William E. Lee

Microstructural evolution during high-temperature oxidation of Ti2AlC ceramics

Abstract Microstructural development during high-temperature oxidation of Ti 2 AlC below 1300xa0°C involves gradual formation of an outer discontinuous TiO 2 layer and an inner dense and continuous α-Al 2 O 3 layer. After heating at 1400xa0°C, an outer layer of mixed TiO 2 and Al 2 TiO 5 phases and a cracked α-Al 2 O 3 inner layer were formed. After heating to 1200xa0°C and cooling to room temperature, two types of planar defect were identified in surface TiO 2 grains: twins with (2xa00xa00) 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 2 O 3 and Al 2 TiO 5 ) in the oxide scale. After heating to 1400xa0°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 2 TiO 5 and the volume changes associated with the reactions during Ti 2 AlC 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 2 AlC, and diffuses outward to form a protective inner α-Al 2 O 3 layer between 1100 and 1300xa0°C. However, the α-Al 2 O 3 layer becomes cracked at 1400xa0°C, providing channels for rapid ingress of oxygen to the body, leading to severe oxidation.

TEM investigation of hot pressed ‐10 vol.%SiC–ZrB2 composite

Abstract Abstract The polytypism of SiC, phase transformation of ZrB2 and the interfaces between SiC and ZrB2 were investigated using high resolution TEM in a hot pressed 10u2005vol.%SiC‐ZrB2 composite. In most cases, no grain boundary interphases between hexagonal ZrB2 and 6H‐SiC phases were observed with SiC being both inter‐ and intragranular. Occasionally, 6H‐SiC transformed into 3C and 15R and hexagonal ZrB2 transformed into cubic ZrB. High resolution TEM showed no grain boundary interphases in most regions. Energy dispersive X‐ray spectroscopy and electron energy‐loss spectroscopy analyses showed the presence of oxygen throughout the sample. The phase transformation of SiC and ZrB2, and the interphase formation between SiC and ZrB2 grains are discussed.