Five parameter distribution of planes in AA 5182 aluminium alloys with relevance to intergranular corrosion resistance

Abstract

Alloys of AA 5182 are commonly used in the automotive industry to provide weight reductions in vehicle chassis. The strength of such alloys is based on the Mg content, with Mg causing the precipitation of a β-Al₈Mg₅ phase along the length of the grain boundaries in the microstructure. These precipitates are vulnerable to intergranular corrosion (IGC) in acidic environments. For this reason, the proportions of Mg in the microstructure must be limited, with >3 wt-Mg causing intergranular attack. The alloys used in this study had an Mg content of 4·43, so were vulnerable to IGC. Standard nitric acid mass loss tests were applied to determine the level of corrosion resistance in two samples subjected to various treatments. Using ‘five parameter’ analysis, the importance of the grain boundary plane and thermomechanical history to IGC resistance was determined. ‘Special’ planes were identified in the microstructure and their impact on IGC resistance was assessed. From such analysis, it was found that a high deformation level of 50 together with a simulated batch annealing heat treatment yielded the highest proportions of ‘special’ boundaries within the microstructure. ‘Special’ boundaries in this study were identified to be low angle boundaries, <111> twist boundaries and <110> tilt boundaries, which are associated with a reduced energy compared to random high angle boundaries. As precipitation generally occurs along high energy boundaries, ‘special’ boundaries reduce precipitate formation, thus reducing IGC intensity. Future implications include the introduction of a higher proportion of ‘special’ boundaries into the microstructure, allowing for an increase in Mg content in these alloys.