Micron 65 (2014) 10–14

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Interfacial microstructure and growth mechanism of Al4 C3 in Grf /Al composites fabricated by liquid pressure method Wang Xu a,∗ , Wang Chenchong b , Zhang Zhichao a , Liang Ping a , Shi Yanhua a , Zhang Guofu a a b

School of Mechanical Engineering, Liaoning Shihua University, Fushun, PR China School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, PR China

a r t i c l e

i n f o

Article history: Received 17 March 2014 Received in revised form 8 April 2014 Accepted 8 April 2014 Available online 19 April 2014 Keywords: Graphite fiber Aluminum matrix composites Diffusion layer Al4 C3

a b s t r a c t In this study, Grf /Al composite was fabricated by liquid pressure method. The diffusion layer and the nucleation and growth of Al4 C3 were observed at the interface of Grf /Al composites by TEM and HRTEM. The growth mechanism of Al4 C3 was analyzed in detail by crystallography theory. It was found that Al4 C3 had no phase relations with the carbon fiber. (0 0 0 1) layer of Al4 C3 was parallel with main growth direction. Both the diffusion layer at the interface and crystal structure of Al4 C3 affected the shape of Al4 C3 . At a certain position, Al4 C3 could connect two fibers when the fibers were close to each other. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Recently, fiber-reinforced (especially continuous carbon fiber) aluminum matrix composites have been receiving considerable attention due to their high specific strength and modulus for application in automobile, aerospace, and electrical cable industries (Daoud, 2005; Parakalan et al., 2011; Xiu et al., 2013; Suzuki and Umehara, 1997). Many studies have been conducted on the interfacial microstructure of Cf/Al composites and it is well established that interface reaction has great effect on the property of Cf/Al composites (Zhang and Wu, 2010; Jung et al., 2013; Montagniero Hochard, 2013). Metal matrix composites have different degrees of interface reaction during the manufacturing process. Slight interfacial reaction can effectively improve wetting and combination between metal matrix and reinforcement. Serious interfacial reaction will cause damage to the reinforcement and form brittle interface, which can affect the mechanical properties of composites. Therefore, the study of the interface structure is essential for composite materials. During the preparation process of carbon fiber reinforced metal matrix composites by liquid pressure method, the high temperature will lead to the reaction between C and Al at the interface. This reaction will lead to excessive combination of the interface.

∗ Corresponding author. Tel.: +86 2456868042. E-mail address: [email protected] (W. Xu). http://dx.doi.org/10.1016/j.micron.2014.04.007 0968-4328/© 2014 Elsevier Ltd. All rights reserved.

In the microstructure studies on Cf/Al composites, it was realized that Al4 C3 was a very important interfacial reactant. Its formation and size could significantly affect the interface combination and the property of composites (Li and Chao, 2004; Wang et al., 2008). Wang et al. (2012) also calculated the thermodynamics and kinetics conditions for the formation of Al4 C3 . TEM studies were also used to compare the size of Al4 C3 in calculation and observation. It was suggested that the formation of Al4 C3 was controlled by the activity of Al element and it explained the formation mechanism of Al4 C3 by thermodynamic. In this present work, the diffusion layer and Al4 C3 were observed at the interface of Grf /Al composites by TEM and HRTEM. Different kinds of Al4 C3 was observed and analyzed. The growth mechanism of Al4 C3 was explained in detail by crystallography theory. 2. Materials and methods Grf /Al composite was fabricated by liquid pressure method using M40 graphite fiber (Fig. 1) to reinforce high purity aluminum (99.99 wt%); the volume fraction of the M40 graphite fibers in the composites was about 60 vol%. The temperature for melted Al alloy and mold was respectively 1023 and 773 K. During the infiltration process, a pressure of 25 MPa was applied and kept for 600 Seconds, and then composites were solidified at the air. The phases in Grf /Al composite were also analyzed by a MAXima X XRD-7000 X-ray diffraction (XRD) and the samples were subjected

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Fig. 1. The photos of graphite fiber. (a) SEM photos of graphite fiber; (b) TEM photos of graphite fiber; (c) SAD pattern of graphite fiber.

Fig. 2. Metallographic photos of Grf /Al composites.

to Cu-K radiation with a scanning speed of 2◦ /min. The morphology of composite was observed by scanning electron microscopy (SEM Quanta 200FEG). The microstructure of interface was investigated by transmission electron microscopy (TEM FEI TENCNAI G2 F30 and JEOL-2010CF) with a voltage of 200 kV. The diameter for TEM observation specimen was 3 mm and the thickness was about 10 ␮m.

Fig. 4 showed the interface structure of Grf /Al composites. The needle-like Al4 C3 was observed at the interface. Al4 C3 was distributed in different directions of carbon fibers (Fig. 4a and c). In Fig. 4(a), Al4 C3 was 830 nm in length and 50 nm in width, the ratio

3. Results and discussion Fig. 2 shows typical micrographs of aluminum matrix composite which indicated that the composite was well infiltrated with good fiber dispersion without apparent porosity, or significant casting defects. It seemed to be beneficial to composite properties. There was Al4 C3 phase in the composite according to XRD pattern (Fig. 3), which was consistent with many authors’ research results (Wang et al., 2008; Liu et al., 2013). Most results showed that M40 carbon fiber with high degree of graphitization have low chemical activity, which is suitable as reinforcement for aluminum matrix composite. The carbon forming interfacial reactions that happened in the Cf/Al are listed at the following text: 3C(s) + 4A1(1) = A14 C3 (s) The Gibbs function of this reaction is: G = −265,000 + 95.06 T. At T = 298 K, G = −236, 672 J/mol.

