A TEM study of the precipitates in a 6082 Al-Mg-Si alloy system

Abstract

The purpose of this work has been to gain a better understanding of the details of the precipitation sequence in Al-Mg-Si alloys. Because of the complexity of such ternary systems is has been necessary to limit the study to one representative composition. An industrially important, widely used alloy was therefore chosen: AA6082, which contains as alloying elements 0.6 wt% Mg, 0.9 wt% Si, 0,5wt% Mn and 0.2wt% Fe. The alloys of the 6xxx series have increasingly been used for construction and automotive purposes due to their good mechanical properties. Their main characteristic is the tremendous increase in hardness, due to the precipitation of Mg-Si metastable phases upon annealing at intermediate temperatures. Our experience is that the main strengthening phase is β”, at least for alloys where the Si/Mg atomic ratio does not differ considerably from unity. This phase is present in the form of needles oriented parallel with the three <100> directions in the A1 matrix. Therefore an important part of this research has been focused on nucleation and growth of the β”-phase and other Mg-Si phases related to β” in the precipitation sequence. The type of microstructure developed as a result of heat-treatment and the relation with the hardness have also been investigated. A big step forward was achieved, as we were able to gain an insight into the atomic structures of the GP-zones and the relationship with the β”-phase. This also enables a better understanding of the difference in the hardness curves caused by RT storage prior to ageing. The crystal structure of β’, the second important phase in this alloy, which is often found together with the β”-phase, is under current investigation.