A STRUCTURAL STUDY OF OXIDATION IN A ZIRCONIA-TOUGHENED ALUMINA FIBER-REINFORCED NIAL COMPOSITE

A composite of NiAl reinforced with continuous zirconia-toughened alum ina (PRD-166) fibers was fabricated by pressure casting. The chemical stability of the composite at 1100-degrees-C in vacuum and air was inv estigated by optical and transmission electron microscopy and energy-d ispersive spectroscopy (EDS). Exposure of the fiber to the molten meta l caused ZrO2 Particles in the fiber to move to the surface, thus perm itting dissolution of ZrO2 into the molten metal. The dissolved Zr rea cted with Al2O3 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 pre cipitation of ZrO2 within the matrix near the fiber/matrix interface. A thin layer of Al2O3 was observed to envelop the ZrO2 particles and c over the fiber. During air annealing, Al oxidized preferentially, ther eby continually reducing the Al content of the beta-NiAl. This caused a progressive change in the microstructure of the matrix from beta-NiA l to pre-martensitic microstructure, to martensitic structure, followe d by nucleation and growth of Ni3Al, to the development of a two-phase microstructure consisting of Ni3Al cuboids dispersed in a disordered alpha-Ni(Al) and, subsequently, the formation of single-phase alpha-Ni (Al). The orientation relationship between Ni3Al and NiAl was {110}NiA l//{111}Ni3Al, [111BAR]NiAl//[011BAR]Ni3Al. Internal oxidation of alph a-Ni(Al) led to precipitation of Al2O3 Particles which subsequently re acted with Ni, in the presence of O, to form NiO . Al2O3 spinel. The N i was oxidized to form beta-NiO. Titanium-containing, platelike precip itates with a {111} habit plane were occasionally observed in NiO. Som e larger NiTiO3 Particles were also formed within NiO. Diffusion of O through the interphase and grain boundaries of the fiber is believed t o be responsible for the rapid oxidation of the composite.