Passivating oxidation behavior of Ti_0.12Al_0.21B_0.67 coatings investigated by scanning transmission electron microscopy and chemical environment dependent density functional theory simulations

The oxidation behavior of stoichiometric Ti0.12Al0.21B0.67 coatings is investigated by scanning transmission electron microscopy (STEM) after oxidizing for 1, 4 and 8 h at 700 degrees C and at 800 and 900 degrees C. In the as deposited state, a similar to 4 nm thick, native, amorphous oxide layer covers the surface of the coating, while the magnitude of incorporated O along the column boundaries decreases with depth. During oxidation, the formation of scale layers consisting predominantly of Al, O and B is observed, that appear to be amorphous at 700 degrees C, while after oxidation at 900 degrees C for 8 h, a (nano-)crystalline aluminoborate layer forms. Concurrently, within the unoxidized coating, the formation of Al- and Ti-rich boride regions, consistent with spinodal decomposition, is observed. Chemical environment dependent density functional theory (DFT) predictions of the energies required for mass transport on the metal sublattice indicate that Al diffusion is initiated before Ti diffusion. Hence, as the temperature is increased, the migration of Al is initiated first, leading to the formation of the oxide scale observed already after oxidation at 700 degrees C for 1 h. Below the oxidized region, the formation of Al-rich and Ti-rich regions by spinodal decomposition require the concurrent migration of Al and Ti. The fact that decomposition takes place at 900 degrees C and hence at larger temperatures than the Al diffusion mediated scale formation is consistent with DFT predictions as the average values of the predicted energies required for both, vacancy formation and migration for Ti, are larger than for Al.