Role of Mn-dispersoid in the fracture toughness enhancement of Al-Zn-Mg-(Mn) alloys

The effects of Mn dispersoid and fabrication methods on the fracture toughness in peak aged Al-Zn-Mg-(Mn) alloys have been studied. Sphere- or rod-shaped Mn dispersoids of the size in range from 0.05 μm to 0.5 μrn are formed by the addition of Mn in Al-Zn-Mg alloy. The extruded alloys containing Mn have higher fracture toughness with higher strength and show transgranular ductile fracture surface. as compared with those of Al-Zn-Mg alloy. These phenomena are found to be obtained from the improved load bearing capacity and the effective accommodation of the applied stress due to the dispersion hardening effect and homogeneous deformation by the Mn-dispersoids. Comparing the mechanical properties between the extruded and the rolled alloys containing 0.8 wt% Mn. the extruded alloy is shown to have higher strength and better fracture toughness than those of the rolled one. This result can be explained by the dispersion of stress concentration and the improved homogeneous deformation attributed to the fine grain structure and the existence of deformation texture for the extruded alloy.