Oxide defects and damage processes in iron-based alumina-formers

In order to examine the nature of incipient defects and damage processes controlling the ultimate breakdown of protective alumina on iron-based alloys, the effects of thermal cycling on scale morphology and adherence were studied for 1200°C exposures that were terminated before the onset of significant spallation. Optical imaging and scanning and transmission electron microscopy were used to compare oxide-metal cross sections from specimens of an oxide-dispersion-strengthened iron aluminide and FeCrAlY that were thermally cycled to those that were isothermally exposed for similar times at temperature. No significant differences in scale and metal-oxide interfacial morphologies/damage were found between the cyclically and isothermally exposed specimens of the respective compositions. These observations suggest that, at least in some cases, the nature of the ultimate scale failure is not set by early-stage oxide or interfacial defect development, but rather by chemical, diffusion, or stress processes affecting interfacial adhesion or by damage that only forms at longer times.