FIBER STRENGTH AND FIBER-MATRIX BOND STRENGTH IN SINGLE-CRYSTAL AL2O3FIBER-REINFORCED NI3AL BASED COMPOSITES

A series of single-crystal Al2O3 fiber (Saphikon), reinforced Ni3Al-ba sed composites were fabricated by a liquid metal infiltration techniqu e, pressure casting. Tensile testing and indentation techniques have b een employed to measure fiber strength and fiber/matrix interfacial de bond shear stress. The Weibull mean strength of the fiber has been obs erved to decrease drastically upon handling, exposure to high temperat ure, and casting. Alloying of Ni3Al with Ti has resulted in a further decrease in fiber strength. Thermal expansion mismatch between the fib er and matrix led to the formation of compressive twins in the fiber. These twins, forming on {1102BAR} planes, produced cracks at their int ersections, which were parallel to the fiber axis, c-axis. Twin-induce d fiber cracking was observed in all cases, but most predominantly whe n Cr was present. While addition of Cr at the 1 at. pct level had no a ppreciable effect on the interfacial debond shear stress, addition of 0.5 at. pct Cf resulted in an approximately threefold increase in debo nd stress, from 19 MPa to about 54.5 MPa. Alloying of Ni3Al with Cr ha s also resulted in partial dissolution of the Al2O3 fiber. Addition of Ti had a moderate effect on increasing the fiber/matrix bond strength .