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I. INTRODUCTION

DISLOCATION plasticity of Mg and its alloys is highly anisotropic at ambient temperature. Concentrated Mg-Li alloys are the exception.[1,2] According to measured data in single-crystal Mg, the critical resolved shear stress (CRSS) of the basal
a slip system is approximately 1/100 of that of other slip systems at room temperature.[3–6] This indicates that the basal
a slip occurs with substantial ease in comparison with the other slip systems. The limited slip on the basal plane provides only two independent slip systems and has been considered as a major reason for the poor ductility of Mg alloys. In spite of the highly anisotropic nature of plastic deformation, commercial alloys of polycrystalline Mg show reasonably good tensile ductility at room temperature.[7] Fine-grained AZ31 Mg alloys show more than 40 pct of elongation to failure.[8] Even superplastic elongation of 180 pct can be attained in pure Mg having a submicron grain size.[9] These results in polycrystalline Mg alloys are not expected from the anisotropic data of single crystals and call for further investigation of deformation mechanisms characteristic to polycrystals. Although the tensile ductility can be improved by grain refinement, failure occurs catastrophically with very little local necking. The brittle nature of failure has been attributed by some researchers to twinning.[10–13] However, the current understanding of twinning is also based on the limited data obtained in single crystals. Since twinning stress and twin types can be substantially altered by a localized stress concentration within polycrystalline grains,[14] renewed J. KOIKE, Professor, is with the Department of Materials Science, Tohoku University, Sendai 980-8579, Japan. Contact e-mail: koikej@ material.tohoku.ac.jp This article is based on a presentation made in the symposium entitled “Phase Transformations and Deformation in Magnesium Alloys,” which occurred during the Spring TMS meeting, March 14–17, 2004, in Charlotte, NC, under the auspices of ASM-MSCTS Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A

attention and detailed work are necessary with regard to twinning and its effects on deformation and fracture mechanisms in polycrystalline Mg alloys. This article reviews the status of the current understanding and its shortcomings in explaining the deformation and fracture behavior in polycrystalline Mg alloys. Our recent results are also presented to discuss possible mechanisms for the good tensile ductility and the catastrophic failure in AZ31 and AZ61 Mg alloys. II. DISLOCATION SLIP In the case of single-crystal Mg, basal dislocation slip dominates the deformation mechanism at room temperature. However, the situation is different in polycrystalline Mg because of constraint by neighboring grains. If only the basal slip occurs in polycrystals, only two independent slip systems are available out of the necessary five, and this causes strain incompatibility at grain boundaries. When the grain boundaries are strong enough, which