Effect of SO2 and water vapour on the low-cycle fatigue properties of nickel-base superalloys at elevated temperature

In this study the effect of SO2+water vapour on strain controlled low cycle fatigue resistance of three different nickel based superalloys has been studied at 450 °C and 550 °C. A negative effect was found on both the crack initiation and crack propagation process. The effect increases with increasing temperature and is likely to be influenced by both the chemical composition and the grain size of the material. In general the negative effect decreases with decreasing strain range even if this means that the total exposure time increases. This is explained by the importance of the protective oxide scale on the specimen surface, which is more likely to crack when the strain range increases. When the oxide scale cracks, preferably at the grain boundaries, oxidation can proceed into the material, causing preferable crack initiation sites and reduced fatigue resistance.