Growth stress - Microstructure relationships for alumina scales

Focused monochromatic synchrotron undulator radiation was used to measure stresses in growing α-Al₂O₃ on FeCrAl-based and NiAl alloys during the first few hours of high-temperature air exposures at 1000-1200°C. Selected specimens were then sectioned and examined by transmission electron microscopy to characterize the microstructure of the oxides grown during the in situ stress measurement experiments. In several cases at 1100 and 1200°C, rapid decreases in an initial tensile stress were noted within the first hour of exposure. It was found that differences in high-temperature stress development in the alumina were at least partially reflected in the observed scale microstructures. Relaxation processes limited the magnitude of growth stresses at the higher temperatures such that after a few hours, stresses were typically quite low.