Simultaneously improving strength and ductility of cast Mg-Gd-Zn alloy through Er addition

Abstract

Precipitation strengthening typically enhances strength at the expense of ductility, giving rise to the strength-ductility trade-off in magnesium-rare earth (Mg-RE) alloys. Erbium (Er), which exhibits relatively high solid solubility in Mg, offers great potential for strengthening and toughening Mg alloys. This study systematically investigates the influence of Er addition on the microstructure, mechanical properties, strengthening and toughening mechanisms of Mg-10Gd-xEr-0.2Zn-0.4Zr (x = 0, 1, 2, 4) (wt.%) alloys. The addition of Er causes a transformation of the eutectic phase from Mg₅Gd to Mg₃(Gd, Er) in the as-cast alloys, accompanied by an increase in the content of unexpected rare earth hydrides. Following appropriate heat treatment, a substantial amount of Er dissolves into the Mg matrix, resulting in pronounced solid solution strengthening. Additionally, a portion of Er participates in the precipitation process, significantly increasing the number densities and sizes of the β’ and β₁ phases. Slip trace analysis indicates that Er addition remarkably promotes the activation of non-basal slip, thereby enhancing homogeneous plastic deformation capability and contributing to improved ductility. First-principles calculations further confirm a substantial reduction in generalized stacking fault energy (GSFE) upon Er addition. The novel-developed Er-containing alloys demonstrate an excellent strength-ductility synergy, with the peak-aged alloy containing 2 wt.% Er showing superior room temperature mechanical properties: 257 MPa yield strength, 382 MPa ultimate tensile strength, and 8.2% elongation. This research provides important scientific insights into evading the strength-ductility trade-off in Mg alloys, which is critical for their broader application.