The use of cryogenic milling to prepare high

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Materials and Design 114 (2017) 373–382

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The use of cryogenic milling to prepare high performance Al2009 matrix composites with dispersive carbon nanotubes Tianbing He ⁎, Xiaolei He, Pengjun Tang, Desheng Chu, Xingyuan Wang, Peiyong Li Beijing Institute of Aeronautic Materials, Beijing 100095, China Beijing Engineering Research Center of Advanced Aluminum Alloys and Application, Beijing 100095, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Cryogenic milling can shorten CNTs with less time. • CNT distributed highly homogeneously in Al2009 powder and the composites. • CNTs had minimal damage and most of them retained tubular structure. • Interface bonding enhanced by CNT partially reacted with Al. • 1.0 wt.% CNT/2009-T4 composite exhibited high strength and ductility.

a r t i c l e

i n f o

Article history: Received 28 August 2016 Received in revised form 14 October 2016 Accepted 2 November 2016 Available online 6 November 2016 Keywords: Carbon nanotube Cryogenic milling Dispersion Aluminum matrix composite Microstructure Mechanical property

a b s t r a c t Al2009 matrix composites reinforced with highly dispersive carbon nanotubes (CNTs) were prepared by cryogenic milling combined with powder metallurgy, the structure evolution of CNTs during preparation and the mechanical properties of composites were investigated. The results show that short CNTs with uniform distribution were obtained in aluminum powders through cryomilling. Most CNTs retained tubular structure in composites except only minimal ones reacted with aluminum and formed Al4C3. The dispersity of CNTs in composites gradually decreased with rising addition, and limited dispersion was found when CNTs content reached at 1.5 wt.%. The ultimate tensile and yield strength of 1.0 wt.% CNTs/2009 composite at natural aged condition were 560 MPa and 443.3 MPa respectively, which enhanced 25% and 24% compared to Al2009 aluminum alloy prepared by the same process. Moreover, it still showed good ductility with elongation of 10.2%. The cryogenic milling is promising to prepare homogeneously distributed carbon nanotubes reinforced composites with high performance. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Aluminum matrix composites exhibit high specific strength and modulus, good electrical and thermal conductivity as well as elevated ⁎ Corresponding author. E-mail address: [email protected] (T. He).

http://dx.doi.org/10.1016/j.matdes.2016.11.008 0264-1275/© 2016 Elsevier Ltd. All rights reserved.

temperature properties, which have been used in aerospace, automobile and electronic fields and increasingly aroused extensive attention. Carbon nanotube (CNT) has distinct structure and excellent physical and chemical properties (e.g. Young's modulus up to 1–1.8 TPa, tensile strength up to 150 GPa [1,2], low density of 1.2–1.8 g/cm3 [3], and neglected coefficient of thermal expansion [4], as well as good ductility and plastic deformation capacity [5]), i.e. the comprehensive properties

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Table 1 The composition of Al2009 alloy powder. composition

Cu

Mg

Fe

Si

O

Al

wt.%

3.9

1.5

b0.1

b0.1

b0.1

Bal.

far outweigh its particle and fiber counterparts, becoming a superb reinforcement for composites. However, since its high specific surface area, CNT usually adheres with each other (the Van der Waals' force of two adherent CNTs is ~ 500 eV/μm [6]) and forms agglomerates. Furthermore, CNT is hydrophobic and highly inert, leading to low solubility in most solvents. These two factors cause great difficulties in dispersing CNTs into metal matrix as well as the interface bonding between them and metal. For the sake of achieving CNTs with homogeneous distribution in aluminum matrix and composites with high performances, a great deal of fabrication methods such as ultrasonic-assisted solvent dispersion [7,8], high energy ball milling [9–11], in-situ synthesis [12–14], molecule level mixing [15,16] and melt infiltration [17,18] etc. have been exploited by researchers in the past decades, and have met with much success. While comparing with theoretical analysis, the experimental properties still indicate great difference and require further elaboration. At present, most of CNTs used in aluminum matrix composites are prepared by catalytic chemical vapor deposition (CVD), which have advantages of high yield and low cost comparing to arc discharge and laser ablation method [19]. However, such CNTs usually show curly morphology and entangle with each other, which obviously result in greater challenge for dispersion. The uniform distribution of CNTs in aluminum matrix always relates to their length reduction (whether by pre-shortening CNTs before adding to the matrix or shortening in the blending process with aluminum alloy), and high energy ball milling has become one of the most common methods owing to its simple

