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 °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 sub- sequent 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)2O4grains. Diffusion of oxygen through the fiber is believed to be responsible for the rapid oxidation of the composite.
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