Deformation Heterogeneity on the Cross-Sectional Planes of a Magnesium Alloy Processed by High-Pressure Torsion
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INTRODUCTION
SEVERE plastic deformation (SPD) processes have been studied extensively as convenient methods for the production of ultrafine-grained and nanostructured metals and their alloys.[1] Processing by SPD is generally imposed using one of three different basic techniques: accumulative roll bonding (ARB),[2] equal-channel angular pressing (ECAP),[3] and high-pressure torsion (HPT).[4] Processing by ARB involves the cold rolling of sheets with a thickness reduction of about 50 pct per pass, cutting the samples transversally at their midlength between passes, and then placing these two halves on top of each other and rolling again. In practice, the success of ARB depends on an adequate bonding of the two samples during the cold rolling operation and, if successful, the process permits the production of materials having a sheet geometry. The most widely studied and fundamental SPD process is ECAP, where the sample is generally in the form of a rod or bar and a plane strain shearing is achieved inside a closed channel within a die. When processing by ECAP, high strains are attained by pressing through a series of successive passes. An important advantage of ECAP is that the process can be easily scaled-up for use with relatively large bulk materials.[5] Much recent attention has been devoted to ROBERTO B. FIGUEIREDO, Postdoctoral Researcher, is with the Department of Metullurgical and Materials Engineering, Universidade Federal de Minas Gerais, Belo Horizonte, 31270-901, Minas Gerais, Brazil. MARIA TERESA P. AGUILAR, Associate Professor, is with the Department of Materials and Construction, Universidade Federal de Minas Gerais. PAULO R. CETLIN, Professor, is with the Department of Mechanical Engineering, Universidade Federal de Minas Gerais. Contact e-mail: [email protected] 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 Materials Research Group, School of Engineering Sciences, University of Southampton, Southampton SO17 1BJ, United Kingdom. Manuscript submitted December 20, 2010. Article published online February 1, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
HPT, because it generally produces greater grain refinement than ECAP[6] and the processed grains are often within the nanometer range, which is defined formally as grain sizes less than 100 nm.[7] In HPT, the sample is usually in the form of a thin disk that is subjected to a high imposed pressure and concurrent torsional straining. Some experimental results are now available where HPT was conducted using larger cylindrical samples.[8,9] Processing by SPD was applied to a very wide range of materials. However, there is a special interest in using SPD processing for magnesium alloys, because these materials have an excellent strength/density ratio and there is a potential for achieving additional strengthening using SPD processing. In practice, however, it is now recognized that there are often significant difficu
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