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INTRODUCTION

THE possibility of taking best advantage of metallic and ceramic materials characteristics has long been the motivation behind the development of metal matrix composites (MMCs).[1,2] Metal matrix composites can be synthesized using a number of processing routes,[3] among which the in-situ method is well known due to its scientific innovativeness.[4] In the in-situ method, reinforcement particles are synthesized in the matrix through one or more chemical reactions. Considering the physical state of the matrix, the in-situ processing methods are categorized into solid and liquid ones.[2] Aluminum matrix composites with different reinforcements have been produced via the in-situ method. In the case of Al/Al2O3 composites, two different liquid state methods are generally reported. The first is by direct oxidation of molten aluminum alloy and the second is by reduction of a metallic oxide in the melt.[5] The general term used for the substitution reaction that takes place between aluminum and metallic oxides is the aluminothermic reaction. The reaction can be written as 2xAl þ 3MyOx ! 3yM þ xAl2 O3

½1


A. MALEKI is Ph.D Student, Department of Materials Engineering, Isfahan University of Technology, and Research Scholar, Department of Mechanical Engineering, National University of Singapore. M. MERATIAN, Assistant Professor, and B. NIROUMAND, Associate Professor, are with the Department of Materials Engineering, Isfahan University of Technology, Isfahan, IRAN, 84156-83111. Contact e-mail: [email protected] M. GUPTA, Associate Professor, is with the Department of Mechanical Engineering, National University of Singapore, Singapore 117576. Manuscript submitted March 4, 2008. Article published online October 2, 2008 3034—VOLUME 39A, DECEMBER 2008

Different metallic oxides such as TiO2,[6] MnO2,[7] SiO2,[8] CuO,[9,10] B2O3,[11] MoO3,[12] Fe2O3,[13] and ZnO[14,15] have been used to synthesize the in-situ Al/Al2O3 composites. As the aluminothermic reaction occurs, element M is reduced and dissolved in the matrix. The reduced element M is potentially a useful alloying element. However, considering the acceptable ranges of alloying elements in the standard alloys, there will be limits for the alloying elements added by the reaction. When higher content of alumina is desired, a larger amount of M is inevitably reduced. Thus, in many cases, the alloying element content of the matrix may surpass the standard range of a given alloy. For example, to form 5 pct Al2O3 in an aluminum matrix by reduction of CuO, about 9.34 pct Cu is simultaneously produced and dissolved in the matrix.[16] One way to control the quantity of the alloying element is by using a metallic oxide as the reactant, which produces M in a vapor form. A good candidate for this purpose is zinc oxide (ZnO). The boiling temperature of zinc is 907 °C. Therefore, by using zinc oxide as the reactant, the alumina content can be further increased while the extra alloying element (zinc) is kept low in the matrix because of its vaporization. Several researchers ha