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 th e underlying metal to renewed oxidation. A transmission X-ray diffract ion technique has been used to measure strains in both the scale and t he metal during all phases of the oxidation process. Strains were meas ured in thin Ni foils, 25-125 mu m thick, before, during and after oxi dation at 900-940 degrees C in oxygen. Strains were measured in the at tached oxide scales during and after oxidation. Compressive residual s trains exist in both the oxide and in the metal at room temperature af ter oxidation is complete. Application of the sin(2)(Psi) technique pr ovides 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 stress es exist in the metal prior to oxidation and are likely due to grain t o grain variations in the large-grained samples used in this study. Th is variation may help explain the large residual tensile strains, typi cally 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-consis tency because the measured strains do not represent appropriate averag es for balance of force calculations. Evidence for stress relaxation i n the metal due to creep was observed in the 125 mu m foils.