MICROSTRUCTURAL OBSERVATIONS ON THE OXIDATION OF GAMMA'-NI3AL AT LOW OXYGEN PARTIAL-PRESSURE

Microstructural development during the oxidation of (001)-oriented gamma'-Ni3Al single crystals at 1223 K under an oxygen partial pressure of 4 x 10(-19) atm has been studied by electron microscopy. 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. Pre-thinned foils oxidized for 6 min and studied in plan view in the TEM showed that the scale consisted of 20 nm gamma-Al2O3 grains oriented to the metal such that (111) gamma-Al2O3 parallel-to (001) gamma'-Ni3Al. 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'-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 at the metal/oxide interface were never observed.