Fig. 3. XRD pattern of Grf /Al composite.

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Fig. 4. TEM observation of Al4 C3 in Interface Grf /Al composite. (a) Interface with reactant of Al4 C3 ; (b) Al4C3 growing between the fibers and appears “lap”; (c) Al4 C3 ; (d) SAD pattern of Al4 C3 .

of length and width was about 16.6. At a certain position, Al4 C3 connected two fibers, which was called ‘Carbide Bridge’ (Yang et al., 1996), could be observed (Fig. 4b). It was formed because these two fibers were close to each other and the carbon atoms were diffused from both of the fibers to the matrix. Fig. 5 showed the interface of Grf /Al composites. No significant interface reactant was observed at most interfaces. But apparent diffusion layer (about 8 nm) between the carbon fiber and aluminum alloy matrix was observed at interface. This diffusion layer provided best nucleation region for the non-homogeneous nucleation of Al4 C3 . As non-homogeneous nucleation, Al4 C3 usually formed at the attachments as residual coke and defects. In theory, the tensile strength value of PAN carbon fiber was 188 GPa, but the value of the fibers we used was only 5 GPa. This was because of the defects in the fibers, which cause the different structure between the surface and core of carbon fibers. The carbon atoms in the core of the fiber suffered symmetrical gravity, so its activity was low. However, the carbon atoms at the surface of the fiber suffered asymmetrical gravity, which made its activity relatively higher and was easier to form Al4 C3 . From Fig. 5a, the diffusion direction of carbon (from fiber to matrix) could be clearly observed and Fig. 5b showed the diffusion area between matrix and fibers. Fig. 6a showed the HRTEM photos of needle-like Al4 C3 , which was 200 nm in length, 15 nm in width and the ratio of length

and width was about 13.3. It had the tendency to grow in the thickness direction of the end portion. Fig. 6b showed the end portion of Al4 C3 . The interface between the end portion of Al4 C3 and matrix was coarse. But the interface between Al4 C3 and matrix was smooth (Fig. 6c and d). At the smooth interface, the structure of Al4 C3 was not completely stable and (0 0 0 1) layer which was not fully grown was observed in HRTEM photo. The interface between Al4 C3 and matrix was smooth and its structure was coherent or semi-coherent. However, the interface between the end portion of Al4 C3 and matrix was incoherent. Therefore, the growth speed of Al4 C3 in different direction was also different. The longitudinal growth speed of Al4 C3 was controlled by diffusion. Diffusion speed was affected by the diffusion of not only the elements in matrix, but also Al and C in Al4 C3 . Lateral growth speed was mainly controlled by two-dimensional nucleation formed at the interface. The different growth speed made the shape of Al4 C3 needle-like. Electron diffraction calibration results showed that Al4 C3 had no phase relations with the carbon fiber. (0001) layer of Al4 C3 was parallel with main growth direction and [0 0 0 1] was vertical with main growth direction. Al4 C3 had three-layer hexagonal structure ˚ (Space group: R-3 m). Its lattice constant was a = 3.3398 ± 0.0019 A, ˚ c = 25.006 ± 0.024 A.

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Fig. 5. TEM observation of the interface of Grf /Al composite. (a) Diffusion direction of carbon; (b) HRTEM observation of diffusion area.

Crystal structure also affected the shape of Al4 C3 (Fig. 7). Crystal structure of Al4 C3 was constituted by the layers of Al and C, which are alternately stacked with each other. According to the different structure of the layers, Al4 C3 could be divided into two parts: [Al2 C2 ]n and [AlC2 ]n . In [Al2 C2 ], the carbon atoms were in the gap

of tetrahedral. But in [AlC2 ], the carbon atoms were in the gap of octahedral. [Al2 C2 ]n and [AlC2 ]n stacked with each other and constituted the whole structure of Al4 C3 (Fig. 7), and this kind of structure was also observed in carbon nanotube reinforced aluminum silicon composites (Srinivasa et al., 2009).

Fig. 6. Electron showing formation of Al4 C3 at the fiber/matrix interface (HRTEM). (a) Al4 C3 at the fiber/matrix interface; (b) high resolution photo of Al4 C3 (head parts); (c) high resolution photo of Al4 C3 (middle parts); (d) High resolution photo of Al4 C3 (root parts).

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the sedimentary of carbon atoms. During the growth process, Al4 C3 embed into the interface between matrix and carbon fibers with the shape of needle-like. The main growth direction of Al4 C3 was longitudinal growth. At a certain position, Al4 C3 could connect two fibers like ‘Carbide Bridge’ when the two fibers were close to each other. References

Fig. 7. Crystal structure of Al4 C3 .

4. Conclusion As the result of TEM and HRTEM observation, the formation of Al4 C3 could be divided into two stages: nucleation and growth. For the nucleation stage, the carbon atoms diffused quickly during the fabrication process and Al4 C3 nucleared at the defects of the surface of carbon fibers. For the growth stage, the carbon atoms diffused to both matrix and the surface of Al4 C3 . Therefore, Al4 C3 grew with

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Al composites fabricated by liquid pressure method.

In this study, Grf/Al composite was fabricated by liquid pressure method. The diffusion layer and the nucleation and growth of Al4C3 were observed at ...
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