In this study, a gradientstructured (GS) AZ31 Mg alloy sheet with high stretch formability is fabricated using turned bearing extrusion (TBE). The mechanism by which the gradient structure contributes to the improvement in formability is elucidated. The Erichsen index of the GS sheet reaches 5. 51 mm, representing an increase of up to 89. 3% compared to conventional extruded (CE) sheets. During the Erichsen cupping test, when the coarsegrained (CG) layer of the GS sheet is positioned on the inner side, the large grains promote the activation of deformation twins, thereby effectively enhancing the strain accommodation capacity in the thickness direction. Meanwhile, the finegrained (FG) outer layer effectively suppresses the formation of 101-1 and 101-1-101-2 twins, reducing local strain concentration.
Hua et al. (Tue,) studied this question.