OXIDATION OF A ZIRCONIA-TOUGHENED ALUMINA FIBER-REINFORCED NI3AL COMPOSITE

A Ni3Al composite reinforced with zirconia-toughened alumina fiber, PRD-166 fiber, was produced by pressure casting. The thermochemical stability of the composite at 1100-degrees-C in vacuum and air was investigated by optical and transmission electron microscopy and energy-dispersive spectroscopy (EDS). Vacuum annealing resulted in precipitation of needle-shaped ZrB and cuboid-shaped Cr-rich, tentatively identified as Cr7C3, PartiCles. Annealing in air led to the precipitation of large ZrO2 particles at the fiber/matrix interface and fine ZrO2 platelets in the matrix. A thin layer of A12O3 was observed to cover the fiber/matrix interface and to envelop the ZrO2 platelets. Diffusion of Ni through the A12O3 layer and its oxidation and subsequent reaction with A12O3 resulted in the formation of NiAl2O4 spinel around the ZrO2 Particles at the fiber/matrix interface. Further annealing resulted in the formation of a thin layer of Cr2O3 on the A12O3 layer and precipitation of A12O3 and Cr2O3 particles within the matrix. These particles subsequently reacted with NiO to form Ni(AlxCr1-x)204 grains. Diffusion of oxygen through the fiber is believed to be responsible for the rapid oxidation of the composite.