Modeling of Residual Stress in Oxide Scales

Abstract

The magnitude of the residual stress in an oxidescale, and how this varies with temperature, is of majorimportance in understanding the failure mechanisms ofoxide scales. This stress encompasses both growth stresses introduced at the oxidationtemperature and thermal-expansion-mismatch stressesinduced on heating and cooling, as well as anyexternally applied stresses or stress relaxation whichtakes place in the scale/substrate system. Althoughsome of these components are reasonably well understood(e.g., thermal stresses), growth stresses and therelaxation of the total scale stress by creep orfracture processes are much less well understood. Inthis study a model has been developed to predict stressgeneration and relaxation in oxide scales as a functionof time and temperature for both isothermal exposure and cooling to room temperature. The modeldetermines growth stress and thermal-stress generationin the scale and how this is balanced by stresses in thesubstrate. The substrate stresses are then allowed to relax by creep and the scale stressesrecalculated. This model accurately predicts theroom-temperature scale stresses for a range ofscale/alloy systems. The model can be used to show howthe scale stress depends on oxidation temperature,cooling rate, substrate, and scale thickness. The modelpredictions are discussed in light of experimentalobservations for alumina scales on FeCrAlY.