An evaluation of the flow behavior during high strain rate superplasticity in an Al-Mg-Sc alloy
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EQUAL-channel angular (ECA) pressing is a processing procedure in which samples are subjected to very intense plastic strains.[1,2] It is now well established that this procedure and other intense plastic straining techniques such as high-pressure torsion (HPT) are capable of producing a substantial reduction in the grain size of metals to the submicrometer or even the nanometer level.[3,4] However, an important advantage of ECA pressing by comparison with HPT is that there is a potential for scaling the procedure for use in the fabrication of relatively large bulk samples. Experiments on aluminum-based alloys have shown that, provided the ultrafine grain sizes introduced by ECA pressing are retained at elevated temperatures through the presence of small precipitates, it is possible to achieve superplastic flow in these materials at strain rates at and above 10⫺2 s⫺1;[5–12] these rates are within the range normally designated as high strain rate superplasticity (HSR SP).[13] Despite the numerous reports of the occurrence of HSR SP after ECA pressing in aluminum alloys, the results reported to date have been insufficient to permit a detailed evaluation of the flow characteristics of these materials. This difficulty has arisen because of the limited numbers of samples that are generally processed using the ECA pressing procedure. Accordingly, the present investigation was initiated with two primary objectives. First, to obtain a detailed set of experimental data on samples of an aluminum alloy subjected to ECA pressing and with the subsequent tensile testing conducted over a reasonably wide range of strain rates and temperatures. Second, to use these data for a SHOGO KOMURA, Graduate Student, and ZENJI HORITA, Associate Professor, are with the Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 812-8581, Japan. MINORU FURUKAWA, Professor, is with the Department of Technology, Fukuoka University of Education, Munakata, Fukuoka 811-4192, Japan. MINORU NEMOTO, formerly Professor, Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, is Principal, Sasebo National College of Technology, Sasebo 857-1193, Japan. TERENCE G. LANGDON, Professor, is with the Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453. Manuscript submitted June 5, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
detailed analysis to determine the flow characteristics associated with these materials. An additional secondary objective was to make a direct comparison with the flow behavior and the elongations obtained in the same alloy after conventional cold rolling (CR) without ECA pressing. The experiments were conducted using an Al-3 pct Mg0.2 pct Sc alloy. This alloy was selected because it is well established that the presence of small amounts of Sc in aluminum-based alloys increase the recrystallization temperature,[14,15] and it was shown earlier that it is possible to retain a
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