Applied Stress Effects on the Isothermal and Cyclic High-Temperature Oxidation Behavior of Superalloys

The effects of applied tensile stresses on the isothermal and cyclic oxidation behavior of two nickel-based superalloys, HAYNES^® 75^® and HAYNES^® 230^®, were investigated through measurements of specific mass change, oxide morphology and thickness, degree and depth of internal oxidation, compositional changes in and around the oxide scales, and x-ray diffraction. Here, an applied tensile stress generally led to larger Cr/Ni ratios in the oxide scales, greater chromium depletion in areas directly under the oxide scales, lower specific-mass-change values, and thinner oxide scales. These improvements in oxidation resistance are attributed to a greater concentration of dislocations, defects, etc. in the grain boundaries caused by the applied stress. These defects could act as fast diffusion paths for the Cr atoms to diffuse to the surface, causing a reduction in the duration of the less-protective transient-oxidation period and promoting faster formation of the more protective Cr₂O₃ layer.

Overall, the 230-alloy specimens demonstrated better oxidation and creep resistance than the 75-alloy specimens, likely due to their compositional differences, and the 230 alloy’s larger average grain size. The oxide scales of the 230-alloy specimens were generally thinner and formed at slower rates than those of the 75-alloy specimens. The 230-alloy specimens also demonstrated fewer internal oxidation penetrations than those of the 75 alloy under all conditions.

The effects of the applied stresses on the residual-stress states of substrates and surface oxides were characterized using x-ray diffraction. For both substrates and oxides, residual strains were compressive. Substrate residual strains were smaller in magnitude than those in the oxides due to relative thickness effects between oxide and substrate, and possibly to recovery and creep surface-residual-stress-relief processes in the substrate. For stressed specimens, these processes were possibly enhanced by dislocation creep. The residual stresses in the substrates were still compressive likely due to internal oxidation effects as well as a shallow x-ray penetration depth.