Unveiling the Microstructure Evolution and Mechanical Strengthening Mechanisms in Mg–2Y–xZn Alloys

This work systematically investigates the Zn-content-dependent phase evolution (1-12 at.%) and its correlation with mechanical properties in as-cast Mg-2Y-<i>x</i>Zn alloys. A sequential phase transformation is observed with the Zn content increasing: the microstructure evolves from X-phase dominance (1-2 at.% Zn) through W-phase formation (3-6 at.% Zn) to I-phase emergence (12 at.% Zn). Optimal mechanical performance is attained in the 2 at.% Zn-containing alloy, with measured tensile properties reaching 239 MPa UTS and 130 MPa YS, while maintaining an elongation of 12.62% prior to its gradual decline at higher Zn concentrations. Crystallographic analysis shows that the most significant strengthening effect of the X-phase originates from its coherent orientation relationship with the α-Mg matrix and the development of deformation-induced kink bands. Meanwhile, fine W-phase particles embedded within the X-phase further enhance alloy performance by suppressing X-phase deformation, revealing pronounced synergistic strengthening between the two phases. Notably, although both the I-phase and W-phase act as crack initiation sites during deformation, their coexistence triggers a competitive fracture mechanism: the I-phase preferentially fractures to preserve the structural integrity of the W-phase, effectively mitigating crack propagation. These dynamic interactions of second phases during plastic deformation-synergistic strengthening and competitive fracture-provide a novel strategy and insights for designing high-performance Mg-RE-Zn alloys.