Strength and Ductility with Dual Grain-Size and Texture Gradients in AZ31 Mg Alloy
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INTRODUCTION
MAGNESIUM (Mg) alloy, the lightest among structural materials, is an attractive material for reducing vehicle weight thus increasing the efficiency in terms of fuel consumption and reducing the emission of greenhouse gas.[1,2] However, their applications have been limited due to their low strength and poor deformability.[1] According to the fundamental principle of tailoring the relative activities of basal slips, non-basal slips, and twins, several approaches to improving the deformability and strength of Mg alloys have been proposed, such as increasing processing temperature,[3,4] alloying with appropriate solutes,[5–8] refining grain sizes,[9–11] weakening textures,[12–14] and forming metallic laminates.[15–17] Refining grain sizes of magnesium alloys can be accomplished by severe plastic deformation (SPD) processes, such as equal channel angular pressing (ECAP)[10,18] and high-pressure torsion (HPT).[19,20] Refining grain sizes in the surface layer has been demonstrated to enhance mechanical properties of the bulk materials, and corresponding techniques are easy to scale up. For instance, surface mechanical attrition treatment (SMAT) can refine grains in the surface layer of different materials[21–24] and develop the grain-size gradient from the surface. Corresponding to the gradient of grain size, the incompatibility of plastic deformation
JIAWEI YAN, JIAWEI MA, and YAO SHEN are with the State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. Contact e-mail: [email protected] JIAN WANG is with the Department of Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588. Manuscript submitted March 14, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
will develop in different depths, and results in an extra strain hardening and a long-range back stress. Compared with homogeneous grain refining,[22,24] refining grain sizes in the surface layer could result in a superior ductility–strength combination of surface-treated materials. In addition to refining grain sizes with a gradient, texture can also influence the tensile strength and ductility because of the strong plasticity anisotropy in Mg alloys.[13] Recently, Chen et al.[25] has found that, in addition to refining grain sizes, SMAT can also modify textures with a gradient distribution along depth. Such texture gradient significantly improves the ductility–strength combination compared with the grain-size gradient, e.g., in steel,[22,26] aluminum,[27] and magnesium alloys without strong basal textures.[28] In our experiment, we modified grain size and texture in the surface layer of AZ31 sheets on both sides by surface rotation rolling (SRR) technique. Compared with SMAT[25,29] and other similar surface treatments,[23,30,31] SRR treatment can be utilized for large plates and result in fine surface finishing. By changing vertical loads, we tailored the microstructure with gradients in grain size and basal texture contents, and conducted tensile tes
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