Effect of reinforcement volume fraction on mechanical alloying of Al–SiC nanocomposite powders S. Kamrani 1,3 , A. Simchi* 1,2 , R. Riedel 3 and S. M. Seyed Reihani 1 Mixtures of aluminium powder and nanoscaled SiC particles (n-SiC) at various volume fractions of 0, 1, 3, 5, 7 and 10 are comilled in a high energy planetary ball mill under an argon atmosphere to produce nanocrystalline Al–SiC nanocomposites. High resolution scanning electron microscopy (HRSEM), X-ray diffraction (XRD) method, laser particle size analysis and powder density measurement were used to study the morphological changes and microstructural evolution occurred during mechanical alloying. Al–SiC composite powder with microscaled SiC particles (1 mm) was also synthesised and characterised to examine the influence of reinforcement particle size on the milling process. It was found that with increasing volume fraction of n-SiC, a finer composite powder with more uniform particle size distribution is obtained. The morphology of the particles also became more equiaxed at shorter milling times. Furthermore, the analysis of XRD patterns by Williamson–Hall method indicated that the crystallite size of the aluminium matrix decreases with increasing reinforcement volume content while the lattice strain changes marginally. As compared with microscaled SiC particles, it appeared that the effect of n-SiC on the milling stages is more pronounced. The results clearly show that the reinforcement particles influence the work hardening and fracture of the metal matrix upon milling, affecting the structural evolution. With decreasing size of the ceramic particles to nanoscale, this influence becomes more pronounced as the surface to volume fraction increases. Keywords: Mechanical alloying, Al–SiC nanocomposite, Structural evolution, Effect of reinforcement particles Introduction Aluminium matrix composites (AMCs) reinforced with ceramic particles offer several advantages over conven- tional alloys including high modulus and strength to weight ratio, superior creep and wear resistance. 1 It is well known that the characteristics of reinforcement particles significantly influence the mechanical proper- ties of AMCs. For instance, yield and tensile strength increase whereas toughness and ductility decrease with increasing volume fraction and/or decreasing reinforce- ment particle size. 2–5 It has recently been reported 6–9 that decreasing the reinforcement particle size to nanoscale range is favorable for achieving higher strength. Smaller particles are less prone to have internal defects and thus are more difficult to be fractured. 10 The stress concentration level on each particle is also lower, because there are more particles bearing applied load, which in fact lowers the probability of fracturing. Nevertheless, fabrication of nanocomposites is difficult because uniform dispersion of nanometric hard particles throughout the metal matrix is a challenging task. 10 A very high surface to volume ratio of nanoparticles promotes agglomeration and clustering, deteriorating the mechanical properties. 11–14 This feature is due to the fact that, the reinforcement clusters have less ability to transfer shear and tensile stresses. 15–17 As the volume fraction of reinforcement particles increases, particularly beyond the percolation threshold, achieving a uniform distribution of the ceramic particles throughout the metal matrix becomes more crucial due to severe clustering. One of the common procedures for fabrication of particulate reinforced metal matrix composites is mechanical alloying (MA). The process enables uniform distribution of the reinforcement particles into the metal matrix without the typical segregation of casting composites. 18–22 Fracturing of the hard particles and refining the metal matrix are also feasible, which may result in synthesising nanocrystalline composites. 23–26 It has been shown that, for example in Refs. 24, 27 and 28, a uniform distribution of nanoscaled hard particles 1 Department of Material Science and Engineering, Sharif University of Technology, Azadi Avenue 14588 Tehran, Iran 2 Institute for Nanoscience and Nanotechnology, Sharif University of Technology, PO Box 11365-9466, Azadi Avenue 14588 Tehran, Iran 3 Dispersive Materials Group, Technical University of Darmstadt, Petersenstr. 23, D-64287, Germany *Corresponding author, email simchi@sharif.edu 276 ß 2007 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 7 March 2007; accepted 14 April 2007 DOI 10.1179/174329007X189621 Powder Metallurgy 2007 VOL 50 NO 3