HIGH-TEMPERATURE OXIDATION BEHAVIOR OF PURE NI3AL

The high-temperature oxidation behaviour of pure Ni3Al alloys in air w as studied above 1000 degrees C. In isothermal oxidation tests between 1000 and 1200 degrees C, Ni3Al showed parabolic oxidation behavior an d displayed excellent oxidation resistance. In cyclic oxidation tests between 1000 and 1300 degrees C, Ni3Al exhibited excellent oxidation r esistance between 1000 and 1200 degrees C, but drastic spalling of oxi de scales was observed at 1300 degrees C. When Ni3Al was oxidized at 1 000 degrees C, Al2O3 was present as theta-Al2O3 in a whisker form. But , at 1100 degrees C the gradual transformation of initially formed met astable theta Al2O3 to stable alpha-Al2O3 was observed after oxidation for about 20 hr. After oxidation at 1200 degrees C for long times, th e formation of a thick columnar-grain layer of alpha-Al2O3 was observe d beneath a thin and fine-grain outer layer of alpha-Al2O3. The oxidat ion mechanism of pure Ni3Al is described.