| Al-Zn-Mg-Cu series super-high strength aluminum alloys under the specific heat treatment states exhibit the higher hardness, toughness and fracture strength as well as the stronger fatigue resistance and better corrosion resistance, and are the main structural materials in aerospace and transportation fields. The application of aluminum alloys in large aircraft occupies 60% to 80% of structural materials in the weight. As the main materials for the force-bearing members, the usuage proportion of high performance Al-Zn-Mg-Cu series aluminum alloys accounts for 35%～50% of total aluminum usuage amount on the aircraft. According to the structure design criterion of A380 aircraft, the improvement in the performances can be achieved with developing new aluminum alloys and correlative processing technologies. It is obvious that the microalloying and heat treatments are the important methods to enhance the performces of Al-Zn-Mg-Cu series aluminum alloys. There are many research reports on the fatigue behavior of various aluminum alloys, including the cyclic stress-strain behavior, stress corrosion behavior, fatigue crack initiation behavior and fatigue crack growth behavior. The researches on the Al-Zn-Mg-Cu alloy with Sc and Zr are mainly concentrated on the microstructure, tensile properties and stress-corrosion resistance. However, the study in its fatigue performance and microstructurial evolution during the fatigue is few. Consequently, the microstructure and low-cycle fatigue behavior for the Al-7.2Zn-2.5Mg-1.5Cu-0.08Zr-0.12Sc alloy with different heat treatment states are investigated, in order to provide the theoretical basis for the practical engineering application and anti fatigue design of the present alloy.The microstructure of as-cast Al-7.2Zn-2.5Mg-1.5Cu-0.08Zr-0.12Sc alloy is composed of equiaxed grains, and a large amount of non-equilibrium eutectic can be found. After the homogenization treatment, the non-equilibrium eutectic in as-cast Al-7.2Zn-2.5Mg-1.5Cu-0.08Zr-0.12Sc alloy dissolves, and a large number of Al3 (Zr, Sc) phase precipitates in the matrix. For the extruded Al-7.2Zn-2.5Mg-1.5Cu-0.08Zr-0.12Sc alloy subjected to the double stage solution treatment, the dissolution of residual second phases is much more complete. Compared with the single stage solution treatment, the double stage solution treatment can effectively increase the degree of super saturation and thus improve the effect of aging strengthening. For the extruded Al-7.2Zn-2.5Mg-1.5Cu-0.08Zr-0.12Sc alloy subjected to the different aging treatments, the GP zones, η’ transition phase, η equilibrium phase and grain boundary precipitate free zone (PFZ) can be observed except for the α-Al matrix phase. Under the single stage aging state, the precipitates inside the grains are fine, the discontinuous equilibrium phase precipitates at the grain boundaries, and there exist the precipitate free zones near the grain boundaries. For the alloy subjected to the double stage aging treatment, the precipitates inside the grains and at the grain boundaries coarsen, and the precipitate free zone widens. Under the retrogression and reaging state, the precipitates inside the grains slightly grow compared with the single stage aging treatment, and the microstructure near the grain boundaries is similar to those in the double stage aging state.Under the low-cycle fatigue loading condition, the Al-7.2Zn-2.5Mg-1.5Cu-0.08Zr-0.12Sc alloy with different heat treatment states exhibits mainly the stable cyclic stress response behavior at the total strain amplitudes ranged from 0.4% to 0.7%. However, at the total strain amplitude of 0.8%, the alloy shows mostly the cyclic strain softening followed by the cyclic strain hardening.Under the single stage aging state, the Al-7.2Zn-2.5Mg-1.5Cu-0.08Zr-0.12Sc alloy subjected to the aging treatment at 150℃ for 36h exhibits the higher cyclic stress amplitudes and longer low-cycle fatigue lives. Compared with the single stage aging treatment at 150℃ for 36h, the double stage aging treatment at 150℃ for 18h plus at 170℃ for 12h can effectively improve the low cycle fatigue lives of the alloy, but the fatigue deformation resistance of the alloy decreases. At the total strain amplitudes from 0.4% to 0.6%, the retrogression and reaging treatment can effectively prolong the low-cycle fatigue lives of the alloy. At the total strain amplitudes from 0.4% to 0.8%, the fatigue deformation resistance of the alloy with the regression and reaging state is close to that of the alloy subjected to the aging treatment at 150℃ for 36h. The relationships between the plastic strain amplitude, elastic strain amplitude and reversals to failure are linear, and can be described separately with the Coffin-Manson formula and Basquin equations. In addition, the relationship between the cyclic stress amplitude and plastic strain amplitude of the alloy with different aging states is also linear.Under the low-cycle fatigue loading condition, the dislocation substructures of extruded Al-7.2Zn-2.5Mg-1.5Cu-0.08Zr-0.12Sc alloy with different heat treated states at the lower total strain amplitudes are the dislocation dipole array, and the typical plane slip feature is noted. At the higher total strain amplitudes, the cross slip of dislocations occurs, and the fine GP zone and η’ phase are sheared repeatedly which will lead to the cyclic strain softening. And then the dislocations are pinned and tangled, which will lead to the cyclic strain hardening. And these correspond to the cyclic stress response behavior. In addition, under the low-cycle fatigue loading condition controlled with total strain amplitude, the fatigue cracks initiate transgranularly at the free surface of fatigue samples and propagate transgranularly for the Al-7.2Zn-2.5Mg-1.5Cu-0.08Zr-0.12Sc alloy with different heat treated states. |
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