Stable and Unstable Flow in Materials Processed by Equal-Channel Angular Pressing with an Emphasis on Magnesium Alloys
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EQUAL-CHANNEL angular pressing (ECAP) is an attractive processing method for refining the grain size of bulk polycrystalline metals, typically to the submicrometer level.[1] The processing procedure involves pressing a billet through a die constrained within a channel that is bent through an abrupt angle. Early experiments documented the basic experimental principles of grain refinement by ECAP[2,3] and, subsequently, this approach was used to critically examine the evolution of homogeneity and the mechanism of grain refinement during the pressing operation.[4,5] The plastic flow of the billet during processing by ECAP has been studied by means of several different methods, including slip line theory,[6,7] slip line experiROBERTO B. FIGUEIREDO, formerly Research Assistant, with the Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, is Postdoctoral Research Associate, Materials Research Group, School of Engineering Sciences, University of Southampton, Southampton, SO17 1BJ, United Kingdom. PAULO R. CETLIN, Professor, is with the Department of Metallurgical and Materials Engineering, Federal University of Minas Gerais, Belo Horizonte 30160-030, MG, Brazil. TERENCE G. LANGDON, Professor, is with the Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, and the School of Engineering Sciences, University of Southampton. Contact e-mail: [email protected] This article is based on a presentation given in the symposium entitled ‘‘Mechanical Behavior of Nanostructured Materials,’’ which occurred during the TMS Spring Meeting in San Francisco, CA, February 15–19, 2009, under the auspices of TMS, the TMS Electronic, Magnetic, and Photonic Materials Division, the TMS Materials Processing and Manufacturing Division, the TMS Structural Materials Division, the TMS Nanomechanical Materials Behavior Committee, the TMS Chemistry and Physics of Materials Committee, and the TMS/ASM Mechanical Behavior of Materials Committee. Article published online November 17, 2009 778—VOLUME 41A, APRIL 2010
ments,[8] and the use of plasticine,[9,10] but the majority of reports have used finite element modeling (FEM). In general, the most characteristic condition examined in these studies is the frictionless pressing of a material with perfect plastic behavior when pressed through a die having an internal angle of 90 deg between the two parts of the channel and no curvature at the external point of intersection of the two channels. For these conditions, it is now well established that relatively homogeneous shear deformation occurs throughout the cross section of the billet at the plane of intersection between the two channels of the die. Nevertheless, this ideal distribution of strain may be influenced by additional factors, including the overall material behavior or variations in the die geometry and pressing conditions. Several reports have used FEM to analyze the distribution of strain within the billet and th
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