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Cu-Ti composites fabricated by Molecular Level Mixing
Journal of Alloys and Compounds 726 (2017) 81e87
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Well-dispersion of CNTs and enhanced mechanical properties in CNTs/ Cu-Ti composites fabricated by Molecular Level Mixing Liang Liu, Rui Bao*, Jianhong Yi**, Caiju Li, Jingmei Tao, Yichun Liu, Songlin Tan, Xin You Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming, 650093, China
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a b s t r a c t
Article history: Received 30 May 2017 Received in revised form 11 July 2017 Accepted 28 July 2017 Available online 31 July 2017
Micron-sized CNTs/Cu2O composite powder prepared by Molecular Level Mixing (MLM) was mixed with flaked Cu-1.0 wt% Ti (Cu-1.0 Ti) matrix powders by low-energy ball milling. After reduction and hot pressing, CNTs/Cu-Ti composites were fabricated. Microstructure and mechanical properties of the composites were characterized by SEM, EDS, HRTEM, hardness and tensile tests, etc. The results showed that mechanical properties of the CNTs/Cu-Ti composites were enhanced compared with alloy matrix, which were ascribed to the well-distributed of CNTs and strong bonding interface with the matrix by the formation of a transition layer of TiC. Finally, strengthening mechanisms were discussed. © 2017 Elsevier B.V. All rights reserved.
Keywords: CNTs Molecular level mixing Interface microstructure Mechanical properties
1. Introduction Advanced metal matrix composites (MMCs) like copper (Cu), aluminium (Al) reinforced by carbon nanotubes (CNTs) have been investigated intensively in the past few years [1e3]. However, fabrication of CNTs reinforces MMCs remains a challenge for many reasons [4,5]. To start with, a leaning towards agglomerates of CNTs impedes the densification during the sintering process in a powder metallurgy technique routine [6], which deteriorates the mechanical and electrical properties of composites severely [7,8]. Secondly, weak interfacial bonding between CNTs and Cu matrix will impede the load transfer responsibility effectively of the CNTs as a reinforcement [9e11]. Some studies have been concentrating on addressing these issues. For example, Tan et al. [12] presented a method named Electrostatic Adsorption (EA) to disperse CNTs effectively. Hisashi et al. [13] mixed CNTs, Cu powders and amphoteric surfactant in solution and ultrasonic blended to obtain a homogeneous mixture. Kwon et al. [14] found that Al4C3 compounds generated in the interface of CNTs and Al, and the strength of CNTs/Al composite has
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (R. Bao), [email protected] (J. Yi). http://dx.doi.org/10.1016/j.jallcom.2017.07.297 0925-8388/© 2017 Elsevier B.V. All rights reserved.
been improved greatly. However, these experiments introduced organic functional groups on the surface of flaked powders by means of imidazoline and polyvinyl alcohol, which would decrease the densification of the sintered bulks. Moreover, an interface reaction of CNTs and Cu binary system cannot take place and the large coefficient of thermal expansion (CTE) mismatch between Cu and CNTs will deteriorate the interface bonding strength [11,15]. Recently, Molecular Level Mixing (MLM) method was successfully employed due to the fact that the method not only achieves uniform dispersion of CNTs and good wetting with Cu oxide powders but also avoids the structural damage of CNTs during ball milling process [16e19]. In addition, the carbide forming elements like titanium (Ti), chromium (Cr), etc. can enhance the wetting between Cu and CNTs, this is beneficial to the interface bonding [11,20]. However, limited studies are paid attention to the synergistic effect of the MLM and the interface modification in the process of preparing CNTs/Cu composites. Hence, synergism between the MLM and Ti element introducing was employed in this study to improve the dispersion of CNTs and improve the interfacial bonding between CNTs and matrix. Furthermore, mechanical properties of the CNTs/Cu-Ti composites were measured and compared with the performances of composites prepared by other methods to evaluate the effectiveness of the synergism between the MLM and the addition of Ti.
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2. Experimental details
speed of 0.1 mm/min at room temperature.
2.1. Pretreatment of CNTs
3. Result and discussion
The pristine CNTs (purity 99.9%, O.D. I.D. L: 15 nm ± 5 nm 5.5 nm ± 1.5 nm 2 mm ± 0.5 mm, Chengdu Organic Chemistry Co. Ltd., China) was impregnated in 3:1(v/v) mixture containing concentrated sulfuric acid (H2SO4) (98%)/nitric acid(HNO3) (61%) and heated in a water bath at 333 K for 4 h. The resulting suspension was diluted with deionized water until the PH ¼ 7.0. Subsequently, they were dried thoroughly in an oven at 323 K for 24 h. 2.2. Preparation of micron-sized CNTs/Cu2O composite powders CNTs was dispersed in deionized water by ultrasonic dispersion to get an ink-like solution. Copper acetate monohydrate (CuAc) was dissolved in water and magnetically stirred for 30 min. After that, CNTs solution was added into CuAc solution and the mixed solution was heated at 348 K in a water bath for 5 min, then 4 M NaOH solution was injected. After heating and stirring for 10min, 2 M glucose solution was added into the mixture. When the color of the mixture transformed into brick red, stopped stirring. Finally, micron-sized CNTs/Cu2O composite powders was obtained after filtering and vacuum drying. 2.3. Preparation of CNTs/Cu-Ti alloy composite powders Cu-1.0 wt% Ti (Cu-1.0 Ti) alloy powders (Ganzhou Jingke Technology Co. Ltd., China) produced by gas atomization was employed in this study. Cu-1.0Ti alloy powders was impregnated in 300 ml alcohol and ball-milled (ⅠⅠ) for 10 h, 300 rotating speed per minute in an one-way manner. The ratio of ball to powders was 10:1. After filtering and drying, flaked alloy powders was obtained. The micron-sized CNTs/Cu2O composite powders with different content of CNTs was mixed with the flaked alloy powders by low-energy ball milling(Ⅱ Ⅱ) for 2 h. In this research, according to the different mass fraction (wt.%) of CNTs in the mixed powder, the samples were labeled as 0%, 0.2%, 0.4%, 0.6%, 0.8%, 1.0%. After that, CNTs/CuTi composite powders was obtained by reduction at 573 K for 6 h under hydrogen atmosphere. 2.4. Hot pressing CNTs/Cu-Ti composite powders was put into a cylindrical graphite die with an inner diameter of 30 mm. The compact powders were sintered at 1073 K for 30 min in a vacuum atmosphere. The heating rate was 10 K/min and a pressure of 50 MPa was implemented from the start to the end of the sintering process. For comparison purposes, the sintered Cu-Ti alloy specimen without CNTs was also fabricated under the same processing conditions. 2.5. Material characterization The morphologies of the raw alloy materials, CNTs/Cu-Ti composites, fracture surface and the interface microstructure of CNTs/ Cu-Ti composites were observed by field-emission scanning electron microscopy (FE-SEM, Nova Nano-450, FEI) and transmission electron microscopy (TEM, Tecnai G2-TF30, S-Twin). The density of samples was measured by the principle of Archimedes. Hardness test was carried out using a Vickers tester (MC010, Yanrun Company, Shanghai) at a load of 0.98 N with a dwell time of 15 s. Tensile test was measured with a AG-IS10KN apparatus under a crosshead
3.1. Raw materials To improve the wetting between CNTs and Cu, alloying element of titanium (Ti), is contained in matrix powders. Fig. 1(a) shows the approximately spherical Cu-1.0 Ti alloy powders with an average particle size is less than 30 mm. The energy dispersive spectroscopy (EDS) element mapping shows that Ti disperses uniformly in the metal matrix. After ball milling for 10 h (the first time, Ⅰ), the pristine alloy powder is transformed into flaked powders (in Fig. 1(b)) with small thickness (<2 mm, in Fig. 1(c)). These flaked powders have high apparent volumes than spherical powders, which can realize high compatibilities of matrix powders with CNTs in terms of both surface properties and geometries [7,10,12]. 3.2. Composite powders Fig. 2(a) displays the color change of CNTs/Cu2O composite powders with 0.8 wt% CNTs prepared by the MLM. With the addition of NaOH solution, the color of the mixture changed from black to blue, and CNTs/Cu(OH)2 flocculent precipitation appeared at the bottom of the bottle. After dehydration reaction, CNTs/ Cu(OH)2 transformed into gray-black CNTs/CuO gradually, and CNTs/Cu2O appeared by the reduction of glucose. The basic reaction principle is shown as Equations (1) and (2) and the phase composition of the reaction products is illustrated in Fig. 2(g). Cu2þ þ 2OH / CuO þ H2O
