Dynamic deformation behavior of ultrafine-grained low-carbon steels fabricated by equal-channel angular pressing
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I. INTRODUCTION
RECENTLY, there have been many active investigations to obtain very fine grains in materials smaller than submicrometer in size.[1–14] These works are motivational, because fine-grained materials show improvements in mechanical properties such as elastic modulus,[2,5] strength,[1–4,6,11] toughness,[2,6,11] superplasticity,[1,7,8] as well as in physical properties such as the Debye and Curie temperatures.[2,5] Ways to achieve grain refinement include thermomechanical processing,[11] mechanical attrition,[12] rapid solidification, and crystallization of amorphous alloys. However, they have been shown to have limited application to actual mass production, because sufficient bulk production is not easy to obtain from these methods and a considerable amount of pores are often present inside the produced materials. Various methods with new concepts to overcome these shortcomings are currently under development, in which grain refinement is achieved by applying severe plastic deformation to materials without BYOUNGCHUL HWANG, Postdoctoral Research Associate, and YANG GON KIM, Research Assistant, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. HAN SANG LEE, Assistant Engineer, Production Engineering Team, Information Technology Display Center, LCD Business, Samsung Electronics Co., Ltd., Cheonan 330-300, Korea, is also Research Assistant, Center for Advanced Aerospace Materials, Pohang University of Science and Technology. SUNGHAK LEE, Professor, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, is also with the Materials Science and Engineering Department, Pohang University of Science and Technology. Contact e-mail: shlee@ postech.ac.kr BYOUNG DOO AHN, Research Assistant, and DONG HYUK SHIN, Professor, are with the Department of Metallurgy and Materials Science, Hanyang University, Ansan 425-791, Korea. CHANG GIL LEE, Principal Researcher, is with the Materials Processing Department, Korea Institute of Machinery and Materials, Changwon 641-010, Korea. Manuscript submitted April 14, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
heat treatment or composition change. Representative methods are severe torsional straining,[1,10] equal-channel angular pressing (ECAP),[2,5–9] equal-channel angular rolling,[3] and accumulative roll-bonding.[4] Among them, ECAP is highly effective in obtaining ultrafine grains sized smaller than 1 m, depending on the amount of strain and processing path. Although ultrafine-grained materials fabricated by ECAP show a high level of yield strength and flow strength, the ductility and fracture toughness can be substantially reduced under dynamic loading conditions, as they show low elongation and work hardenability. There have been studies on the deformation and fracture behavior of ultrafine-grained materials under either static or quasistatic loading, but few studies have been done on the deformation behavior under dynamic loading conditions.[12,13,14] Since the ultrafin
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