Stresses in alumina scales on high-temperature alloys measured by X-ray and optical methods

The effect of silicon and titanium on the spallation resistance of alumina scales formed on NiCrAlY-type alloys has been investigated using model alloys with different additions of Si or Ti. For this purpose cyclic oxidation experiments were carried out at temperatures between 950 and 1100 °C. For evaluation of the growth and temperature induced stresses in the oxide scales selected samples were analysed by X-ray stress evaluation (XSE) at ambient temperature after various oxidation treatments. The compressive strains in the scales were found to increase with increasing oxidation time, tending to become constant after longer oxidation times. The strains reduced remarkably, when scale damage, such as cracking, started. The developed strains in the oxide scales appeared to be affected by the microstructure of the alloy and the phase distribution in the subsurface depletion layer beneath the oxide. This distribution is affected by the presence of Si and Ti. Apart from the X-ray method, laser Raman spectroscopy (LRS) was calibrated for strain measurement by XSE results aiming at applying this method for stress analysis at higher temperatures.