Study of fracture toughness and crack tip deformation behaviors of highly plastic Mg-2Y-0.6Zr alloys

Abstract

The influence of recrystallization degree on the tensile mechanical properties, impact toughness, and fracture toughness of the Mg-2Y-0.6Zr alloy was investigated. The microstructure and deformation mechanisms of Mg-2Y-0.6Zr alloys at different extrusion ratios were analyzed using optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The fracture toughness testing is conducted through fatigue crack propagation and three-point bending methods. The findings indicate that the extruded alloy displays a heterogeneous structure, featuring three distinct microstructural characteristics: unrecrystallized grains, equiaxed recrystallized grains, and fine-grained bands. As the extrusion ratio decreases, the proportion of unrecrystallized grains increases, while the size of the recrystallized grains decreases. Samples with smaller extrusion ratios have lower plasticity (EL of 31%) and impact toughness (α_k of 16.19 J·cm^-2), higher yield strength (YS of 153 MPa) and fracture toughness (K_ⅠC of 27.1 MPa·m^1/2) than samples with the largest extrusion ratios (EL of 44%, YS of 105 MPa, α_k of 18.35 J·cm^-2, K_ⅠC of 22.1 MPa·m^1/2). The rare earth texture and the fine-grained bands in samples with large extrusion ratios ensure the homogeneity of their plastic deformation and are the main reasons for their excellent plasticity. The deformation behavior of grains at the crack tip of the material was subjected to twin variant Schmid factor analysis and in-grain misorientation axis (IGMA) distribution, which showed that tensile twinning activity was higher in alloys with larger recrystallized grain sizes, whereas plastic deformation was dominated by basal slip in alloys with smaller recrystallized grain sizes. In the fracture toughness testing, cracks mainly propagated along grain boundaries, exhibiting deflections and branching. The presence of large, hard unrecrystallized grains significantly hinders crack propagation, which is a key factor for the high fracture toughness observed in the ER12 samples. Increased activity of tensile twins provides additional potential pathways for crack growth, and the initiation of tensile twins within the unrecrystallized grains facilitates easier crack propagation. Consequently, higher tensile twin activity is considered to reduce the material's fracture toughness.