Ultrafine Grain Formation in Ferritic Stainless Steel during Severe Plastic Deformation
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INTRODUCTION
THE growing interest in studies on ultrafine-grained metallic materials is motivated by engineering requirements for production of fine-scale parts mainly for electromechanical and medical devices as well as by attractive mechanical properties, such as high strength with sufficient ductility,[1] enhanced impact toughness,[2] low-temperature or high-strain-rate superplasticity,[3] and many others occasionally reported for submicrocrystalline metals and alloys.[4,5] One of the most promising methods for production of ultrafine-grained materials is based on severe deformation, i.e., processing by large strain plastic working at relatively low temperatures.[5,6] Several special techniques were developed to achieve very large strains. Among those are mechanical milling,[7,8] torsion under high pressure,[9] equal channel angular extrusion,[10,11] accumulative roll bonding,[12] etc., which are applicable to a wide variety of structural metals and alloys. These processing methods, however, require costly equipment and have to use some additional consolidation working to make a bulky product. On the other hand, some conventional processing methods such as multiple forging, which is well known from ancient times,[13] can also provide a very large strain in forged sample. Recently, the multidirectional or three-dimensional (3-D) cross forging was successfully used as a method of severe deformation.[14,15] The T. SAKAI, Professor, and H. MIURA, Associate Professor, are with the Department of Mechanical Engineering and Intelligent Systems, UEC Tokyo (The University of Electro-Communications), Chofu, Tokyo 182-8585, Japan. Contact e-mail: [email protected] A. BELYAKOV, Research Associate, formerly with the Structural Metals Center, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan, is with Belgorod State University, Belgorod 308034, Russia. Manuscript submitted January 1, 2008. Article published online May 17, 2008 2206—VOLUME 39A, SEPTEMBER 2008
advantages of this method are not only standard facilities and processing of sizeable samples, but also provision of flow stress behaviors during severe deformation.[16] It is generally agreed that the structural changes leading to submicrocrystalline structures during severe deformation follow a common sequence of microstructure evolution irrespective of differences in the processing methods.[14–21] The new fine grains result from a kind of strain-induced continuous reaction, which is the formation of dislocation sub-boundaries, especially geometrically necessary sub-boundaries, subdividing microvolumes with different operating slip systems, and gradual rise in the sub-boundary misorientations up to typical values of ordinary high-angle boundaries (HABs) in large strain. Any deformation heterogeneities, therefore, are expected to play an important role in the grain refinement by severe deformation. Large shear strains were shown to result in a rapid increase of misorientations among strain-induced sub-boundaries.[16,21–25] However, regulations of such a
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