operation and can be used in batch processing. Undoubtedly, the shortening of CNTs causes open tips increase, structural integrity declines, and may also damages the side wall in carbon nanotubes during ball milling since the intense collision. To reduce the damage of reinforcement is also an important issue for composites preparation. Lee et al. [20,21] have reported adopting ball milling to shorten carbon nanotubes at liquid nitrogen temperature, and showed the length of CNTs can be reduced in a short time with only minimal side wall defects, by this method the CNTs exhibit good dispersity in solvent without adding any surfactant. Some researchers also reported manufacturing carbon nanotubes reinforced aluminum or polymer matrix composites through cryogenic milling [22–25], but their main aim lay in obtaining nanosized matrix via deformation under low temperature. In addition, the CNTs or compound powder were not directly milled in liquid nitrogen but immersing sealed vials which filled with milling balls and powder in cryogenic agent. In order to investigate the influence of cryogenic milling on dispersity of CNTs in aluminum matrix and the resulting structure evolution during composites fabrication process, Al2009 alloy powder with different additions of CNTs were directly milled in liquid nitrogen, then consolidated by powder metallurgy. The microstructure and mechanical properties of CNTs/2009 composites were characterized in this article. 2. Experimental 2.1. Preparation of CNTs/2009 powder 2009 aluminum alloy powders were prepared by nitrogen gas atomization, and the particle size was b45 μm, the composition and micrograph were shown in Table 1 and Fig. 1(a) respectively. Multi-walled carbon nanotubes with nominal diameter of 40–60 nm, length of 5–

Fig. 1. Morphologies of raw materials: (a) nitrogen atomized Al2009 powder observed by SEM, (b) CNT powder by SEM, (c) and (d) TEM micrographs of CNTs at low and high magnification, respectively.

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2.2. Synthesis of CNTs/2009 composites The milled CNTs/2009 powders were placed into Ф80 × 180 mm aluminum container, and degassed at 480 °C until the vacuum pressure was b2 × 10− 3 Pa, then sealed. The degassed can was subsequently hot isostatic pressed (HIP) at 465 °C under a pressure of 120 MPa for 3 h. After HIP, the aluminum layer was machined then CNTs/2009 billet was obtained. The hot extrusion was conducted using a 1000 t horizontal extruder with temperature and extrusion ratio of 460°Cand 18:1, the ram speed was 2 mm/s, and the diameter of extruded bar was 15 mm. Finally, the CNTs/2009 extrusion bars were solution treated at 498 °C for 4 h, quenched in room temperature water, and natural aged for N 96 h (T4 state). 2.3. Characterization of CNTs/2009 composites

Fig. 2. Schematic of cryogenic milling device.

15 μm, and purity N 97 wt.% were purchased from Shenzhen Nanotech Port Co., China. The microstructures of CNTs were illustrated in Fig. 1(b)–(d). CNTs/2009 powders were prepared by cryogenic milling with ball-to-powder weight ratio of 39:1, milling time and rotation speed of 2 h and 180 rpm respectively. The milling chamber was made of 316L steel, and the material of milling ball was ZrO2. Fig. 2 schematically describes the milling device. Al2009 alloy powders with different CNTs loading (0, 0.3 wt.%, 0.7 wt.%, 1.0 wt.% and 1.5 wt.%, respectively) were put into the stirring jar, then filled with liquid nitrogen and agitated after the milling balls were completely immersed. The mixed powders and milling balls were always sunk in the liquid nitrogen during the milling process.

