Microstructure and mechanical properties of gradient ultrafine-grained Mg-Gd-Zr alloy

Fabricating gradient structure (GS) is increasingly adopted in structural engineering materials due to the unique combinations of mechanical properties. In the present work, a gradient ultrafine-grained structure with a change in the grain size up to three orders of magnitude from nano to micrometer is prepared on Mg-2Gd-0.4Zr (wt.%) alloy sheet with coarse grains (CG) by sliding friction treatment (SFT). Compared with homogeneous CG structure, the yield strength of GS + CG + GS is improved from 126 MPa to 243 MPa, acquiring an increase of 93%, and maintains an excellent ductility of 25%, showing an extraordinary strength-plasticity combination among the reported GS-containing Mg alloys. Two-beam condition TEM and EBSD analyses reveal that the refined grain structure and grain orientation modification can promote the activation of multiple dislocation slip systems such as and slipping in the GS area, which improves the coordinate ability of plastic deformation. In addition, GS + CG + GS owns higher strength, ductility and better strain hardening ability than that of the GS + CG sample. This is because the symmetrical sandwich structure can alleviate the strain localization so exhibits a better work hardening effect and resistance for failure. This work sheds light on the deformation behavior of heterostructured material and provides a new strategy to improve the mechanical properties of wrought Mg alloys.