Ultrafine-Grain Structure Formation in an Al-Mg-Sc Alloy During Warm ECAP
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NTRODUCTION
DURING the last decade, the fabrication of bulky ultrafine-grained (UFGed) structural materials by severe plastic deformation (SPD) has been one of the hottest subjects in the materials research field.[1–33] Metallic alloys processed by SPD are composed of UFGs with sizes below 1 lm and generally exhibit enhanced mechanical, physical, and chemical properties, which can be successfully utilized in commercial applications.[3,9,20,28] Several SPD techniques, such as equal channel angular pressing (ECAP),[1,3–8,10,13–16,18–21,23–26,29,31–33] accumulative roll bonding (ARB),[2,12] and multi-directional forging (MDF)[1,3,9,11,17,22,27,30] are mostly used for development of UFGed structures in numerous metals and alloys. These SPD techniques can also be a very valuable scientific tool for studying the microstructural development taking place during SPD.[1,4–8,11,15–33] The characteristics of SPD methods and bulky UFGed materials produced currently have reasonably been well known. Fine-grained Al alloys among the materials with high stacking fault energy have been frequently studied on the processing and their properties at low-to-high
OLEG SITDIKOV, Senior Research Associate, and ELENA AVTOKRATOVA, Research Associate, are with the Institute for Metals Superplasticity Problems, Russian Academy of Sciences, Khalturina 39, Ufa 450001, Russia. Contact e-mail: [email protected] TAKU SAKAI, Professor, is with UEC Tokyo (the University of Electro-Communications), Chofu, Tokyo 182-8585, Japan. KANEAKI TSUZAKI, Professor, is with the National Institute for Materials Science, Tsukuba 305-0047, Japan. Manuscript submitted September 2, 2011. Article published online September 26, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
temperatures.[4–6,8,10,12–21,23–26,29–33] It has been well understood that strain localization taking place in some Al alloys during SPD can result in significant intrinsic structure instability and lead to formation of deformation bands such as microshear bands (MSBs) at relatively low strains.[16,17,19,22,24–27,30,31,33] Their number and boundary misorientation increase with deformation, finally followed by a full development of fine-crystalline assemblies at high strains. Such a evolution mechanism of UFGed microstructure is often discussed as continuous dynamic recrystallization (cDRX).[11,17,22,27,34,35] The exact mechanisms of new grain formation, however, remain unclear yet, despite considerable studies being conducted on the development of UFGed structures. For instance, it has been emphasized in the recent studies[7,15,16,19–21,32] that our understanding of the processes occurring during severe deformation is still very superficial, and there have been widespread discussions regarding the grain refining mechanisms operating during SPD. This may be related to the fact that many important data regarding the microstructural development and various factors affecting grain refinement during SPD are still lacking, especially for heavily alloyed Al alloys. It is also not clear how the processing conditions, s
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