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

Abstract

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\(\langle 1\bar 11\rangle _{{\text{NiAl}}} //\langle 0\bar 11\rangle _{{\text{Ni}}_{\text{3}} {\text{Al}}} \). 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 formβ-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.