 15°,
red and green lines in Fig. 1c) increases remarkably. On the otherhand, the differences in average grain size (1.6–1.7 lm) and texture (h1 1 1i dominant) are quite small in the two CNTs/Al composites [13] and thus they have small effect on the mechanicalproperties. Comparing the mechanical properties of the CNTs/Al
composite sintered at 900 K to that of 800 K, there are simultaneous enhancement in tensile strength and ductility (Table 1). The
strength improvement mechanism has been clarified as theFig. 1. Misorientation angle maps of grain boundaries in pure Al sintered at 900 K(a), CNTs/Al at 800 K (b), and CNTs/Al at 900 K (c). (d) is a summary of
misorientation angle frequency. Inset of (d) shows the fraction of low-angle grainboundaries (LAGBs, <15°). (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)enhanced bonding conditions of Al-CNTs interface at elevated sintering temperatures in the previous study [13]. The enhancement
in interface will contribute to improved load transfer from matrix
to CNTs, i.e., increased restriction of matrix deformation, leading toexpected reduced ductility [14]. Opposite to prediction, there isremarkable improvement (over 80%) of elongation in the CNTs/Alcomposite sintered at 900 K compared to that of 800 K (Table 1).
Since the effect of grain size and texture has been proved to besmall, it is deduced that the ductility improvement is associate tothe orientation angles of GBs.
The detailed information of misorientation angle is further
examined by its frequency distribution in Al and CNTs/Al materials.
It is observed that in the CNTs/Al composite sintered at 900 K, thefrequencies of misorientation angles less than 20° increase noticeably; contrarily those of angles larger than 30° reduce much(Fig. 1d). As a result, the length fraction of LAGBs occupies 0.59in CNTs/Al sintered at 900 K, while the value of 0.2–0.3 in the pureAl and CNTs/Al sintered at 800 K (inset of Fig. 1d, Table 1). Therefore, the sintering temperature plays a significant role in determining the GB features of CNTs/Al composites. It is also noted that thepure Al sintered at 900 K possesses a slightly increased value of0.30 compared with that of pure Al sintered at 800 K, which isassociate to the in-situ formed alumina nanoparticles [15].
The grain and dislocation features are examined by TEM in theCNTs/Al composites. In the composite sintered at 800 K, large
quantity of dislocations as well as dislocation bands are observed(Fig. S1a). The formation of a high density of dislocations is attributed to the stored strains in the prepared flaky Al powders thatexperienced severe plastic deformation in the high-energy ball
milling process [16]. With the CNTs/Al composite sintered at900 K, except for a high dislocation density, the entangled dislocations and the LAGB are also detected (Fig. S1b).
To understand the formation mechanism of LAGBs, in-situ hightemperature TEM study is performed on the TEM sample of theCNTs/Al composite sintered at 800 K (Fig. S1a). Well-dispersed
and unreacted CNTs are observed in Al matrix at the initial state
before heating-up (inset of Fig. 2a). There is no obvious microstructure change at temperature of 700 K (Fig. 2b) and 800 K (Fig. 2c). As
the temperature is increased to 850 K, it is seen that a dislocation
band (as black arrows indicated in Fig. 2c) moves ahead to theposition as a red arrow indicated in Fig. 2d. It is seen that the dislocation is pinned by CNTs, as a white arrow indicated in Fig. 2d.
The grey contracts surrounding CNTs should be a result of strainat Al-CNT interface because of the large difference of the thermalexpansion coefficients between Al (~106 K1) and CNTs
(23.6  106 K1) [17]. Even at higher temperature of 900 K, thedislocation no long moves forward and it is steady at the place contacting with CNTs (Fig. 2e). After cooling down, a LAGB withmisorientaiton angle of ~4° forms as revealed in Fig. S2.
Fig. 3 shows the formation scheme of the LAGBs based on thein-situ study and microstructural observations. With Al, as the process temperature is increased, the dislocation immigration is activated under the energy input. As a result, many dislocations preferto sink at GBs, which corresponds to the commonly observed
recovery process of metals at high annealing temperatures [18].
However, in CNTs/Al composites, because of the existence of long

Table 1
Tensile properties and grain features in Al and CNTs/Al composites consolidated at different temperatures.
SampleYield strength (MPa)Tensile strength (MPa)Elongation (%)

Grain size (lm)TextureLength fraction of LAGBs

Al-SPS800K
Al-SPS900K
CNTs/Al-SPS800K
CNTs/Al-SPS900K

122
121
127
150

172
166
186
212

21.1
24.1
11.2
20.4

1.92
2.34
1.67
1.75

h1 1 1ih1 1 1ih1 1 1ih1 1 1i

0.21
0.31
0.20
0.59

B. Chen et al. / Materials Letters 264 (2020) 127323

3Fig. 2. In-situ TEM observation on CNTs/Al composite at different temperatures. (a) Target temperature-time curve. (b-f) Recorded TEM images during the stages in (a) as
marked by corresponding captial letters.

Fig. 3. Formation scheme of a CNT-induced low angle grain boundary in Al MMCs with abundant dislocations at elevated temperatures.

CNTs, the movement of dislocations is restricted as they encounterthe CNTs. The dislocations finally settle down (Fig. 3). As more
CNTs or CNT bands stop there, LAGBs will form in the area adjoining to CNTs. From the viewpoint of the whole sample, large fractionof LAGBs are thus observed (Fig. 1d).

Declaration of Competing Interest

4. Conclusions

Acknowledgements

This study investigates the formation mechanism of high fraction LAGBs at elevated temperatures in Al MMCs. From in-situ
high-temperature TEM observations on CNTs/Al composite sintered at 800 K, the abundant CNTs can be trapped by long CNTs
and evolve to LAGBs. The abundant dislocations and long CNTs
are internal factors and high temperature is the external one toproduce high fraction LAGBs and consequently outstanding tensile
properties. Our study provides new insight into designing high performance MMCs reinforced with large-aspect ratio CNTs orgraphene.

This study is supported by the Fundamental Research Funds for
the Central Universities (G2018KY0301), from the National Natural
Science Foundation of China (No. 51901183), and the Research
Fund of the State Key Laboratory of Solidification Processing(2019-TS-13).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared
to influence the work reported in this paper.

Appendix A. Supplementary data
Supplementary data to this article can be found online athttps://doi.org/10.1016/j.matlet.2020.127323.

4B. Chen et al. / Materials Letters 264 (2020) 127323

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