  Atomic-scale simulations are performed to study the effect of solute
segregation on the shear-induced motion of select grain boundaries in the
classical $\alpha$-Fe/C system. At shear rates larger than the solute diffusion
rate, we observe a transition from coupled motion to sliding. Below a critical
solute excess, the boundaries break away from the solute cloud and move in a
coupled motion. At smaller shear rates, we observe extrinsic coupled motion at
small stresses indicating that the coupling is aided by convective solute
diffusion along the boundary. Our studies underscore the role of solutes in
modifying the bicrystallography, temperature and rate dependence of shear
accommodation at grain boundaries.
