Enhanced tensile properties of aluminium matrix composites reinforced with graphene encapsulated SiC nanoparticles

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Abstract

Due to a high propensity of nano-particles to agglomerate, making aluminium matrix composites with a uniform dispersion of the nano-particles using liquid routes is an exceptionally difficult task. In this study, an innovative approach was utilised to prevent agglomeration of nano-particle by encapsulating SiC nano-particles using graphene sheets during ball milling. Subsequently, the milled mixture was incorporated into A356 molten alloy using non-contact ultrasonic vibration method. Two different shapes for graphene sheets were characterised using HRTEM, including onion-like shells encapsulating SiC particles and disk-shaped graphene nanosheets. This resulted in 45% and 84% improvement in yield strength and tensile ductility, respectively. The former was ascribed to the Orowan strengthening mechanism, while the latter is due primarily to the fiber pull-out mechanism, brought about by the alteration of the solidification mechanism from particle pushing to particle engulfment during solidification as a consequence of high thermal conductive graphene sheets encapsulating SiC particles.

Introduction

The achievement of both strength and ductility for aluminium alloys reinforced with ceramic particulates is very useful for a wide range of safety applications. However, although aluminium alloys reinforced with ceramic particulates enhance the tensile strength, they suffer from inadequate ductility [1], [2]. This is, in fact, a well-known bottleneck that limits the widespread engineering application of micro-composites especially when there is high ceramic particle content [3], [4].

To attain a higher strength and retain ductility of the composite, nano-sized ceramic particles are often used [5], [6]. The result is a metal matrix nano-composite (MMNC) [7], [8]. In such cases, it is extremely challenging to distribute and disperse the nano-particles uniformly in the metal matrix. This is especially true for using liquid based processing routes such as casting, because of the large surface to volume ratio and the poor wettability of nano-particles in most metallic melts [9], [10]. It was found that composites manufactured specifically by liquid methods such as stir casting suffer from particle pushing [11], resulting in rejection and agglomeration of particles from the growing solid/liquid interface during solidification. These problems easily induce the agglomeration and clustering of the nano-particles in the matrix, prompting low tensile properties, especially ductility, associated with intergranular fracture mode in the final solidified material [12], [13], [14].

The key to prolonged ductility is, therefore, to disperse these nano-particles into the grain interior, rather than having them agglomerated and concentrated at the grain boundaries, which can endanger elongation by causing cracks in nano-particles settled at the grain boundaries [13]. The former is accomplished by manipulating the shape of the interface and its curvature, controlling the interaction between the particle and the interface. It has also been suggested that the shape of the interface behind the particle is also dependent on the thermal conductivity of the particle and the melt [15], [16]. Khan and Rohatgi [16] showed that when the thermal conductivity of the particle is greater than that of the melt, the shape of the interface behind the particle changes from convex to concave. This difference in interface curvature brings about engulfment of particles in the matrix instead of segregation and pushing into the interdendritic regions, conferring improved tensile properties on the produced composites [17].

Many researchers have focused on using innovative methods to incorporate nano-particles into the molten aluminium, making an MMNC with a suitable dispersion and avoiding nano-particle agglomeration [10]. The milling of nano-particles with metallic powders such as aluminium and ultrasonic assisted casting methods, especially in the semi-solid state, are the most important methods which have been proposed in this regard. However, these methods are still in development, especially where a high reinforcement loading is sought [3], [14], [18] in the implementation of thermal models to predict the interaction at the solid/liquid interface during solidification.

In addition to the unprecedented characteristic of graphene, i.e. a single-atom-thick sheet of sp2 hybridised carbon atoms, as a strengthening nanofiller in the world of polymer matrix composites [19], [20], recent studies have shown that graphene can also be considered as an effective reinforcement for metal matrices [21]. However, to the best of our knowledge, most of these studies [21], [22] have only considered the mechanical property changes after the implantation of graphene nanosheets (GNSs) as a new nano filler for metal matrix composites, but exploiting the promising characteristics of GNSs, such as the high thermal conductivity and anchoring nano particles, to harness the structural uniqueness and full potential of utilising nano-particles during the fabrication process remains largely unexplored. For instance, it has been reported that graphene sheets possess the unique feature of having a two-dimensional shell(s) which can nucleate and anchor nano-particles on the edges and surface [23], [24], however, the ability of graphene sheets to alleviate the agglomeration of nano-particles during solid and liquid processing for the production of meal matrix composites has hitherto not been reported.

In this study, an innovative nanocapsulating route is implemented to exploit the high thermal conductivity and flexibility of graphene sheets as wrapping shells to diminish the pushing and agglomeration of SiC nano-particles during milling and the subsequent semi-solid casting process. The effect of using 1 vol.% GNSs to alleviate the agglomeration of nano SiC particles and subsequent enhancement in tensile properties of the composite, produced using a process encompassing milling and semi-solid stir casting, was investigated using high resolution transmission electron microscopy (HRTEM) and tensile tests, respectively. In order to gain insight into the relationship between the microstructure of the fabricated composite and its tensile properties, two mathematical models were developed, making use of the Orowan strengthening mechanism, and the models were further validated with tensile test results.

Section snippets

Experimental procedures

In order to prepare a mixture containing nano SiC and GNSs reinforcements, high purity aluminium powder (45 μm, supplied from Alpha Aesar Company with 99.5% purity), a sufficient amount of nano-β-SiC particles (45 nm, supplied from Nanostructured & Amorphous Materials, Inc.) and pure pristine monolayer graphene with the average lateral size of 550 nm (supplied from Graphene Supermarket) were used.

The ball milling process was performed in a Fritsch Pulverisette P5 planetary ball mill without

Morphological characterization of powders

Fig. 1(a) shows the morphology of Al–55 wt.% SiC–12 wt.% graphene powder (preform), set to reach the final composition after casting (A357–3 vol.% SiC-1 vol.% graphene). As shown in Fig. 1(a), the size of the agglomerated particles after milling is smaller than 1 μm, which can accommodate better dissolution and lower agglomeration during subsequent semi-solid stir casting. Basically, the dissolution of finer milled powders in the melt is easier than that of larger agglomerated ones. Three important

Conclusions

This study proposes an innovative fabrication method, making use of encapsulating and high thermal conductivity promising features of graphene, for manufacturing aluminium-based composite reinforced with nano SiC particles. The former enables the agglomeration of nano SiC particles during powder milling and subsequent liquid processing to be diminished, and the latter made a useful change in the solidification mechanism from pushing to the engulfment of particles.

The augmented tensile

Acknowledgments

The authors would like to acknowledge the use of facilities (ARC-LE0237478) within the University of Wollongong (UOW) Electron Microscopy Centre and especially the great assistance of Dr. Gilberto Casillas. Authors also thank Dr. David Mitchel and Mr. Mitchel Nancarrow for their valuable support.

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