Texture Modification During Extrusion of Some Mg Alloys
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TRODUCTION
DEFORMATION structures produced during hightemperature extrusion of Mg rods recrystallize to produce a ring fiber texture with the c-axes of the grains perpendicular to the extrusion direction (ED).[1,2] Such extrusions exhibit poor ductility and strong anisotropy[3] in their mechanical properties. Texture weakening or randomization is a desirable approach to obtain high ductility Mg alloys, and a number of cases of weak or nonbasal textures were reported recently for Mg alloy sheets and extrusions.[3–7] Various authors have suggested that the texture could be weakened via dynamic recrystallization during processing with the addition of Y, Ce, and other rare earth elements. Mishra et al.[5] showed that tensile ductility of extruded Mg-0.2 wt pct Ce is significantly enhanced due to texture randomization, where the basal planes become oriented at 40 to 50 deg to the ED in the recrystallized microstructure. Mackenzie and Pekguleryuz pointed out[7] the weakening of the basal texture in Mg-Zn-Ce alloy sheet with Ce addition. No weakening of the basal texture in Mg-AlCe tubes was observed.[8] Texture weakening in Mg alloy was associated with various phenomena, including abnormal grain growth,[9,10] particle-stimulated nucleation (PSN),[3,6] particle pinning,[11] solute drag,[4] dynamic strain aging,[12] and heterogeneous deformation leading to ease of shear band formation.[6,7] To clarify the effect of strain on the recrystallization texture after extrusion and to optimize the processing conditions for producing random texture in the Mg-Ce extrusions, the microstructure of Mg-0.2 wt pct Ce alloy rod at various extrusion ratios was investigated in this study. In addition, at the LI JIN, Associate Professor, is with the National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. Contact e-mail: j_jinli@sjtu. edu.cn RAJA K. MISHRA, Staff Researcher, and ANIL K. SACHDEV, Lab Group Manager, are with General Motors Global Research & Development, Warren, MI 48090-9055. Manuscript submitted December 22, 2010. Article published online January 5, 2012 2148—VOLUME 43A, JUNE 2012
optimized extrusion ratio identified for Mg-Ce, the microstructures of Mg-Ca, Mg-Al, and Mg-Al-Ce alloy rods were processed to develop solution-strengthened and texture-modified alloys for better strength-ductility combinations. The approach for developing wrought Mg alloys with texture randomization and strengthening via alloying and extrusion process optimization is discussed on the basis of the results. The choice of extrusion ratio used in this study was determined using the findings reported by Jiang et al.,[12] who show that Mg-Ce alloy exhibits dynamic strain aging in a narrow window of temperature and strain rate; and if the extrusion conditions correspond to this window of strain rate and temperature, texture randomization can occur. If dynamic strain aging is suppressed by extruding the material outside this strain rate–temperature window, the same alloy should exhibit no texture
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