Modeling of residual stress in oxide scales

The magnitude of the residual stress in an oxide scale, and how this varies with temperature, is of major importance in understanding the failure mechanisms of oxide scales. This stress encompasses both growth stresses introduced at the oxidation temperature and thermal-expansion-mismatch stresses induced on heating and cooling, as well as any externally applied stresses or stress relaxation which takes place in the scale/substrate system. Although some of these components are reasonably well understood (e.g., thermal stresses), growth stresses and the relaxation of the total scale stress by creep or fracture processes are much less well understood In this study a model has been developed to predict stress generation and relaxation in oxide scales as a function of time and temperature for both isothermal exposure and cooling to room temperature. The model determines growth stress and thermal-stress generation in the scale and how this is balanced by stresses in the substrate. The substrate stresses ape then allowed to relax by creep and the scale stresses recalculated. This model accurately predicts the room-temperature scale stresses for a range of scale/alloy systems. The model can be used to show how the scale stress depends on oxidation temperature, cooling rate, substrate, and scale thickness. The model predictions are discussed in light of experimental observations for alumina scales on FeCrAlY.