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

Oxygen incorporation in the MAX phases Ti2AlC, V2AlC and Cr2AlC was studied by ab initio calculations. Comparing the calculated energies for oxygen incorporation indicates that oxygen substitutes for carbon in Ti2AlC and V2AlC, but is incorporated interstitially in the Al layer of Cr2AlC, even for carbon-deficient Cr2AlC. To evaluate these predictions, combinatorial d.c. sputtering was used to deposit thin films with different oxygen concentrations. Two phase regions of Cr2AlC and Cr2Al were investigated in order to study oxygen incorporation in carbon-deficient Cr2AlC. 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 Cr2AlC. 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.