In Situ Measurement of Growth Stress in Alumina Scale

n the early stages of growth has been measured in α-Al2O3 (alumina) scales formed on and NiAl-based alloys during heating in air at 1000 to 1200°C. Scale thickness rom 0.5 to 5 μm, times from 5 to 720 min. Stress was measured using the multiple-tilt . In order to measure the thinnest scales at the earliest times, focused, monochromatic tron radiation was used for high intensity, and a fixed, small angle of incidence was ng with an appropriate wavelength to maximize scattering from the film relative to und from the substrate. Depending on the composition, transient tensile stresses of up Pa were observed, with maximum stress at times ranging from >10 h at 1000°C to at 1200°C. Thermal stresses induced by an abrupt temperature change were found to uch more quickly, suggesting that the kinetics observed during isothermal growth dynamic competition between stress generation and stress relaxation. These results e commonly-accepted models of growth stress in scales which predict that a ssive stress will be generated as the metal converts to a larger-volume oxide in a ned location such as an interface. The observed tensile stress may be due to another ism altogether (e.g. grain coalescence), or to the conversion of a transitional Al2O3 to ilibrium α-Al2O3 phase. For one composition, transitional Al2O3 is observed during the f tensile stress.