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

The oxidation behaviour of gamma'-Ni3Al in air has been studied by transmission electron microscopy (TEM). Gamma'-Ni3Al single crystals (surfaces parallel to (001)) were oxidized and cross-sections were prepared for TEM examination. After 1 min oxidation simultaneous formation of an external NiO scale and internal oxidation of gamma'-Ni3Al were observed. The internal oxide particles were identified as gamma-Al2O3, which possesses a cube-on-cube orientation relationship with respect to the Ni matrix. After 6 min oxidation a continuous gamma-Al2O3 scale has formed between the internal oxidation zone (IOZ) and the gamma'-Ni3Al single crystal. Oxidation for 30 min resulted in a microstructure similar to the 6 min oxidation, but the Ni in the two-phase zone has been oxidized to NiO. Oxidation for 50 h resulted in a scale consisting of an outer layer of NiO, an intermediate layer of NiAl2O4 and an inner layer of Al2O3. The spinel must have formed by a solid state reaction between NiO and gamma-Al2O3. The gamma-Al2O3 in the inner Al2O3 layer transforms to the thermodynamically stable alpha-Al2O3. A crystallographic orientation relationship exists between both modifications of Al2O3, whereby (0001)[1100BAR]alpha-Al2O3 parallel-to (111)[110BAR]gamma-Al2O3, i.e. close packed planes and close packed directions of alpha-Al2O3 are parallel to the close packed planes and close packed directions in gamma-Al2O3.