In-situ strain measurements in the Ni/NiO system during high temperature oxidation

Stresses generated during the high temperature oxidation of metals and alloys can cause scale fracture, buckling and spallation, exposing the underlying metal to renewed oxidation. A transmission X-ray diffraction technique has been used to measure strains in both the scale and the metal during all phases of the oxidation process. Strains were measured in thin Ni foils, 25-125 mu m thick, before, during and after oxidation at 900-940 degrees C in oxygen. Strains were measured in the attached oxide scales during and after oxidation. Compressive residual strains exist in both the oxide and in the metal at room temperature after oxidation is complete. Application of the sin(2)(Psi) technique provides an independent confirmation of this conclusion and demonstrates in addition that an equiaxial plane stress state is operative in the oxide, but not in the metal. Substantial variations of residual stresses exist in the metal prior to oxidation and are likely due to grain to grain variations in the large-grained samples used in this study. This variation may help explain the large residual tensile strains, typically 0.4%, that have been measured in the NiO scales during oxidation. Simultaneously, residual tensile strains of approximately 0.08% have been measured in the metal. The condition of balance of forces across the metal/scale interface cannot be applied as a check on self-consistency because the measured strains do not represent appropriate averages for balance of force calculations. Evidence for stress relaxation in the metal due to creep was observed in the 125 mu m foils.