Production of Al–20 wt.% Al 2 O 3 composite powder using high energy milling S.S. Razavi Tousi ⁎, R. Yazdani Rad, E. Salahi, I. Mobasherpour, M. Razavi Ceramic Department, Materials and Energy Research Center, P.O. Box 31787/316, Karaj, Iran abstract article info Article history: Received 18 April 2008 Received in revised form 30 December 2008 Accepted 26 January 2009 Available online 4 February 2009 Keywords: Metal–matrix composites Nano-structured materials Steady state High energy ball milling was used to produce a nanostructured Al matrix composite reinforced by submicron α-alumina particles. Scanning electron microscopy analysis as well as tap and green density measurements were used to optimize the milling time needed for the completion of the mechanical milling process. Results show that addition of alumina particles as the reinforcement has a drastic effect on the size, morphology and pressability of the powder. Scanning electron microscopy shows that distribution of alumina particles in the Al matrix reaches a full homogeneity after steady state. This would increase the hardness of powder due to a nano-structured matrix and oxide dispersion strengthening. © 2009 Elsevier B.V. All rights reserved. 1. Introduction An optimum combination of high strength and ductility gives Al based metal matrix composites (MMCs) a wide range of possible advanced applications [1,2]. A survey of the previous studies indicates that a homogenous dispersion of fine particles in a fine grained matrix is beneficial to the mechanical properties of MMCs [2–6]. Mechanical alloying (MA) is a simple and useful technique for attaining a homogeneous distribution of the inert fine particles within a fine grained matrix [7]. Addition of ceramic reinforcements into a ductile matrix has a great effect on the structural evolution during ball milling. Many researchers focused on the addition of low percentages of the ceramic phases to the Al matrix by mechanical alloying [8–14]. This study shows that addition of 20% wt. Al 2 O 3 markedly influences the structural evolution of the Al matrix during milling process. The time needed to reach the steady state also depends on the distribution of alumina particles in the Al matrix. In spite of the absence of alloying elements, the ultimate powder has an excellent hardness and acceptable morphology for the powder metallurgy process. 2. Experimental procedure Commercial purity Al powder (Merck, Art. No: 1056) as a mono- lithic system and a mixture of Al–20 wt.% alumina powder (Martins- werk, MR70, d50: 0.5–0.8 μm) were separately milled in a P5 planetary mill for various periods of time up to 25 h. The ball to powder ratio was approximately 15:1 and the mill speed was maintained at 250 RPM. 3 wt.% of stearic acid as process control agent (PCA) was added to retard excessive welding. The milling atmosphere was Ar which was purged into the cups before milling. Product sampling was performed in the glove box in the Ar atmosphere to prevent oxidation. The powders produced after different stages of milling were examined using a Cambridge (Stereo Scan s360) scanning electron microscope (SEM) operating at a voltage of 30 kV. Particle size and its mean deviation were obtained by a visual basic software using several SEM images. The mean deviation from the average particle size was used as a criterion for the estimation of particle size distribution: d = X n i =1 j D i - D Av D Av j ð1Þ where n is the number of particles, d mean deviation from the average particle size, D i the diameter of the particle i and D Av is the average particle size. X ray diffraction (XRD) patterns of powders were taken in air atmosphere using a Philips (PW3710) X ray diffractometer (30 kV and 25 mA) with Cu Kα radiation. Grain size and lattice strain changes during milling stages were calculated by the Williamson–Hall method for at least three peaks [15]: B cos θ =0:9λ = D +2η sin θ ð2Þ where B, λ, θ, D and η are full width at half maximum (FWHM), the wave length, peak position, crystallite size and lattice strain, respec- tively. A Philips CM 200 FEG transmission electron microscopy (TEM) was used to investigate grain size and dispersed particles. The powders were mounted, cross sectioned and polished in preparation for the microhardness test. Micro-hardness values were obtained averaging 5–10 indents of 50 g force; error bars indicate the positive/negative deviation from the average hardness. To study the green density changes, the powders were first pressed by an isostatic press in the air atmosphere (500 MPa) and then the BS1902A standard was applied using methanol via the Archimedes method. The density values were Powder Technology 192 (2009) 346–351 ⁎ Corresponding author. Tel./fax: +98 2616201888. E-mail address: s.razavitousi@gmail.com (S.S. Razavi Tousi). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.01.016 Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec