| Microstructures as well as their influence on the properties of advanced super-high strength AA 7055 Al alloy have been investigated systematically. The main contents includes: a study of microstructures and properties of the as-received AA 7055-T77 Al alloy plate using optical microscope(OP), scanning electron microscope(SEM), transmission electron microscope(TEM) and electronic back-scattered diffraction(EBSD) technique; a study of microstructure evolution during ageing process by means of (HR)TEM and small angle X-ray scattering(SAXS); strength model of the alloy during the ageing process using the quantitative information obtained by the former studies; and a study of the effect of ageing conditions on the fatigue crack growth(FCG) rate of the AA 7055 Al alloy.Microstructure analyses of the as-received AA 7055-T77 plate show that the degree of recrystallization as well as the size of the subgrains decreases from the surface to the mid-thickness of the plate. The texture near the center of the plate is rolling type, consisted of {011}<211>, {123}<634> and {112}<111>. However, shear type textures near the surface included {001}<110>, {112}<110> and {111}<110> are dominant. The main strengthen precipitate of the AA 7055-T77 Al alloy plate is plate-likeη' phase, which is fully coherent with the Al matrix with a coherent strain of 0.0133. The average radius ofη' precipitate is 3.7 nm and its thickness is about 1~3 nm.The tensile test results show that the yield strength in different positions along the thickness direction of the AA 7055 Al alloy plate is mainly depended on the size of the grain as well as the texture. The variation of the yield strength in the longitudinal direction along the thickness direction of the plate is approximately linear, fitting an equation ofσy =-38.7S + 604.8 (0≤S≤1). Using this equation, one can understand the changes of the yield strength along the thickness direction of the plate without testing every layer of the plate. The equation therefore can be used to guide the application of the AA 7055 Al alloy plate in some certain circumstances. Combintional analysis using TEM, HRTEM, DSC and SAXS shows that the main precipitation sequence of the AA 7055 Al alloy at 120 and 160 oC is: SSS→Small GPI zones→η' phase→ηphase. Some GPI zones can exist stably through the ageing process. The morphology of theη' precipitates is similar to a flat ellipsoid with an axis ratio between 0.2 and 0.3.The study of the precipitation kinetics shows that at the early stage of ageing, the evolutions of the radius and the half thickness of the plate-like precipitates are both linear with the square root of the ageing time. While at the later stage of ageing, they are linear with the cubical root of the ageing time. The evolution of the volume fraction of the precipitates follows a JMA-type equation.Based on the quantitative results of the microstructures, the strength contributions from both GPI zones andη' precipitates are inverstigated. Strength models considered both modulus strengthening mechanism and coherency strain strengthening mechanism of these two kinds of precipitates have been built for the AA 7055 Al alloy aged at 120 and 160 oC. Therefore, the yield strength during ageing can be predicted.The results of fatigue crack growth(FCG) rate test show that the magnitude of the FCG resistance of the alloy is depended on the ageing conditions, from great to small, being arranged as T77> under-ageing > peak ageing> over ageing. The coherency degree between the precipitate and the matrix is the main factor determing the difference of the FCG resistance for different ageing conditions. The higher the coherency between precipitates and Al matrix is, the greater the FCG resistance is. Furthermore, when the precipitate is coherent with the Al matrix, the larger the precipitate is, the greater the FCG resistance is.
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