FIBER STRENGTH AND FIBER-MATRIX BOND STRENGTH IN SINGLE-CRYSTAL AL2O3 FIBER-REINFORCED NI3AL BASED COMPOSITES

A series of single-crystal Al2O3 fiber (Saphikon), reinforced Ni3Al-based composites were fabricated by a liquid metal infiltration technique, pressure casting. Tensile testing and indentation techniques have been employed to measure fiber strength and fiber/matrix interfacial debond shear stress. The Weibull mean strength of the fiber has been observed to decrease drastically upon handling, exposure to high temperature, and casting. Alloying of Ni3Al with Ti has resulted in a further decrease in fiber strength. Thermal expansion mismatch between the fiber and matrix led to the formation of compressive twins in the fiber. These twins, forming on {1102BAR} planes, produced cracks at their intersections, which were parallel to the fiber axis, c-axis. Twin-induced fiber cracking was observed in all cases, but most predominantly when Cr was present. While addition of Cr at the 1 at. pct level had no appreciable effect on the interfacial debond shear stress, addition of 0.5 at. pct Cf resulted in an approximately threefold increase in debond stress, from 19 MPa to about 54.5 MPa. Alloying of Ni3Al with Cr has also resulted in partial dissolution of the Al2O3 fiber. Addition of Ti had a moderate effect on increasing the fiber/matrix bond strength.