Effect of dopants on alumina grain boundary sliding: implications for creep inhibition

We investigate by means of periodic density functional theory the mechanism of grain boundary sliding along the α-alumina Σ11 tilt grain boundary. We identify minimum and maximum energy structures along a preferential sliding pathway for the pure grain boundary, as well as for grain boundaries doped with a series of early transition metals, as well as barium, gadolinium, and neodymium. We predict that the segregation of those dopants results in a considerable increase in the grain boundary sliding barrier. Grain boundary sliding occurs by a series of bond breaking and forming across the grain boundary. Our results suggest that the presence of large cations inhibits the regeneration of bonds during sliding, which results in a decrease in total number of bonds across the grain boundary interface, thereby raising the barrier to sliding. Trends in predicted grain boundary sliding energies are in good agreement with recently measured creep activation energies in polycrystalline alumina, lending further credence to the notion that grain boundary sliding plays a dominant role in alumina creep.