Microstructural evolution during high-temperature oxidation of Ti 2AlC ceramics

Abstract

Microstructural development during high-temperature oxidation of Ti 2AlC below 1300 °C involves gradual formation of an outer discontinuous TiO 2 layer and an inner dense and continuous α-Al 2O 3 layer. After heating at 1400 °C, an outer layer of mixed TiO 2 and Al 2TiO 5 phases and a cracked α-Al 2O 3 inner layer were formed. After heating to 1200 °C and cooling to room temperature, two types of planar defect were identified in surface TiO 2 grains: twins with (2 0 0) twin planes, and stacking faults bounded by partial dislocations. Formation of planar defects released the thermal stresses that had generated in TiO 2 grains due to thermal expansion mismatch of the phases (TiO 2, α-Al 2O 3 and Al 2TiO 5) in the oxide scale. After heating to 1400 °C and cooling to room temperature, crack propagation in TiO 2 grains resulted from the thermal expansion mismatch of the phases in the oxide scale, the high anisotropy of thermal expansion in Al 2TiO 5 and the volume changes associated with the reactions during Ti 2AlC oxidation. An atomistic oxidation mechanism is proposed, in which the growth of oxide scale is caused by inward diffusion of O 2− and outward diffusion of Al 3+ and Ti 4+. The weakly bound Al leaves the Al atom plane in the layered structure of Ti 2AlC, and diffuses outward to form a protective inner α-Al 2O 3 layer between 1100 and 1300 °C. However, the α-Al 2O 3 layer becomes cracked at 1400 °C, providing channels for rapid ingress of oxygen to the body, leading to severe oxidation.