Effect of aging treatment on the microstructure, fracture toughness and fracture behavior of the extruded Mg-7Gd-2Y–1Zn-0.5Zr alloy

A commercial scale Mg-7Gd-2Y–1Zn-0.5Zr alloy extrusion rod with synergetic high yield strength of 350 MPa and fracture toughness KIC of 22.4 MPa m1/2 was fabricated by direct-chill casting, hot extrusion and subsequent aging treatment at 200 °C for 108 h. The microstructures and mechanical properties of the as-extruded and peak-aged alloys were investigated, and the fracture toughness and fracture mechanisms before and after aging treatment were also analyzed. The results showed that the as-extruded alloy consisted of long period stacking ordered (LPSO) phase and bimodal matrix microstructure composed of fine dynamic recrystallized (DRXed) grains and coarse deformed grains. Microstructural analysis near fracture surfaces indicted that twinning was activated in the coarse deformed grains during fracture toughness test, which may be conducive to the improvement of the fracture toughness of the alloys. The lamellar-shaped LPSO phases enhanced the fracture toughness since the formation of secondary cracks parallel to the lamellar LPSO can cause significant energy dissipation and deflection of main crack. The micro-cracks were initiated and propagated in the bulk LPSO phases, which may deteriorate the fracture toughness. Large amount of nano β' phase precipitated in the T5 peak-aged alloy significantly improved the strength of the alloy. Fractography analysis indicated that fracture mechanism changed from ductile fracture of the as-extruded alloy to quasi-cleavage fracture of the peak-aged alloy. The precipitation of β' would suppress twin growth and induce the microcracks initiated at the DRXed grain boundaries and in the deformed grains, which led to slight decrease of KIC.