The dispersity of CNTs and microstructure of composites were characterized by optical microscope (OM, Olympus GX 51), scanning electron microscope (SEM, Nova Nano SEM 450) and transmission electron microscope (TEM, JEOL 2100). The Keller's reagent (2 ml HF + 3 ml HCl + 5 HNO3 + 190 ml H2O) was used in etching OM samples. TEM samples were made by Gantan 691 precision ion polishing system. The X-ray diffraction (XRD) was measured using a Bruker D8 Advance diffractometer with a Cu Kα radiation source (50 kV and 30 mA) in the 2θ range from 20° to 90°, the step size and scan rate were 0.02 and 4°/min. Raman spectrometer (Jobin Yvon HR800) with a laser wavelength of 532 nm was used to evaluate the disorder and structure evolution in CNTs. Tensile test of dog-bone cylindrical sample with a diameter of 5 mm and gauge length of 25 mm was carried out using an Instron testing machine at room temperature. The tensile test results were the average of three specimens. 3. Results and discussions 3.1. Dispersity of CNTs in Al2009 powder Fig. 3 shows the SEM micrographs of CNTs/2009 powder. It can be seen that the spherical shape of atomized powder (Fig. 1a) disappeared after cryogenic milling. Severe cold welding was found in Al2009 powder, which showed pebble-like shape with size of ~300 μm. In contrast, the majority of CNTs/2009 powder was flake, and the particle size became smaller with CNT content increased. Fig. 3(f)–(h) show the surface morphology of the compound powder, carbon nanotubes with length of 0.1–1.5 μm were clearly seen on the surface, which was

Fig. 3. SEM micrographs of milled CNTs/2009 powder with (a) and (e) 0 wt.%, (b) and (f) 0.3 wt.%, (c) and (g) 1.0 wt.%, (d) and (h) 1.5 wt.% CNTs loading (the white arrows show single CNT).

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effectively avoids the probable contaminant of adding other substances (e.g. steric acid). The rigidity and inflexibility of CNTs in cryogenic condition makes them more inclined to fracture during milling [20,21], then the length of CNTs become short and entangles unpacked as well. With the strong mechanical stirring, the CNTs and metal powder experience continuous mixing. At the same time, the liquid nitrogen comes to boil under ambient pressure, thus the mixture is further promoted via this agitation. Moreover, the CNTs previously distributed on the Al powder surface will be embedded into the matrix because of cold welding [27], then new metal surface generates and provides more space for the spreading out of CNTs. Hence it can be seen that the cryogenic milling for CNTs and Al2009 powder is the integration of CNTs shortening, mechanical stirring, liquid agitation and cold welding. Therefore, the homogeneous distribution of CNTs is able to be obtained in a short time. 3.2. XRD pattern of CNTs/2009 powder and composites Fig. 4. XRD pattern of 1.0 wt.% CNTs/2009 as-milled powders and as-T4 state composite.

remarkably shorter than as-received CNTs (nominal length is 5–15 μm, also can be seen in Fig. 1b). At the same time, the CNTs distributed fairly uniformly on the powder surface without any agglomeration. With the increasing of CNTs, their distribution density in the matrix also went up (Micrographs of 0.7 wt.% CNTs/2009 powder along with more images show CNTs distribution on a large scale can be seen in Fig. S1 in Supplementary data). Comparing with milling at room temperature, cryogenic milling is also a competing process with powder fracture and cold welding. However, the deformation behavior of the powder has been changed in some extent, e.g. the enhanced micro-hardness, reduced ductility and decreased heat accumulation between powders, thus cold welding minimized under cryogenic temperature [26]. Despite this, the matrix alloy powder still formed severe cold welding resulted from its excellent ductility. Carbon nanotube has good self-lubricity and thermal conductivity, the lubrication function reduces the friction force, thus the powder bonding becomes difficult. Moreover, the heat generated in collision process can be rapidly transferred in liquid nitrogen, hence the cold welding is inhibited. These effects boost with the CNTs content increases, and that is the reason of constantly decrease in particle size. It is well documented that process control agent (PCA) is required to reduce cold welding during ball milling [10,11,27], while the result in this research shows CNT itself can be functioned as a PCA, which

