Effects of interrupted aging T6I4 on hardness and fracture toughness of aeronautic AA7050 alloy

Al–Zn–Mg–Cu alloys have been extensively researched for aeronautical uses due to their good mechanical properties. Among these alloys, AA7050 is notable for its impressive fracture toughness. Alternative heat treatments have been developed to improve fracture toughness without compromising AA7050 strength. T6I4, a method involving underaging for short times and secondary aging, is one of them. The main objective of this research was to investigate the influence of T6I4 temper on the hardness and fracture toughness of AA7050 alloy. A 75-mm thick plate of AA7050-T7451 alloy was microstructurally characterized in three directions regarding the rolling direction. Nine interrupted aging conditions were statistically evaluated for their Vickers microhardness measurements of the treated samples, from which, two of them were chosen for Chevron-notch (CN) tests. Chevron-shaped specimens with L-S orientation were produced from the T7451, T6I4-24, and T6I4-72 conditions. Charpy V-notch impact tests were performed to obtain alternative estimates of the fracture toughness of AA7050 alloy in the T7451 and T6I4-24 states. The results from the CN and Charpy tests indicated that the T6I4-24 temper provides to this alloy a higher capacity to accumulate plastic deformation. Furthermore, the analysis of fracture surfaces of Chevron-shaped specimens suggested that the fracture topography of T6I4-24 samples is more irregular with more ductile features than that of the T7451 and T6I4-72 conditions.