A structural study of oxidation in a zirconia-toughened alumina fiber-reinforced NiAl composite

A composite of NiAl reinforced with continuous zirconia-toughened alumina (PRD-166) fibers was fabricated by pressure casting. The chemical stability of the composite at 1100 °C in vacuum and air was investigated by optical and transmission electron microscopy and energy-dispersive spectroscopy (EDS). Exposure of the fiber to the molten metal caused ZrO₂ particles in the fiber to move to the surface, thus permitting dissolution of ZrO₂ into the molten metal. The dissolved Zr reacted with A1₂O₃ of the fiber and formed ZrO₂ particles in some regions at the fiber/matrix interface. Vacuum annealing did not result in any noticeable change in the microstructure. Air annealing led to the precipitation of ZrO₂ within the matrix near the fiber/matrix interface. A thin layer of A1₂O₃ was observed to envelop the ZrO₂ particles and cover the fiber. During air annealing, Al oxidized preferentially, thereby continually reducing the Al content of the β-NiAl. This caused a progressive change in the microstructure of the matrix from β-NiAl to premartensitic microstructure, to martensitic structure, followed by nucleation and growth of Ni₃Al, to the development of a two-phase microstructure consisting of Ni₃Al cuboids dispersed in a disordered α-Ni(Al) and, subsequently, the formation of single-phase α-Ni(Al). The orientation relationship between Ni₃Al and NiAl was {Mathematical expression}. Internal oxidation of α-Ni(Al) led to precipitation of A1₂O₃ particles which subsequently reacted with Ni, in the presence of O, to form NiO · A1₂O₃ spinel. The Ni was oxidized to form β-NiO. Titanium-containing, platelike precipitates with a {111} habit plane were occasionally observed in NiO. Some larger NiTiO₃ particles were also formed within NiO. Diffusion of O through the interphase and grain boundaries of the fiber is believed to be responsible for the rapid oxidation of the composite.