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.

The fracture toughness of extruded Mg-1Zn-2Y (at. %) alloys, featuring a multimodal microstructure containing fine dynamically recrystallized (DRXed) grains with random crystallographic orientation and coarse-worked grains with a strong fiber texture, was investigated. The DRXed grains comprised randomly oriented equiaxed α-Mg grains. In contrast, the worked grains included α-Mg and long-period stacking ordered (LPSO) phases that extended in the extrusion direction (ED). Both types displayed a strong texture, aligning the 〈1010〉 direction parallel to the ED. The volume fractions of the DRXed and worked grains were controlled by adjusting the extrusion temperature. In the longitudinal-transverse (L-T) orientation, where the loading direction was aligned parallel to the ED, there was a tendency for the conditional fracture toughness, K_Q, tended to increase as the volume fraction of the worked grains increased. However, the K_Q values in the T-L orientation, where the loading direction was perpendicular to the ED, decreased with an increase in the volume fraction of the worked grains. This suggests strong anisotropy in the fracture toughness of the specimen with a high volume fraction of the worked grains, relative to the test direction. The worked grains, which included the LPSO phase and were elongated perpendicular to the initial crack plane, suppressed the straight crack extension, causing crack deflection, and generating secondary cracks. Thus, these worked grains significantly contributed to the fracture toughness of the extruded Mg-1Zn-2Y alloys in the L-T orientation.

The mechanisms of multimodal microstructure evolution and the effects of microstructural factors on mechanical properties must be elucidated to design new alloys with superior properties. In this study, high-fracture-toughness and ductile Mg_96.75Zn_0.85Y_2.05Al_0.35 alloys were developed using rapidly solidified (RS) ribbon-consolidation technique, and the inherited multimodal microstructure evolution during plastic flow consolidation of the RS ribbons was investigated. The use of extrusion for plastic flow consolidation of the heat-treated RS ribbons produced a multimodal microstructure consisting of the worked grains with high Kernel average misorientation (KAM) angles (Group 1), the ultrafine dynamically recrystallized (DRXed) grains with intermediate KAM angles (Group 2), and the fine DRXed grains with low KAM angles (Group 3). Groups 1 and 2 contribute to the alloy strengthening, while Group 3 contributes to improving ductility with strain-hardening, resulting in enhancement of the fracture toughness. To form the multimodal microstructure, it was necessary to apply plastic flow with equivalent strains of >2.3 to the heat-treated RS ribbons possessing duplex microstructures with different dispersions of the long-period stacking ordered phase.