  Recent experiments suggest that Bi impurities segregate to form bilayer films
on Ni and Cu grain boundaries but do not segregate in Fe. To explain these
phenomena, we study the total energies of Bi films on transition metal (TM)
$\Sigma$3(111) and $\Sigma$5(012) GBs using density functional theory. Our
results agree with the observed stabilities. We propose a model to predict Bi
bilayer stability at Ni GBs which suggests that Bi bilayer is not stable on
(111) twist CSL GBs but is stable in most (100) twist CSL GBs. We investigate
the interaction and bonding character between Bi and TMs to explain the
differences among TMs based on localization of orbitals and magnetism.
