  Based on ab-initio simulations, we report on the nature of principally
different mechanisms for interaction of Mg and Zn atoms with grain boundaries
in Al alloys leading to different morphology of segregation. The Mg atoms
segregate in relatively wide GB region with heterogeneous agglomerations due to
the deformation mechanism of solute-GB interaction. In contrast, in the case of
Zn atoms an electronic mechanism associated with the formation of directional
bonding is dominating in the solute-GB interaction. As a result, for Zn atoms
it is energetically beneficial to occupy interstitial positions at the very GB
and to be arranged into thin layers along the GBs. The results obtained show
the essential role of elements chemistry in segregation formation and explain
the qualitative features in morphology of GB segregation observed in Al-Mg and
Al-Zn alloys with ultrafine grains.
