Oxygen incorporation in M2AlC (M=Ti, V, Cr)

Abstract Oxygen incorporation in the MAX phases Ti 2 AlC, V 2 AlC and Cr 2 AlC was studied by ab initio calculations. Comparing the calculated energies for oxygen incorporation indicates that oxygen substitutes for carbon in Ti 2 AlC and V 2 AlC, but is incorporated interstitially in the Al layer of Cr 2 AlC, even for carbon-deficient Cr 2 AlC. To evaluate these predictions, combinatorial d.c. sputtering was used to deposit thin films with different oxygen concentrations. Two phase regions of Cr 2 AlC and Cr 2 Al were investigated in order to study oxygen incorporation in carbon-deficient Cr 2 AlC. X-ray strain analysis data indicate that the a and c lattice parameters increase with oxygen content. These trends are in good agreement with the change in lattice parameters predicted by ab initio calculations and therefore corroborate the notion of interstitial oxygen incorporation in Cr 2 AlC. A metastable solubility limit for oxygen of 3.5 at.% was determined experimentally. This is the first report on interstitial oxygen in a MAX phase and may be of relevance during the initial stages of oxidation.