Journal of Alloys and Compounds 536S (2012) S35–S40 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds jou rn al h om epage: www.elsevier.com/locate/jallcom Synthesis of nanostructured Al–Mg–SiO 2 metal matrix composites using high-energy ball milling and spark plasma sintering J. Bhatt a,∗ , N. Balachander a , S. Shekher a , R. Karthikeyan b , D.R. Peshwe a , B.S. Murty b a Department of Metallurgical and Materials Engineering, Visvesvaraya National Institute of Technology, Nagpur 440010, India b Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India a r t i c l e i n f o Article history: Received 23 June 2011 Received in revised form 14 December 2011 Accepted 15 December 2011 Available online 24 December 2011 Keywords: Metal matrix composites Nanocomposites Spark plasma sintering Microhardness a b s t r a c t Mechanical alloying by high-energy ball milling is successfully used to produce a metal matrix com- posite of Al–Mg reinforced with amorphous silica particulate. Four different compositions are chosen with varying Mg content (0.5, 1, 2.5 and 5 by wt.%) by keeping SiO 2 content constant at 5 wt.% to make nanocomposites by high energy ball milling and microcomposites by mechanical mixing. No new phases are found in 20 h mechanically alloyed Al–Mg–SiO 2 metal matrix composite. XRD study showed Mg is completely dissolved into the Al matrix. XRD observation also showed decrease in crystallite size and increase in lattice strain with progress of mechanical alloying. SEM micrographs indicate decrease in particle size via fracture and cold welding phenomena. The powders are made in the form of cylindrical pellets of 20 mm diameter by Spark Plasma Sintering. X-ray diffraction analysis of the pellets obtained after sintering indicates the evolution of MgAl 2 O 4 spinel structure along with Al 2 O 3 . Vickers hardness values observed for nanocomposites are more than twice as high as that of microcomposites. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Metal matrix composites (MMCs) have been developed to meet the specific engineering properties which cannot be achieved by monolithic material. Different types of reinforcement in form of particulate, whiskers, or fiber have been used to alter the properties of MMC for specific application. One of the important compos- ites which have received enormous attention is aluminum metal matrix composites (AMCs) reinforced with particulates. Different types of particulates such as SiC, Al 2 O 3 , AlN, TiB 2 and TiC dis- persed in commercial Al alloy have been studied for interfacial effects to improve wetting and decrease degradation of reinforce- ment [1]. Processing techniques such as powder metallurgy, spray deposition and various casting techniques, namely, squeeze cast- ing, rheocasting and compocasting [2] have been used to produce MMC. Ceramic reinforcement to matrix material can either be done by ex situ [3] or in situ [4] method depending upon the process- ing route. One of the important drawbacks of ex situ MMCs is the interfacial reaction between reinforcement and matrix resulting in poor wettability and bonding [5]. To overcome this, in situ pro- cess has been widely recognized because of its advantages such as formation of thermodynamically stable reinforcements in the matrix, clean reinforcement–matrix interfaces resulting in a strong ∗ Corresponding author. Tel.: +91 712 2801513. E-mail addresses: jatinbhatt@mme.vnit.ac.in, jatin1411@yahoo.com (J. Bhatt). interfacial bonding, finer particle size of reinforcement yielding better mechanical properties and potential for lower cost of pro- duction [4]. It is well known that properties of MMC are controlled by size and volume fraction of reinforcement and matrix material [4]. Enhanced mechanical properties are observed when dimension of the reinforcement is reduced to make it thermodynamically stable and homogenously distributed in matrix material [6]. Mechanical alloying (MA) processes have been widely used to produce nanos- tructured materials and MMC [7,8] followed by sintering to make bulk nanostructured MMC [9]. In the present investigation attempt is made to synthesize Al–Mg reinforced with amorphous silica par- ticulate by varying Mg content (0.5, 1, 2.5 and 5 by wt.%) to form MMC. Spark plasma sintering (SPS) is used to form bulk microcom- posites and nanocomposites. Vickers microhardness study is done on both microcomposites and nanocomposites to understand the effect of structure on hardness. 2. Experimental procedure High energy ball milling is carried out in a planetary ball mill (Fritsch pulverisette P-5) at room temperature using WC vials and balls as milling media and toluene as process controlling agent (PCA) for the four different compositions as shown in Table 1. The materials used in this study are 99.7% pure Al powder, 99% pure Mg powder and 99.8% pure amorphous SiO2 powder with a particle size of <45 m (325 mesh). The milling speed and ball-to-powder weight ratio are maintained at 300 rpm and 10:1, respectively. Samples taken out of the vial at regular intervals of 5 h for X-ray diffraction (XRD) analysis using a PANalytical X’pert-PRO diffractome- ter with Cu K ( = 1.54 ˚ A) radiation. Single peak approximation method is used to determine crystallite size and lattice strain by drawing Williamson–Hall plot [10] 0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2011.12.062