Deformation Properties And Fracture Mechanism Of High-strength 7075 Aluminum Alloy

7075 aluminum alloy has been a good choice for the application of the aircraft and traffic structures for the high specific strength, stiffness, good resistance to stress corrosion and easy processing. As the structures are always cyclic loaded, it is important to study the fracture mechanism and cyclic deformation properties of the 7075 material.Three thermomechanical treatments, that is T651, T6 and H112 were chosen in this paper. Optical Microscopy （OM）, Scanning Electron Microscopy （SEM） and Transmission Electron Microscopy （TEM） were used to observe the as-received as well as fatigued microstructures, and phase analysis of the alloy. At the meanwhile, the cyclic deformation properties and micro-mechanism of fracture damage of 7075 alloy, fatigue tests of hot extrusion state 7075-T651 and hot forging state 7075-T6 alloy have been tested to better understand fatigue characteristics.The results are as follows:Different as-received microstructures and mechanical properties could be found after H112, T73, T736 and RRA heat treatments, as well as T651, T6 and H112 treatments. The 7075 solid solution has less Al₇Cu₂Fe particles and smaller MgZn2 precipitates. The precipitations surrouding grain boundary become more discrete, with reduced strength. The uniform distributed precipitates can be observed both in T651 and T6 heat treatments. The fine precipitates in 7075-H112 matrix has lower microscopic intensity. The decarbonization situation are observed on all the surfaces of 7075-T651,7075-T6 and 7075-H112 aluminum alloys.The as-received 7075 aluminum alloy have many Al₇Cu₂Fe particles distributed in the grain boundary, as well as amounts of Si-rich oxide particles and MgZn2 phases dispersed in the matrix. This alloy has ultra-high fatigue life, when it is stress loaded, it cyclic softening first, following hardening and saturation finally. The though ratcheting phenomenon palys an important role in the initial cycles in the stress control condition. It is found that obvious plastic deformation could not be found in the initial region of cyclic loading, and the chief character is similar to cleavage pattern. Upon cycling, the typical patterns, such as fatigue tire, grooves, furrows, depressions and so on could be observed on the fracture surface induced by the dislocation reversibility. It is interested that the fracture zone is similar to the tensile tested simple.In the low stress amplitudes （60%ay,70%σy）, the main damage is the micro-cracking in the matrix. Whereas in the high stress amplitudes （80%ay,90%σy）, the coarse Al₇Cu₂Fe particles become the main crack initiation and propagation source.There are two fracture mechanisms of 7075 aluminum alloy. On the one hand, the inclusions or second phases would be broken or separated with the matrix, leading to crack initiation subsequently. This mechanism occurs primarily at high stress loading conditions. On the other hand, the crack initiation would be induced by the local stress concentration. In this alloy, the T651 treatments always have microporous dimples aggregating and propagating to the microcracks, whereas the fatigue fracture mechanism of 7075-T6 alloy is found to be quasi-cleavage fracture.