Effect of Ca Addition on the Intensity of the Rare Earth Texture Component in Extruded Magnesium Alloys
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TRODUCTION
BECAUSE of the intrinsic plastic anisotropy of hcp crystals, Mg wrought alloys with strong crystallographic textures generally exhibit limited formability. Basal textures, which are the most common deformation texture in wrought Mg alloys, are associated with very low Schmid factors (SFs) when the forming load is essentially applied along the c-axis. Under such conditions, high loads are required to promote slip and the concentration of orientations contributes to the high anisotropy. The formability of wrought Mg alloys can be improved by weakening the intensity of the basal texture in two ways. One is to increase the volume fraction of various components located some distance from the ideal basal orientation.[1–7] The second is to introduce some specific softer orientations associated with higher SFs, such as the rare earths (RE) texture component.[3–12] In extrusion, appropriate levels of RE additions lead to the formation of high intensities of the h1121i parallel to the extrusion direction (ED), referred to as the RE texture extrusion component.[12] The presence of this orientation, by replacing the conventional basal fiber texture, significantly improves the formability at ambient temperatures. This is accomplished by increasing the average SF due to the greater volume fraction of grains in which basal glide can take place. Furthermore, by
TONG WANG, Ph.D. Student, LAN JIANG, Research Associate, and JOHN J. JONAS, Professor, are with the Department of Materials Engineering, McGill University, Montreal, QC H3A 2B2, Canada. Contact e-mail: [email protected] RAJA K. MISHRA, Research Manager, is with the General Motors Global Research & Development Center, Warren, MI 48090. Manuscript submitted March 10, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
decreasing the intensity of the basal fiber texture, the plastic anisotropy of the workpiece is also reduced. The formation of the RE texture component can be linked in part to the large atomic radius difference between Mg and the RE elements and therefore to a strong solute drag effect. However, it is of interest that Ca also has a large atomic radius and is therefore a candidate to replace the rare earths, as has already been pointed out by Stanford.[13] In that work, the addition of Ca to Mg-1.3Mn-xCa alloys led to the introduction of RE textures during extrusion. More recently, it has been found that the addition of Ca to some Mg-Zn-Ca alloys can modify the rolling texture as well and in this way leads to increased elongation and higher stretch formability.[14,15] In our previous study of the extrusion behavior of Mg-0.5Ce and Mg-1.5Gd alloys,[16,17] a relationship was established between the formation of RE textures and the occurrence of dynamic strain aging (DSA). It was found that the strength of the RE component could be optimized by extruding under temperature and strain rate conditions that favor the occurrence of DSA. In addition, increasing the solute content broadened the range of conditions so that the RE texture component coul
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