Diffusional reactions during processing of TIMETAL 21S/Al2O3 composites

The stability of an Al2O3 reinforcement in TIMETAL 21S has been investigate d by annealing diffusion couples and consolidated fiber composites at 1100 degrees C, 900 degrees C, and 750 degrees C. Diffusion couple studies indic ate that gamma-TiAl, alpha(2)-Ti3Al, and alpha-Ti(Al,O) phases can form upo n annealing above the beta transus of TIMETAL 21S, but gamma-TiAl, alpha(2) -Ti3Al, and a ternary T phase form during annealing below the beta transus. The phases developed during diffusional interaction define a diffusion pat h between TIMETAL 21S and Al2O3. A coating of Nb, Mo, or Ta between TIMETAL 21S and Al2O3 acts as a diffusion barrier, but the coatings can diffuse in to TIMETAL 21S at high temperature. In agreement with a kinetics analysis, a 2-mu m-thick interface coating of Nb, Mo, or Ta in the TIMETAL 21S/Al2O3 composite can prevent the reaction during processing (2 hours at 850 degree s C or 900 degrees C) with no detectable diffusion into the matrix. If ther e are imperfections such as pinholes or cracks present in the diffusion bar rier, the reaction quickly starts at the interface and does not remain conf ined at the imperfection; rather, it progresses along the interface. The me chanism for progressive development of interface reaction at a discontinuit y in the diffusion barrier has been proposed. The analysis of the diffusion al interface reactions in this work has identified some of the governing de sign concepts for development of robust high-temperature titanium-based com posites.