Diffusional reactions during processing of TIMETAL 21S/Al2O3 composites

Abstract

The stability of an Al{sub 2}O{sub 3} reinforcement in TIMETAL 21S has been investigated by annealing diffusion couples and consolidated fiber composites at 1100 C, 900 C, and 750 C. Diffusion couple studies indicate that {gamma}-TiAl, {alpha}{sub 2}-Ti{sub 3}Al, and {alpha}-Ti(Al,O) phases can form upon annealing above the {beta} transus of TIMETAL 21S, but {gamma}-TiAl, {alpha}{sub 2}-Ti{sub 3}Al, and a ternary T phase form during annealing below the {beta} transus. The phases developed during diffusional interaction define a diffusion path between TIMETAL 21S and Al{sub 2}O{sub 3}. A coating of Nb, Mo, or Ta between TIMETAL 21S and Al{sub 2}O{sub 3} acts as a diffusion barrier, but the coatings can diffuse into TIMETAL 21S at high temperature. In agreement wit h a kinetics analysis, a 2-{micro}m-thick interface coating of Nb, Mo, or Ta in the TIMETAL 21S/Al{sub 2}O{sub 3} composite can prevent the reaction during processing (2 hours at 850 C or 900 C) with no detectable diffusion into the matrix. If there are imperfections such as pinholes or cracks present in the diffusion barrier, the reaction quickly starts at the interface and does not remain confined at the imperfection; rather, it progresses along the interface. The mechanism for progressivemore » development of interface reaction at a discontinuity in the diffusion barrier has been proposed. The analysis of the diffusional interface reactions in this work has identified some of the governing design concepts for development of robust high-temperature titanium-based composites.« less