Balanced strength and ductility by asymmetric gradient nanostructure in AZ91 Mg alloy

Abstract

High-strength Mg alloys have historically suffered from a challenge in achieving good ductility. Here, we report an asymmetric gradient nanostructure design prepared by ultrasonic severe surface rolling (USSR) at room temperature. Unlike conventional gradient-nanostructured materials that employ a hard-soft-hard sandwich structure, this new design incorporates a combined gradient distribution of grain microstructure and nanoprecipitates throughout the entire sample along the thickness direction. The nanoprecipitates are identified as the β-Mg17Al12 phase and are primarily generated through In-situ precipitation promoted by the USSR-induced high-density dislocations and temperature increment. Benefiting from this unique microstructure, an outstanding strength-ductility synergy is achieved, with a yield strength of 372.8 MPa, an ultimate tensile strength of 453.3 MPa, and an elongation of 11.5%. The enhanced strength can be attributed to several mechanisms, including grain boundary strengthening, dislocation strengthening, precipitation strengthening, twin strengthening, and hetero-deformation induced (HDI) strengthening. The HDI hardening and activation of multiple deformation modes also contribute to good ductility. This work provides a promising and effective method for overcoming the longstanding strength-ductility trade-off dilemma in Mg alloys.