The effect of high-temperature oxidation on the creep behavior of a superalloy (NIMONIC-105)

Enhanced creep damage occurring in a NIMONIC 105 superalloy at 750 °C due to prior heat treatment at 1150 °C was investigated. The medium of heat treatment and creep testing was air. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) examination indicated that the heat treatment resulted in a surface region consisting of oxide casing, γ′ depleted zone, and grain boundary oxides. Rule of mixture-based mathematical analyses and experiments revealed that this region (referred to here after as the oxygen-affected zone (OAZ)) extended much deeper than that perceived by the SEM and EPMA observations. The formation of OAZ reduced the creep ductility drastically while its influence on the minimum creep rate was low. The probable mechanisms affecting the secondary and tertiary regimes are discussed. Prolonged heat treatments did not damage the creep properties any more significantly than the shorter heat treatments, indicating that all the important structural variations occurred during the initial stages of exposure.