(1)
2CuO þ RCHO / Cu2O þ RCOOH
(2)
Fig. 2(b) shows the morphology of CNTs/Cu2O composite powders, the size distribution is uniform (1~2 mm) and the shape is like regular cubes. It is worth noting that CNTs is embedded in Cu2O and even has some CNTs is like the bridges that connect the two adjacent Cu2O particles together (in Fig. 2(c)). CNTs disperses uniformly and implants into the Cu2O matrix deeply, as shown in Fig. 2(d). A reasonable explanation is that defect sites as well as some dangling carbon atoms on the wall of processed CNTs provided plenty of nucleation sites for Cu2O, CNTs embedded into Cu2O grains gradually after the grains growing up. All above results suggest that the composite powders prepared by the MLM can effectively disperse CNTs and achieve better interface bonding with Cu2O. In the MLM process, CNTs/Cu2O powders was fabricated as an intermediate product prior to obtaining CNTs/Cu-Ti composites. Such an intermediate product plays an important role for distributing evenly of CNTs. As noted in the above analysis, CNTs was mixed with the flaked alloy powders in the form of micron-sized composite powders (in Fig. 2(e)) rather than easily agglomerated CNTs, the difference of density and size between CNTs and matrix can be narrowed by this process. On the other hand, the flaked Cu-1.0 Ti alloy powders possesses large apparent volume and contact area [10], thus, a good dispersion of CNTs/Cu2O composite powders and high compatibilities of flaked powders with CNTs/Cu2O powders has be achieved after the low-energy ball milling (Ⅱ Ⅱ). The implant-type structure of CNTs and matrix is maintained after reducing the mixed powders, the morphology and the phase composition of the reductive mixed powders are shown in Fig. 2(f) and (g). Since the content of Ti and CNTs in the composite powder has a small mass ratio, there is no obvious peak intensity in the XRD pattern.
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Fig. 1. SEM images of (a) Cu-1.0 Ti alloy powders with EDS element mapping, (b) Cu-1.0 Ti alloy powders, and (c) the thickness measurement of the powders after the first ball milling (ⅠⅠ).
3.3. CNTs/Cu-Ti alloy composites After reducing for 6 h, the mixed powders were consolidated by hot pressing. In order to allow the diffusion of the solute Ti atoms into the surface of CNTs, the matrix and CNTs must be contacted directly and the relative density of consolidated composites must be high enough [1,11]. Fig. 3 shows the relative density of CNTs/CuTi composites sintered at 1073 K as a function of the wt.% of CNTs. The relative density of consolidated samples is about 97% in the case where the wt.% of CNTs is less than 0.8%. However, the relative density of the consolidated sample decreases when the wt.% of CNTs reaches to 1.0%. Interestingly, the measurement of microhardness has similar trends with the relative density. It is reasonable to assume that the decrease of relative density attributes to the agglomeration of CNTs when the wt.% reaches to 1.0% [17,21]. To verify the assumption, the metallographic analysis (longitudinal section, parallel to the pressure axis) of 0.8% and 1.0% samples are shown in Fig. 4(a) and (b). Since matrix powders is flaked observed in SEM, the sintered composites have distinct lamellar structure. In addition, there are many black spots in the 1.0% sample, which attribute to the agglomeration of CNTs leading to the composites cannot be under densification. On the contrary, this phenomenon is not obvious in the 0.8% sample. Fig. 4(c) shows the surface morphology of 0.8% CNTs/Cu-Ti alloy composite. The images are obtained in the nominal plane of the hot compression axis. The rough surface, especially the position of the grain boundary is a result of chemical etch. The magnified image in Fig. 4(d) shows that CNTs are dispersed individually in the grain boundary of the matrix and a shape change of CNTs (white arrows in Fig. 4(d)) is observed, indicating the possibility of the interface reaction and the
formation of carbide [11]. The detailed microstructures of 0.8% CNTs/CueTi composites are investigated by HRTEM (Fig. 5). Although there is no visible gap at CNTs/CueTi interface, the interface boundary can be clearly distinguished, as shown the white dotted line. As seen in Fig. 5(a), CNTs shows an irregular morphology of partially destroyed graphite layers in CueTi matrix, suggesting that the structural change of CNTs occurs after sintering. HRTEM image in Fig. 5(a) shows the existence of a transition interfacial layer with the thickness of 10 nm, which is attached to the CNTs surface closely. Further inspection (Fig. 5(a) inset) indicates the interfacial layer zone displaying the lattice fringes of the titanium carbide (TiC) (111) face with a spacing of 0.26 nm. The typical Fast Fourier Transform (FFT) image (Fig. 5(a) inset) shows some diffraction spots, indicating that the interfacial layer has a crystalline structure. The indexation of the diffraction spots shows the presence of TiC, which has a face-centered cubic structure of space group Fm-3m corresponding to a unit cell parameters of a ¼ 0.46 nm [15]. Both the analysis of the lattice fringes and the FFT suggest that the interfacial phase is TiC. Interestingly, it is extremely difficult to detect the lattice fringes of CNTs in the interface zone of the composite in Fig. 5(b), and the diffraction pattern reveals the amorphous-like structure of the CNTs. Similar to Fig. 5(a), there is also a transition layer with a thickness of 12 nm at the interface between CNTs and matrix. By analyzing the diffraction pattern of the FFT, it can be determined that the interfacial transition layer is TiC. In a recent study of Chen et al. [9], even all CNTs in metal matrix composites (AMCs) was transformed to Al4C3nanorods under SPS temperature of 903 K (95% Tm) holding for 120 min. In summary, reaction situations can
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Fig. 2. Images of varied solution color during the preparation process (a), SEM microstructures of 0.8% CNTs/Cu2O composite powders: (b) morphology of composite powders in low magnifications, (c) the “bridging phenomenon” of CNTs between two adjacent Cu2O particles, (d) CNTs embedded in the Cu2O matrix deeply, (e) 0.8% CNTs/Cu2O composite powders mixed with flaked Cu-1.0 Ti alloy powders by the low-energy ball milling (Ⅱ Ⅱ), (f) CNTs implant in the matrix after reducing the mixed powders and (g) XRD pattern of 0.8% CNTs/ Cu2O composite powders and the mixed powders reduced at 573 K for 6 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
be divided into two categories considering the structural damage state of CNTs. The slightly defective CNTs which has highly crystalline multi-walled structure are found still stable in the composite; however, severely-damaged CNTs which has poorly crystalline multi-walled structure are found easy to completely transform to carbide nanorods at high temperatures, which has been observed in many studies [9,22e26]. It is needed to be pointed out that due to the CNTs is a nano-size precursor for carbide formation, the TiC phase attached to the non-crystalline CNTs would be also a nano-products and scattered in the matrix [27]. 3.4. Mechanical properties and the strengthening mechanisms Fig. 6(a) shows the engineering stress-strain curves of 0%, 0.8% and 1.0% CNTs/Cu-Ti alloy composites. Ultimate tensile strength (UTS) of Cu-Ti alloy matrix (0% sample) is 421 MPa, about 2 times that of pure Cu which was fabricated by Kim with similar
processing [28]. Obviously, Ti plays a role in solid solution strengthening, but this is also at the expense of Cu plasticity in some extent, thus, the elongation (EL) of CueTi alloy is only 8.33%. However, 0.8% CNTs/Cu-Ti alloy composites significantly improve the UTS and EL compared with 0% sample, the UTS and EL increases by about 35.15% and 41.74%, respectively. In addition, the elastic modulus (EM) of 0.8% CNTs/Cu-Ti alloy composites has also increased. Fig. 6(b) shows the fracture surfaces of 0.8% CNTs/Cu-Ti alloy composites after the tensile test. It is obvious that lots of dimples which are associated with ductile fracture display on the fracture surfaces, and CNTs is pulled out from the matrix. However, when the wt.% of CNTs reaches to 1.0%, the UTS and EL decrease significantly. A reasonable explanation is that CNTs clusters (as shown in Fig. 6(c)) give rise to the degradation in the densification, and the result is consistent with the metallographic analysis (in Fig. 4(a)). Compared with the Cue1.0 Ti alloy matrix, 0.8% CNTs/Cu-Ti alloy
L. Liu et al. / Journal of Alloys and Compounds 726 (2017) 81e87
sc ¼ sm þ Vf 1 þ
Fig. 3. Relative density and micro-hardness of the sintered composites as a function of the CNTs content.
