Observations of unique plastic behavior in micro-pillars of an ultrafine-grained alloy
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esearch Letters
Observations of unique plastic behavior in micro-pillars of an ultrafine-grained alloy Nguyen Q. Chinh and Tivadar Gyo˝ ri, Department of Materials Physics, Eötvös Loránd University, H-1117 Budapest, Hungary Ruslan Z. Valiev, Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, Ufa 450000, Russia Péter Szommer, Gábor Varga, and Károly Havancsák, Department of Materials Physics, Eötvös Loránd University, H-1117 Budapest, Hungary Terence G. Langdon, Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089-1453, and Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK Address all correspondence to Nguyen Q. Chinh at [email protected] (Received 8 May 2012; accepted 1 June 2012)
Abstract When ultrafine-grained (UFG) samples are deformed plastically, it is necessary to consider the role of grain boundaries even at the micrometer scale in sample size. We report here the occurrence of intensive grain boundary sliding (GBS) at room temperature in micro-pillars of a UFG aluminum alloy having an unusually high strain rate sensitivity. A consequence of this GBS is that the intermittent flow with detrimental strain avalanches characterizing micro-sized conventional crystals is not present in UFG materials, thereby illustrating a potential for effectively applying these UFG materials in micro-devices.
Recently, the size-scale effects in plastic deformation have been studied both experimentally[1–3] and theoretically[4,5] for several different materials. In the case of conventional crystalline materials, as with metals, it is well established that the decreasing sample size may significantly affect the fundamental mechanisms of plasticity: for example, in dislocation storage, multiplication, motion, pinning, etc. Therefore, although the plasticity of macroscopic samples appears as a smooth process, in the plastic deformation of microcrystals specific dislocation avalanches are formed leading to an uncontrolled deformation process including also the possibility of catastrophic failure. Accordingly, micrometer-sized samples of coarse-grained metals are not suitable for use in the fabrication of micro-devices. It is well established that bulk ultrafine-grained (UFG) materials may be achieved by using severe plastic deformation (SPD) techniques,[6,7] decreasing the grain sizes into the submicrometer and nanometer ranges. Typically, the materials produced by SPD exhibit tensile ductilities below 10% at ambient temperature due to exhaustion in their work hardening capacity. Furthermore, this low ductility is correlated with extremely low strain rate sensitivity (SRS) of ~0.01–0.03 characterizing the mechanical behavior of these materials.[8] It was suggested that the mechanical properties of UFG metals may be improved if grain boundary sliding (GBS) is achieved at relatively low temperatures.[9–11] It was recently shown that SPD is capable of i
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