JOURNAL OF MATERIALS SCIENCE 39 (2 0 0 4 ) 127 – 132 6061 Al reinforced with zirconium diboride particles processed by conventional powder metallurgy and mechanical alloying J. B. FOGAGNOLO Department of Manufacturing Engineering, Mechanical Engineering Faculty, State University of Campinas, Cidade Universit ´ aria, 13083-970, Campinas-SP, Brazil; Materials Science and Engineering Department, Universidad Carlos III de Madrid, Avenida de la Universidad, 30, E-28911, Legan ´ es, Spain E-mail: jfoga@iris.ufscar.br M. H. ROBERT Department of Manufacturing Engineering, Mechanical Engineering Faculty, State University of Campinas, Cidade Universit ´ aria, 13083-970, Campinas-SP, Brazil E. M. RUIZ-NAVAS, J. M. TORRALBA Materials Science and Engineering Department, Universidad Carlos III de Madrid, Avenida de la Universidad, 30, E-28911, Legan ´ es, Spain The homogenous distribution of the reinforcement phase is an essential condition for a composite material to achieve its superior performance. Powder metallurgy (PM) can produce metal matrix composites in a wide range of matrix reinforcement compositions without the segregation phenomena typical of casting processes. Particularly, mechanical alloying can be used to mix the matrix and reinforcement particles, enhancing the homogeneity of the reinforcement distribution. This work investigates the production of aluminium 6061 reinforced with zirconium diboride by mechanical alloying followed by cold pressing and hot extrusion, and compares the results with the same composite produced by conventional PM and hot extrusion. The incorporation of the ZrB 2 particles produces only a small increase in the material hardness, but a small decrease in the UTS when conventional PM is employed. Mechanical alloying breaks the reinforcement particle clusters, eliminates most of the cracks present in the surface of the reinforcement particles, decreases its size and improves its distribution. This enhancement of the composite structure, in addition to the metallurgical aspects promoted by mechanical alloying in the matrix, brings approximately 100% improvements in the composite UTS and hardness, compared with the composites obtained by PM. C  2004 Kluwer Academic Publishers 1. Introduction Metal matrix composites (MMC) reinforced with ce- ramic materials usually have better service temperature, strength, creep resistance, wear resistance, and thermal stability, than the unreinforced matrix [1]. Particulate- reinforced composites, though not achieving the level of improvement of continuous fiber-reinforced compos- ites, give isotropic materials with a better property/cost relation. Particularly in the case of discontinuous MMCs, the homogeneous distribution of the reinforcing phase is an essential requirement [2]. Defects such as clusters of reinforcement particles impair the mechanical prop- erties of the composite. Differences in particle sizes, densities, geometries, flow or development of an elec- trical charge all contribute to particle agglomeration [3]. Powder Metallurgy (PM) provides a better reinforce- ment distribution for a wide of reinforcement contents, when compared with casting process. In PM, the mixing of the matrix and reinforcement powders is the critical step towards a homogeneous distribution throughout the consolidated composite material, although subse- quent processes, such as powder extrusion, can help to optimise the reinforcement distribution [4–6]. High-energy ball milling or mechanical alloying has been successfully used to improve particle distribu- tion throughout the matrix [7–13]. Mechanical alloy- ing, in which mixtures of powders are milled together in a high-energy mill, involves repeated deformation/ welding/fracture mechanisms [14, 15]. Ceramic particles, mainly SiC and Al 2 O 3 , are the most widely used materials for reinforcement of alu- minium alloys. More recently, new families of particle reinforcement have been used with promising re- sults: intermetallic compounds (Al Ni, Al Fe, Al Nb systems) [16, 17] and nitrides (Si 3 N 4 , AlN) [18–20]. 0022–2461 C  2004 Kluwer Academic Publishers 127