Investigation On The Strengthening Behavior And Mechanism Of Stress-aging Of 7075 Aluminum Alloy

The precipitation hardening Al-Zn-Mg-Cu aluminum alloys are widely used in the aerospace and automotive structural material. But the traditional T6 and other aging treatments can not make the alloy obtain high strength and excellent corrosion resistance at the same time. In the perspective of the thermodynamics, stress is another thermodynamic parameter that can control the organization, structure and performance of materials, except the temperature and composition. The systematic researches on the microstructural and properties evolutions of 7075 alloy aged under elastic stress are helpful to accurately control the second phase structure of the alloy, which can provide new experimental basis and theoretical guiding for the preparation of high performance aluminum alloys.In this thesis, the effect of external stress on the microstructures and properties of7075 alloy after aging was systematically studied by scanning electron microscopy(SEM),X-ray diffraction(XRD) and transmission electron microscopy(TEM) methods, combined with mechanical test. Double peaks occur in 7075 alloy after aging treatments at 160 oC for 1 h under various stresses. In the 25 MPa, 100 MPa tensile stress and 25 MPa, 112.5MPa compressive stress-aged conditions: the hardness is the highest; the yield strength and tensile strength have been obviously improved with a slightly decreased elongation.Compared with the stress-free aged condition, the dispersions of the aging precipitates are higher and the average sizes of the aging precipitates are lower after four stress-aging treatments. The tensile and compressive stresses promote the growing of the larger-size MgZn2 phase and the precipitation of the η′ metastable phase. The compressive stress promotes the precipitation of the stable η phase, while the tensile stress suppresses the formation of the η phase. Grain boundary precipitations after tensile stress-aging are discontinuous distributed in the grain boundaries.The effects of aging temperature on the microstructures and properties of 7075 alloy after 25 MPa stress aged for 1 h were studied. At 120-180 oC, the hardness, yield strength and tensile strength are higher in the compressive stress aged samples than those in thestress-free aged conditions. The hardness of the compressive stress aged samples reaches the maximum value(178 HV) at 150 o C; at lower temperatures(120 oC), the hardness,yield strength and tensile strength are lower and the elongation is higher in the tensile stress aged samples than those in the stress-free aged conditions. At higher temperatures(160 oC), the hardness, yield strength and tensile strength are higher and the elongation is lower in the tensile stress aged samples than those in the stress-free aged conditions. The hardness of the tensile stress aged samples reaches the maximum value(180 HV) at 165 oC. The applied 25 MPa stresses suppress the growing of the larger-size MgZn2 phase after aging at 120 oC for 1 h; a lot of GPII zones are found in the stress-free aged specimen;many edge-on η′ platelets are found in the tensile stress aged sample; a lot of η′ phases are found in the compressive stress aged specimen. The descending order of the precipitate dispersion after 25 MPa stress(-free) aging treatments are: compressive stress aging >tensile stress aging > stress-free aging. The average precipitate sizes of the three aging treatments are 3.1 nm, 6.3 nm and 12.5 nm respectively.The effects of aging time on the microstructures and properties of 7075 alloy after 25 MPa stress-aged at 120 oC and 160 oC were studied. Compared with the stress-free aged condition, the hardness, yield strength and tensile strength are obviously improved after compressive stress aged at 120 oC for 1-32 hours and at 160 oC for 1-10 hours. The hardness increases faster in the 8-24 hours after tensile stress aged at 120 oC, and reaches the maximum value(191 HV) at 24 h, while the yield strength and tensile strength are the same to the stress-free condition. At 160 oC, the hardness, yield strength and tensile strength are obviously improved after tensile stress aged for 1-10 hours. The ability of intergranular corrosion and exfoliation corrosion resistance is improved after 25 MPa tensile stress aged at 120 oC for 24 hours.The stress aging mechanism of 7075 alloy was studied. Compared with the stress-free aged condition, the curled threading dislocations and dislocation rings change into the straight dislocations after 25 MPa tensile and compressive stress-aging. The applied stress increases the nucleation rate of the aging precipitates and improves the dispersions of the precipitates, resulting in an increased mechanical properties of the alloy. The dislocations slip after 50 MPa tensile stress and 75 MPa compressive stress-aging, which lowers thedislocation density. The dislocation motion destroys the smaller-size nuclei of the precipitates and reduces the dispersion of the precipitates to a minimum, making little changes in the mechanical properties of the alloy. The dislocations slip and produce a lot of value-added after 100 MPa tensile stress and 112.5 MPa compressive stress-aging. The dispersions of the precipitates are increased, resulting in another increased mechanical properties of the alloy. Compared with the stress-free aged condition, the Zn/Mg value of the larger-size MgZn2 phase is reduced after 25 MPa tensile stress-aging, while the Zn/Mg value of the larger-size MgZn2 phase is increased after 25 MPa compressive stress-aging.