Fatigue Properties Test And Evaluation Of2A97Al-Li Alloy

Today in the21st century, people have very deep understanding of traditional static load and defect-free materials with the further study of fracture mechanics, damage mechanics. In this situation, fatigue comes to one of the main factors of metal components failure. It’s our definite work to determine the fatigue life accurately of aluminum alloys widely used in aerospace industry before practical use.Aluminum alloys are mainly used as structural materials in airplanes, such as skin, frame, propeller, fuel tank, siding and pillars of the landing gear and so on. During the1980s, the material was demanded to be lighter as a result of the rise of fuel price; since the1990s, people are aiming at improving the durability and damage tolerance of aluminum as well as the weight control. For example, they try to develop new aluminum with high-strength, high toughness, high corrosion resistance. In the present work, a new aluminum named as2A97(Al-Cu-Li-X) was studied. It’s superior to the traditional aluminums on heat resistance and corrosion resistance as well as on the strength, modulus, weldability and density.In this study, the fatigue limit and S-N curve of2A97alloy at different rolling directions and different strain factors were given according to the axial fatigue test. Optical microscope, Scanning electron microscope and Transmission electron microscope were used to determine the crack initiation and growth of2A97alloy and the plasticity at different directions, and the main conclusions are as follows:1. The yielding strength at LT direction of2A97is lower than that at L direction, while the plasticity and fatigue performance are a little better.2. The conditions fatigue limit at LT direction of2A97alloy round sample is higher than that of L direction at lower strain level, when the strain reached a certain value, the conditions fatigue limit at L direction exceeded that of LT direction.3. The fatigue crack of2A97mainly initiates at the surface of sample, the second phase particles, matrix interface, defects and grain boundaries.4. The fatigue crack of2A97deflects at early stage, and extends along the slip plane preferentially. If the slip planes of two grains are in consistent, the crack doesn’t deflects; if there is angle between the slip planes, the crack deflects and form the crystallographic morphology of fracture surface.5. The2A97alloy has the characteristics of planar slip, and the dislocations could cut the coherent δ, particles, which would make the crack deflects inside the grain. The deflection of cracks at LT direction is severer than that of L direction, which results in that the crack growth is slower.6. The precipitates in2A97alloy are mainly δ’ phase, T1phase and θ’ phase. Among them, the δ’ phase is coherent with the matrix and could be cut through by dislocations; the T1phase is hexagonal flakes at the {111} plane and semi-coherent with the matrix, and the distortional strain energy is so large that the dislocations could just pass through by Orowan mechanism rather than cut through; the θ’ phase is flake phase at {100} plane and is not conducive to the crack deflection and the reduction of cumulative damage from the point view of crack growth.