Thermochemical Calculations of the Stability of Ti2AlC in Various Atmospheres

摘要

The thermochemical stability of the MAX phase Ti2AlC under vacuum, inert Ar, pure O-2 and N-2 and a reactive air atmosphere was explored based on thermochemical equilibrium calculations. A dataset on the enthalpy of formation, standard molar entropy and the temperature variation of the heat capacities of Ti2AlC and TiC0.5 was derived from thermochemical properties given in the literature and implemented in the software package HSC Chemistry. In inert atmosphere Ti2AlC decomposes peritectically into solid TiC0.5 and a liquid Al-rich phase at approximately 1673 degrees C whereas in vacuum (1 mPa) dissociation into solid TiC0.5 and an Al gas phase is observed at a significantly lower temperature of 984 degrees C. When Ti2AlC is exposed to air as well as pure O-2, solid Al2O3 and TiO2 will form with the carbon being oxidized to gaseous CO and CO2. At very low air content, however, the calculations show TiC0.5 and AlN as the prevailing reaction products with small fractions of Al2O3. In a pure nitrogen atmosphere Ti2AlC is nitrided to AlN, TiN and residual carbon.