Grain boundary segregation and precipitation in aluminium alloys

The solute enrichment at the grain boundary determined by TEM with a nanoprobe can only be understood qualitatively. EDS-analysis in combination with electron diffraction proves that precipitates with a Mg/Si-ratio of approx1.3 and a structure similar to the Q-phase (at larger aging times) are present at the grain boundaries of AA6061. The Cu-content is lower than the stoichiometric value, which may indicate that the Q-phase is preceded by the beta -phase. The model originally developed for 7XXX-series aluminium alloys is applied to AA6061 and AA7050. Predicted values of the GB precipitate growth are in fair agreement with experimental ones. Calculated inter-distances are in between measured longitudinal and transveral inter-distances. This is explained by the model assuming a square collector plate while a rectangular shape is expected.