(Scanning) Transmission Electron Microscopy Studies of Grain Boundary Segregation relevant to Intergranular Corrosion in Al-Mg-Si-Cu Alloys

Abstract

The effect of the alloy composition on the susceptibility to intergranular corrosion in Al-Mg-Si-Cu alloys has been investigated by Transmission Electron Microscopy. The main focus of this thesis has been the effect of Mg:Si-ratio and the effect of Mn-content on the intergranular corrosion, since Mg and Si are the main alloying elements of the 6xxx alloys and Mn is a common alloying element. The addition of 0.40 wt% Cu to a 6082 alloy increases the hardness of the alloy by approximately 10 HV in all ageing conditions. The susceptibility to intergranular corrosion is also increased in a slightly underaged condition due to the formation of an enriched layer of Cu at the grain boundary. By adjusting the Mg:Si-ratio from 0.87 in the 6082 alloy to 2.00, the susceptibility to intergranular corrosion is reduced. In this Mg-rich alloy, an enriched layer of both Mg and Cu was observed at the grain boundary. The presence of Mg reduces the cathodic potential created by the Cu and reduces the potential difference between the grain boundary layer and the adjacent depleted zone. In leaner alloys, a reduced susceptibility to intergranular corrosion is observed when the Mg:Si-ratio is increased from 0.80 to 2.90. The alloy with a high Mg:Si-ratio has the same amount of excess Mg as the 6082-type alloy with a ratio of 2.00. The Mg-rich lean alloy shows an enriched layer of both Mg and Cu at the grain boundary, while the Si-rich only shows an enriched layer of Cu. Once again, the Mg+Cu has a reduced electrochemical potential compared to the pure Cu layer in the Si-rich alloy and the resistance to intergranular corrosion is increased. By removing the Mn from the 6082-type alloy with excess Mg, a fully recrystallized structure with small grains and an aspect ratio close to 1 is achieved. This alloy show significant susceptibility to intergranular corrosion, even though the enriched grain boundary layer contain both Mg and Cu. It is likely that the grain structure makes it easier for the corrosion attack to penetrate more deeply into the sample since most of the grain boundaries are approximately perpendicular to the surface. In the Mn-containing alloy, most of the interior grains are very large and have an aspect ratio >> 1. The intergranular corrosion attacks can only propagate parallel to the surface and the penetration depth is low. This underlines the geometrical aspects of intergranular corrosion in alloys where the grain boundary chemistry is otherwise similar. In a very Si-rich version of the 6082-type alloy, there is still only observed an enriched layer of Cu. The absence of an enriched layer of Si is not yet understood, but more quantitative work needs to be done. The Si might all disappear to the grain boundary precipitates and dispersoids, but that investigation has been beyond the scope of this work. It has not been found any periodicity or structure to the enriched layer at the grain boundary, so the main theory at the moment is that it is not periodic. The equipment needed to investigate this have only recently become available and there are some issues with orienting the sample perfectly, meaning that so far it is too early to conclude whether there is any periodicity in the enriched boundary layer or not.