Unveiling the diffusion pathways under high-temperature oxidation of Cr2AlC MAX phase via nanoscale analysis

The MAX phase chromium aluminium carbide (Cr2AlC) is a potential candidate for high-temperature structural and energy applications due to the unique combination of properties, including excellent oxidation and corrosion resistance in harsh environmental conditions. In this work, the oxidation behaviour of bulk Cr2AlC was studied at temperatures of 1000 degrees C and 1200 degrees C in humid synthetic air, mimicking a realistic application environment. An alpha-Al2O3 scale forms at both temperatures, consisting of both inward and outward-growing layers, with a continuous Cr7C3 layer beneath the oxide. A special focus was put on employing atom probe tomography (APT) to analyse the oxide and carbide layers at the nanoscale. Decomposition of Cr2AlC to Cr7C3 is the main source (>70%) of aluminium needed for the oxide scale formation, while Al depletion in the underlying MAX phase remains marginal (<1 at%), exhibiting a pattern different from the common high-temperature oxidation of alloys. Al diffusion to the oxide scale proceeds predominantly through grain boundaries (GBs) in Cr2AlC and through the bulk in Cr7C3. Ionic diffusion through GBs of the large-grained inner alumina layer seems to be the rate-limiting step for the oxide scale growth, resulting in the apparent parabolic oxidation kinetics. No segregation of Cr, C or any impurities, which could affect ionic transport, was found at these alumina GBs by APT, rendering the results of this study as characteristic of a pure Cr2AlC oxidation.