Sample size dependence of high-temperature thermal stability of Ti2AlN MAX phase

Abstract

The thermal stability of Titanium Aluminum Nitride (Ti2AlN) microcrystalline powder in vacuum has been studied by in-situ high-temperature X-ray diffraction. The limit of thermal stability for Ti2AlN phase is T < 850 °C. The process of thermal decomposition consists of the Al atoms sublimation and the formation of a new TiNx phase on the sample surface. An increase in sample size leads to an increase in thermal stability limit (850 °C for microcrystals and 1550 °C for dense bulk sample). Based on crystal chemical analysis and quantum chemical calculations, a mechanism of thermal dissociation was proposed. Kinetics of thermal dissociation of the Ti2AlN phase was modeled using the Avrami equation. Avrami exponent parameters (n = 1.09; 1.28) indicate that the process of germs formation occurs at a subgrain boundaries of the sample. The observed temperature dependence of the parameter n can be associated with a probability change of crosslinking formation during the growth of a new phase. The size dependence of Ti2AlN thermal stability was explained by the ratios of Al diffusion coefficients in the crystallite bulk and along the subgrain boundaries.