Synergistic strength and ductility enhancement of ultrasonically solidified Mg-Gd-Y-Zn quaternary alloy

The microstructural evolution and strengthening-toughening mechanism of directionally solidified Mg-6wt%Gd-4wt%Y-2wt%Zn alloy within ultrasonic field were investigated. The application of ultrasound refined the primary α-Mg phase from coarse columnar dendrites to the mixed heterogeneous structure, which were composed of both fine equiaxed and short columnar grains. The synergy of ultrasonic cavitation and thermal flow led to the formation of two types of grains, in which equiaxed grains originated from cavitation-promoted nucleation while short columnar grains formed under temperature gradient. High-frequency vibration induced local stress within α-Mg phase, increasing the geometrically necessary dislocation density. This promoted the nucleation and coarsening of long-period stacking ordered (LPSO) phase during solid-state phase transformation, resulting in its morphological transition from rods to flakes with increased volume fraction. The ultimate tensile strength and elongation of heterogeneous structure were enhanced by 1.55 and 1.51 times, respectively. The strengthening effect was mainly ascribed to hetero-deformation induced (HDI) hardening, which induced more fine twin boundaries during deformation. The enhanced plasticity originated from stress coordination through the HDI effect and more kink bands in rod and flake shaped LPSO phase.