Abstract:The influence of microstructures, matrix precipitates, grain boundary precipitates and precipitate free zone on the strength and fracture toughness of 7055 aluminum alloy after peak aging and secondary T7951 heat treatments was studied through mechanical property tests, microstructures and fractography analysis. The results showed that strength of 7055-T7951 would not decrease dramatically compared to the peak aging material because of the competition between the softening effect of η phase precipitating and the strengthening effects of dislocation and fine grains. The main reason for the improvement of fracture toughness after secondary aging was the aggregation and coarsening of grain boundary precipitates. Fractographic analysis showed that the fracture mechanism was mixed fracture containing both ductile and brittle features, but with more ductile features in the secondary aging materials.