Mechanical reinforcement of a high-performance aluminium alloy AA5083 with homogeneously dispersed multi-walled carbon nanotubes
Introduction
Carbon reinforced materials have been widely developed in the last decades. Carbon fibre (CF) and short carbon fibre (SCF) reinforced composites have been successfully produced and led to interesting applications [1]. Since their observation by Iijima [2], carbon nanotubes (CNTs) appeared as the ultimate carbon nano-fibres with high potential for reinforcement of composite materials, due to their high aspect ratio and outstanding mechanical stiffness. The Young’s modulus for CNT of high crystallinity, typically those prepared by electric arc or laser ablation, was calculated and measured to be as high as 1 TPa [3], [4], [5]. By contrast, multi-walled carbon nanotubes (MWCNTs) prepared by chemical vapour deposition (CVD) techniques often present higher defect densities and their Young’s modulus shows strong diameter-dependence in the range of 10–200 GPa [4], [6]. As far as tensile strength is concerned, the mean value for well-crystallised CNT was found to range between 30 GPa [7] and 150 GPa [5] while that of CVD ‘‘herringbone’’ MWCNTs [8], [9] was found surprisingly to be at least as high (about 110 GPa). This high strength was assigned to efficient stress transfer mechanisms between walls in these irregular structures [10].
Because of these exceptional mechanical properties associated with high electrical and thermal conductivities and their ability to be chemically functionalised [11], CNT have been actively investigated as reinforcements in polymers [12], [13], [14]. In addition, the low density of CNT, varying from ∼1.3 g/cm3 for single-walled carbon nanotubes (SWCNTs) up to 1.8 g/cm3 for MWCNTs, may lead to interesting weight lightening in ceramic [15] or metal matrix composites (MMCs). In contrast to polymers, only limited research was achieved so far on CNT–MMC (for a recent review, see [16]). Furthermore, no real improvement of the mechanical properties of CNT/metal was reported before 2005 [17], [18], [19], [20], [21], [22] essentially because of the challenge to disperse CNT in a metal matrix homogeneously [23], [24] but also because of the low bonding expected between CNT and metal matrices [25]. To overcome these difficulties, the melting route was used as an alternative, but no evidence of a much better dispersion was reported so far and the formation of carbides may be a problem at such high temperatures [26]. On the other hand, high-performance composites were prepared by ‘‘molecular-level mixing’’, i.e. chemical reactions between functionalised CNT and metal ions [27], but such a process is hardly compatible with industrial production routes. Therefore, powder metallurgy (PM) appears as the most promising route for preparing homogenous composites [23], [28], [29]. Moreover, PM involves lower processing temperatures which limits uncontrolled chemical reactions at the reinforcement/matrix interface [30].
PM typically involves three processing steps i.e. mixing, pressing and solid state sintering, and most usually an additional final forming process step such as extrusion, forging or rolling. Mixing is generally achieved by ball-milling, sometimes after a first step of acid-treatment of the CNT [31] or ultrasonication [26], [32]. The damage of nanotubes during this step remains an important question. Esawi et al. [23] underlined that there are two competitive effects during ball-milling of CNT/Al powder mixture: (i) metal cold working/grain size refinement and possible destruction of nanotubes and (ii) cold welding leading to dispersion and protection of the CNT inside the welded nanoparticles [24]. Perez-Bustamante et al. [32] reported the formation of an amorphous layer at the CNT/Al interface. Kwon et al. [33] checked by Raman spectroscopy and TEM that the nanotubes were not damaged during the process. Sintering and/or extrusion of CNT/Al composites are generally achieved below 600 °C in order to avoid the formation of carbides [31], [33]. However, the formation of carbides above 600 °C was reported by some groups to play a positive role in the enhancement of the mechanical properties of the composites by strengthening bonding between CNT and the matrix [33]. Recent studies, where CNT–MMC were prepared by PM, reported significant improvement of the mechanical properties of light metals, especially aluminium [18], [26], [31], [32], [33], [34], [35]. However, so far, the reported studies were essentially limited to pure aluminium matrices, except Deng et al. [36] who worked with the AA2024 aluminium alloy. Such matrices are usually not eligible for applications due to their low mechanical strength, although the poor mechanical properties can be enhanced through strain hardening or grain size refinement. Both mechanisms likely occur during the different steps of powder metallurgy so that it is quite difficult to separate strengthening due to CNT from that related to the microstructural change of the Al matrix during the process. On the other hand, to our best knowledge, the influence of the dispersion parameters on the homogeneity of the mixture and the mechanical properties of the composites was not carefully studied so far.
In this paper, we use two different mixing processes for dispersing MWCNTs in an aluminium alloy powder AA5083, namely sonication in a liquid phase and dry ball-milling. MWCNT/AA5083 composites are prepared by hot isostatic pressing (HIP) of the mixtures. We study the influence of the dispersion parameters on the homogeneity of the mixtures and we discuss the relationship between homogeneity and mechanical properties of the composites. We show that the mechanical properties of high-performance alloys of light metals can be enhanced using CNT as reinforcements, providing the nanotubes are well-dispersed not only at the surface but also in the bulk of the metallic particles.
Section snippets
Materials and methods
The matrix precursor was an aluminium alloy powder from the AA5083 series (about 4 wt.% Mg, 0.8 wt.% Mn and 0.2 wt.% Cr for the main alloy elements – N2 atomised) with an average particle size of about 25 μm. This non age-hardenable Al-alloy exhibits good specific mechanical characteristics, good corrosion resistance as well as a good potential for superplasticity at low temperatures [40] and is widely used in the aerospace industry. A typical scanning electron microscopy (SEM) picture of the Al
CNT dispersion
The mixtures can be classified in four different categories corresponding to the process and parameters used for mixing: (i) 60 min ultrasonication (US), (ii) low energy milling at 300 rpm during 30 min (BM1), (iii) medium energy milling at 600 rpm during 180 min (BM2) and (iv) high energy milling at 600 rpm during 420 min (BM3). Fig. 4 shows typical SEM images of the mixtures for the different categories. The backscattered electron mode allows identifying CNT clusters easily (dark-grey zones in the
Conclusion
MWCNT/AA5083 high-performance composites were prepared by powder metallurgy. The mechanical properties of the composites are not enhanced with respect to raw alloys, and even slightly decreased, when CNT are dispersed heterogeneously as well as when they are dispersed homogeneously on the surface of the alloy particles (at the scale of 25 μm). Indeed, high energy in the milling process is required to deform and reshape the AA5083 particles so that the CNT can be distributed homogeneously inside
Acknowledgements
We thank Raul Arenal for the HRTEM measurements and Vincent Jourdain for fruitful discussions.
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