composites can significantly improve the yield strength (YS) and the UTS. The reason can be accounted for from two aspects. TiC interfacial transition layer forms at the defective sites on the surface of CNTs, which tightly locks the tube walls inside the matrix [29]. The formation of TiC interfacial layer greatly improves the interfacial bonding between CNTs and matrix which responsible to a stress transfer mechanism inside the CNTs/CueTi alloy composites during loading. According to the model for the strength of CNTs composite related to the interfacial region, which has the following equation [1,30]:
2b d
85
l d
ss 1 þ
2b sm d
(3)
where ss is the shear strength of the interface; sc and sm are the yield strength of CNTs composite and alloy matrix, the values are 400 MPa and 325 MPa, respectively. Vf is the volume fraction of CNTs, 4.0% for this study; b and d are the thickness of the interfacial region and the diameter of CNTs, 10 nm and 15 nm, respectively. Given these materials parameters, ss can be back-calculated as 14.5 MPa for CNTs/CueTi composites at 4.0 vol% CNTs loading based on the yield strength measurements. In this study, the interface strength of 0.8% CNTs/Cu-Ti alloy composites exceeds that of CNTs/ Cu composites by approximately 50% fabricated by Chu [1]. Hence, it is concluded that the formation of an intermediate TiC transition layer between CNTs and matrix greatly improves the interfacial bonding of CNT/CueTi composites [20], which in turn helps to distribute the external load from matrix to CNTs and further enhances the strength of the composites. In addition, in the area where CNTs is severely damaged, the formation of carbides destroys the multi-walled structure of the CNTs completely. However, due to the CNTs is a nano-size precursor for carbide formation, the TiC phase attached to the non-crystalline CNTs would be also a nano-products and scattered in the matrix [31]. These fine precipitate could strengthen alloys matrix by the precipitation hardening mechanism [27]. The “bridge mechanism” of CNTs observed in Fig. 2(c) may be the main reasons for improving the EL of the CNTs/CueTi alloy composites [32]. Compared with the research of K. Chu [15], CNTs/CueTi alloy composites in this research display higher yield strength than the alloy composites fabricated by ball milled, as shown in Table 1. These superior performance is fully demonstrated that CNTs/CueTi alloy composites
Fig. 4. Metallographic images in longitudinal section of (a) 0.8% sample and (b) 1.0% sample, (c) SEM images of 0.8% sample in the nominal plane of the hot compression axis and (d) is the enlarged image of (c) in white dotted box.
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Fig. 5. Microstructure of the 0.8% CNTs/CueTi composites: (a) HRTEM image of the composite interface with partially destroyed graphene layers of CNTs (b) HRTEM image of the composite interface with non-crystalline CNTs; and the insets in (a), (b) were obtained by FFT acquired from the marked areas.
Fig. 6. Stressestrain curve of 0%, 0.8% and 1.0% CNTs/Cu-Ti alloy composites (a) and fracture surfaces of 0.8%, 1.0% CNTs/Cu-Ti alloy composites after the tensile test (b), (c).
are successfully fabricated by the synergistic effect of the MLM and the introducing of Ti in the process of preparing CNTs/Cu composites which are favorable for the uniform dispersion of CNTs and strong interfacial bonding.
4. Conclusion In summary, CNTs/CueTi alloy composites with individual distribution of CNTs are fabricated by the synergistic effect of the MLM
L. Liu et al. / Journal of Alloys and Compounds 726 (2017) 81e87 Table 1 Comparison of the best mechanical properties for CNTs/CueTi alloy composites in this article and the research of K. Chu and Hu by ball milled and electroless plated [1,15,32].
Cu [32] CueTi alloy CNTs/CueTi CNTs/CueTi [15] CNTs/CueCr [1]
UTS (MPa)
YS (MPa)
EL (%)
Hardness (HV)
248 421 569 e e
136 325 400 365 390
19.6 8.33 11.82 e e
e 197 216 e 160
and modificatory interface of TiC precipitated phase. CNTs/Cu2O powders is prepared as an intermediate product and CNTs is mixed with the flaked alloy powders in the form of micron-sized composite powder, which can serve to disperse the CNTs and improve the compatibilities with matrix. The good interfacial bonding can be satisfactorily achieved through the formation of TiC interfacial transition layer at the interface between CNTs and alloy matrix. Better dispersion, higher compatibilities of matrix with CNTs, and improved interfacial bonding make 0.8% CNTs/CueTi composites exhibit the increments of UTS being 35.15%, in relation to CueTi alloy matrix. Therefore, the present study indicates that CNTs is an effective reinforcement to significantly improve the mechanical properties of CueTi alloy matrix by the synergistic effect of the MLM and the third phase precipitation. Acknowledgment The authors would like to thank the financial support from the Major Project of Scientific and Technical Department in Yunnan Province (2014FC001), and the Applied Basic Research Projects of Scientific and Technical Department in Yunnan Province (2015FB127). This work was partially supported by the National Natural Science Foundation of China (51401098). References [1] K. Chu, C.C. Jia, L.K. Jiang, W.S. Li, Improvement of interface and mechanical properties in carbon nanotube reinforced CueCr matrix composites, Mater. Des. 45 (2013) 407e411. [2] J. Tao, X. Chen, P. Hong, J. Yi, Microstructure and electrical conductivity of laminated Cu/CNT/Cu composites prepared by electrodeposition, J. Alloy. Compd. 717 (2017) 232e239. [3] J.L. Song, W.G. Chen, L.L. Dong, J.J. Wang, N. Deng, An electroless plating and planetary ball milling process for mechanical properties enhancement of bulk CNTs/Cu composites, J. Alloy. Compd. 720 (2017) 54e62. [4] C.N. He, N.Q. Zhao, C.S. Shi, S.Z. Song, Mechanical properties and microstructures of carbon nanotube-reinforced Al matrix composite fabricated by in situ chemical vapor deposition, J. Alloy. Compd. 487 (2009) 258e262. [5] G. Kou, L.J. Guo, Z.Q. Li, J. Peng, J. Tian, C.X. Huo, Microstructure and flexural properties of C/C-Cu composites strengthened with in-situ grown carbon nanotubes, J. Alloy. Compd. 694 (2017) 1054e1060. rez-Bustamante, M.J. Gonza lez-Ibarra, J. Gonz [6] R. Pe alez-Cantú, I. Estrada-Guel, J.M. Herrera-Ramírez, M. Miki-Yoshida, R. Martínez-S anchez, AA2024eCNTs composites by milling process after T6-temper condition, J. Alloy. Compd. 536 (2012) S17eS20. [7] L. Jiang, Z. Li, G. Fan, L. Cao, D. Zhang, The use of flake powder metallurgy to produce carbon nanotube (CNT)/aluminum composites with a homogenous CNT distribution, Carbon 50 (2012) 1993e1998. [8] L. Jiang, Z. Li, G. Fan, L. Cao, D. Zhang, Strong and ductile carbon nanotube/ aluminum bulk nanolaminated composites with two-dimensional alignment of carbon nanotubes, Scr. Mater. 66 (2012) 331e334.
