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INTRODUCTION

EQUAL-CHANNEL angular pressing (ECAP) is now widely used to refine the grain structure of metallic materials to the submicrometer range. The grain refinement imposed by ECAP not only leads to improved strength at low temperatures but also introduces superplastic properties provided the fine grain structure is stable at high temperatures. An early publication summarized the reports of superplasticity in materials processed by ECAP[2] and an examination of this tabulation shows that the numbers of reports of superplasticity in aluminum alloys is significantly higher than for magnesium alloys. This lower number of reports of superplastic flow in magnesium alloys is generally attributed to the difficulty of processing these materials by ECAP. Specifically, the first attempt to process pure magnesium by ECAP was conducted at very high temperatures due to the occurrence of cracks at lower temperatures[3] and these high temperatures compromised the ability to achieve significant grain refinement by ECAP. Later, processing routes were developed, involving a combination of prior extrusion and ECAP[4] in order to overcome the lack of ductility in magnesium alloys and enable processing at lower temperatures. The successful processing methods were summarized in a recent report[5] and analyzed in [1]

ROBERTO B. FIGUEIREDO, Assistant Professor, is with the Department of Materials Engineering and Civil Construction, Universidade Federal de Minas Gerais, Belo Horizonte, MG 30160-030, Brazil. Contact e-mail: fi[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 also with the Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, U.K. Manuscript submitted May 30, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A

terms of the mechanism of grain refinement in magnesium. Basically, it appears that successful processing of magnesium alloys at lower temperatures follows two different possible strategies. First, cracking may be avoided by preventing the development of damage through an increase in the channel angle in the ECAP die[6,7] or by incorporating a back-pressure into the pressing operation.[8] Second, shear banding and segmentation may be avoided by breaking up the coarse grain structure of magnesium through preliminary processing by extrusion[9] or rolling[10] or by using intermediate steps of ECAP at a high temperature followed by ECAP at a lower temperature.[11–14] The development of procedures for processing magnesium alloys at moderate temperatures led to significant grain refinement in these materials and to the introduction of superplastic capabilities. Nevertheless, inspection shows that most of the reports of superplasticity in magnesium alloys relate to materials containing precipitates. Thus, the first report of enhanced elongation in the AZ31 (Mg-3 pct Al-1 pct Zn) single-phase alloy described achieving a ma