Fig. 4 shows the XRD pattern of 1.0 wt.% CNTs/2009 as-milled powders and the extruded sample in T4 state. Except for the α (Al) and S (Al2CuMg) phases, other diffraction peak was not observed. The disappearance of CNTs may be resulted from the relatively low content (1 wt.%) and the small atomic number (6) of carbon. The Al4C3 peak was also not found in the CNTs/2009 composite, indicating no remarkable reaction between CNTs and Al2009. 3.3. Microstructure of CNTs/2009 composites Fig. 5 shows the optical micrographs of CNTs/2009-T4 composites. No void observed in the images corroborated the composites had dense structure. When the CNTs content was b0.7 wt.%, there was nearly no agglomerate in the composites. With the amount reached at 1.0 wt.%, a small number of non-uniformly distributed CNTs appeared with width and length of about 1 and 10 μm respectively. Further increasing to 1.5 wt.% CNTs, the size of agglomerates became large and the quantity significantly ascended. For the samples after etching, the fibrous structure can be seen in Fig. 5 (e)–(h). Comparing with Al2009 alloy, the composites displayed a much smaller fiber size. It is proved that the CNTs in composites can inhibit the motion of grain boundary [28], which restrains the recrystallization and grain growth during hot extrusion and solution treatment, hence finer structure can be obtained (The TEM images showing grain structure of Al2009 alloy and 1.0 wt.% CNTs/2009 composite at T4 state can be seen in Fig. S3 in Supplementary data).

Fig. 5. Optical micrographs of CNTs/2009-T4 composites with (a) and (e) 0 wt.%, (b) and (f) 0.7 wt.%, (c) and (g) 1.0 wt.%, (d) and (h) 1.5 wt.% addition of CNTs before (a–d) and after (e–h) etched (the circles mark the CNTs cluster ).

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Fig. 6. Schematic of CNTs redistribution in CNTs/2009 composite.

Although quite uniform distribution of CNTs was observed in CNTs/ 2009 powder (Fig. 3), but we reckoned a few clusters probably were not observed with the limitation of sampling and viewing range. These clusters will be elongated during deformation and tend to arrange along the extrusion direction. The distribution of CNTs in the matrix before and after extrusion are schematically illustrated in Fig. 6. For another, with the CNTs content increasing, distribution density of them in the matrix raised as well, while cold welding of particles declined (Fig. 3), i.e. an increased number of CNTs dispersed on the surface of metal powder instead of being embedded into it. Because the CNTs and aluminum alloy exhibit different flow rate in extrusion process, the CNTs on

Al2009 surface (especially areas have dense distribution) are inclined to form segregation zone when two flakes bind. As for this point, slower external deformation rate probably required to minimize the gap of flow rate between CNTs and metal during hot working. Therefore, it can be concluded that the distribution of CNTs in the composites is controlled by their dispersion in matrix powder as well as the consolidation and forming process. For lower CNTs, a good dispersity of them in metal powders can be relatively easily obtained, and the influence of deformation process on dispersion is either not significant (low distribution density of CNTs and high fraction of matrix, even the CNTs re-agglomerate, the size of cluster is small). Conversely, the above two facts are all

Fig. 7. TEM micrographs of 1.0 wt.% CNTs/2009-T4 composite: (a) grain structure, (b) and (c) the distribution of CNTs on grain boundaries and in grains, (d) the magnification area in (b).

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prominent when the CNTs content is high. That is the reason why clusters begin to be observed in composites when the CNTs content increases. Fig. 7 shows the TEM micrographs of 1.0 wt.% CNTs/2009-T4 composite at low and high magnification. From Fig. 7(a), it can be seen that the composite demonstrated relative fine grain with rectangular shape which had average length and width of 1.5 and 0.5 μm, respectively. A multitude of second phases and dislocations were also found in the composite. Fig. 7(b)–(d) further show the microstructure of composite on the grain boundaries and in the grains. Carbon nanotubes with several hundred nanometers length (which is consistent with the morphology after cryogenic milling) clearly displayed in the images. Some of the CNTs showed singly dispersed while most of them revealed several ones adhered with each other. No matter how they existed, the CNTs had fairly homogeneous dispersion at the sub-micron scale. Fig. 7 (b) and (d) describe dense distribution of CNTs or even cluster formation on the grain boundaries. It can be known that this kind of grain boundary derived from powder particle boundaries, i.e. the flake powder boundaries experienced metallurgical bonding then transformed to grain boundaries, thus the previous CNTs on the flakes surface pinned at these sites. This phenomenon and process also can be illustrated via Fig. 6. Fig. 8 shows the interface microstructure of the 1.0 wt.% CNTs/2009T4 composite. In Fig. 8(a), we can clearly see the tubular carbon nanotube and a rod-like phase existed. Fig. 8(c) reveals some CNTs located

