ALCHEMI studies of site occupancies in Cr-, Ni-, and Fe-substituted manganese cobaltite spinels

The effects of Cr, Ni, and Fe substitution into manganese cobaltite (MCO) spinels are of great interest due to the roles that the diffusion of these cations play in reaction layer development during high temperature exposure of MCO-coated alloys. Here we report a study on a series of model Cr-, Ni-, and Fe-substituted MCO spinel ceramics produced by consolidation of combustion-synthesized oxide powders. The cation site occupancies in these samples have been studied by X-ray spectrometry-based Atom Location by CHanneling Enhanced MIcroanalysis (ALCHEMI) experiments in the transmission electron microscope, with the data being analyzed using the ordering tie-line approach. In Cr-substituted samples, the Cr ions lie on the octahedral B sites and the Co ions reside on the tetrahedral A sites with Mn occupying the remaining sites. In Ni-substituted samples, all of the Ni ions occupy the B sites and the Co and Mn ions tend to lie on A and B sites, respectively. In contrast to the Cr-substituted samples, there is some mixing of the Co and Mn ions on the two types of sites at lower Ni contents. In Fe-substituted samples with lower Fe contents, all of the Mn ions occupy the B sites, roughly equal proportions of Fe ions occupy the A and B sites, and Co ions fill the remaining sites. With increasing Fe content, the degree of order decreases which ultimately results in the High Fe sample exhibiting no channeling evidence for preferred site occupation. These ALCHEMI data could provide a useful insight into the role of cation sub-lattice site preference in the formation of reaction layers in MCO-coated stainless steels and superalloys.