Microstructural development of α alumina scales developed on FeCrAl alloys

A scanning electron microscope (SEM) study was conducted of α alumina scales developed on the surface of FeCrAl alloys during high temperature oxidation, using backscattered electron imaging, cathodoluminescence (CL) imaging, electron backscatter diffraction (EBSD) and X-ray microanalysis. CL variations relate to trace element distributions and to crystallographic orientations. The compositional variations suggest that grain boundary diffusion was dominant in the scale. The α alumina grains are equiaxed at the outer scale surface and columnar between here and the metal oxide interface; both grain types have a very strong crystallographic preferred orientation (CPO): a fibre texture of r axes perpendicular to the sheet. The CPO cannot be explained by substrate control or by plastic deformation. There is a correlation of the orientation of the anisotropic elastic properties of the scale and the likely thermal stress systems: one possible explanation is that the CPO may relate to oriented oxide nucleation during heating.