Determination of Oxide Growth Stress by a Novel Deflection Method

A new technique has been developed to monitorthe growth stresses in oxide scales by measuringspecimen deflection. Compared to the conventionaldeflection method, the new technique does not require acoating for protecting one side of the specimen fromoxidation. This new method does not have temperaturelimits and has improved precision for stress measurementin alumina scales. Two alumina-forming alloys, Fe-22Cr-5Al-0.3Y and Fe-22.6Cr4.42Al-0.21Ti,were studied with this method. During oxidation at1000°C in air, the average compressive stresses inthe alumina scales were found to decrease from ~1000 to ~100 MPa with increasing scale thickness on thetwo alloys. The growth stress in alumina scales wasdirectly proportional to the oxide growth rates. Thestresses in the substrate alloys were much lower, on the order of 1 MPa. Creep occurred in thesubstrate alloys and was taken into consideration in thestress analysis. It is believed that creep in the alloysubstrates allowed stresses in the oxide scales to relax. Annealing tests showed that thestresses relaxed much easier in thin than in thickscales.