Mechanism for breakaway oxidation of the Ti2AlC MAX phase

The good oxidation resistance of MAX phases up to temperatures around 1200 degrees C can be compromised for long exposure due to the breakaway of the protective alumina layer. Herein, we unveil a mechanism of breakaway oxidation of the Ti2AlC MAX phase, identifying the main trigger and the solutions to avoid it. It is caused by excessive rumpling of the oxide scale on surfaces with arithmetical mean roughness (R-a) > 3 mu m and constitutes a key factor in subsequent consumption of Ti2AlC. First, the oxide scale experienced rumpling due to significant radial stresses generated at the Ti2AlC/oxide interface. Second, scale blistering resulted from substantial buckling due to the evolution of in-plane stresses and lateral lengthening. Third, blister collapse and exposure of the underlying Al-depleted Ti2AlC surface led to rapid ingress of oxygen and oxide/substrate interface recession. The self-healing ability of Ti2AlC has been restrained and breakaway oxidation kinetics following a linear trend have been initiated. Similarly, breakaway oxidation was observed on micro-damaged surfaces. A mixed oxide layer with high porosity mainly composed of rutile titanium dioxide (TiO2) promptly formed on these surfaces, gradually consuming the base Ti2AlC material. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.