Diffusion path during internal displacement reactions in multi-component oxides: Reaction between Fe and (Co,Mg)TiO₃ solid solution at 1273 K

The reaction between Fe foil and a disc of ilmenite solid solution (Co _0.48 Ni_0.52) TiO₃ was studied at 1273 K. At the metal/oxide interface, the displacement reaction, Fe + (Co,Mg)TiO₃ = Co + (Fe,Mg)TiO₃ occurs, resulting in an ilmenite solid solution containing three divalent cations. Ferrous ions diffuse into the oxide solid solution and cause the precipitation of Co-Fe alloy as discrete particles inside the oxide matrix. The morphology of the product layer was characterized by SEM. Only two phases, alloy and ilmenite, were detected in the reaction zone. This suggests that the local flux condition imposed by ilmenite stoichiometry [(Co + Fe + Mg):Ti = 1:1] was satisfied during the reactive diffusion: (J_Co + J_Fe + J_Mg) = J_Ti. The composition of the alloy and the oxide was determined using EPMA as a function of distance in the direction of diffusion. Although Mg does not participate in the displacement reaction, its composition in the ilmenite phase was found to be position dependent inside the reaction zone. The up-hill diffusion of inert Mg is caused by the development of chemical potential gradients as a result of displacement reaction. The evolution of composition gradients inside the reaction zone and the diffusion path in a ternary composition diagram of the system CoTiO ₃-FeTiO₃-MgTiO₃ are discussed.