87
[9] B. Chen, L. Jia, S. Li, H. Imai, M. Takahashi, K. Kondoh, In situ synthesized Al4 C3 nanorods with excellent strengthening effect in aluminum matrix composites, Adv. Eng. Mater. 16 (2014) 972e975. [10] X.Z. Kai, Z.Q. Li, G.L. Fan, Q. Guo, D.B. Xiong, W.L. Zhang, Y.S. Su, W.J. Lu, W.J. Moon, D. Zhang, Enhanced strength and ductility in particulatereinforced aluminum matrix composites fabricated by flake powder metallurgy, Mater. Sci. Eng. A 587 (2013) 46e53. [11] S. Cho, K. Kikuchi, T. Miyazaki, A. Kawasaki, Y. Arami, J.F. Silvain, Epitaxial growth of chromium carbide nanostructures on multiwalled carbon nanotubes (MWCNTs) in MWCNTecopper composites, Acta Mater. 61 (2013) 708e716. [12] Z. Tan, Z. Li, G. Fan, W. Li, Q. Liu, W. Zhang, D. Zhang, Two-dimensional distribution of carbon nanotubes in copper flake powders, Nanotechnology 22 (2011) 225603. [13] H. Imai, K. Kondoh, S. Li, J. Umeda, B. Fugetsu, M. Takahashi, Microstructural and electrical properties of copper-titanium alloy dispersed with carbon nanotubes via powder metallurgy process, J. Powd. Metall. 59 (2012) 609e615. [14] J.Z. Liao, M.J. Tan, I. Sridhar, Spark plasma sintered multi-wall carbon nanotube reinforced aluminum matrix composites, Mater. Des. 31 (2010) 96e100. [15] K. Chu, C.C. Jia, W.S. Li, P. Wang, Mechanical and electrical properties of carbon-nanotube-reinforced Cu-Ti alloy matrix composites, Phys. Sta. Sol. 210 (2013) 594e599. [16] S.I. Cha, K.T. Kim, S.N. Arshad, C.B. Mo, S.H. Hong, Extraordinary strengthening effect of carbon nanotubes in metal-matrix nanocomposites processed by molecular-level mixing, Adv. Mater. 17 (2005) 1377e1381. [17] S.I. Cha, K.T. Kim, K.H. Lee, C.B. Mo, S.H. Hong, Strengthening and toughening of carbon nanotube reinforced alumina nanocomposite fabricated by molecular level mixing process, Scr. Mater. 53 (2005) 793e797. [18] K.T. Kim, J. Eckert, G. Liu, M.P. Jin, B.K. Lim, S.H. Hong, Influence of embeddedcarbon nanotubes on the thermal properties of copper matrix nanocomposites processed by molecular-level mixing, Scr. Mater. 64 (2011) 181e184. [19] B.K. Lim, C.B. Mo, D.H. Nam, S.H. Hong, Mechanical and electrical properties of carbon nanotube/Cu nanocomposites by molecular-level mixing and controlled oxidation process, J. Nano. Sci. Nanotechnol. 10 (2010) 78e84. [20] K. Chu, C.C. Jia, W.S. Li, Thermal conductivity enhancement in carbon nanotube/CueTi composites, Appli. Phys. A 110 (2013) 269e273. [21] Z.W. Xue, L.D. Wang, P.T. Zhao, S.C. Xu, J.L. Qi, W.D. Fei, Microstructures and tensile behavior of carbon nanotubes reinforced Cu matrix composites with molecular-level dispersion, Mater. Des. 34 (2012) 298e301. [22] Z.Y. Liu, B.L. Xiao, W.G. Wang, Z.Y. Ma, Singly dispersed carbon nanotube/ aluminum composites fabricated by powder metallurgy combined with friction stir processing, Carbon 50 (2012) 1843e1852. [23] W. Zhou, S. Bang, H. Kurita, T. Miyazaki, Y. Fan, A. Kawasaki, Interface and interfacial reactions in multi-walled carbon nanotube-reinforced aluminum matrix composites, Carbon 96 (2016) 919e928. [24] C. Deng, X. Zhang, D. Wang, Chemical stability of carbon nanotubes in the 2024Al matrix, Mater. Lett. 61 (2007) 904e907. [25] S.R. Bakshi, A.K. Keshri, V. Singh, S. Seal, A. Agarwal, Interface in carbon nanotube reinforced aluminum silicon composites: thermodynamic analysis and experimental verification, J. Alloy. Compd. 481 (2009) 207e213. [26] C. Ke, C.C. Jia, W.S. Li, W. Ping, Mechanical and electrical properties of carbonnanotube-reinforced Cu-Ti alloy matrix composites, Phys. Sta. Sol. 210 (2013) 594e599. [27] L. Ci, Z. Ryu, N.Y. Jin-Phillipp, M. Rühle, Investigation of the interfacial reaction between multi-walled carbon nanotubes and aluminum, Acta Mater. 54 (2006) 5367e5375. [28] K.T. Kim, S.I. Cha, S.H. Hong, S.H. Hong, Microstructures and tensile behavior of carbon nanotube reinforced Cu matrix nanocomposites, Mater. Sci. Eng. A 430 (2006) 27e33. [29] K. Kondoh, T. Threrujirapapong, H. Imai, J. Umeda, B. Fugetsu, CNTs/TiC reinforced titanium matrix nanocomposites via powder metallurgy and its microstructural and mechanical properties, J. Nanomater. 12 (2008) 538e541. [30] J.N. Coleman, U. Khan, W.J. Blau, Y.K. Gun’ko, Small but strong: a review of the mechanical properties of carbon nanotubeepolymer composites, Carbon 44 (2006) 1624e1652. [31] E. Asmatulu, J. Twomey, M. Overcash, Life cycle and nano-products: end-oflife assessment, J. Nanopart. Res. 14 (2012) 1e8. [32] H. Wang, Z.H. Zhang, Z.Y. Hu, F.C. Wang, S.L. Li, E. Korznikov, X.C. Zhao, Y. Liu, Z.F. Liu, Z. Kang, Synergistic strengthening effect of nanocrystalline copper reinforced with carbon nanotubes, Sci. Rep. 6 (2016) 258e262.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Well-dispersion of CNTs and enhanced mechanical properties in CNTs/ Cu-Ti composites fabricated by Molecular Level Mixing Liang Liu, Rui Bao*, Jianhong Yi**, Caiju Li, Jingmei Tao, Yichun Liu, Songlin Tan, Xin You Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming, 650093, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 May 2017 Received in revised form 11 July 2017 Accepted 28 July 2017 Available online 31 July 2017
Micron-sized CNTs/Cu2O composite powder prepared by Molecular Level Mixing (MLM) was mixed with flaked Cu-1.0 wt% Ti (Cu-1.0 Ti) matrix powders by low-energy ball milling. After reduction and hot pressing, CNTs/Cu-Ti composites were fabricated. Microstructure and mechanical properties of the composites were characterized by SEM, EDS, HRTEM, hardness and tensile tests, etc. The results showed that mechanical properties of the CNTs/Cu-Ti composites were enhanced compared with alloy matrix, which were ascribed to the well-distributed of CNTs and strong bonding interface with the matrix by the formation of a transition layer of TiC. Finally, strengthening mechanisms were discussed. © 2017 Elsevier B.V. All rights reserved.