at the side of this phase, while the CNTs adhered to each other and became a miniature cluster. The rod-like phase showed both amorphous zone which can be supposed to amorphous carbon and crystalline part which known as aluminum carbide [29]. This ascribes to the partially reacted of CNT and aluminum. Fig. 8(d) demonstrates the HR-TEM image of a bar-shape phase on the grain boundary in the Fig. 8(b). The inter-lamellar spacing of the phase is 0.83 nm which is consistent with the (003) lattice face of Al4C3. The matrix reveals lattice fringe space of 0.202 nm corresponding to the (200) face. Besides, the correlation between Al4C3 and aluminum matrix can be found, i.e. (003) Al4C3// (200)Al, which is also confirmed by the selected area electron diffraction pattern from the inset in Fig. 8(b). A great deal of papers regarding CNTs/Al composites have reported the existence of Al4C3 phase. Carbon nanotube can be seen as a tube which rolls from the basal plane of graphite. In theory, the reactivity of CNTs is quite low (the surface energy is 0.15 J/m2 [30]). However, with the CVD preparation method, there are defects in CNTs usually, together with the damage during dispersion process, the reactivity between CNTs and aluminum has been improved. Because of the limited supply of carbon source (the carbon nanotube is around tens of nanometer), the size of reaction products, Al4C3, is always at nanoscale. Controlling the interface structure is equally as important as the dispersion of CNTs themselves for CNT-Al composites. Generally, CNT and aluminum is a non-wettable system (the contact angle is ~127° [18]), while the generation of carbide reduces the contact angle remarkably (~45°)

Fig. 8. HR-TEM images of 1.0 wt.% CNTs/Al2009-T4 composite: (a) and (b) Al4C3, (c) and (d) the interface between CNT and Al2009.

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Fig. 9. Raman spectrum of raw CNTs, 1.0 wt.% CNTs/2009 as-milled powder and as-T4 composite.

[31], then the wettability boosted. In other words, the Al and CNTs are able to link together, the formation of covalent bonds (nano-Al4C3) enhance the interfacial shear resistance, resulting in the load to be efficiently transferred from the Al matrix to the CNTs reinforcement [32, 33].

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The ID/IG of as-received CNTs, 1.0 wt.% CNTs/2009 powder and T4 state composite were 0.71, 1.33 and 1.17 respectively. Furthermore, the latter two samples show an up-shift in G-band. It indicates the defects increased and a higher degree of disorder in CNTs [35]. In milling process, the intense collision may cause a certain extent of deterioration in tube walls, and the markedly shortening of CNTs resulted in numerous open tips. Therefore, a rising D-band intensity was measured in the milled powder and CNTs/2009 composite compared with raw CNTs. However, it should be noted that ID/IG of the composite decreased in comparison with the milling powder. In the previous research, we usually detected a corresponding (or even a little deteriorated) defects degree in the composites compared to the compound powder. From the Raman curve of 1.0 wt.% CNTs/2009-T4 composite, we can also observe a small peak with Raman shift of ~850 cm−1, which assigned to the formation of Al4C3 [24,32,39]. It can be supposed that in the consolidation process (HIP and hot extrusion), the heavily damaged CNTs reacted with aluminum and formed carbides, i.e. the amount of CNTs with higher defects declined, thereby the lower defects ones correspondingly raised up, so the Raman scattering intensity enhanced. Raman spectroscopy originates from the vibration and rotation of the molecules, which is quite sensitive and trace amount of substance can be detected. In contrast, XRD is the transition radiation of the inner electrons in atoms, and its intensity is proportional to the content of the materials. When the phase amount is minimal, the signal is always shaded. That is the reason Al4C3 was not detected in Fig. 4. 3.5. Mechanical properties

3.4. Raman spectrum Fig. 9 shows the Raman spectrum of structure evolution in CNTs during the composites fabrication process. Two significant peaks can be seen from the Raman spectrum curves, i.e. the G-band and D-band with Raman shift of ~ 1580 cm− 1 and ~ 1350 cm−1 respectively. The G-band origins from the vibration mode corresponding to the movement in opposite directions of two neighboring carbon atoms in a graphite sheet, which is a good measure of the graphitization of the sample. D-band usually assigned to the presence of disorder in graphic materials. The disorder in CNT can be ascribed to the structural defects in it (the vacancies, dopant atoms and pentagon-heptagon rings), nano-scaled graphic sheets and other allotropies of carbon (sp3 or incomplete sp2 hybridization of carbon atoms) [34–36]. Generally, the lower ratio of peak intensity between D and G band (ID/IG), the higher graphitization and more integrated structure of CNTs [37,38]illustrated.

