Oxidation of a zirconia-toughened

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(Al_xCr_1-x)₂O₄grains. Diffusion of oxygen through the fiber is believed to be responsible for the rapid oxidation of the composite.