Influence of pressing speed on microstructural development in equal-channel angular pressing
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I. INTRODUCTION
EXPERIMENTS have shown that intense plastic straining procedures, such as equal-channel angular (ECA) pressing, provide the capability for refining the grain size of polycrystalline materials down to, typically, within the submicrometer range. In ECA pressing, a very large plastic strain is introduced into the sample through simple shear by a procedure of repetitive pressings through a special die.[1,2] An important advantage of using this procedure, which has potential industrial applications, is that the grains may be refined to a sufficiently small size that it is possible to achieve superplastic ductilities at high strain rates above ,1022 s21; there are several recent examples of this effect in various aluminum-based alloys containing Zr or Sc additions.[3–6] In addition, several reports have documented significant differences in the physical properties of samples which had ultrafine grain sizes produced by ECA pressing, as compared to the same materials in the unpressed condition.[7,8,9] All of these results serve to confirm the potential utility of the ECA pressing procedure in making controlled and beneficial changes in the properties of materials. It is now apparent that any significant utilization of the ECA pressing method will require a detailed understanding of the experimental parameters which dictate the development of the ultrafine-grained structures. The requisite experimental parameters associated with ECA pressing fall into two distinct categories. On the one hand, it has been shown that the nature of the microstructural development PATRICK B. BERBON, formerly Research Assistant, Departments of Materials Science and Mechanical Engineering, University of Southern California, is Member of Technical Staff, Structural Metals Division, MSA25, Rockwell Science Center, Thousand Oaks, CA 91360. MINORU FURUKAWA, Associate Professor, is with the Department of Technology, Fukuoka University of Education, Munakata, Fukuoka 811-4192, Japan. ZENJI HORITA, Associate Professor, and MINORU NEMOTO, Professor, are with the Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 812-8581, Japan. TERENCE G. LANGDON, Professor, is with the Departments of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453. Manuscript submitted November 23, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
depends upon the precise testing procedure, such as the total strain imposed on the sample during pressing[6,10–12] and the nature of the shearing imposed in each passage through the die, which is a consequence of any rotation of the sample between consecutive pressings.[2,10,11,13–17] On the other hand, there are also important variables associated with the nature of the ECA pressing facility including, for example, the angle subtended between the two channels within the die and the speed at which the pressing is conducted. An earlier report examined the influence of the channel angle and demonstrated that an array of
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