An investigation of microstructure and grain-boundary evolution during ECA pressing of pure aluminum
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THE properties of metallic materials may be improved through grain refinement using processes that include severe plastic deformation. Recent studies have shown that ultrafine grain sizes in the submicrometer or even nanometer range can be achieved by imposing extremely large plastic strains during deformation processing.[1–4] Methods such as high-pressure torsion loading,[1,5–8] highenergy ball milling,[9] sliding wear,[10] or equal-channel angular pressing (ECAP)[11–14] are required in order to impart strains which are sufficiently large to produce such grain refinement. During ECAP, a billet of material is pressed through a die having two channels, of equal cross section, which intersect at an angle; the arrangement of these channels is illustrated in the schematic of Figure 1 for a 90 deg die. Ideally, a billet experiences simple shear without any change in cross-sectional area upon passage through the intersection of the die channels, and the process is, therefore, amenable to repetition. For the coordinate axes shown in Figure 1, the billet is pressed downward in S.D. TERHUNE, formerly Graduate Student with the Department of Mechanical Engineering, Naval Postgraduate School, is with the United States Navy. D.L. SWISHER, Graduate Student, is with the Department of Mechanical Engineering, Naval Postgraduate School. K. OH-ISHI, Postdoctoral Research Associate, and Z. HORITA, Professor, are with the Department of Materials Science and Engineering, Kyushu University, Fukuoka, 812-8581, Japan. T.G. LANGDON, Professor, is with the Departments of Aerospace, Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453. T.R. McNELLEY, Professor, is Chairman, Department of Mechanical Engineering, Naval Postgraduate School, Monterey, CA 93943-5146. Contact e-mail: [email protected]. mil Manuscript submitted July 6, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
the ⫺z direction through the vertical channel and then exits the die in the ⫺x direction. The x, y, and z planes are the transverse, flow, and longitudinal planes, respectively, and the shape change in each of these planes is indicated in the insets in Figure 1 for a cubic element of material that is pressed through the die. The sense of the shear in the y plane is also illustrated in the inset at the lower-right-hand side. Iwahashi et al.[13] have demonstrated that pure aluminum may be refined from an initial grain size of ⬃1.0 mm to an apparent grain size of ⬃1.3 m after ECAP for four pressings at room temperature through a die with a 90 deg angle between the die channels. As successive pressings were performed, the microstructure evolved from one consisting of subgrains and low-angle boundaries to an apparently recrystallized state, with grains surrounded by highangle boundaries. However, the conclusions regarding the grain boundaries were obtained by use of selected-area electron diffraction (SAED) methods in a transmission electron microscope. The SAED analysis used an aperture with a diameter of 12
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