Microstructural study of the theta-alpha transformation in alumina scales formed on nickel-aluminides

Luminescence spectroscopy and scanning electron microscopy have been used to study the transformation from theta-Al2O3 to alpha-Al2O3 on the surface of platinum-modified nickel aluminide, (Ni,Pt)Al, bond coats on superalloys during initial stages of oxidation at 1,000-1,200 degrees C. The transformation can proceed in a number of ways, depending on the surface roughness, leading to different microstructures of the stable alpha-alumina scale. The use of the luminescence method makes it possible to correlate microstructural features of the oxide with the local stress through piezospectroscopic shifts of both alpha- and theta-alumina optical spectra.