Fig. 10 shows the mechanical properties of CNTs/2009-T4 composites. The ultimate tensile strength, yield strength and elongation of Al2009 alloy were 448 MPa, 357.3 MPa and 14.4% respectively. With the CNTs content increasing, the strength of the composites firstly showed an upward trend then declined, while the elongation displayed a gradually descent. 1.0 wt.% CNTs/2009 composite revealed the maximum ultimate tensile and yield strength (560 MPa and 443.3 MPa), which were 25% and 24% improved compared to the Al2009 matrix. At the same time, it still exhibited good ductility with elongation of 10.2%. With a further exploration of Fig. 10(b), it can be found that with the CNTs content increased from zero to 0.7 wt.%, the strength of the composite boosted dramatically, whereas the upward trend was slight when this loading went from 0.7 wt.% up to 1.0 wt.%. The main reason can ascribe to highly uniform distribution of CNTs and good bonding between reinforcement and matrix when its content under 0.7 wt.%. As for the 1.0 wt.% CNTs/2009 composite, a few of clusters

Fig. 10. Mechanical properties of CNTs/2009-T4 composites with different content of CNTs, showing (a) the engineering stress - strain curves, (b) variation of tensile properties with addition of CNTs.

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Table 2 Tensile properties of CNTs/2 × × × composites. Materials

The main preparation methods

σb/MPa

σ0.2/MPa

δ/%

Ref.

1.0 1.0 1.0 4.0

Cryogenic milling Friction stir processing Mechanical stirring, ultrasonic dispersion and ball milling Molecular level mixing

560 ± 5.2 477 474 601 ± 15

443.3 ± 0.6 385 336 450 ± 21

10.2 ± 1.2 8 3 not reported

This work [45] [40] [46]

wt.% CNTs/2009 (T4) wt.% CNTs/2009 (T4) wt.% CNTs/2024 vol.% CNTs/Al-4Cu (T6)

formed (Fig. 5) and the resulting defects (weak bonding in CNTs and aluminum, or micro-voids) are detrimental to the mechanical properties. Even though, the strength effect of the CNTs is prior at this point, so the tensile strength of composite still show an increase. When the CNTs content reaches at 1.5 wt.%, the number of clusters arise, the detrimental effect become more dominant, therefore the mechanical properties begin to decline, especially the sharply decrease in elongation. The CNTs/2009 composites demonstrate elastic modulus of 72GPa, which is similar to the aluminum alloy. With the variation of CNTs content, the elastic modulus shows nearly constant. The elastic modulus of composites can be roughly calculated by the rule of mixture (ROM): EC ¼ ECNT V CNT þ Em V Where Ec, ECNT and Em are the elastic modulus of the composite, CNTs and matrix respectively, VCNT and Vm are the volume fraction of the CNTs and matrix. According to the estimation, the elastic modulus of composites should be enhanced, especially with relative good dispersion and low damage of the CNTs (verified by microstructures in the previous results). From the published papers, some researchers reported significantly enhancement of elastic modulus (79–93 GPa) [15,29,40] as well as nearly no difference [37,41–43] between the composites and matrix aluminum alloys. Stein et al. [37] have ascribed little reinforcement of elastic modulus to significant damage of the CNTs. However,

