Modeling the Break-up of Nano-particle Clusters in Aluminum- and Magnesium-Based Metal Matrix Nano-composites
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METAL matrix composites (MMC) form a class of advanced materials typically based on light metals such as Al and Mg and ceramic reinforcements including but not limited to Al2O3, AlN, SiC, etc. Combining the light weight and ductility of Al and Mg with high strength and high modulus of ceramic materials makes MMC desirable for applications in aerospace and automotive industries. A good review of the development of MMCs is given in Reference 1. Metal matrix nano-composites (MMNC) are a recently developed subclass of MMCs based on nano-particle reinforcements. Recent papers showed a clear increase in aluminum Young’s modulus (by up to 100 pct) and in hardness (by up to 50 pct) with the addition of carbon nano-particles.[2] Another study indicated a slight enhancement in Brinell hardness of aluminum-, magnesium-, and copper-based MMNCs with Al2O3 and AlN nano-particles.[3] The study suggested that a better dispersion of nano-particles is needed. Other researchers also report agglomerations of nano-particles made visible using high-definition scanning electron microscopy (SEM).[4] The effect of uneven distribution of NPs on the final properties of MMNCs is explained by the fact that large-size clusters no longer act ANTON MANOYLOV, Post-Doctoral Research Associate, VALDIS BOJAREVICS, Reader - Research, and KOULIS PERICLEOUS, Professor of Computational Fluid Dynamics and Leader of Computer Science and Engineering Group, are with the Department of Mathematical Sciences, Centre of Numerical Modelling and Process Analysis, University of Greenwich, Old Royal Naval College, Park Row, London SE10 9LS, U.K. Contact e-mail: [email protected], [email protected] Manuscript submitted November 18, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
as dislocation anchors, but instead become defects, leading to inconsistent properties. The agglomeration of particles in MMNCs is related to the fact that nano-sized inclusions have a larger ratio of surface area to the volume than, e.g., micro-sized particles. This causes surface forces such as van der Waals interaction and adhesive contact to dominate over the volume forces such as, e.g., inertia or elastic repulsion in the case of nano-particles. Various mechanisms of detachment of adhered particles have been reported in the literature,[5] which includes turbulent flow. It is expected that drag and shear forces in turbulent flow can improve separation of the particles and thus contribute to de-agglomeration. However, the drag force alone is not sufficient to de-agglomerate the nanoparticles. This can be qualitatively illustrated by comparing the Stokes equation for the drag force with the force required to break two spherical particles apart, known as the pull off force, given by, e.g., Bradley:[6] 6plf Rvf ¼ 4pRcsl ;
½1
where lf and vf are the velocity and dynamic viscosity of the melt and csl is the solid–liquid interfacial energy. For the case of aluminum melt, the dynamic viscosity is lf = 0.0013 Pa s. Assuming the interfacial energy csl = 0.2 to 2.0 J/m2, Eq. [1] yields vf = 10
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