Ultra-strong and ductile magnesium alloy enabled by ultrafine grains with nano-spacing solute-enriched planar defects

Mg_96.3Ho_1.6Y_1.2Zn_0.8Zr_0.1 (at.%) alloy with sub-micron ultrafine grains containing nano-spacing solute-enriched planar defects is developed to exhibit high strengths (yield strength = ∼ 382 MPa and ultimate tensile strength = ∼ 426 MPa) and good ductility (fracture elongation = 19%), compared to the as-homogenized counterpart (yield strength = ∼ 160 MPa, ultimate tensile strength = ∼ 225 MPa, and fracture elongation = 7.5%). Ultrafine grains with an average grain size of ∼ 940 nm is attained via particle-stimulated nucleation mechanism induced by the second Mg₁₂(Ho,Y)Zn phase during hot extrusion. A substantial number of ultrafine grains are formed surrounding these second-phase grains. The addition of Ho/Y/Zn elements lowers the I₁ stacking fault energy, facilitating the formation of I₁-type fault loops and promoting the activity of <c+a> dislocations. Meanwhile, the nano-spacing solute-enriched planar defects (including long-period stacking order structure and I₂-type stacking faults) effectively hinder the motion of <c+a> dislocations, increasing flow stress while simultaneously promoting the activation of new <c+a> dislocations. As a result, the synergistic effect between ultrafine grains and solute-enriched planar defects significantly enhances the yield strength and facilitates the numerous non-basal dislocation activity responsible for significantly improved ductility. In addition, the refined second deformable Mg₁₂(Ho,Y)Zn phase further strengthens the alloy and effectively delays the formation of macrocracks to improve the ductility. This study not only present an efficient strategy for developing high-strength, high-ductility Mg alloys but also provides new insights into the interplay between planar defects and dislocations.