it may be not tenable in this work. It also has been found that no increment of elastic modulus in the graphene reinforced aluminum matrix composites prepared by cryomilling [44]. A study on this problem is underway by the authors. Table 2 compares the tensile properties of 1.0 wt.% CNTs/2009 composite in this work with those reported CNTs/2 × × × composites. In this work, the content of reinforcement, matrix alloy and heat treatment are corresponding to Liu's et al. [45] study, while the former shows much higher ultimate tensile and yield strength as well as elongation. The main reasons attribute to excellent dispersity and low damage of CNTs via cryogenic milling. For the dispersion of CNTs in copper (or metals can be reduced from their oxides easily) alloy, the molecule level mixing [47] is an effective routine. However, aluminum is quite active, thus the Al2O3 cannot be reduced with H2 etc., which inhibits the application of this method in preparing aluminum matrix composites to some extent. In Nam's et al. [46] study, the volume fraction of CNTs is 4% (equal to ~ 2.6 wt.%, where the ρMWCNT = 1.8 g/cm3 [3], ρAl-4Cu = 2.8 g/cm3 [48]) and the composite is artificial aged, which means a higher strength should be achieved for this composite itself. Even though, our experiment shows similar yield strength, and only 7% less in ultimate tensile strength. Moreover, the deviation of strength in this research is minimal for every tested sample. It can be concluded that cryogenic milling is quite promising in fabrication of carbon nanotubes reinforced composites. It also should be noted that if well controlled in hot working

Fig. 11. SEM fractography of 1.0 wt.% CNTs/2009-T4 composite after tensile test: (a) and (b) pulled-out and bridged CNTs, (c) and (d) CNTs agglomerations on fracture surface.

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procedures, the nanocrystal grains formation in cryomilling can be retained in composites. Generally, nanocrystalline materials usually exhibit limited ductility, while CNTs have good deformation capacity, which can improve the elongation [42]. Overall, considering these factors as a whole, high strength and good ductility bulk nanocrystalline materials with homogeneous CNTs distribution can be obtained. Fig. 11 illustrates the SEM fractographs of 1.0 wt.% CNTs/2009-T4 composite. Relative rough fracture surfaces comprising tear ridges which mingled with a few microscopic voids can be viewed. On the side of the tear ridges, scores of fine ductile dimples exist. Since lots of CNTs located on the grain boundaries (the previous particle boundaries), growth and coalescence of micro-voids were inhibited. Fig. 11(b) shows CNTs with different morphology on the surface: pullingout or bridging during loading, and trapping in dimples, which indicate good interface bonding between CNTs and aluminum, as well as effectiveness in load transfer and crack reflection of CNTs [41]. Fig. 11(c) and (d) also reveal the clusters of CNTs impairing the bonding strength in composite and aggravating the crack propagation. Therefore, how to further improve the dispersion of CNTs in metal matrix is still an urgent issue. 4. Conclusions (1) The shortening and homogeneous distribution of CNTs in aluminum powder can be achieved in a short time through cryogenic milling. Carbon nanotube itself functioned as a PCA reduced the cold welding of metal powder in milling process since its good self-lubricity. (2) After cryomilling, the majority of CNTs retained well in consolidation process, only a few heavily damaged CNTs reacted with aluminum and formed Al4C3. The generation of nano-sized carbides increased the wettability of carbon nanotube and aluminum, which boosted the interface bonding strength. (3) The dispersity of CNTs in composites decreased with the CNTs content increasing. A few micro-clusters can be observed in 1.0 wt.% CNTs/2009 composite, further adding of CNTs resulted in significant deterioration of their dispersion. The CNTs mainly distributed on the grain boundaries, which markedly inhibited the grain growth in hot working procedures. (4) 1.0 wt.% CNTs/2009-T4 composite exhibited excellent tensile properties. Its ultimate tensile and yield strength were 560 MPa and 443.3 MPa respectively, which enhanced 25% and 24% compared to the matrix alloy. At the same time, it still had good ductility with elongation of 10.2%. Carbon nanotubes effectively transferred force and reflected cracks under loading, which showed pulling-out, bridging and trapping in dimples on the fracture surfaces. Cryogenic milling is a promising method to prepare homogeneously distributed carbon nanotubes reinforced composites with high performance. Acknowledgements The authors are thankful to financial support of Key Laboratory of Advanced Composite Materials Foundation, and also grateful to Dr. Jiongli Li for the use of his milling machine. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.matdes.2016.11.008. References [1] M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, Exceptionally high Young's modulus observed for individual carbon nanotubes, Nature 381 (1996) 678–680. [2] B.I. Yakobson, P. Avouris, Mechanical properties of carbon nanotubes, Carbon Nanotubes, Springer 2001, pp. 287–327. [3] R. George, K.T. Kashyap, R. Rahul, S. Yamdagni, Strengthening in carbon nanotube/ aluminium (CNT/Al) composites, Scr. Mater. 53 (10) (2005) 1159–1163.

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