Precipitation in the high strength AA7449 aluminium alloy: implications on internal stresses on different length scales

Al-Zn-Mg(-Cu) (AA7xxx) alloys are widely used structural materials owing to their excellent mechanical properties and low corrosion susceptibility. The high strength of these materials is obtained by the formation of secondary phases, e.g. precipitation. This present thesis aims at contributing to the understanding of the influence of precipitation in the high strength AA7449 alloy on the internal stress formation at different length scales. Two different cases are studied in this thesis. In the first case, the influence of precipitation during quench on macroscopic residual stresses (RS) is studied in an industrial 75 mm AA7449 thick plate. For some investigations the behaviour of the AA7449 alloy is compared with the medium strength AA7040 alloy. In the second case, the influence of different precipitation states on the internal strain formation is investigated at the microstructural length scale. For the first case, in situ small angle X-ray scattering evidenced that during quenching the heterogeneous eta phase forms at high temperatures in contrast to homogeneous GP(I) zones, which form at low temperatures. The eta formation is influenced by the cooling conditions but also by macrosegregation in the thick plate. Yet, the volume fraction of eta is very low and has a negligible effect on the macroscopic RS in thick plates. The homogeneous GP(I) zones form during quench with radii in the subnanometre range. Their formation is strongly influenced by excess vacancies. Fast cooling rates can lead to higher radius and volume fraction of GP(I) zones compared to slower cooling. The GP(I) zone formation increases the yield strength during quench and is responsible for the high RS in industrial thick plates. Therefore, the GP(I) zone formation during quench needs to be taken into account in macroscopic finite element (FE) residual stress simulations by using precipitation modelling. The GP(I) zone formation during quench is simulated by using a thermodynamic based Eulerian multi-class model. Therefore, a thermodynamic description for GP(I) zones is derived from reversion heat treatments by using the solubility product. The influence of excess vacancies is taken into account by adapted effective diffusion coefficients. This simplified approach allows reproducing reasonably well the measured GP(I) zone formation during rapid coolings. The coupling of the precipitation model with FE residual stress calculations (performed by the other PhD studentt) allows predicting very well the RS in two AA7449 thick plates. For the second case, in situ mechanical testing during tensile deformation using X-rays and neutrons evidence that the presence of different precipitate types change the internal strain formation in the aluminium matrix at the microstructural length scale. The presence of small GP(I) zones leads to an internal strain formation that is dominated by the intergranular strains of the aluminium matrix. When large eta precipitates are embedded in the aluminium matrix the interphase strains clearly dominate the internal strain formation. Further, it is shown that the eta phase has a higher elastic modulus than the aluminium matrix and shows an anisotropic behaviour during elastic and plastic deformation. In addition, it is pointed out that the internal strain formation at the microstructural length scale can have a significant effect on macroscopic RS measurements performed by XRD when the plastically deformed sample contains eta' and eta.

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