Keywords: CNTs Molecular level mixing Interface microstructure Mechanical properties
1. Introduction Advanced metal matrix composites (MMCs) like copper (Cu), aluminium (Al) reinforced by carbon nanotubes (CNTs) have been investigated intensively in the past few years [1e3]. However, fabrication of CNTs reinforces MMCs remains a challenge for many reasons [4,5]. To start with, a leaning towards agglomerates of CNTs impedes the densification during the sintering process in a powder metallurgy technique routine [6], which deteriorates the mechanical and electrical properties of composites severely [7,8]. Secondly, weak interfacial bonding between CNTs and Cu matrix will impede the load transfer responsibility effectively of the CNTs as a reinforcement [9e11]. Some studies have been concentrating on addressing these issues. For example, Tan et al. [12] presented a method named Electrostatic Adsorption (EA) to disperse CNTs effectively. Hisashi et al. [13] mixed CNTs, Cu powders and amphoteric surfactant in solution and ultrasonic blended to obtain a homogeneous mixture. Kwon et al. [14] found that Al4C3 compounds generated in the interface of CNTs and Al, and the strength of CNTs/Al composite has
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (R. Bao), [email protected] (J. Yi). http://dx.doi.org/10.1016/j.jallcom.2017.07.297 0925-8388/© 2017 Elsevier B.V. All rights reserved.
been improved greatly. However, these experiments introduced organic functional groups on the surface of flaked powders by means of imidazoline and polyvinyl alcohol, which would decrease the densification of the sintered bulks. Moreover, an interface reaction of CNTs and Cu binary system cannot take place and the large coefficient of thermal expansion (CTE) mismatch between Cu and CNTs will deteriorate the interface bonding strength [11,15]. Recently, Molecular Level Mixing (MLM) method was successfully employed due to the fact that the method not only achieves uniform dispersion of CNTs and good wetting with Cu oxide powders but also avoids the structural damage of CNTs during ball milling process [16e19]. In addition, the carbide forming elements like titanium (Ti), chromium (Cr), etc. can enhance the wetting between Cu and CNTs, this is beneficial to the interface bonding [11,20]. However, limited studies are paid attention to the synergistic effect of the MLM and the interface modification in the process of preparing CNTs/Cu composites. Hence, synergism between the MLM and Ti element introducing was employed in this study to improve the dispersion of CNTs and improve the interfacial bonding between CNTs and matrix. Furthermore, mechanical properties of the CNTs/Cu-Ti composites were measured and compared with the performances of composites prepared by other methods to evaluate the effectiveness of the synergism between the MLM and the addition of Ti.
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2. Experimental details
speed of 0.1 mm/min at room temperature.
2.1. Pretreatment of CNTs
3. Result and discussion
The pristine CNTs (purity 99.9%, O.D. I.D. L: 15 nm ± 5 nm 5.5 nm ± 1.5 nm 2 mm ± 0.5 mm, Chengdu Organic Chemistry Co. Ltd., China) was impregnated in 3:1(v/v) mixture containing concentrated sulfuric acid (H2SO4) (98%)/nitric acid(HNO3) (61%) and heated in a water bath at 333 K for 4 h. The resulting suspension was diluted with deionized water until the PH ¼ 7.0. Subsequently, they were dried thoroughly in an oven at 323 K for 24 h. 2.2. Preparation of micron-sized CNTs/Cu2O composite powders CNTs was dispersed in deionized water by ultrasonic dispersion to get an ink-like solution. Copper acetate monohydrate (CuAc) was dissolved in water and magnetically stirred for 30 min. After that, CNTs solution was added into CuAc solution and the mixed solution was heated at 348 K in a water bath for 5 min, then 4 M NaOH solution was injected. After heating and stirring for 10min, 2 M glucose solution was added into the mixture. When the color of the mixture transformed into brick red, stopped stirring. Finally, micron-sized CNTs/Cu2O composite powders was obtained after filtering and vacuum drying. 2.3. Preparation of CNTs/Cu-Ti alloy composite powders Cu-1.0 wt% Ti (Cu-1.0 Ti) alloy powders (Ganzhou Jingke Technology Co. Ltd., China) produced by gas atomization was employed in this study. Cu-1.0Ti alloy powders was impregnated in 300 ml alcohol and ball-milled (ⅠⅠ) for 10 h, 300 rotating speed per minute in an one-way manner. The ratio of ball to powders was 10:1. After filtering and drying, flaked alloy powders was obtained. The micron-sized CNTs/Cu2O composite powders with different content of CNTs was mixed with the flaked alloy powders by low-energy ball milling(Ⅱ Ⅱ) for 2 h. In this research, according to the different mass fraction (wt.%) of CNTs in the mixed powder, the samples were labeled as 0%, 0.2%, 0.4%, 0.6%, 0.8%, 1.0%. After that, CNTs/CuTi composite powders was obtained by reduction at 573 K for 6 h under hydrogen atmosphere. 2.4. Hot pressing CNTs/Cu-Ti composite powders was put into a cylindrical graphite die with an inner diameter of 30 mm. The compact powders were sintered at 1073 K for 30 min in a vacuum atmosphere. The heating rate was 10 K/min and a pressure of 50 MPa was implemented from the start to the end of the sintering process. For comparison purposes, the sintered Cu-Ti alloy specimen without CNTs was also fabricated under the same processing conditions. 2.5. Material characterization The morphologies of the raw alloy materials, CNTs/Cu-Ti composites, fracture surface and the interface microstructure of CNTs/ Cu-Ti composites were observed by field-emission scanning electron microscopy (FE-SEM, Nova Nano-450, FEI) and transmission electron microscopy (TEM, Tecnai G2-TF30, S-Twin). The density of samples was measured by the principle of Archimedes. Hardness test was carried out using a Vickers tester (MC010, Yanrun Company, Shanghai) at a load of 0.98 N with a dwell time of 15 s. Tensile test was measured with a AG-IS10KN apparatus under a crosshead
3.1. Raw materials To improve the wetting between CNTs and Cu, alloying element of titanium (Ti), is contained in matrix powders. Fig. 1(a) shows the approximately spherical Cu-1.0 Ti alloy powders with an average particle size is less than 30 mm. The energy dispersive spectroscopy (EDS) element mapping shows that Ti disperses uniformly in the metal matrix. After ball milling for 10 h (the first time, Ⅰ), the pristine alloy powder is transformed into flaked powders (in Fig. 1(b)) with small thickness (<2 mm, in Fig. 1(c)). These flaked powders have high apparent volumes than spherical powders, which can realize high compatibilities of matrix powders with CNTs in terms of both surface properties and geometries [7,10,12]. 3.2. Composite powders Fig. 2(a) displays the color change of CNTs/Cu2O composite powders with 0.8 wt% CNTs prepared by the MLM. With the addition of NaOH solution, the color of the mixture changed from black to blue, and CNTs/Cu(OH)2 flocculent precipitation appeared at the bottom of the bottle. After dehydration reaction, CNTs/ Cu(OH)2 transformed into gray-black CNTs/CuO gradually, and CNTs/Cu2O appeared by the reduction of glucose. The basic reaction principle is shown as Equations (1) and (2) and the phase composition of the reaction products is illustrated in Fig. 2(g). Cu2þ þ 2OH / CuO þ H2O
(1)
2CuO þ RCHO / Cu2O þ RCOOH
(2)
