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EASY accessibility of an ultrafine-grained (UFG) regime by the current processing techniques have spurred research and development activities to better understand the physical, chemical, and mechanical properties of UFG materials.[1–5] Researchers all across the world are actively pursuing the research to understand the entire spectrum of properties of UFG materials. Among others, mechanical behavior is of paramount importance for structural application. The microstructure and mechanical properties are intertwined. Hence, equal efforts are being made to develop microstructures in a controlled way to meet a desired level of mechanical property requirements. The techniques to obtain UFG materials are classified in two basic categories: the bottom-up approach and the top-down approach. The details of these can be found elsewhere.[6,7] Severe plastic deformation (SPD) is a topdown approach and has been widely explored as a means to develop bulk ultrafine-grained materials.[6,8–14] Equalchannel angular pressing (ECAP),[6,8] high-pressure torsion (HPT),[9] accumulative roll bonding (ARB),[10,11] friction-stir processing (FSP),[12–14] etc., are some prime examples of SPD techniques. Among these, FSP is relatively new and slightly different from other SPD techniques. Unlike others, it can be considered as a hightemperature SPD technique due to the high temperature NILESH KUMAR, Postdoctoral Research Associate, and RAJIV S. MISHRA, Professor, are with the Department of Materials Science and Engineering, Center for Friction Stir Processing, University of North Texas, Denton, TX 76203. Contact e-mail: Rajiv.Mishra@ unt.edu Manuscript submitted May 23, 2012. Article published online October 17, 2012 934—VOLUME 44A, FEBRUARY 2013

involved because of frictional and adiabatic heating during processing of a material. Also, in comparison to other techniques, this is a relatively less explored technique, and efforts are being made to process UFG materials using FSP and understand their mechanical properties. FSP has emerged as a generic microstructure modification tool in the last decade. All the initial attempts to refine grain size by FSP were limited to fine-grained microstructure (1 to 10 lm).[15] Mishra and Ma[15] have tabulated the grain sizes obtained by FSP or friction-stir welding (FSW) under various processing conditions. It is clear from the experimental conditions and resulting grain size that an external cooling medium or a special design of tool was required to obtain UFG microstructure. Of late, there have been some efforts to obtain UFG microstructure by changing the processing parameters such as tool rotational rate (x) at constant tool traverse velocity (m) or the ratio x/m.[12,16,17] Su et al.[18] and Rhodes et al.[19] demonstrated the possibility of achieving grains as small as 25 to 100 nm by employing special cooling arrangement during FSP. But as mentioned earlier, most of the mean grain sizes tabulated by Mishra and Ma were larger than 1 lm under ordinary processing conditions.[15] Hence, these observations ar