OXIDATION BEHAVIOR OF SINGLE-CRYSTAL AL2O3-FIBER-REINFORCED NI3AL-BASED COMPOSITES

A series of single-crystal Al2O3-fiber-reinforced Ni3Al-based intermet allic matrix composites were fabricated by pressure casting. The matri ces employed were binary Ni3Al, Ni3Al-0.5 at. pct Cr, and Ni3Al-0.34 a t. pct Zr. The development of microstructure upon oxidation in air at either 1100-degrees-C or 1200-degrees-C was investigated by optical, s canning, and transmission electron microscopy. In air-oxidized binary Ni3Al, some of the fibers were fully or partially covered with a layer of oxide. A weak fiber/matrix bond in this system, which led to fiber debonding during composite processing, is believed to be responsible for the ingress of O into the composite and oxidation of the matrix in the debonded regions at the fiber/matrix interface. Addition of Cr to Ni3Al resulted in an almost threefold increase in fiber/matrix bond s trength. No oxidation of the interface was observed. A thick layer of oxide was formed around all the fibers when the composite was thermall y cycled prior to isothermal annealing. Addition of Zr to Ni3Al result ed in the formation of a layer of ZrO2 on the surface of the fibers du ring composite processing. The ZrO2 layer provided a fast path for the diffusion of O, which led to the formation of a rootlike oxide struct ure around the fibers. The rootlike structure consisted of a network o f Al2O3-covered ZrO2.