Fig. 2(b) shows the morphology of CNTs/Cu2O composite powders, the size distribution is uniform (1~2 mm) and the shape is like regular cubes. It is worth noting that CNTs is embedded in Cu2O and even has some CNTs is like the bridges that connect the two adjacent Cu2O particles together (in Fig. 2(c)). CNTs disperses uniformly and implants into the Cu2O matrix deeply, as shown in Fig. 2(d). A reasonable explanation is that defect sites as well as some dangling carbon atoms on the wall of processed CNTs provided plenty of nucleation sites for Cu2O, CNTs embedded into Cu2O grains gradually after the grains growing up. All above results suggest that the composite powders prepared by the MLM can effectively disperse CNTs and achieve better interface bonding with Cu2O. In the MLM process, CNTs/Cu2O powders was fabricated as an intermediate product prior to obtaining CNTs/Cu-Ti composites. Such an intermediate product plays an important role for distributing evenly of CNTs. As noted in the above analysis, CNTs was mixed with the flaked alloy powders in the form of micron-sized composite powders (in Fig. 2(e)) rather than easily agglomerated CNTs, the difference of density and size between CNTs and matrix can be narrowed by this process. On the other hand, the flaked Cu-1.0 Ti alloy powders possesses large apparent volume and contact area [10], thus, a good dispersion of CNTs/Cu2O composite powders and high compatibilities of flaked powders with CNTs/Cu2O powders has be achieved after the low-energy ball milling (Ⅱ Ⅱ). The implant-type structure of CNTs and matrix is maintained after reducing the mixed powders, the morphology and the phase composition of the reductive mixed powders are shown in Fig. 2(f) and (g). Since the content of Ti and CNTs in the composite powder has a small mass ratio, there is no obvious peak intensity in the XRD pattern.
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Fig. 1. SEM images of (a) Cu-1.0 Ti alloy powders with EDS element mapping, (b) Cu-1.0 Ti alloy powders, and (c) the thickness measurement of the powders after the first ball milling (ⅠⅠ).
3.3. CNTs/Cu-Ti alloy composites After reducing for 6 h, the mixed powders were consolidated by hot pressing. In order to allow the diffusion of the solute Ti atoms into the surface of CNTs, the matrix and CNTs must be contacted directly and the relative density of consolidated composites must be high enough [1,11]. Fig. 3 shows the relative density of CNTs/CuTi composites sintered at 1073 K as a function of the wt.% of CNTs. The relative density of consolidated samples is about 97% in the case where the wt.% of CNTs is less than 0.8%. However, the relative density of the consolidated sample decreases when the wt.% of CNTs reaches to 1.0%. Interestingly, the measurement of microhardness has similar trends with the relative density. It is reasonable to assume that the decrease of relative density attributes to the agglomeration of CNTs when the wt.% reaches to 1.0% [17,21]. To verify the assumption, the metallographic analysis (longitudinal section, parallel to the pressure axis) of 0.8% and 1.0% samples are shown in Fig. 4(a) and (b). Since matrix powders is flaked observed in SEM, the sintered composites have distinct lamellar structure. In addition, there are many black spots in the 1.0% sample, which attribute to the agglomeration of CNTs leading to the composites cannot be under densification. On the contrary, this phenomenon is not obvious in the 0.8% sample. Fig. 4(c) shows the surface morphology of 0.8% CNTs/Cu-Ti alloy composite. The images are obtained in the nominal plane of the hot compression axis. The rough surface, especially the position of the grain boundary is a result of chemical etch. The magnified image in Fig. 4(d) shows that CNTs are dispersed individually in the grain boundary of the matrix and a shape change of CNTs (white arrows in Fig. 4(d)) is observed, indicating the possibility of the interface reaction and the
formation of carbide [11]. The detailed microstructures of 0.8% CNTs/CueTi composites are investigated by HRTEM (Fig. 5). Although there is no visible gap at CNTs/CueTi interface, the interface boundary can be clearly distinguished, as shown the white dotted line. As seen in Fig. 5(a), CNTs shows an irregular morphology of partially destroyed graphite layers in CueTi matrix, suggesting that the structural change of CNTs occurs after sintering. HRTEM image in Fig. 5(a) shows the existence of a transition interfacial layer with the thickness of 10 nm, which is attached to the CNTs surface closely. Further inspection (Fig. 5(a) inset) indicates the interfacial layer zone displaying the lattice fringes of the titanium carbide (TiC) (111) face with a spacing of 0.26 nm. The typical Fast Fourier Transform (FFT) image (Fig. 5(a) inset) shows some diffraction spots, indicating that the interfacial layer has a crystalline structure. The indexation of the diffraction spots shows the presence of TiC, which has a face-centered cubic structure of space group Fm-3m corresponding to a unit cell parameters of a ¼ 0.46 nm [15]. Both the analysis of the lattice fringes and the FFT suggest that the interfacial phase is TiC. Interestingly, it is extremely difficult to detect the lattice fringes of CNTs in the interface zone of the composite in Fig. 5(b), and the diffraction pattern reveals the amorphous-like structure of the CNTs. Similar to Fig. 5(a), there is also a transition layer with a thickness of 12 nm at the interface between CNTs and matrix. By analyzing the diffraction pattern of the FFT, it can be determined that the interfacial transition layer is TiC. In a recent study of Chen et al. [9], even all CNTs in metal matrix composites (AMCs) was transformed to Al4C3nanorods under SPS temperature of 903 K (95% Tm) holding for 120 min. In summary, reaction situations can
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Fig. 2. Images of varied solution color during the preparation process (a), SEM microstructures of 0.8% CNTs/Cu2O composite powders: (b) morphology of composite powders in low magnifications, (c) the “bridging phenomenon” of CNTs between two adjacent Cu2O particles, (d) CNTs embedded in the Cu2O matrix deeply, (e) 0.8% CNTs/Cu2O composite powders mixed with flaked Cu-1.0 Ti alloy powders by the low-energy ball milling (Ⅱ Ⅱ), (f) CNTs implant in the matrix after reducing the mixed powders and (g) XRD pattern of 0.8% CNTs/ Cu2O composite powders and the mixed powders reduced at 573 K for 6 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
be divided into two categories considering the structural damage state of CNTs. The slightly defective CNTs which has highly crystalline multi-walled structure are found still stable in the composite; however, severely-damaged CNTs which has poorly crystalline multi-walled structure are found easy to completely transform to carbide nanorods at high temperatures, which has been observed in many studies [9,22e26]. It is needed to be pointed out that due to the CNTs is a nano-size precursor for carbide formation, the TiC phase attached to the non-crystalline CNTs would be also a nano-products and scattered in the matrix [27]. 3.4. Mechanical properties and the strengthening mechanisms Fig. 6(a) shows the engineering stress-strain curves of 0%, 0.8% and 1.0% CNTs/Cu-Ti alloy composites. Ultimate tensile strength (UTS) of Cu-Ti alloy matrix (0% sample) is 421 MPa, about 2 times that of pure Cu which was fabricated by Kim with similar
processing [28]. Obviously, Ti plays a role in solid solution strengthening, but this is also at the expense of Cu plasticity in some extent, thus, the elongation (EL) of CueTi alloy is only 8.33%. However, 0.8% CNTs/Cu-Ti alloy composites significantly improve the UTS and EL compared with 0% sample, the UTS and EL increases by about 35.15% and 41.74%, respectively. In addition, the elastic modulus (EM) of 0.8% CNTs/Cu-Ti alloy composites has also increased. Fig. 6(b) shows the fracture surfaces of 0.8% CNTs/Cu-Ti alloy composites after the tensile test. It is obvious that lots of dimples which are associated with ductile fracture display on the fracture surfaces, and CNTs is pulled out from the matrix. However, when the wt.% of CNTs reaches to 1.0%, the UTS and EL decrease significantly. A reasonable explanation is that CNTs clusters (as shown in Fig. 6(c)) give rise to the degradation in the densification, and the result is consistent with the metallographic analysis (in Fig. 4(a)). Compared with the Cue1.0 Ti alloy matrix, 0.8% CNTs/Cu-Ti alloy
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sc ¼ sm þ Vf 1 þ
Fig. 3. Relative density and micro-hardness of the sintered composites as a function of the CNTs content.
