Achieving enhanced mechanical properties of extruded Mg-Gd-Y-Zn-Zr alloy by regulating the initial LPSO phases

In this paper, "bimodal microstructure" and enhanced mechanical properties of extruded Mg-Gd-Y-Zn-Zr alloy is achieved by regulating the initial long-periodic stacking-ordered (LPSO) phases, the effect of blocky LPSO phase and lamellar LPSO phase on the dynamic recrystallization (DRX) behavior and the formation mechanism of bimodal microstructure are systematically investigated, the deformation modes, strengthening and toughening mechanism and fracture behavior of the "bimodal microstructure" are analyzed by slip traces. Blocky LPSO phase has significant particle stimulated nucleation (PSN) effect to promote DRX. The effect of lamellar LPSO phase on DRX depends on the size, fragmented lamellar LPSO with small size promotes numerous DRXed grains through the PSN effect, while continuous intragranular lamellar LPSO with large size inhibits DRX by hindering slip and grain boundary rotation, which is the key factor to form the bimodal microstructure. The almost fully DRXed H510 sample has the lowest strength, and the H-525 sample with "bimodal microstructure" achieves enhanced comprehensive mechanical properties (ultimate tensile strength, yield strength and elongation of 383 MPa, 308 MPa and 9.2 %, respectively), which is related to the significant texture strengthening and dislocation strengthening of large-sized unDRXed grains, as well as strain coordination of higher DRXed grains fraction. The deformation mode of the DRXed regions is grain boundary sliding, while the unDRXed regions are dominated by prismatic slip, the strain incompatibility between the two is the decisive factor causing microcracks propagation and final fracture. This paper will provide theoretical guidance for the initial microstructure design of strengthplasticity synergistic Mg-Gd-Y-Zn-Zr alloys with bimodal microstructure.