Microscopic Mechanism Of Fatigue Damage And Cyclic Deformation Characteristic Of High-strength 7075 Aluminum Alloy

7075 aluminum alloy has been a good choice for the application of the aircraft structures for the high specific strength, stiffness and good resistance to stress corrosion. As the structures are always cyclic loaded, it is important to study the fatigue behavior and cyclic deformation response of the 7075 material.The fracture toughness tests, the tensile tests and fatigue tests of 7075 alloy were done to pursue the mechanical properties. At the meanwhile, the Optical Microscopy(OM), Scanning Electron Microscopy(SEM) and Transmission Electron Microscopy(TEM) were used to built the bridge to better understand the relationship between marco-and micro-mechanics. Three heat treatments, that is T651, T6 and H112, were chosen in this paper. Furthermore, the fracture morphology and fatigue characteristic of 7075-T651 alloy have been tested. The results are as follows:(1) The as-received micro-structures and macro-mechanisms of 7075-T651, 7075-T6 and 7075-H112 alloys have been tested respectively. The uniform distributed precipitates can be observed both in 7075-T651 material and 7075-T6 alloy.(2) The hot extruded 7075-T651 alloy has excellent mechanic properties. The TEM as-received microstructure, diffraction pattern and XRD results present the existence of precipitated MgZn2, hard Al7Cu2Fe particle, as well as silicon oxide in Al matrix.(3) The P-V curves (I model) were analyzed according to the results of fracture toughness test. The fatigue fracture toughness Kic value equals to 41.59 MPa.m1/2. The fracture surface of compact tension (C(T)) specimen were observed, which was found that the chief character of fracture is striation and dimple (deep and disordered).(4) In static tensile test for 7075-T651 alloy, tensile strength is 529 MPa and yield strength is 450 MPa. The elongation, section shrinkage and volume plastic volume are high. Fracture occurs when the external load is higher than a certain value.(5) The fatigue tests demonstrate that the fatigue life of 7075-T651 alloy is ultra high under low stress cycling (60%σy,70%σy), and prefer to earlier cyclic saturated and failure when the applied load is up to the critical value (80%σy,90%σy).(6) The fracture surface of fatigued specimens were observed by SEM.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, microcracks initiated and propagated, and the typical fatigue striation induced by dislocation movement could be observed on the fracture surface. It is interested that the fracture zone is similar to the tensile tested simple, and the chief character in this region is fatigue dimple.(7) Pattern recognition of fractographs is very important for analysis of fracture surface. In 7075-T651 alloy, the failure is always due to microcrack initiation and propagation, especially inside the grains. The dislocation pile-up and the Fe-, Si-rich particles, as well as MgZn2, Al7Cu2Fe precipitates play the role of coordination in obvious cyclic deformation.