Analytical electron microscopy of grain boundary segregation: Application to Al-Zn-Mg-Cu (7xxx) alloys

Grain boundary segregation is important in many metallurgical phenomenon. A technologically important example is in the performance of high strength Al-Zn-Mg-Cu (7xxx) alloys. These alloys can be susceptible to environmentally assisted cracking related to the local microstructure and microchemistry at grain boundaries. In particular, the concentration of alloying elements on grain boundaries relative to the matrix is thought to be important. This paper demonstrates the important effect of the resolution of the electron probe on measurements of grain boundary segregation in these alloys. A convolution model has been applied to demonstrate that a sub-nm probe diameter is necessary to accurately capture the composition profile at grain boundaries. Larger diameter probes will produce profiles that are in significant error, not only in concentrations measured but also in the shape of the profile. These errors arise from both smearing effects and sampling of precipitates. Contradictory reports regarding excess or deficiency of solute on grain boundaries can be explained by these effects.