Effect of bimodal microstructure on the tensile properties and fracture toughness of extruded Mg-9Gd-4Y-0.5Zr alloy

The effects of bimodal microstructure on the tensile properties and fracture toughness of Mg-9Gd-4Y-0.5Zr alloys were investigated. The Mg-9Gd-4Y-0.5Zr alloy was extruded at different temperatures to produce samples with bimodal and fully dynamic recrystallized (DRXed) grain structures. The results show that the bimodal grain structure exhibits a better combination of yield strength (YS of 277 MPa) and plane strain fracture toughness (K_Ic = 21.5 MPa m^1/2), compared to the fully DRXed microstructure (YS of 207 MPa, K_Ic = 16.3 MPa m^1/2). The fracture mechanisms for the bimodal and fully DRXed microstructures are the ductile fracture mode and quasi-cleavage fracture mode, respectively. Both hetero-deformation induced (HDI) strengthening effect and strong basal texture can contribute to high tensile strength in the bimodal grain structured sample. As compared to the sample with fully DRXed microstructure, the stronger strain hardening capacity and larger hardened region around the crack tip in the bimodal grain structured sample can bring about the improvement in the fracture toughness. The tortuous crack propagation path and the hindrance of propagating crack by the un-DRXed grains result in larger fracture resistance and more energy dissipation. The increase of geometrically necessary dislocation (GND) density and the activation of non-basal dislocations also have positive effects on the fracture toughness of the samples with bimodal grain structure. The fracture toughness is effectively improved via intrinsic and extrinsic toughening mechanisms associated with the bimodal microstructure.