Correlation Analysis Of Microstructure And Fracture Toughness Of 7xxx Aluminum Plates

7085 and 7055 aluminum alloy were chosen as the experimental materials in this thesis. Transmission electron microscopy(TEM), optical microscopy(OM), scanning electron microscopy(SEM) and EBSD were used to comprehensively study the microstructure evolution under several traditional heat treatments of 7xxx aluminum alloy. Fracture toughness of the alloy was recessed by tear tests. Fracture toughness is experimentally approached mainly by tear test, and verified by testing the KIC value Extensive fractographic analysis by scanning electron microscopy(SEM) were carried out by observation of the broken tear test specimen or CT specimen surfaces. And then the correlations of fracture toughness and characteristics of precipitates, microstructure were investigated. The main jobs and results were drawn as follows:The UPE value by tear test showed consistence with KIC value of the compact tension sample on recessing the fracture toughness of 7085 and 7055 alloys under typical aging conditions. The UPE value can qualitatively evaluate the fracture toughness of 7xxx series aluminum alloy. The results suggested the variation trend of UPE values tested by L-T direction and T-L direction tear specimens were similar over the aging time.The correlations of precipitates and fracture toughness of 7085 aluminum alloy were investigated under several typical aging conditions. During one step aging at 140°C,the main precipitates were GP zones and η′ phases inside grains. Prolonging the aging time, the precipitates inside grains slowly grew up, grain boundaries precipitates(GBPs) gradually became coarser, and the width of the precipitate-free zone(PFZ) increased. Intergranular fracture behavior of the alloy was not increasing until the aging time was long enough. The fracture toughness of the alloy decreased first and then increased over the aging time. The main matrix precipitates were GP zones and η’ phases in the grains and the GBPs were continuous after 118℃/6h pre-aging treatment. With further secondary aging treatment at 152°C, the precipitates grew up, η phases became more numerous and PFZ became wider. The proportion of intergranular failure were increased first and then decreased showing on the fracture morphologies over the secondary aging time. The fracture toughness was first decreased and then increased with the time variation.The correlations of precipitates and fracture toughness of 7055 aluminum alloy were analyzed after three-step aging heat treatment. With prolonging retrogression aging treatment time at 175°C, the GBPs became coarser and discontinuous, and the width of the precipitate-free zone(PFZ) gradually increased. And then the fracture mode transmit from the intergranular fracture to transgranular fracture, and the direction of crack propagation changed. The fracture toughness of the alloy increased gradually.The correlations of grain structures and fracture toughness of 7085 aluminum alloy under solution heat treatment were discussed. In the range of this experiment, the results showed there weren’t remarkable changes in fracture toughness of the alloy with the recrystallization changes under different temperature or time of solution heat treatment.