The effect of Mn dispersoid and thermomechanical treatment on the fracture toughness in peak aged Al-Zn-Mg-(Mn) alloy have been studied. Sphere- or rod-shaped Mn dispersoids in the size of range from 0.05μm to 0.5μm are formed by the addition of Mn in Al-Zn-Mg alloy. As the content of Mn is increased, the amount of Mn dispersoids is increased without changing the morphology and the size of Mn dispersoids. But Mn dispersoids do not affect the precipitation phenomena during the aging treatment. These dispersoids result in the increasing of the strength without sacrificing of ductility by dispersion hardening effect and homogeneous deformation.
The extruded alloy containing Mn has higher fracture toughness and transgranular ductile fracture surface with micro-dimples. These phenomena are due to the improved load bearing capacity and the effective accommodation of the applied stress. That is, the homogeneous deformation at the crack tip caused by the Mn dispersoids disperses the slip and prevents the stress and/or strain concentration. Meanwhile, in the Mn-free extruded alloy, partially intergranular fracture by inhomogenous planar slip is observed.
In comparison between the extruded and the rolled alloy containing 0.8wt% Mn, the extruded alloy is known to have higher strength and better fracture toughness than the rolled one. This result can be explained by dispersion of stress concentration and improved homogeneous deformation attributed to finer grain structure and existence of deformation texture.
The final thermomechanical treatment(FTMT) of the rolled alloy containing 0.8wt% Mn improves remarkably yield strength and tensile strength but decreases fracture toughness with the increasing of the amount of rolling. Increase of the strength is related with the precipitation hardening and the work hardening effect by FTMT. However, cold rolling during FTMT causes severely deformed structure which reduces the density and the moving distance of mobile dislocation. Therefore, decrease of elongation and lower work hardening capacity is occured and fracture toughness is decreased.
