Preparation of ultra-high strength Mg-Gd-Y-Zn-Zr alloy by pre-ageing treatment prior to extrusion

In this paper, the effect of pre-ageing treatment on the mechanical properties, microstructure, and fracture behavior of the extruded Mg-9.5Gd-4Y-2.2Zn-0.5Zr (wt%) alloys was studied. The results show that the poor mechanical properties of the solution-extruded (SE) sample with a bimodal microstructure are attributed to the deformation incompatibility between different regions (deformed grain region, fine grain region and bulk-shaped long period stacking ordered (LPSO) phases), which is prone to crack initiation and propagation. In contrast, pre-aged sample with various morphological phases (lamellar-shaped LPSO phases, granular-shaped β phases and bulk-shaped LPSO phases) promotes the dynamic recrystallization (DRX) by the particle-stimulated nucleation (PSN) mechanism during extrusion, which results in the pre-aged-extruded (PE) sample having a more homogeneous microstructure with a finer and higher percentage of dynamically recrystallized grains. The densely distributed fine precipitates in the dynamically recrystallized region and homogeneous microstructure can enhance the capacity of the different regions to accommodate strain, and avoid premature fracture of the alloy without sufficient work hardening. Therefore, the PE sample has excellent strength-ductility synergy, and its yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) were 408 MPa, 476 MPa and 8.3%, respectively. The YS and UTS of the PE sample after further ageing treatment (PEA sample) increased to 461 MPa and 535 MPa, respectively.