Grain refinement and superplasticity in a magnesium alloy processed by equal-channel angular pressing

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I. INTRODUCTION

EQUAL-CHANNEL angular pressing (ECAP) is now an accepted processing technique for the development of an ultrafine-grained structure in bulk materials.[1–4] The grain sizes produced by ECAP processing are typically in the submicrometer range and this leads to high strength at ambient temperatures.[5,6] In addition, and provided the ultrafine grains are reasonably stable at elevated temperatures, there is a potential for developing superplastic characteristics in tensile testing,[7,8] with the superplastic flow occurring both at rapid strain rates and at relatively lower temperatures by comparison with conventional superplasticity.[9] Processing by ECAP entails pressing a sample, generally in the form of a bar or rod, through a die constrained within a channel which is bent through an abrupt angle usually equal to, or very close to, 90 deg. Since the crosssectional dimensions of the workpiece remain unchanged when it is pressed through the die, repetitive pressings may be conducted to introduce very high strains. It has been shown that, for a die having an angle of 90 deg between the two parts of the channel, a strain of 1 is introduced on each separate pass through the die.[10] The application of ECAP processing has been used successfully with several fcc metals including aluminum and copper. For example, there are numerous reports of high strength and high superplastic elongations in various aluminum alloys after processing by ECAP.[11–14] By contrast, only limited success has been reported when the ECAP procedure is used with hcp metals. An early attempt to achieve significant grain refinement in pure Mg with a grain size of YUICHI MIYAHARA and KIYOSHI MATSUBARA, Graduate Students, and ZENJI HORITA, 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 Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453. Contact e-mail: [email protected] This article is based on a presentation made at the Symposium entitled “Phase Transformations and Deformation in Magnesium Alloys,” which occurred during the Spring TMS meeting, March 14–18, 2004, in Charlotte, NC, under the auspices of the ASM-MSCTS Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A

400 m was unsuccessful, and it was reported instead that the as-pressed grain size was 100 m after pressing for two passes to a strain of 2 at 673 K.[15] Similarly, a relatively large grain size of 17 m was produced in an Mg0.9 pct Al solid solution alloy having an initial grain size of 100 m after pressing through two passes at 473 K,[15] where this and all subsequent compositions are given in weight percent. On the other hand, there are reports of some success in the ECAP processing of more complex Mg-based alloys where the alloys have two phases or dispersions of intermetallic precipitates.[16–27] It was shown very recentl

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