Achieving strength-ductility synergy and low mechanical anisotropy in a Mg-Zn based alloy at 77 K via Gd addition and asymmetric extrusion

Abstract

There is a challenge in preparing low-cost Mg-Zn-based alloys with synergistic strength-ductility at cryogenic condition. In the present work, it is found that asymmetric extrusion (ASE) unexpectedly led to texture strengthening in a Mg-Zn-Gd (ZG21) alloy by promoting a distinct preferred orientation (the $\left[10\bar{1}2\right]$/$\left[10\overline{1}1\right]$ texture), rather than the expected weakening; such texture component causes a significantly reduced mechanical anisotropy across three loading directions (ED, TD and 45°) of the extruded plate. Moreover, the addition of a small amount of Gadolinium (Gd) combined with asymmetric extrusion also resulted in substantially enhanced mechanical properties: yield strength (YS) increased by 113 MPa (179% increment), ultimate tensile strength (UTS) rose by 75 MPa (20% increment), while achieving a fracture elongation of 14% at 77 K. The enhanced yield strength (YS) originated mainly from grain boundary and texture strengthening, where the latter critically governed mechanical anisotropy. Systematic characterization via EBSD assisted slip-traces, MWH-MWA method based dislocation density calculations, and TEM observations reveals the evolution of 〈a〉, 〈c〉 and 〈c + a〉 dislocation densities with strains and intuitive observation of interactions among 〈c + a〉, 〈a〉 and 〈c〉 dislocations. The activation of extensive non-basal 〈c + a〉 slip, which exhibits a higher multiplication rate than <a>-type dislocations at higher strains, primarily contributes to the enhanced cryogenic plasticity. This work provides a novel strategy for synergistic cryogenic strength-ductility enhancement in cost-effective Mg-Zn based alloys.