Fabrication of bulk ultrafine-grained materials through intense plastic straining
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INTRODUCTION
SEVERAL methods are available for the processing of materials with ultrafine grain sizes, including inert gas condensation,[1,2] high-energy ball milling,[3] and sliding wear.[4] These procedures are attractive for producing materials with grain sizes within the nanometer range, but there are disadvantages because some residual porosity remains after fabrication and it is difficult to use these techniques to make large bulk samples. As a consequence of these difficulties, much attention has been given to the alternative procedure of introducing an ultrafine grain size in a material through intense plastic straining. Torsion straining under a high pressure[5] and equal-channel angular (ECA) pressing,[6] in which a sample is pressed through a die with no change in the cross-sectional area, are two methods currently under investigation for the fabrication of materials with ultrafine grain sizes.[7–11] Although it is possible, in principle, to use these procedures to produce materials within the nanometer range, the grain sizes attained by these techniques are often in the submicrometer range of ;0.1 to 1.0 mm. Despite this possible limitation, however, ECA pressing is an especially attractive procedure PATRICK B. BERBON, Research Assistant, Department of Materials Science, and TERENCE G. LANGDON, Professor, Departments of Materials Science and Mechanical Engineering, are with the University of Southern California, Los Angeles, CA 90089-1453. NIKOLAI K. TSENEV, Senior Scientist, is with the Institute of Chemical Technology, Ufa State Petroleum Technical University, Ufa 450062, Russia. RUSLAN Z. VALIEV, Professor, is with the Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, Ufa 450000, Russia. MINORU FURUKAWA, Associate Professor, is with the Department of Technology, Fukuoka University of Education, Murakata Fukuoka 81141, Japan. ZENJI HORITA, Associate Professor, and MINORU NEMOTO, Professor, are with the Department of Materials Science and Engineering, Kyushu University, Fukuoka 812-81, Japan. This article is based on a presentation made in the symposium ‘‘Mechanical Behavior of Bulk Nanocrystalline Solids,’’ presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees. METALLURGICAL AND MATERIALS TRANSACTIONS A
because it is a simple process which may be readily adapted for a wide range of materials, the processed samples are free of any porosity, and there is the potential for fabricating large bulk samples which may be subsequently utilized in a wide range of applications. In the superplastic forming (SPF) industry, for example, it is well established that SPF becomes a viable processing tool only for high-added-value products in medium to low production runs of, typically, ;50 to 5000 pieces.[12] This limitation on SPF technology arises primarily because of the relatively l
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