Strain Rate Sensitivity, Work Hardening, and Fracture Behavior of an Al-Mg TiO 2 Nanocomposite Prepared by Friction Stir

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ALUMINUM matrix nanocomposites (AMNCs) are an important class of new materials for structural and electrical applications.[1] Particulate-reinforced AMNCs are of particular interest because of their isotropic properties and relatively low-cost fabrication procedure.[2] It is well established that the mechanical properties of AMNCs are controlled by the size and volume fraction of the reinforcements as well as the nature of the matrix-reinforcement interface.[1] Superior mechanical properties can be achieved when fine and stable reinforcements with good interfacial bonding are dispersed uniformly in the matrix,[3,4] which can be achieved via in-situ reactions upon processing. Various methods such as mechanical alloying (MA),[5] reactive hot pressing (RHP),[6] reactive squeeze casting (RSC),[7] and combustion synthesis[8] have been used to prepare AMNCs, in which the reinforcement particles are formed in situ during processing. When the fabrication FARZAD KHODABAKHSHI, Ph.D. Student, and AMIRHOSSEIN KOKABI, Professor, are with the Department of Materials Science and Engineering, Sharif University of Technology, Azadi Avenue, P.O. Box 11365-9466, 14588 Tehran, Iran. ABDOLREZA SIMCHI, Professor, is with the Department of Materials Science and Engineering and Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O.Box 11365-9466, 14588 Tehran, Iran. Contact e-mail: [email protected] MARTIN NOSKO, Assistant Professor, is with the Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Racˇianska 75, 83102 Bratislava, Slovak Republic. PETER SˇVEC, Professor, is with the Institute of Physics, Slovak Academy of Sciences, Du´bravska´ cesta 9, 845 11 Bratislava 45, Slovak Republic. Manuscript submitted December 11, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A

process is combined with severe plastic deformation of the metal matrix, the grain structure is also refined yielding another advantage of ultra-fine-grained materials with regard to mechanical properties.[9] Friction stir processing (FSP) is a relatively new solidstate procedure for the fabrication of AMNCs.[10–17] In this process, a rotating tool pin is plunged into the surface of metallic sheets and traversed along their surface. The friction and plastic deformation imposed by the tool heat and soften the workpiece, while the tool pin promotes intermixing of material in a local region.[18] Severe plastic deformation of the metal matrix promotes mixing and refining of the grain structure and constituent phases, while the local temperature rise facilitates the formation of intermetallic phases such as Al3Fe, Al2Cu, Al3Ni, and Al3Ti.[10–12,15,17] Consequently, the fine-grained structure and homogeneous distribution of hard particles yield superior mechanical properties. For instance, Hsu et al.[10] and Zhang et al.[19] reported a considerable enhancement in elastic modulus and yield strength of Al-Al3Ti nanocomposites fabricated by reactive FSP of Al-Ti powder mixtures; similarly, Zhang et al.[17,20] processed Al-TiO2