Asymmetric twinning modes in a triple-textured Mg-12Gd-1.5Yb-0.5Zr alloy yielding high reverse tension-compression asymmetry

Extension twinning has been widely accepted as the underlying cause for the typical tension-compression yield strength asymmetry in traditional <01 ̅ 1 0>-fiber-textured wrought magnesium alloys. However, the twinning modes in the <0001>-textured grains along with the mechanism for alloy’s reverse tension-compression asymmetry were hitherto unclear. In this work, a Mg-12Gd-1.5Yb-0.5Zr in wt.% was prepared by traditional hot-extrusion, and it exhibits three texture components, namely, the basal component (<0001>//ED (extrusion direction)), the conventional fiber component (<01 ̅ 1 0>//ED), and the rare-earth (RE)-type component (<11 ̅ 2 1>//ED). Interestingly, the studied alloy shows a pronounced reversed yield asymmetry, with the compressive yield strength being ∼1.21 times greater than the tensile yield strength. Transmission Kikuchi diffraction (TKD) examinations reveal that near yielding, extension twinning occurred in both <0001>- and RE-textured grains with sizes greater than 3.5 μm under tension, while only in non-recrystallized grains (<01 ̅ 1 0>-fiber-texture) textures under compression. Furthermore, the nucleation of almost all {10 ̅ 1 2} twins was governed by the Schmid law under compression while approximately half of the twins were nucleated by local stress concentration in parent or neighbor grains, which was proven by quasi-in-situ transmission electron microscopy (TEM) examinations. As a result, the yield-strength contributions from these three textured grains are differential under tension and compression, and the difference was calculated to be 59 MPa, which is reasonably agreeable with the experimental value of ∼48 MPa. Therefore, the relatively coarse <0001>-textured grains are key to regulating the tension-compression yield strength asymmetry for wrought Mg alloys.