composites can significantly improve the yield strength (YS) and the UTS. The reason can be accounted for from two aspects. TiC interfacial transition layer forms at the defective sites on the surface of CNTs, which tightly locks the tube walls inside the matrix [29]. The formation of TiC interfacial layer greatly improves the interfacial bonding between CNTs and matrix which responsible to a stress transfer mechanism inside the CNTs/CueTi alloy composites during loading. According to the model for the strength of CNTs composite related to the interfacial region, which has the following equation [1,30]:
2b d
85
l d
ss 1 þ
2b sm d
(3)
where ss is the shear strength of the interface; sc and sm are the yield strength of CNTs composite and alloy matrix, the values are 400 MPa and 325 MPa, respectively. Vf is the volume fraction of CNTs, 4.0% for this study; b and d are the thickness of the interfacial region and the diameter of CNTs, 10 nm and 15 nm, respectively. Given these materials parameters, ss can be back-calculated as 14.5 MPa for CNTs/CueTi composites at 4.0 vol% CNTs loading based on the yield strength measurements. In this study, the interface strength of 0.8% CNTs/Cu-Ti alloy composites exceeds that of CNTs/ Cu composites by approximately 50% fabricated by Chu [1]. Hence, it is concluded that the formation of an intermediate TiC transition layer between CNTs and matrix greatly improves the interfacial bonding of CNT/CueTi composites [20], which in turn helps to distribute the external load from matrix to CNTs and further enhances the strength of the composites. In addition, in the area where CNTs is severely damaged, the formation of carbides destroys the multi-walled structure of the CNTs completely. However, due to the CNTs is a nano-size precursor for carbide formation, the TiC phase attached to the non-crystalline CNTs would be also a nano-products and scattered in the matrix [31]. These fine precipitate could strengthen alloys matrix by the precipitation hardening mechanism [27]. The “bridge mechanism” of CNTs observed in Fig. 2(c) may be the main reasons for improving the EL of the CNTs/CueTi alloy composites [32]. Compared with the research of K. Chu [15], CNTs/CueTi alloy composites in this research display higher yield strength than the alloy composites fabricated by ball milled, as shown in Table 1. These superior performance is fully demonstrated that CNTs/CueTi alloy composites
Fig. 4. Metallographic images in longitudinal section of (a) 0.8% sample and (b) 1.0% sample, (c) SEM images of 0.8% sample in the nominal plane of the hot compression axis and (d) is the enlarged image of (c) in white dotted box.
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Fig. 5. Microstructure of the 0.8% CNTs/CueTi composites: (a) HRTEM image of the composite interface with partially destroyed graphene layers of CNTs (b) HRTEM image of the composite interface with non-crystalline CNTs; and the insets in (a), (b) were obtained by FFT acquired from the marked areas.
Fig. 6. Stressestrain curve of 0%, 0.8% and 1.0% CNTs/Cu-Ti alloy composites (a) and fracture surfaces of 0.8%, 1.0% CNTs/Cu-Ti alloy composites after the tensile test (b), (c).
are successfully fabricated by the synergistic effect of the MLM and the introducing of Ti in the process of preparing CNTs/Cu composites which are favorable for the uniform dispersion of CNTs and strong interfacial bonding.
4. Conclusion In summary, CNTs/CueTi alloy composites with individual distribution of CNTs are fabricated by the synergistic effect of the MLM
L. Liu et al. / Journal of Alloys and Compounds 726 (2017) 81e87 Table 1 Comparison of the best mechanical properties for CNTs/CueTi alloy composites in this article and the research of K. Chu and Hu by ball milled and electroless plated [1,15,32].
Cu [32] CueTi alloy CNTs/CueTi CNTs/CueTi [15] CNTs/CueCr [1]
UTS (MPa)
YS (MPa)
EL (%)
Hardness (HV)
248 421 569 e e
136 325 400 365 390
19.6 8.33 11.82 e e
e 197 216 e 160
and modificatory interface of TiC precipitated phase. CNTs/Cu2O powders is prepared as an intermediate product and CNTs is mixed with the flaked alloy powders in the form of micron-sized composite powder, which can serve to disperse the CNTs and improve the compatibilities with matrix. The good interfacial bonding can be satisfactorily achieved through the formation of TiC interfacial transition layer at the interface between CNTs and alloy matrix. Better dispersion, higher compatibilities of matrix with CNTs, and improved interfacial bonding make 0.8% CNTs/CueTi composites exhibit the increments of UTS being 35.15%, in relation to CueTi alloy matrix. Therefore, the present study indicates that CNTs is an effective reinforcement to significantly improve the mechanical properties of CueTi alloy matrix by the synergistic effect of the MLM and the third phase precipitation. Acknowledgment The authors would like to thank the financial support from the Major Project of Scientific and Technical Department in Yunnan Province (2014FC001), and the Applied Basic Research Projects of Scientific and Technical Department in Yunnan Province (2015FB127). This work was partially supported by the National Natural Science Foundation of China (51401098). References [1] K. Chu, C.C. Jia, L.K. Jiang, W.S. Li, Improvement of interface and mechanical properties in carbon nanotube reinforced CueCr matrix composites, Mater. Des. 45 (2013) 407e411. [2] J. Tao, X. Chen, P. Hong, J. Yi, Microstructure and electrical conductivity of laminated Cu/CNT/Cu composites prepared by electrodeposition, J. Alloy. Compd. 717 (2017) 232e239. [3] J.L. Song, W.G. Chen, L.L. Dong, J.J. Wang, N. Deng, An electroless plating and planetary ball milling process for mechanical properties enhancement of bulk CNTs/Cu composites, J. Alloy. Compd. 720 (2017) 54e62. [4] C.N. He, N.Q. Zhao, C.S. Shi, S.Z. Song, Mechanical properties and microstructures of carbon nanotube-reinforced Al matrix composite fabricated by in situ chemical vapor deposition, J. Alloy. Compd. 487 (2009) 258e262. [5] G. Kou, L.J. Guo, Z.Q. Li, J. Peng, J. Tian, C.X. Huo, Microstructure and flexural properties of C/C-Cu composites strengthened with in-situ grown carbon nanotubes, J. Alloy. Compd. 694 (2017) 1054e1060. rez-Bustamante, M.J. Gonza lez-Ibarra, J. Gonz [6] R. Pe alez-Cantú, I. Estrada-Guel, J.M. Herrera-Ramírez, M. Miki-Yoshida, R. Martínez-S anchez, AA2024eCNTs composites by milling process after T6-temper condition, J. Alloy. Compd. 536 (2012) S17eS20. [7] L. Jiang, Z. Li, G. Fan, L. Cao, D. Zhang, The use of flake powder metallurgy to produce carbon nanotube (CNT)/aluminum composites with a homogenous CNT distribution, Carbon 50 (2012) 1993e1998. [8] L. Jiang, Z. Li, G. Fan, L. Cao, D. Zhang, Strong and ductile carbon nanotube/ aluminum bulk nanolaminated composites with two-dimensional alignment of carbon nanotubes, Scr. Mater. 66 (2012) 331e334.
