High-strength age-hardened aluminum alloys have been used successfully as structural materials due the their unique combination of low density, high strength and high corrosion resistance. For most aerospace applications in recent years, the improvement of the fracture toughness and fatigue resistance, especially in the short transverse direction, has become of crucial importance, towards the incorporation of these materials in more advanced aircraft. In this work the relationships between microstructure, fracture toughness, fatigue crack initiation and fatigue crack propagation in high-strength aerospace aluminum alloys, with focus on the 7X50 series, are investigated. An attempt is made towards the improvement and control of these properties, through the accurate control of the microstructure. Special attention is given to the evolution of the microstructure at the subgrain level and to the processing parameters driving to the occurrence of dynamic and static recrystallization. Low fracture toughness values and low resistance to fatigue crack growth were observed to be associated with a high degree of recrystallization. The resistance to fatigue crack propagation was improved in microstructures consisting of deformed grains with subgrains with high dislocation density. The morphology of various types of particles, the morphology of the hydrogen porosity, the volume fraction and distribution of the recrystallized areas, the grain and sub-grain morphologies were observed to influence considerable the fatigue resistance. These microstructural parameters were effectively controlled during casting and thermo-mechanical processing through the control of variables such as casting parameters, strain and temperature in each deformation stage, the number of deformation stages and the cumulative strain.
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