Residual stresses in alumina scales grown on different types of Fe–Cr–Al alloys: effect of specimen geometry and cooling rate

Abstract

Abstract Residual stress measurements in alumina scales grown on Fe–Cr–Al alloys were conducted using the ruby fluorescence technique and the results compared with common theoretical considerations. The oxidation experiments were carried out at 1200 °C using different Fe–Cr–Al-based materials, different substrate thickness values and varying cooling rates. In case of a conventional wrought alloy the cooling rate significantly influences the residual stress state, whereas in case of a stronger, dispersion strengthened (ODS) alloy hardly any effect is visible. The results indicate at the very beginning of the oxidation process a high oxide growth stress level (up to −1.6 GPa), which does not substantially depend on the alloy type or specimen geometry. The growth stresses relax with time and this relaxation at oxidation temperature is faster in case of the wrought alloy than for the ODS alloy. Additionally, for all studied systems except the 2 mm thick PM 2000 specimens a transition from compressive to apparent tensile growth stresses after longer exposure times occurs. For very thin Aluchrom YHf specimens ( h M = 50 μm) this transition might be explained by nearly total Al-depletion of the substrate material and the resulting decrease of the thermal expansion coefficient. However, the validity of the theoretical calculations based on an ideal flat surface cannot be assured at least for such thin substrates. In fact, geometrical irregularities, a non-even relaxation of the stress or micro-cracking in the scale are likely to affect the calculated residual and/or growth stresses even for thicker specimens.