The relative energies of normally and abnormally growing grain boundaries in alumina displaying different complexions

Chemically induced grain-boundary structural transitions are well documented in a number of ceramic systems. It has been hypothesized that transitions between distinct structural states, referred to as complexions, are driven by a reduction in the free energy of the grain boundaries. The current work tests this hypothesis by measuring the relative energies of boundaries on normal grains and abnormal grains in the same microstructure, which are known to have different complexions. The complexion transition from the most ordered state reduces the average grain-boundary energy by up to 45%. Similar changes in the energy are not observed for cases when the initial state is a higher order complexion that is more disordered. The results indicate that complexion transitions are influenced by both the change in free energy and the activation energy of the transition and that the dominant factor depends on the specific transition.