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 environmen-
tally 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 pa-
per demonstrates the important eect of the resolution of the electron probe
on measurements of grain boundary segregation in these alloys. A convolu-
tion model has been applied to demonstrate that a sub-nm probe diameter is
necessary to accurately capture the composition prole at grain boundaries.
Larger diameter probes will produce proles that are in signicant error, not
only in concentrations measured but also in the shape of the prole. These
errors arise from both smearing eects and sampling of precipitates. Contra-
dictory reports regarding excess or deciency of solute on grain boundaries
can be explained by these eects.