Effects of heterogeneous microstructure evolution on the tensile and fracture toughness properties of extruded AZ31B alloys

• Investigates the effect of heterogeneous microstructure evolution effect on the fracture toughness of extruded AZ31B alloys. • The heterogenous microstructure is classified into three regions: worked grains, fine DRXed grains, and coarse DRXed grains. • Twinning behavior during fracture toughness is dependent on grain size and internal strain within individual grains. • The extruded AZ31B alloys primarily comprise coarse DRXed grains, exhibiting low yield strength but high fracture toughness due to significant energy dissipation during crack extension. This study aims to investigate the extrusion temperature effects on the development of heterogeneous microstructures and mechanical properties, focusing on their impact on the fracture toughness of AZ31B alloys. Magnesium AZ31B (Mg-3wt%Al-1wt%Zn) alloys with high strength and reasonable fracture toughness, featuring heterogeneous microstructures, were fabricated via warm/hot extrusion at temperatures ranging from 523 to 723 K. The AZ31B alloy extruded at 523 K was bimodally grained into coarse worked grains with high Kernel average misorientation (KAM) values and fine dynamically recrystallized (DRXed) grains (< 10 µm) with intermediate KAM values. The 523 K-extruded alloy exhibited a high tensile yield strength of ∼280 MPa and fracture toughness K JIC of ∼26 MPa·m 1/2 . Conversely, the 723 K-extruded AZ31B alloy was trimodally grained into a small amount of worked grains, fine DRXed grains, and coarse DRXed grains (> 10 µm) with low KAM values. The 723 K-extruded alloy exhibited low tensile yield strength but a high K JIC value of ∼36 MPa·m 1/2 owing to the high energy dissipation for crack extension in the coarse DRXed grains.