Microstructural observations on the oxidation of gamma-prime-Ni3Al at low oxygen partial pressure

Electron microscopy is used to investigate microstructural development during the oxidation of the 001-plane-oriented gamma-prime-Ni3Al single crystals at 1223 K under an oxygen partial pressure of 4 x 10 exp -19. After 1-min oxidation, TEM cross sections revealed a continuous 4-nm-thick film of gamma-Al2O3 with equiaxed 20-nm protrusions into the metal. Continued oxidation resulted in thickening of the gamma-Al2O3 scale, no grain growth, and the development of a plane metal/gamma-Al2O3 interface. Depletion of Al from the adjoining metal resulted in a well-defined disordered zone of Ni-Al solid solution between the Al2O3 scale and the gamma-prime-Ni3Al. After 5-h oxidation, large randomly oriented alpha-Al2O3 grains nucleated at the metal/gamma-Al2O3 interface, growing inward and transforming from the gamma-Al2O3 outward. The alpha-Al2O3 contained intragranular and intergranular voids. The gamma-Al2O3 exhibited a high density of planar defects and the interface between the gamma-Al2O3 and alpha-Al2O3 contained many voids. Voids were never observed at the metal/oxide interface. 19 refs.