The effect of grain size, strain rate, and temperature on the mechanical behavior of commercial purity aluminum
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INED materials have recently attracted considerable attention because of their interesting properties, such as high strength, high toughness, and low-temperature superplasticity. However, the conventional methods for producing fine-grained materials, such as rapid solidification and vapor condensation, are difficult to apply to bulk processing. A relatively new technique, equal channel angular extrusion (ECAE), pioneered by Segal[1,2] approximately two decades ago can produce bulk material with high accumulated strains and resultant fine microstructures. It has been demonstrated that pure aluminum with different purities,[3,4] Al-Mg alloys,[5–8] and other aluminum alloys[9,10] can be processed by ECAE to produce submicron-grained (SMG) structures. The mechanical properties of ultrafine-grained (UFG) material (typically 0.2 to 0.9 mm) differ from their coarsegrained (CG) counterpart.[11–14] It has been demonstrated that nanocrystalline (NC), typically ,0.1 mm, and UFG materials have higher strength, but their stress-strain curves display different work hardening behavior than CG materials.[11–14] As the grain size decreases to the SMG or NC range, the fraction of grain boundary to matrix increases. This can cause the deformation mode to change from dislocation dominated in large grains to grain boundary dominated in small grain regimes.[15,16,17] For smaller grain sizes (a few tens of nanometers), the empirical Hall– Petch relationship is expected to break down because it is based on dislocation pileup at boundaries. As the grain size decreases, the number of ‘‘piled up’’ dislocations becomes smaller, and, eventually, at a critical grain size, dislocations can no longer pile up at grain boundaries. Consequently, at P.L. SUN, formerly Postdoctoral Research Associate, MST-8, Los Alamos National Laboratory, is with the R&D Department, China Steel Corporation, Kaohsiung, Taiwan R.O.C. Contact e-mail: [email protected]. com.tw or [email protected] E.K. CERRETA, G.T. GRAY III, and J.F. BINGERT, Staff Members, are with MST-8, Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted December 13, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
this critical grain size, the yield stress no longer increases with decreasing grain size.[18] It is believed that grain boundary processes, such as grain boundary sliding, grain rotation, or interface-controlled plasticity,[19] are some of the mechanisms that can control the deformation at these small grain sizes. For this reason, the dislocation source length and dislocation mean free path need to be considered in ultrafine grains. Recently, studies on UFG/NC materials have principally focused on characterization of the fine grain structure, primarily focused on the grain size. However, the microstructures produced by severe plastic deformation (SPD) have a distribution of low- and high-angle deformation-induced boundaries surrounding nanometer to micrometer scaled volumes. This means that UFG/NC materials produced by SPD exhibit nanometer or submicron length scales but wit
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