Structures and formation mechanisms of dislocation-induced precipitates in relation to the age-hardening responses of Al-Mg-Si alloys

In the slightly deformed Al-Mg-Si alloys, dislocation-induced precipitates are frequently observed, and they usually line up, forming sophisticated precipitation microstructures. Using atomic-resolution electron microscopy in association with hardness measurements, we systematically investigated these precipitates in relation to the age-hardening responses of the alloys. Our study reveals that the majority of dislocation-induced complex precipitates are actually short-range ordered while long-range disordered polycrystalline precipitates and multiphase composite precipitates, including polycrystalline U2 precipitates, B'/U2, B'-2/U2, B'/B'-2/U2 and β'/U2 composite precipitates. It is suggested that the formation of these complex precipitates is mainly owing to a high nucleation rate and rapid growth of different precipitate phases parallel to the associated dislocation lines. Since dislocation-induced precipitates consume more Mg than Si from the matrix and have a high formation kinetics, they will have different impacts on the matrix precipitation in different types of Al-Mg-Si alloys. Our results further demonstrate that for the “normally-β"-hardened” alloy, their formation leads to a coarser precipitate microstructure in the matrix, whereas for the “normally-β'-hardened” alloy, their formation reverses the precipitation pathway in the matrix, resulting in a reduced age-hardening potential of the former alloy and an improved age-hardening potential of the latter alloy.