87
[9] B. Chen, L. Jia, S. Li, H. Imai, M. Takahashi, K. Kondoh, In situ synthesized Al4 C3 nanorods with excellent strengthening effect in aluminum matrix composites, Adv. Eng. Mater. 16 (2014) 972e975. [10] X.Z. Kai, Z.Q. Li, G.L. Fan, Q. Guo, D.B. Xiong, W.L. Zhang, Y.S. Su, W.J. Lu, W.J. Moon, D. Zhang, Enhanced strength and ductility in particulatereinforced aluminum matrix composites fabricated by flake powder metallurgy, Mater. Sci. Eng. A 587 (2013) 46e53. [11] S. Cho, K. Kikuchi, T. Miyazaki, A. Kawasaki, Y. Arami, J.F. Silvain, Epitaxial growth of chromium carbide nanostructures on multiwalled carbon nanotubes (MWCNTs) in MWCNTecopper composites, Acta Mater. 61 (2013) 708e716. [12] Z. Tan, Z. Li, G. Fan, W. Li, Q. Liu, W. Zhang, D. Zhang, Two-dimensional distribution of carbon nanotubes in copper flake powders, Nanotechnology 22 (2011) 225603. [13] H. Imai, K. Kondoh, S. Li, J. Umeda, B. Fugetsu, M. Takahashi, Microstructural and electrical properties of copper-titanium alloy dispersed with carbon nanotubes via powder metallurgy process, J. Powd. Metall. 59 (2012) 609e615. [14] J.Z. Liao, M.J. Tan, I. Sridhar, Spark plasma sintered multi-wall carbon nanotube reinforced aluminum matrix composites, Mater. Des. 31 (2010) 96e100. [15] K. Chu, C.C. Jia, W.S. Li, P. Wang, Mechanical and electrical properties of carbon-nanotube-reinforced Cu-Ti alloy matrix composites, Phys. Sta. Sol. 210 (2013) 594e599. [16] S.I. Cha, K.T. Kim, S.N. Arshad, C.B. Mo, S.H. Hong, Extraordinary strengthening effect of carbon nanotubes in metal-matrix nanocomposites processed by molecular-level mixing, Adv. Mater. 17 (2005) 1377e1381. [17] S.I. Cha, K.T. Kim, K.H. Lee, C.B. Mo, S.H. Hong, Strengthening and toughening of carbon nanotube reinforced alumina nanocomposite fabricated by molecular level mixing process, Scr. Mater. 53 (2005) 793e797. [18] K.T. Kim, J. Eckert, G. Liu, M.P. Jin, B.K. Lim, S.H. Hong, Influence of embeddedcarbon nanotubes on the thermal properties of copper matrix nanocomposites processed by molecular-level mixing, Scr. Mater. 64 (2011) 181e184. [19] B.K. Lim, C.B. Mo, D.H. Nam, S.H. Hong, Mechanical and electrical properties of carbon nanotube/Cu nanocomposites by molecular-level mixing and controlled oxidation process, J. Nano. Sci. Nanotechnol. 10 (2010) 78e84. [20] K. Chu, C.C. Jia, W.S. Li, Thermal conductivity enhancement in carbon nanotube/CueTi composites, Appli. Phys. A 110 (2013) 269e273. [21] Z.W. Xue, L.D. Wang, P.T. Zhao, S.C. Xu, J.L. Qi, W.D. Fei, Microstructures and tensile behavior of carbon nanotubes reinforced Cu matrix composites with molecular-level dispersion, Mater. Des. 34 (2012) 298e301. [22] Z.Y. Liu, B.L. Xiao, W.G. Wang, Z.Y. Ma, Singly dispersed carbon nanotube/ aluminum composites fabricated by powder metallurgy combined with friction stir processing, Carbon 50 (2012) 1843e1852. [23] W. Zhou, S. Bang, H. Kurita, T. Miyazaki, Y. Fan, A. Kawasaki, Interface and interfacial reactions in multi-walled carbon nanotube-reinforced aluminum matrix composites, Carbon 96 (2016) 919e928. [24] C. Deng, X. Zhang, D. Wang, Chemical stability of carbon nanotubes in the 2024Al matrix, Mater. Lett. 61 (2007) 904e907. [25] S.R. Bakshi, A.K. Keshri, V. Singh, S. Seal, A. Agarwal, Interface in carbon nanotube reinforced aluminum silicon composites: thermodynamic analysis and experimental verification, J. Alloy. Compd. 481 (2009) 207e213. [26] C. Ke, C.C. Jia, W.S. Li, W. Ping, Mechanical and electrical properties of carbonnanotube-reinforced Cu-Ti alloy matrix composites, Phys. Sta. Sol. 210 (2013) 594e599. [27] L. Ci, Z. Ryu, N.Y. Jin-Phillipp, M. Rühle, Investigation of the interfacial reaction between multi-walled carbon nanotubes and aluminum, Acta Mater. 54 (2006) 5367e5375. [28] K.T. Kim, S.I. Cha, S.H. Hong, S.H. Hong, Microstructures and tensile behavior of carbon nanotube reinforced Cu matrix nanocomposites, Mater. Sci. Eng. A 430 (2006) 27e33. [29] K. Kondoh, T. Threrujirapapong, H. Imai, J. Umeda, B. Fugetsu, CNTs/TiC reinforced titanium matrix nanocomposites via powder metallurgy and its microstructural and mechanical properties, J. Nanomater. 12 (2008) 538e541. [30] J.N. Coleman, U. Khan, W.J. Blau, Y.K. Gun’ko, Small but strong: a review of the mechanical properties of carbon nanotubeepolymer composites, Carbon 44 (2006) 1624e1652. [31] E. Asmatulu, J. Twomey, M. Overcash, Life cycle and nano-products: end-oflife assessment, J. Nanopart. Res. 14 (2012) 1e8. [32] H. Wang, Z.H. Zhang, Z.Y. Hu, F.C. Wang, S.L. Li, E. Korznikov, X.C. Zhao, Y. Liu, Z.F. Liu, Z. Kang, Synergistic strengthening effect of nanocrystalline copper reinforced with carbon nanotubes, Sci. Rep. 6 (2016) 258e262.