Effect of Nd Additions on Extrusion Texture Development and on Slip Activity in a Mg-Mn Alloy
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INTRODUCTION
MAGNESIUM (Mg) and its alloys are promising structural materials due to their weight-saving possibilities. However, wrought Mg products are typically characterized by an excessive directionality of the mechanical properties,[1] which restricts many of their potential applications. As such, the mechanical properties of Mg alloy extrusions are affected by undesirable yield asymmetry, which results in limited formability and unusual bending characteristics, such as failure initiation on the compressive side of bent parts.[2] Typically, the compressive yield stress can be as little as half the tensile yield stress when tested along the extrusion direction (ED).[1] The yield asymmetry of extruded round bars is due to the formation of the well-known prismatic h10 10i fiber texture or the h10 10i–h11 20i double fiber texture, with the {0001} basal planes being parallel to the ED.[3] Due to the polar nature of mechanical twinning,[4] extension twinning along the {10 12} planes is active only when the deformation conditions are such that an extension along the c-axis takes place.[5] Accordingly, a strong typical extrusion texture favors twinning in compression, but not in tension along the ED, which results in the above-described asymmetry between the tensile and the compressive yield stresses. PALOMA HIDALGO-MANRIQUE, Research Associate, and M.T. PE´REZ-PRADO, Senior Researcher, are with the IMDEA Materials Institute, C/Eric Kandel 2, 28906 Getafe, Madrid, Spain. Contact e-mail: [email protected] S.B. YI and J. BOHLEN, Scientists, and D. LETZIG, Head of Department, are with the Magnesium Wrought Alloys Department, Magnesium Innovation Centre MagIC, Helmholtz-Zentrum Geesthacht, Max-Planck-Strasse 1, 21502 Geesthacht, Germany. Manuscript submitted March 4, 2013. Article published online June 4, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
One way of modifying the deformation behavior of the Mg extrusions, and in particular the twinning response, is to alter the extrusion texture by alloying. It has been shown that the addition of certain rare-earth (RE) elements including yttrium (Y), even in dilute concentrations, promotes weaker extrusion textures or broader distributions of the basal planes toward the ED, leading to reduced yield asymmetries.[6–8] It is currently assumed that the texture weakening in the RE-containing Mg alloys is primarily a recrystallization phenomenon rather than a direct effect on the deformation texture. However, the specific mechanisms are currently under discussion. It has been suggested that the weakened texture can be attributed to particle-stimulation nucleation (PSN).[6] Alternatively, weakened textures have been associated to nucleation at deformation heterogeneities, such as shear bands.[9] It has also been proposed that RE elements have an effect on boundary mobilities leading to oriented growth.[7] Such effects may be related to a form of particle pinning or solute drag that alters orientation relationships for high boundary mobility. In this way, unusual orientation
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