  In alloys, solute segregation at grain boundaries is classically attributed
to three driving forces: a high solution enthalpy, a high size mismatch, and a
high difference in interfacial energy. These effects are generally cast into a
single scalar segregation energy and used to predict grain boundary solute
enrichment or depletion. This approach neglects the physics of segregation at
many competing grain boundary sites, and can also miss electronic effects that
are energetically significant to the problem. In this paper, we demonstrate
that such driving forces cannot explain, nor thus predict, segregation in some
alloys. Using quantum-accurate segregation spectra that have recently become
available for some polycrystalline alloys, we predict strong segregation for
gold in aluminum, a solvent-solute combination that does not conform to
classical driving forces. Our experiments confirm these predictions and reveal
gold enrichment at grain boundaries that is two orders of magnitude over the
bulk lattice solute concentration.
