The aim of the present investigation was to study precipitation of Cr in Al-Mg-Si alloys during high temperature annealing. It has been shown that Cr containing dispersoids precipitate during heating to the homogenisation temperature. During the heating small a-Al(CrMn)Si particles nucleate at the surface of the b-AlFeSi particles and after additional heating to a higher temperature the a-Al(CrFe)Si particles grow and the b-AlFeSi particles dissolve. If the content of Cr is high (~0•3 wt-%Cr) both a-Al(CrFe)Si and a=-AlCrSi dispersoids are present in the matrix during heating while only a-Al(CrFe)Si dispersoids are present when the content is low (~0•15 wt-%Cr). The nucleation mechanisms for the dispersoids are not fully understood but most likely the Cr containing dispersoids nucleate by means of an intermediate phase. This intermediate phase is found to be a 'u phase' precipitate.MST/4489
Stress corrosion cracking (SCC) in Cu-free Al-Zn-Mg (7xxx) aluminium alloys limits its use in many applications. In this work, we study in detail the microstructure of a peak and slightly overaged condition in an AA7003 alloy using transmission- and scanning electron microscopy in order to provide a comprehensive understanding of the microstructural features related to SCC. The SCC properties have been assessed using the double cantilever beam method and slow strain rate tensile tests. Grain boundary particles, precipitate free zones, and matrix precipitates have been studied. A difference in the SCC properties is established between the two ageing conditions. The dominating difference is the size and orientation of the hardening phases. Possible explanations correlating the microstructure and SCC properties are discussed.
The bendability of extruded profiles of an age hardenable aluminium alloy is investigated using mechanical tests on flat tensile specimens and bending specimens. Two profile geometries are considered, where the profiles exhibit different grain structure. The microstructure of the profiles in terms of the crystallographic texture and constituent particles is otherwise comparable. While the tensile properties are not that different for the two profiles, their bendability is strongly dependent on the grain structure and is about twice as high for one profile than for the other. A newly proposed coupled damage and single crystal plasticity model is used in finite element analyses of the mechanical tests to investigate the influence of the grain structure on the bending behaviour, and the numerical results are compared to the experimental tests. The crystallographic texture and the grain morphology of the profiles, found by the electron back-scatter diffraction technique, are explicitly represented in the finite element models. The crystal plasticity simulations capture the difference in the bendability of the two profiles, and in agreement with the experiments predict a considerably higher bendability for one of the profiles. It is found that the grain structure affects the shear band formation in these profiles, but also the local texture where the shear bands are located is important for crack initiation and propagation as grains with certain crystallographic orientations may have a higher fracture resistance.
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