In-Situ Synthesis of Cu/Cr-Al 2 O 3 Nanocomposite by Mechanical Alloying and Heat Treatment
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INTRODUCTION
PARTICLE-REINFORCED copper composites are widely used in electric, automotive, and aerospace industrial fields due to their high strength, good wear resistance, satisfying high-temperature behavior, and low thermal expansion coefficient. They can be produced by dispersing hard particles such as oxides, carbides, or nitrides in the copper matrix either by liquid or solidstate techniques. Copper strengthened with aluminum oxide (Al2O3) has great potential as a class of oxidedispersion-strengthened alloys with relatively high strength and good stability at elevated temperatures.[1–3] Also, in-situ refractory metal-copper composites are candidates for such applications. Extensive studies are currently being carried out on Cu-Nb,[4] Cu-Fe,[5] Cu-Ag,[6] and Cu-Cr[7] in-situ composites. Although such in-situ composites have higher electrical conductivity compared to oxide-dispersed composites, precipitate coarsening during high-temperature applications of these composites may reduce mechanical strength, which is one of the shortcomings in these composites. Mechanical alloying is now an established technique for preparation of nanostructured alloys, oxidedispersion-strengthened alloys, amorphous materials, intermetallic compounds, and ceramics.[8] It has been shown that enhanced reaction rates can be achieved and dynamically maintained during milling as a result of microstructural refinement and mixing processes accompanying repeated fracture, welding, and deformation of particles during collision events.[9,10] Mechanochemical synthesis using displacement reactions in high-energy ball milling has been widely investigated in order to SAEED SHEIBANI, Ph.D. Student, ABOLGHASEM ATAIE, Professor, and SAEED HESHMATI-MANESH, Associate Professor, are with the School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran. Contact e-mail: [email protected] Manuscript submitted July 23, 2009. Article published online June 26, 2010 2606—VOLUME 41A, OCTOBER 2010
study the changes of reactivity of solids either by milling or by postmilling operations. It has been reported that the nanostructured composites with uniform distribution of reinforcement particles could be obtained by mechanochemical synthesis. Dispersion of fine reinforcement particles by mechanical alloying within a nanostructure matrix is beneficial to the mechanical properties of the composite.[11,12] Solid-state reactions with large negative enthalpies generally occur in a self-propagating combustion manner during ball milling and may lead to a coarse microstructure. These reactions can hardly be controlled effectively. For example, with an initial powder mixture of Al and Cr2O3, alumina-based composites are obtained through a displacement reaction induced by reactive ball milling, which is basically a conventional thermite reaction.[13,14] In this case, the following reaction may take place during milling in a combustion manner:[15] Cr2 O3 þ 2Al ¼ 2Cr þ Al2 O3 DG ¼ 555:7 þ 60T kJ mol1
½1
In order to produce nanocomposites, the self-pro
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