On the effects of thermomechanical processing on failure mode in precipitation-hardened aluminium alloys

This paper concentrates on the influences of thermomechanical processing on fracture behaviour of Al-Mg-Si-Cu (AA6061) alloys. Important factors are grain boundary structure and extent of matrix- and grain boundary precipitation. Large grain boundary phases in the as-air-cooled alloy, explains its much smaller fracture strain with respect to the water-quenched alloy. With increasing artificial ageing time, the bulk fracture strain of the air-cooled alloy exhibits a minimum. This is due to grain boundary precipitate growth and coarsening affecting the fraction of strain confined to the grain boundary region. For the recrystallized microstructure aged to peak strength, the fraction of intergranular fracture is much larger for the air-cooled alloy. This can be understood on the basis of a much wider precipitate free zone and a smaller grain boundary precipitate volume fraction for the air-cooled alloy, increasing the fraction of strain confined to the grain boundary region by about one order of magnitude with respect to the water-quenched alloy. A much coarser distribution of intermetallic phases in the extruded microstructure is responsible for a larger degree of slip localization. This enhances the tendency for shear- and intergranular fracture, reducing the ductility and thus the fracture strain with respect to that of the recrystallized microstructure