  The local variation of grain boundary atomic structure and chemistry caused
by segregation of impurities influences the macroscopic properties of
poylcrystalline materials. Here, the effect of co-segregation of carbon and
boron on the depletion of aluminum at a $\Sigma 5\,(3\,1\,0\,) [0\,0\,1]$ tilt
grain boundary in a $\alpha-$Fe-$4~at.~\%$Al bicrystal was studied by combining
atomic resolution scanning transmission electron microscopy, atom probe
tomography and density functional theory calculations. The atomic grain
boundary structural units mostly resemble kite-type motifs and the structure
appears disrupted by atomic scale defects. Atom probe tomography reveals that
carbon and boron impurities are co-segregating to the grain boundary reaching
levels of >1.5 at.\%, whereas aluminum is locally depleted by approx. 2~at.\%.
First-principles calculations indicate that carbon and boron exhibit the
strongest segregation tendency and their repulsive interaction with aluminum
promotes its depletion from the grain boundary. It is also predicted that
substitutional segregation of boron atoms may contribute to local distortions
of the kite-type structural units. These results suggest that the
co-segregation and interaction of interstitial impurities with substitutional
solutes strongly influences grain boundary composition and with this the
properties of the interface.
