Exploring the size effects of Al4C3 on the mechanical properties and thermal behaviors of Al-based composites reinforced by SiC and carbon nanotubes
Graphical abstract
Introduction
Owing to high Young's modulus (∼1 TPa) [1], high strength (∼110 GPa) [2], large aspect ratio (>100), light weight (1.2–2.1 g/cm3) [3] and low thermal expansion coefficient (CTE ∼0) [4,5], carbon nanotubes (CNTs) are expected as a new generation of reinforcements for composites since its discovery by Iijima in 1991 [6]. Initially the attempts using CNTs as reinforcements were carried out on polymer based composites [7] due to their relatively moderate fabrication process comparing with metal matrix composites (MMCs). Until the last two decades, attention has been ever-increasingly paid to the potential of CNTs in fabricating MMCs due to its unique advantage over traditional ceramic particles [[8], [9], [10], [11]]. Among these MMCs, the aluminum (Al) matrix composites (AMCs) reinforced by CNTs draw particular interest because of their light weight, high strength and anti-corrosive properties as promising materials in aerospace and automobile industries [12]. However, the improvement in the mechanical properties of CNTs reinforced AMCs was not commensurate with the extraordinary high tensile strength of carbon nanotubes (∼110 GPa). One major reason was the difficulty in acquiring the uniform dispersion of CNTs in Al matrix [13]. The mechanical properties of CNTs reinforced MMCs degraded at high CNTs content (>2 vol%) due to CNTs agglomeration [[14], [15], [16], [17]].
Aimed to acquire uniform dispersion of reinforcements and high strengthening efficiency, some researchers have attempted to hybridize low content of CNTs with other ceramic particles, instead of using CNTs as single reinforcement [18,19]. For instance, Thakur et al. [18] substituted 1 vol% CNTs with 0.3 vol% CNTs and 0.7 vol% Al2O3 for fabricating magnesium matrix composites, and the yield strength (YS) increased from 112 MPa to 153 MPa. Kim et al. [19] hybridized 1.5 vol% TiC with 0.7 vol% CNTs to reinforce Al matrix, and their study suggested that hybridization of a small number of CNTs significantly enhanced the distribution of TiC nano-particles in the matrix and increased the YS from 203 MPa to 263 MPa. Recently, the AMCs reinforced by the combination of 1 vol% CNTs and 1 vol% SiC particles (Al-SiC-CNTs) were also fabricated through spark plasma sintering (SPS) and hot sheath rolling [20]. It was found that 1 vol% CNTs can promote the uniform dispersion of SiC particles in Al matrix, leading to an enhanced YS (+25%), ultimate tensile strength (+33.5%), ductility (+33%) and decreased CTE (−11.6%).
Besides the initial composition design, the in situ chemical reaction might also play a critical role in the mechanical properties of composites. Due to the low Gibbs free energy (−196 kJ/mol at 298 K) [21], the formation of Al4C3 was widely observed in Al-CNTs composites [[22], [23], [24], [25]]. However, the effects of Al4C3 formation on the mechanical properties of the composites are still under debate [[26], [27], [28], [29], [30], [31], [32]]. On one hand, because of the intrinsic brittleness, some researchers claimed that the formation of Al4C3 between the Al matrix and the CNTs resulted in weakened strengthening efficiency [[26], [27], [28]]. Li et al. [26] demonstrated that the tensile strength of 0.75 wt.% CNTs/Al composite decreased by 24% after forming an Al4C3 layer between the CNTs and the Al matrix. On the other hand, other researchers found that the formation of Al4C3 increased the wettability between Al and CNTs, hence strengthening the interfacial bonding and enhancing the load transfer efficiency [[29], [30], [31], [32]]. Zhou et al. [32] pointed out that the formation of Al4C3 could enhance the interfacial bonding, as well as yield strength (19 MPa) and ultimate tensile strength (12 MPa). Chen et al. [30] revealed that forming Al4C3 can lead to a maximum enhancement of 49 MPa in ultimate tensile strength. The improved interfacial bonding was verified by an in-situ test. However, in these studies, the quantity and size effects of Al4C3 on the mechanical properties of composites were not reported. Moreover, in addition to the mechanical properties, it should also be noted that the CTE of Al4C3 is only 5 × 10−6 K−1 [33], which is much lower than that of Al matrix (25.9 × 10−6 K−1) [34]. The formation of Al4C3 might lower the CTE of the composites for the potential engineering application such as electronic package [20], which was rarely reported.
In present study, the quantity and size effects of Al4C3 on the mechanical properties of Al-SiC-CNTs composites have been investigated. Additionally, the effect of Al4C3 on the CTE of these composites has also been studied [[35], [36], [37]]. In order to control the size and amount of Al4C3, the fabricated Al-1vol% SiC-1vol% CNTs composites were heat treated at various temperatures. The reacted product (Al4C3) was identified using X-ray diffraction, Raman spectroscopy and transmission electron microscopy (TEM). The involved mechanisms were discussed based on microstructures and related models.
Section snippets
Fabrication of Al-SiC-CNTs composites
The raw multi-walled carbon nanotubes (CNTs) (∼10 nm in diameter and 3 μm in length) were supplied by Sigma-Aldrich Co. LLC. Gas-atomized Al powders with a purity of 99.9% and an average particle size of 2 μm were obtained from General Research Institute for Nonferrous Metals (GRinm) in Beijing. The SiC powders (500–700 nm in particle size) with a purity of 99% were purchased from Aladdin Industrial Co. LLC.
To increase the attachment surface of Al particles for CNTs [38], the as received
Macroscopic analysis of Al-SiC-CNTs composites
Fig. 1a shows X-ray diffraction (XRD) patterns of the as prepared and heat treated composites, which indicate that only SiC and Al peaks can be observed for these Al-SiC-CNTs composites. However, Al4C3 and CNTs not being detected is possibly due to the sensitivity of XRD. Fig. 1b presents the Raman spectra of the composites after heat treatment in the temperature range of 530–630 °C, the Raman spectrum of the oxidized CNTs is also presented as comparison. It can be seen that the noticeable peak
Conclusion
The role of Al4C3 on the mechanical properties and thermal behaviors of the Al based composite reinforced by SiC and carbon nanotubes (CNTs) was explored by controlled heat treatment. The following conclusions can be drawn:
- (1)
The homogeneous dispersion of CNTs and SiC particles in the fabricated composites was successfully realized by the chemical oxidization of CNTs and mechanical milling during the composite powder preparation.
- (2)
Al4C3 rods are originated from CNTs instead of SiC particles. The
Acknowledgments
The financial supports from National Natural Science Foundation of China (51531009) and Open Foundation of Key Laboratory of Advanced Materials of Yunnan Province (738010094) are appreciated. One of the author (B. Guo) would also thank the financial support from Hunan Provincial Innovation Foundation for Postgraduate (150140012).
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